# How To: Rear Fill With Helix DSP



## SkizeR (Apr 19, 2011)

this subject has peaked some interest lately and i have been getting questions from others on how to do it, and i have been asking others about implementing it correctly. Lets discuss all of that here i guess. First things first, this is how to set up L-R/R-L rear fill on the helix processors. credit of course to ErinH, as i learned about this in his build log. I just made this so i dont have to direct everyone to a specific post in there.

1) lets assume im using a pair of full range coaxials in the rear just to make it simple. in the input matrix, set your channels to rear left full, and rear right full. drag and drop the appropriate inputs to the matrix like so..














2) next step is to drag and drop the opposite sides rear signal onto the channels, so your essentially running mono rear rill














3) ok, heres where the l-r/r-l part comes in. lets use the rear left channel output for the example.. since were using the rear left output channel, double click on the rear right input signal thats on its input path. that will pull up this menu. in this menu, click invert polarity. hit ok. do the same thing for the rear right channel, but invert the left input signal. this gives you your l-r/r-l














This is what it should look like when done.















you can also adjust the amount of center cancellation by adjusting the percentages. just double click again and adjust..













This is just how to set up the L-R/R-L rear fill. this isnt the end of properly implementing it. that part, i am still researching. If you would like to share any info on that part, please do.


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## seafish (Aug 1, 2012)

IN for this!!!! Thanks again!!!


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## SkizeR (Apr 19, 2011)

Erin also told me there is a good amount of info on rear fill in our former member werewolf's old posts. i still need to dig them up


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## seafish (Aug 1, 2012)

SkizeR said:


> Erin also told me there is a good amount of info on rear fill in our former member werewolf's old posts. i still need to dig them up


I have those threads bookmarked if you want me to post or send you links!!


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## SkizeR (Apr 19, 2011)

seafish said:


> I have those threads bookmarked if you want me to post or send you links!!


post some links in here for all to read


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## seafish (Aug 1, 2012)

OK...here is one of them from diyma--

Werewolf, ErinH and Gary Summers are all posting in it at some point, so it is a worthwhile, though long read!!

http://www.diymobileaudio.com/forum...n/22523-hey-werewolf-confusion-about-l-r.html


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## seafish (Aug 1, 2012)

And here is a simple Werewolf rear fill post to chew on from another diyma thread, with the entire thread to digest linked after the single post--

http://www.diymobileaudio.com/forum/101740-post34.html

http://www.diymobileaudio.com/forum...uipment-my-car/9806-rear-fill-do-you-use.html


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## Arete (Oct 6, 2013)

:snacks:


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## Arete (Oct 6, 2013)

seafish said:


> OK...here is one of them from diyma--
> 
> Werewolf, ErinH and Gary Summers are all posting in it at some point, so it is a worthwhile, though long read!!
> 
> http://www.diymobileaudio.com/forum...n/22523-hey-werewolf-confusion-about-l-r.html


post # 4 ....

"we covered this. your balenced signal already contains the inverse signal. A balenced transmission consists of an L+ and an L- signal. likewise R+ and R-.

On the recieving end, the device subtracts the two, giving you L+ - L- = 2L. Any noise that was induced into the cable will be done equally to both legs and is also subtracted: N+ - N+ = 0.

cool!

So to achieve the L-R signal, simply take whatever device recieves the Balenced transmission and converts it to standard. Rather than the standard method of wiring, Wire L+ to the L+ input, but Wire R+ where L- would go. When the signals are internall subtracted, you end up with L+ - R+ = L-R

cake"


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## BlackHHR (May 12, 2013)

Nick, from what I understand about the "new" software updates, the delay has been increased to as much as double. It could be as much as 25 to 30 ms. 

This was implemented for those who are wanting to use rear fill.


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## bugsplat (Nov 7, 2014)

Good info. thanks. I was almost there with you until you flipped the polls on each L,R/R,L. I'll have to try that. 

Question.... of course I add delay but I don't touch the EQ for the rears and leave them flat. I've always seen it as just fill and only EQ the fronts. How much time do you spend EQ'ing the rears?


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## SkizeR (Apr 19, 2011)

bugsplat said:


> Good info. thanks. I was almost there with you until you flipped the polls on each L,R/R,L. I'll have to try that.
> 
> Question.... of course I add delay but I don't touch the EQ for the rears and leave them flat. I've always seen it as just fill and only EQ the fronts. How much time do you spend EQ'ing the rears?


I see no reason to not eq them

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## brumledb (Feb 2, 2015)

bugsplat said:


> Good info. thanks. I was almost there with you until you flipped the polls on each L,R/R,L. I'll have to try that.
> 
> Question.... of course I add delay but I don't touch the EQ for the rears and leave them flat. I've always seen it as just fill and only EQ the fronts. How much time do you spend EQ'ing the rears?


Are you band passing the rears? They should really only be playing from ~300-400hz up to ~4,000-7,000. Yes you can eq them. You don't have to spend a ton of time getting them perfect but just eq them to match the curve you are using for your fronts. 

If you have the ability, you should level match and eq the rears just like you would the fronts.


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## bugsplat (Nov 7, 2014)

brumledb said:


> Are you band passing the rears? They should really only be playing from ~300-400hz up to ~4,000-7,000. Yes you can eq them. You don't have to spend a ton of time getting them perfect but just eq them to match the curve you are using for your fronts.
> 
> If you have the ability, you should level match and eq the rears just like you would the fronts.


I do have them band passed. 150hz to 5000hz. They are just cheap $100 sony's I picked up at bestbuy I think +15 years ago. I reused them on this car as rear fill because that actually sound quite nice for cheap coax's. I picked up some nicer coax's but returned them because these Sony's sounded better at 1/3 the price. One of those diamonds in the rough.


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## brumledb (Feb 2, 2015)

Yeah, personally I don't think you have to spend a bunch of money on rears. You should really only be able to even notice that they are on when you are turning your head. I had a pair of Alpine SPR coaxials already so I just utilized those. 


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## .69077 (Aug 24, 2013)

Subbed, thanks!


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## Huckleberry Sound (Jan 17, 2009)

Information!!!


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## oabeieo (Feb 22, 2015)

Didn't realize that would do that. Pretty cool.


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## bbfoto (Aug 28, 2005)

At one point or another I've used the following speakers for this type of rear fill in different installs just because that's what I had on-hand and all have worked really well:

Pioneer TS-A878 Coaxial (surprisingly amazing for $40!)
Fountek FR-88
Eton 3-400/A8/25 MG
SB Satori MW13P-4
Pioneer TS-C520PRS 

I suppose that these would be perfect for this as well, but are probably overkill:

Audiofrog GS-42 and the Illusion Audio Carbon C4CX/C3CX.

You'll need to play around with the mix percentage, level, and delay depending on your install and speaker locations.

Just curious...I've never heard it, but is Mark Eldridge using this technique in the NASCAR?

And Greg, it's great to hear that the new software for the Helix DSPs will allow longer delay times.  w00t


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## rockinridgeline (Feb 2, 2009)

Used this on my son's install this afternoon. Ended up with 19-21 ms delay and fairly significant attentuation (don't remember off the top of my head, but I think 7-9 db.

When listening to music you don't hear the rear speakers, even when turning your head. But if you turn them off, you notice a change in presence. My son plays saxaphone in jazz band and concert band. I didn't tell him what I did with the rear speakers. I just asked him if he could hear the difference between having the rears on and off without telling him I was cycling the rears on and off. All he said was "it sounds bigger". 

I would say that there are some imaging cues that change slightly on information that is panned fair left or far right. Since the rear door coaxials are low, those far left and far right panned sounds probably drop just a bit relative to the rest of the soundstage. In my opinion, it is worth it for the extra presence that you sense. Would I use this for a SQ competition? Probably not. But for pleasurable listening? Absolutely!


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## SkizeR (Apr 19, 2011)

rockinridgeline said:


> Used this on my son's install this afternoon. Ended up with 19-21 ms delay and fairly significant attentuation (don't remember off the top of my head, but I think 7-9 db.
> 
> When listening to music you don't hear the rear speakers, even when turning your head. But if you turn them off, you notice a change in presence. My son plays saxaphone in jazz band and concert band. I didn't tell him what I did with the rear speakers. I just asked him if he could hear the difference between having the rears on and off without telling him I was cycling the rears on and off. All he said was "it sounds bigger".
> 
> I would say that there are some imaging cues that change slightly on information that is panned fair left or far right. Since the rear door coaxials are low, those far left and far right panned sounds probably drop just a bit relative to the rest of the soundstage. In my opinion, it is worth it for the extra presence that you sense. Would I use this for a SQ competition? Probably not. But for pleasurable listening? Absolutely!


try messing with the percentages and see if you can "widen" the canceled center info even more. that might help


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## bbfoto (Aug 28, 2005)

^^ & ^ Cool. I've also found that in general, having the rear fill speakers up higher (at or near ear level) and more on-axis improves the effect and/or minimizes the shifts or changes to imaging cues, though you will need to readjust your mix ratios, levels, and delays to compensate.

I'd still like to know if Mark Eldridge's NASCAR system uses this technique?


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## Hillbilly SQ (Jan 26, 2007)

bbfoto said:


> ^^ & ^ Cool. I've also found that in general, having the rear fill speakers up higher (at or near ear level) and more on-axis improves the effect and/or minimizes the shifts or changes to imaging cues, though you will need to readjust your mix ratios, levels, and delays to compensate.
> 
> I'd still like to know if Mark Eldridge's NASCAR system uses this technique?


I'll be using the factory 2.75" speakers up high in the d-pillars of my Grand Cherokee. Damn near on axis tooWill also connect to the processor running the rears when Erin gets in for a demo in April. I never pass up an opportunity to learn by watching someone else tinker and explain what and why they're doing what they're doing


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## Bnlcmbcar (Aug 23, 2016)

I have DSP Pro and love this discovery for rear fill!

I am wondering if this technique for the Helix can be adapted or modified to extract a center signal from stereo input using the Helix IO matrix options?

Something along the lines of subtracting the L-R difference signal from the mono sum R+L signal.

If my logic is right this would leave a signal with only center information behind??


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## James Cole (Dec 6, 2016)

Bnlcmbcar said:


> I have DSP Pro and love this discovery for rear fill!
> 
> I am wondering if this technique for the Helix can be adapted or modified to extract a center signal from stereo input using the Helix IO matrix options?
> 
> ...


Wondering about this as well...


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## Elektra (Feb 4, 2013)

Nick - I presume the connections to the amp is normal left and right and RCA's are also normal left and right - only how you manipulate them on the software is the main change? 


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## Bnlcmbcar (Aug 23, 2016)

Unfortunately it looks like a no. I asked ErinH about it and he broke it down for me:

ErinH:

If you use L-R and R-L to extract center from combined L+R you wind up with L+R again.

(L+R) - [(L-R)+(R-L)] = L+R - L + R - R + L = L+R


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## WhereAmEye? (Jun 17, 2013)

I have a question. L-R makes sense after reading over some of the rear-fill threads on here. However, R-L= -(L-R). This means that both L-R and R-L are the exact same signal, just completely out of phase, right?

So this means there are no special L/R cues in one signal that isn't in the other. Does this mean we just pick which rear speaker (L or R) gets each signal randomly? Or send the same signal to each rear speaker to keep them in phase, or will keeping them in phase mess something up?


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## Bnlcmbcar (Aug 23, 2016)

From what I gather, they are different. One is a hard right and one is a hard left signal. Focus on step 3 of SkizeR's original post on this thread.


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## WhereAmEye? (Jun 17, 2013)

Bnlcmbcar said:


> From what I gather, they are different. One is a hard right and one is a hard left signal. Focus on step 3 of SkizeR's original post on this thread.




They are both a combination of hard right and hard left, the only difference is the phase of one in particular in each set. Mathematically you cannot say they are composed of different signals because the only difference is phase related.


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## SkizeR (Apr 19, 2011)

In the end, the left speaker is playing hard left, and the right speaker is playing hard right

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## WhereAmEye? (Jun 17, 2013)

SkizeR said:


> In the end, the left speaker is playing hard left, and the right speaker is playing hard right
> 
> Sent from my SM-G920V using Tapatalk




I don't think that's accurate. In the end the left is playing L-R and the right is playing the inverse of L-R.

To be fair, what you said is accurate if you send more of the left signal to the left and more of the right signal to the right. But if you send both signals (L and R) to each rear speaker (L and R) in equal magnitudes (e.g. 50% of one and -50% of the other) then it will result in two equal in magnitude, opposite in phase, signals.



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## Bnlcmbcar (Aug 23, 2016)

I'm still researching and learning about it too but maybe these can help..

I was pointed to this post a while back by another user:
http://www.diymobileaudio.com/forum/4307362-post73.html

Also attached some visuals


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## WhereAmEye? (Jun 17, 2013)

You just confirmed what I was saying. There are no specific L/R cues in one back speaker that isn't in the other. They are mathematically the same signals but out of phase. Perhaps the phase of R-L should be on the right side and L-R should be on the left side, but to say they are unique signals is incorrect.

Make separate L-R/R-L signals, send them to two separate speakers, wire one in normal polarity and one in inverse polarity, and I guarantee they'll be exactly the same.


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## WhereAmEye? (Jun 17, 2013)

From this quote from werewolf it seems like there's only one true "rear fill" signal and that's the difference signal, i.e. L-R. The link he talks about at the end sent me to a page not found 



werewolf said:


> The "L-R difference" signal is really just that : the algebraic _difference_ between the left and right signals. Sometimes it's called the "ambience" or "surround" component of the original 2-channel source.
> 
> Consider a _largely_ mono signal, like voice (in most recordings, certainly not all). A mono signal will have about the same level in the left and right channels ... and will therefore _disappear_ in the difference  So immediately we recognize a HUGE difference bewteen standard, L and R rear-fill and a rear-fill signal constructed from the L-R difference : one will have the main vocal in the rear, the other will not  Perhaps that alone is a big incentive to resist lumping _all_ rear-fill possibilities together?
> 
> Anyway, check the link i posted earlier for more about L-R difference signals, and how to construct and manipulate them _without_ using PL II, Neo:6 or Logic7.


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## Bnlcmbcar (Aug 23, 2016)

I hear you but the point I'm trying to make is that the phase difference is what helps creates that rear fill "image". Penteo, Dolby, DTS, Logic 7 all can't be wrong. They designate some version of R-L and L-R to the corresponding right and left rear surround speakers. There was a published paper by Fosgate himself that Andy posted in the past. If I can find it I'll send it to you.

I'll experiment myself but I don't think sending R-L to both speakers will give the same effect as R-L and L-R to each corresponding rear speaker.


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## WhereAmEye? (Jun 17, 2013)

Bnlcmbcar said:


> I hear you but the point I'm trying to make is that the phase difference is what creates that rear fill "image". Penteo, Dolby, DTS, Logic 7 all can't be wrong. They designate some version of R-L and L-R to the corresponding right and left rear surround speakers. There was a published paper by Fosgate himself that Andy posted in the past. If I can find it I'll send it to you.




That'd be awesome, please do. I was under the impression that rear fill should not have any difference between left and right (basically mono) so as not to mess up the front stage. But I'm just trying to learn like the rest of us 


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## brumledb (Feb 2, 2015)

I think this is a good quote on the topic. It is from this thread: http://www.diymobileaudio.com/forum/technical-advanced-car-audio-discussion/71218-rear-fill-l-r-stereo-mono-question-2.html



Gary S said:


> I'm a little late to this thread, but thought I would set the record straight.
> 
> The rear speakers should be wired out of phase for best results with the L-R signal... so they cannot be localized.
> 
> ...


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## WhereAmEye? (Jun 17, 2013)

brumledb said:


> I think this is a good quote on the topic. It is from this thread: http://www.diymobileaudio.com/forum...218-rear-fill-l-r-stereo-mono-question-2.html




Sweet that's the answer I was looking for. Therefore if you don't need a center then you can do this with a basic helix dsp and not need the dsp pro. Channel 7 for the sub, channel 8 for L-R and just wire the right rear with inverted polarity. Ultimately that's what I was trying to make sure of.


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## brumledb (Feb 2, 2015)

WhereAmEye? said:


> Sweet that's the answer I was looking for. Therefore if you don't need a center then you can do this with a basic helix dsp and not need the dsp pro. Channel 7 for the sub, channel 8 for L-R and just wire the right rear with inverted polarity. Ultimately that's what I was trying to make sure of.
> 
> 
> Sent from my iPhone using Tapatalk




Before I had the Helix Pro I was using the regular Helix. I sent one L-R into the miniDSP that was running the rear-fill plugin and then inverted the right output. The main reason I was using the miniDSP was for the added delay. My rear-fill is located in the bottom of the rear doors which is about the worst spot. 

The only time it ever sounds weird is if I play a test track with distinct LCR. Like on a track where just the L side should be playing, both rears still play. 


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## lasakro (May 21, 2017)

brumledb said:


> ...The main reason I was using the miniDSP was for the added delay....


I just got my DSP.2 installed this weekend after removing my 3 Audiocontrol pieces that it replaced. I've got the IO and Crossovers now set and it is functional. Tomorrow starts RTA. My rear fill is also in my lower rear doors but I'm using the latest software. I think i read that one of the versions increase the delay. How much delay did you find was required with rears located in the less than desirable locations? I understand that 20ms is a good starting point.


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## disconnected (May 10, 2017)

Would this work in IASCA? Two weeks away.


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## SkizeR (Apr 19, 2011)

disconnected said:


> Would this work in IASCA? Two weeks away.


If done right, yeah

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## Gump_Runner (Aug 2, 2014)

SkizeR said:


> If done right, yeah
> 
> Sent from my SM-G950U1 using Tapatalk


Nick, Whats your thoughts on using coax vs (2-way) 6.5-Tweeters for rear fill?


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## SkizeR (Apr 19, 2011)

Gump_Runner said:


> Nick, Whats your thoughts on using coax vs (2-way) 6.5-Tweeters for rear fill?


no need for either, especially components. Just use a midrange. Bandpass from 250-3000 with 12db slopes.


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## Gump_Runner (Aug 2, 2014)

SkizeR said:


> no need for either, especially components. Just use a midrange. Bandpass from 250-3000 with 12db slopes.


One reason I asked is the guy I purchased this truck (crewcab) from drilled holes in the door panels for Tweeters and my thinking was (if it's realistic) to go back with Tweeters to cover those holes. The "kill 2 birds with one stone" logic.


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## disconnected (May 10, 2017)

How many db of attenuation? I don't want to sacrifice my front stage.


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## ominous (Apr 21, 2017)

disconnected said:


> How many db of attenuation? I don't want to sacrifice my front stage.


It will probably vary by vehicle, but my rears are set to -8.5. And it doesn't mess with my front stage at all. 

You can verify how well it's done by simply turning your head. Sit in the driver's seat with your head facing the passenger side door. Your left ear should hear music, while your right ear should barely hear something that's very vague. You get a sense that something is there, but it's diffuse to the point that you can't hear specific details. If your focus is drawn to the rear, simply lower the rear speaker levels.

I like the effect. It provides a sense of space without drawing attention to the rear speakers. Without it, your attention is drawn to the fact that there is nothing there, just a huge void (to my ears, it sound's incomplete).


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## e46SQ (Apr 18, 2016)

Ok I guess it's too late in this thread to ask this but why would you do l-r/r-l rear fill :blush: ? I mean what is the audible difference you will get if you do this instead of just putting the rear l/r signals straight? This seems kinda interesting to me but can't understand its effect.


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## flgfish (Jan 17, 2019)

This thread's a little old, so I'll apologize for going all Necromancer now...

So I have a Helix DSP input question. Does the DSP need both front and rear audio inputs to set up rear fill? IE, with a factory radio with FR, FL, RR, RL outputs, do you really only need to use the FR and FL, or do you need to use all 4?

I assume the answer is only the front 2, and that'll you'll lose front/back fading on the head unit, but hoping someone who has used a Helix can confirm.

Thanks!


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## SkizeR (Apr 19, 2011)

flgfish said:


> This thread's a little old, so I'll apologize for going all Necromancer now...
> 
> So I have a Helix DSP input question. Does the DSP need both front and rear audio inputs to set up rear fill? IE, with a factory radio with FR, FL, RR, RL outputs, do you really only need to use the FR and FL, or do you need to use all 4?
> 
> ...


you can still do it with only 2 inputs. You just wont have fader control from the radio


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## Nik_Ind (May 24, 2017)

flgfish said:


> This thread's a little old, so I'll apologize for going all Necromancer now...
> 
> 
> 
> ...


Hi! I use a Helix V Eight, and only use the front two inputs. They're the only ones available on my factory HU, actually! So, I didn't really have a choice. But you rightly mentioned that I lost the fader control to the rear from the HU.

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## flgfish (Jan 17, 2019)

Thanks, folks. Appreciate the fast response.

I'll be doing this with a Helix V Eight DSP. Tweeter, 4" and 8" in the front, and 4" coaxials in the rear. The radio has FR, FL, RR, RL outputs (line level), but the only reason I can think to use all 4 is to keep fader control... which I'm pretty sure I've never used in a car (since my children were infants anyway).

Looking forward to doing the install & playing with the toys.


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## SkizeR (Apr 19, 2011)

if you have the option to do it, might as well just so you have the flexibility. its just a few extra wires


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## rockinridgeline (Feb 2, 2009)

flgfish said:


> Thanks, folks. Appreciate the fast response.
> 
> 
> 
> ...




Rear fill in your 911 is interesting and fun to listen to but a different experience altogether than when your rear fill is farther away. In my Jeep the rear fill is high and in the rear and it does some really cool things to create a sense of space. 

In the 911 you might find that you wish you could delay the rear fill more. Since the speakers are low and close to you they give more of a headphone effect than a sense of special ambiance. 

For competition I wouldn’t use rear fill on the 911 because it messes with the soundstage on info that was recorded hard left or right. However I will say that driving down the road I really enjoy the effect and sense of having the stage wrap around and envelope me. 

Just wanted you to know what to expect.


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## flgfish (Jan 17, 2019)

rockinridgeline said:


> Just wanted you to know what to expect.


That makes sense. Appreciate you sharing your experience. In the beginning, I wasn't going to bother with the rear fill; I actually screwed up, thought I could run the 8's bridged, but I can't on that amp. 75W should be fine with the 8's I chose, so I had two extra channels... and so I ended up with this rear fill possibility. 

I'm not sure how much time delay you can do in the Helix DSP; I should go look at that in the software. I'd think it would have plenty of range to move them back pretty far in time, if I'd like to though....


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## rockinridgeline (Feb 2, 2009)

I forget the setting, but I have the delay at the maximum on the Helix. Like I said, I really enjoy it. If I were to compete, however, I would turn it off because it will take midrange information that is panned hard left or right in the recording and pull it towards the back. What you end up with is sonic cues that place things midway between the front speakers and the rear. It is almost like having headphones on but with much better up front center staging than what headphones would offer.


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## preston (Dec 10, 2007)

I recently finished a big install in my crew cab Sierra. Always planned to do rear fill but didn't follow through because I felt like I'd have to go out and buy a Helix Pro to get 10 channels instead of teh 8 on my Helix basic.
The stock head unit is feeding the helix via a NavTV interface box, but I had left the rear door speakers connected to the deck. If I took the fader off of full front, it would bring them into play. Although there was a lot of problems with the sound, I still found that I kept turning them on and off, abck and forth because I have always liked the ambience of rear fill. 


Had to take some stuff apart to install a Director so I ended up putting a little class D amp in the console where the factory "subwoofer" used to go, and then I pulled the doors apart and did the full soundproofing and built baffles and replaced the truly crappy OEM (Bose) speakers with some Legatia L4's I had laying around. It actually turned into quite a project. 

I had been summing two channels to the sub, so i changed that over to a mono feed and I had one mono feed left for rear fill. I know its not the proper L-R, and R-L processing but like I said I've always enjoyed rear fill of any sort. I put the maximum delay I could on it - in my system the longest delay is about 5ms, and the most I could put in the rear fill is 15mS, so the overall delay is only about 10ms. I went ahead and set up the mic and level set them and even EQ'ed them, and run them from 350Hz and up. 

So - mono, not differential. Low in doors, fairly close to the front seat and completely off axis. Less than ideal compared to the theory. 

And - I absolutely love it. Adds so much "space" to the sound with no loss of cohesion. Stage isn't necessarily wider or deeper, its "thicker". Ha ha stupid audiophile words but that' how it sounds to me. Instruments are richer and take up more individual space. It really made my favored jazz combos feel more "there" and orchestra stage is vastly improved. I spent about 3 hours just raising the rear volume up and down and muting the rears on and off and 100% enjoyed it more with rear fill even if I'm not following all the Hasselhoff principles (ha don't remember the actual name). And needless to say it is a huge improvement over whatever the oem speakers were doing, and another upside the rear passengers are getting a much nicer sound as well.


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## dobslob (Sep 19, 2011)

preston said:


> I recently finished a big install in my crew cab Sierra. Always planned to do rear fill but didn't follow through because I felt like I'd have to go out and buy a Helix Pro to get 10 channels instead of teh 8 on my Helix basic.
> The stock head unit is feeding the helix via a NavTV interface box, but I had left the rear door speakers connected to the deck. If I took the fader off of full front, it would bring them into play. Although there was a lot of problems with the sound, I still found that I kept turning them on and off, abck and forth because I have always liked the ambience of rear fill.
> 
> 
> ...


L-R and R-L are the same thing signal wise, run two speakers to that channel and run one of them with reversed polarity and you should have a great effect channel. I have been playing with this a bit lately in my Audi. Not ready for demos yet, but I will have several different presets stored in the Director soon.


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## preston (Dec 10, 2007)

I was going to start arguing with you but I actually see you're right about that. I could set the channel in the Helix to be L-R instead of L+R (mono). Then if the right speaker is wired in reverse polarity, it should be R-L. 

Unfortunately with the amp buried in the console and the speakers behind the door cards and being fed a mono signal, its not a trivial experiment to try, but I'll give it a go. I'd be very curious if I can hear a difference or if it would be better. I know it wouldn't be better for the rear seat passengers though.

In my last vehicle I ran them in series off of a mono channel with one wired out of phase to the other. Then, if I flipped polarity on the DSP for that channel, they were the opposite ! It was one of those cases where I would think, well I can hear a difference but I can't say one is better than the other. Also it seems to be in that case that one of them is always out of polarity with the rest of the system.


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## troy.lawrence.58 (Jan 9, 2019)

I'm new here guys so here is the info on my build. I will be a bit long winded until I talk about my rear fill experience because I want you all to know everything about my system. About a month ago I had a shop install a Linkswell T style radio in the dash of my 2017 F250. I immediately noticed a huge difference in sound quality over my stock Sony radio. 

Two weeks ago I returned to the same shop in which they installed Focal KX2 6.5 components in the front powered by a Helix P Six Dsp Mk2. I also had the cover both of the front doors with Soundskin as well as the using the soundskin speaker cover. In addition, under my rear seat they installed 2 10 inch Focal Flax P25 subwoofers(inside of a Foxbox) powered by a JL Audio HD 600/1 amplifier. Initially I had a JL Audio JX1000/1D but was told that would have been waaaaay too much power so I decided to return the JX1000/1D and utilize the HD 600/1. 

Ok initially I wasn't impressed with the sound quality of the Focal Speakers so I was told give them a week or two so they could break in. In addition, I went along with their tune and never really tweaked anything. 

REAR FILL EXPERIENCE:

Ok.........after the last install, I was told that until my speakers(Focal EC 165 coaxial) for the rear doors arrive, they would turn off my rear factory Sony speakers from the Helix DSP because the factory speakers will mess up my sound stage. So up until this past weekend I wasn't impressed with the midbass coming from the front speaker as they sounded too flat. So, I decided to use my laptop to access the Helix DSP and play around with the eq settings. After reading numerous different post and forums, I took my time and began adjusting some settings one speaker at a time and they began to WAKE UP! Then I decided to turn on my rear Sony stock speakers from the Helix and I was shocked as to how good they made my system sound. After spending several minutes adjusting their EQ settings and I was floored as to how well they sounded! 

Afterwards, I spent time turning off and on the rear speakers to make a decision for myself as to the rear speakers messing up the sound stage. WOW, THEY SOUND SOOOOO DARN GOOD WITH THE REAR SPEAKERS! It was a very noticeable difference in sound quality. I had several people listen as I turned on and off my rear speakers and everyone quickly agreed that the rears made a huge difference. Keep in mind these are the stock rear speakers because my Focal's for the rear will not arrive until today. But Holy Moly...........I'm glad I turned those rear speakers on and tuned them as well as the front speakers! I can only imagined what the system will sound like once I install the Focals in the rear. 

I can't thank this website/forum enough for some of the good information that I have acquired over the past 2 weeks! 

BTW, I don't have any echoing as reported by some and I didn't have to toggle with the Time Alignment settings for the rear speakers. Not sure if the installation shop previously set all of that up before turning off the speakers from the DSP. And yes my fader works as it should. I must say that the shop did an awesome job with everything.

Sent from my SM-N950U using Tapatalk


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## Nik_Ind (May 24, 2017)

Okay! So, I tried doing the l-r/r-l with 50-50% and opposite outputs inverted approach as mentioned here. Muted all other channels, and only kept the rear channel open. I found that this makes music sound as if I'm using a 3" inch mid-range, band-passed from 700hz to 8000hz, and fairly hollow. Is this how it's supposed to sound? On the contrary, if I simply use the opposite inputs (Rightin-Left Out, vice-versa) at 100% without inverted polarity for the rear channel, it sounds much better.

Am I doing something wrong here? Details of my setup as follows:

Helix V Eight DSP.
Focal 165 Coaxials crossed at 80hz HPF till 8000hz LPF. Time-aligned using distance, and running attenuated at -4db.

I have only two inputs available from the source unit.


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## rockinridgeline (Feb 2, 2009)

keep in mind that the purpose of rear fill as described here is to create an ambiance effect or try to recreate a sense of room reflections. Using this technique the only information that comes out of rear fill is information that isn't part of the center of the soundstage, in other words it is part of the recording that you wouldn't hear coming out of the middle of the soundstage. It is going to sound diffuse and hollow because that is what echo or reverberation will sound like.

If you do it the way that you described changing it, the rear speakers will sound better if you are listening to them alone. However, the effect will be that your front soundstage will be pulled towards the back and you will lose focus and detail on the front soundstage.

From a competition standpoint this would be the last thing you would want to do. However, if you like the sound to wrap around you and don't care about the soundstage being accurately created in front of you, then go for it! It is a matter of what you want from the system.


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## Nik_Ind (May 24, 2017)

rockinridgeline said:


> keep in mind that the purpose of rear fill as described here is to create an ambiance effect or try to recreate a sense of room reflections. Using this technique the only information that comes out of rear fill is information that isn't part of the center of the soundstage, in other words it is part of the recording that you wouldn't hear coming out of the middle of the soundstage. It is going to sound diffuse and hollow because that is what echo or reverberation will sound like.
> 
> 
> 
> ...


Thank you! That was just the answer I was looking for 

Sent from my BBF100-6 using Tapatalk


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## audiocholic (Dec 5, 2016)

Hi Guys quick question,



is this by anymeans possible off a head unit rather than the dsp? I mean consider the head unit can bandpass say for example 300-3000hz 12db slope and has 20ms time aligment,can obviously attenuate also this leaves the L-R/R-L arrangement to be done.

but can this be done physically by the way we wire the speakers? If so can someone help me understand how  ?


basically I plan to sell my P six and bought a Helix Dsp mini but this is only 2 way front,since I already have a Pioneer DMH-2550Nex head unit I can perhaps use the 2 channels output (front channels) from the headunit into the dsp for full 2way front

and then the 2 channels rear out from the head unit (hi level) bandpassed (network mode) and time aligned to the rear speakers as rear fill but want to know if I can get that L-R/R-L arrangement or how ever its written


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## rockinridgeline (Feb 2, 2009)

No way to wire the speakers to get the differential signal that you are looking for and no HU that I am aware of that will do it either. Why selling the P Six if it does what you want? Personally I've been down the road of rear fill in two high end builds. I've ended up doing away with it because I think it smears the image. Also, it sounds terrible for rear seat passengers. When a good SQ system is set up properly there should be plenty of ambient information but YMMV.


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## audiocholic (Dec 5, 2016)

rockinridgeline said:


> No way to wire the speakers to get the differential signal that you are looking for and no HU that I am aware of that will do it either. Why selling the P Six if it does what you want? Personally I've been down the road of rear fill in two high end builds. I've ended up doing away with it because I think it smears the image. Also, it sounds terrible for rear seat passengers. When a good SQ system is set up properly there should be plenty of ambient information but YMMV.



thanks for the reply mate!

to be honest I only need and want 2way front and I'am selling the P six for financial reasons, It goes for around 850-900 dollars here used,I got the helix mini dsp inc hec usb for dirt cheap at roughly 230 usd as a demo unit from a friend who is a dealer.

I also got the sony hi res amps GS4 & GS100 mono for a total 200 usd so for 430 usd I now have a full 3 way setup complete and can keep the extra 420-470 usd for myself.


though I'am worried as I'am getting very very odd user reviews/reports of the GS4 (GS400) amp as some state its crazy good and performs excellent especially for the money while others state it performs no where near a high end amp like the P six both in terms of staging,sq and especially output while the other bunch who stated it was good reported back that this was BS and that it has plenty balls to perform enough unless Iam in search for crazy SPL


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## rockinridgeline (Feb 2, 2009)

You are welcome. Ok, makes sense. Did you run rear fill with the P6 before? I didn't explain in my previous email that the reason you can't do it with wiring is that the L-R signal is only playing the information that is specific to one side and none of the center info. It takes DSP to do that. I've heard competition vehicles with rear fill that I really liked. It sounded pretty cool in my Jeep but with certain music I felt like I got unwanted changes to the imaging. For example, strong ambient info on loud piano passages would pull the image toward me instead of it staying put over the dash. On the other hand, when I had it in my old 911 the rear speakers were so close that it created a bit of a wraparound headphone effect. That was cool driving down the road, but from a competition perspective it would probably cost you points.


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## LBaudio (Jan 9, 2009)

you need to bandpass midranges heavily and dont use tweeters on Rear fill + they need to be heavily delayed - 50-60ms. also output level should be very low so they dont pull image to the rear.


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## Ge0 (Jul 23, 2007)

SkizeR said:


> Erin also told me there is a good amount of info on rear fill in our former member werewolf's old posts. i still need to dig them up


Look at my posts from way back. I was all over this subject. I provided tons of experiments and data. I knew Werewolf. We were good friends. We learned together. I learned prior to that from some odd ball at DIYMA at the time. I can't quite remember his tag but respected his research.. He had a a lot of crazy ideas that just worked. The truth is that this rear fill method makes a 2 channel stereo setup absolutely rock. It livens your listening space to make it seem more realistic without impacting your front stage.


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## Ge0 (Jul 23, 2007)

LBaudio said:


> you need to bandpass midranges heavily and dont use tweeters on Rear fill + they need to be heavily delayed - 50-60ms. also output level should be very low so they dont pull image to the rear.


Yep. Trust me as an SQ award winner back then. Band-pass your rear fill steep (24dB/octave) on the lower end (200 --> 300Hz) and high-pass with a shallow slope (6dB/octave). A mid-range is perfect for this task. Don't need mid-bass or tweeters.

The L-R / R-L stuff just cancel's center channel info that could confuse your frontal sound-stage.

Lower your gain to the point where your stage is not drawn backward. Delay time carefully. To little time delay the listening space seems small an un-lively.. Too much time delay and it starts to sound too big with echo. Balance it right and it will open the borders of your vehicle. No more feeling like you are listening to music in a small box.

If implemented properly this rear fill method should increase room size. Not create special effects. Use it in moderation and you'll see.

FYI. I also use this method in my home systems to bring more life to 2 channel music. I like it that much.

Ge0


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## Mauian (Jul 25, 2019)

Just curious how you guys are doing time alignment with a dsp like the helix p six. I have entered in the actual measured values (in inches), so when I try to delay the rear speakers by 20-30ms I “run out of room” within the helix software and I can’t set the distance of the rear speakers any closer than zero inches away (which results in ~6ms delay). 

Hopefully that makes sense. But are you guys entering in delay values NOT by distance in order to get a delay of the rears that is 20-30ms?

thanks!


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## Ge0 (Jul 23, 2007)

rockinridgeline said:


> keep in mind that the purpose of rear fill as described here is to create an ambiance effect or try to recreate a sense of room reflections. Using this technique the only information that comes out of rear fill is information that isn't part of the center of the soundstage, in other words it is part of the recording that you wouldn't hear coming out of the middle of the soundstage. It is going to sound diffuse and hollow because that is what echo or reverberation will sound like.


Absolutely Rockinridgeline, you nailed it!!! 

L-R rear fill can be used to mimic a larger listening area inside your car (or small listening space). Think of the delayed and attenuated signal as sound you would hear if it passed you then bounced off walls at a larger distance behind you and then reflected back. There is not a lot of distinct info in reflections (vocals or instruments). They are there due to a side effect of physics. Your brain is trained to decipher this delay and attenuation as being in a certain sized listening space. You can artificially make your listening space seem larger / more alive by implementing these few changes.

Read On...

Ge0


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## Ge0 (Jul 23, 2007)

Mauian said:


> Just curious how you guys are doing time alignment with a dsp like the helix p six. I have entered in the actual measured values (in inches), so when I try to delay the rear speakers by 20-30ms I “run out of room” within the helix software and I can’t set the distance of the rear speakers any closer than zero inches away (which results in ~6ms delay).
> 
> Hopefully that makes sense. But are you guys entering in delay values NOT by distance in order to get a delay of the rears that is 20-30ms?
> 
> thanks!


First of all. Read through this thread carefully. You don't want to set your rear fill closer to you with time alignment. Zero inches? You want to push it back by delaying it. Make it 10 or 20 ft. behind you and listen.

Ge0


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## Ge0 (Jul 23, 2007)

LBaudio said:


> you need to bandpass midranges heavily and dont use tweeters on Rear fill + they need to be heavily delayed - 50-60ms. also output level should be very low so they dont pull image to the rear.


Yep . Experimentation with signal amplitude and time delay is highly recommended.

Ge0


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## Ge0 (Jul 23, 2007)

rockinridgeline said:


> No way to wire the speakers to get the differential signal that you are looking for and no HU that I am aware of that will do it either. Why selling the P Six if it does what you want? Personally I've been down the road of rear fill in two high end builds. I've ended up doing away with it because I think it smears the image. Also, it sounds terrible for rear seat passengers. When a good SQ system is set up properly there should be plenty of ambient information but YMMV.


Come on RRL. Who sits in the back seat anyway besides kids who wear their damn headphones watching their phone. They don't care about your sound quality. If you really nail L-R rear fill it will do nothing but enhance the front stage.

The only reason to implement full range rear fill is to implement a full 5.1 channel surround system. However, this concept died years ago due to the .MP3 and streaming service infestation.

Straight 2 channel front stage without rear fill sounds nice in a car but not realistic / "life like". You need a little artificial physics to help.

Ge0


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## Ge0 (Jul 23, 2007)

audiocholic said:


> Hi Guys quick question,
> 
> is this by anymeans possible off a head unit rather than the dsp? I mean consider the head unit can bandpass say for example 300-3000hz 12db slope and has 20ms time aligment,can obviously attenuate also this leaves the L-R/R-L arrangement to be done.
> 
> ...


Does your head unit have speaker level outputs or only line level?

This can be done to a limited extent if your head unit has speaker level outputs. However, an important part of this solution includes time delay which you probably won't have access to.

Ge0


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## Mauian (Jul 25, 2019)

Ge0 said:


> First of all. Read through this thread carefully. You don't want to set your rear fill closer to you with time alignment. Zero inches? You want to push it back by delaying it. Make it 10 or 20 ft. behind you and listen.
> 
> Ge0


Haha. Yep. I’ve read the thread. The way the helix processor interprets distances is relative to the speaker that is farthest away (the subwoofer). So, in order to delay the speakers more than the front stage (which is closer to me), they have to be set closer than the front speakers (so that they are delayed more); as the helix processor thinks you are trying to have the sound arrive at the same time (like you normally would in time alignment).

does that make sense?

I know what you are saying...of course I want the rear to seam like they are farther away but in “distance mode” in the helix if I actually set them to be the farthest speaker they are delayed the least.


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## Ge0 (Jul 23, 2007)

Mauian said:


> Haha. Yep. I’ve read the thread. The way the helix processor interprets distances is relative to the speaker that is farthest away (the subwoofer). So, in order to delay the speakers more than the front stage (which is closer to me), they have to be set closer than the front speakers (so that they are delayed more); as the helix processor thinks you are trying to have the sound arrive at the same time (like you normally would in time alignment).
> 
> does that make sense?
> 
> I know what you are saying...of course I want the rear to seam like they are farther away but in “distance mode” in the helix if I actually set them to be the farthest speaker they are delayed the least.


That gave me a minor headache reading a few times. But, I finally grasp the concept 

Ge0


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## seafish (Aug 1, 2012)

FYI all DSPs (not just the Helix) by nature need to set time alignment of channels relative to whichever speaker is farthest from the listening position, (which as you mentioned in a "classic" install is the rear subwoofer) which then is the "zero delay" channel relative to all the other channels. IF you happen to have other speaks that are further away from the listening position then the subwoofer, those speakers would then be become the "zero" delay channel.

Rear fill wants to be delayed at east an additional 20ms from all the other channels in order to be effective in adding ambience. As you not4d, most DSPs do NOT have enough ms delay to achieve that. One way to do so is to use an additional DSP to achieve the delay. As others have pointed out, rear fill channel info also should be band passed and at the very least contain only mono information (though using proper steering algorithms is even better).


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## jtrosky (Jul 19, 2019)

That is why I just use the "Delay Mode" in the Helix (instead of "Distance Mode"). In delay mode, you literally just set how much _actual_ delay you want for each speaker. 

I use EricH's web site to convert distances to delay times (https://www.erinsaudiocorner.com/timedelaycalc/)

This way, if I want to delay my rear speakers 20ms, I just actually set the delay time to 20ms in the Helix. Easy that way.


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## Bnlcmbcar (Aug 23, 2016)

seafish said:


> As you not, most DSPs do NOT have enough ms delay to achieve that. One way to do so is to use an additional DSP to achieve the delay.


This is where the virtual channel processing on the Helix Ultra come in handy.

With virtual channel processing, users can set up to 20.82ms of delay on desired output channels. Then in addition, user can add more delay on top of that on the virtual channel (up to an additional 20.82ms).

So Helix products with VCP features (just Ultra right now) can technically have up to 41.64ms delay.


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## Mauian (Jul 25, 2019)

jtrosky said:


> That is why I just use the "Delay Mode" in the Helix (instead of "Distance Mode"). In delay mode, you literally just set how much _actual_ delay you want for each speaker.
> 
> I use EricH's web site to convert distances to delay times (https://www.erinsaudiocorner.com/timedelaycalc/)
> 
> This way, if I want to delay my rear speakers 20ms, I just actually set the delay time to 20ms in the Helix. Easy that way.


Ah! Awesome. I was hoping this was the way to do it. Glad to hear this is how you’re doing it. I’ll give it a shot soon. Thanks!


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## Picassotheimpaler (Sep 21, 2014)

Nothing like resurrecting a dead thread, but if you have a head unit that also has TA you can use rear channels outputs to add an extra amount of delay. Then stack that with the Helix delay to get more than the 20 that the helix pro offers.


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