# MiniDSP: Intelligent Rear Fill Plugin



## highly (Jan 30, 2007)

This thread is a result of the discussion going on *here* about the use of intelligent rear fill to increase stage width and provide ambiance. I've posted in the _Space: the Final Frontier_ thread and linked back to this thread in order to minimize threadjacking the original thread. 

The *MiniDSP* is a small form factor standalone DSP module with a $99 price point. It has two analog inputs and four analog outputs, and is configured through a $10 plugin that defines its functionality. I've posted a request to the MiniDSP forum about a plugin for intelligent rear fill, and was thrilled to see the interest it immediately generated from the developers. 

It is hoped that this thread will generate discussion on the idea. I request that we limit posts to the MiniDSP forum thread to encouraging posts. If there is a need to change the requested specs for the plugin to accommodate a wider user base (or because I've just got it all wrong...!), I submit that we make that decision HERE and post ONCE to the MiniDSP forum with that request. Posting 10 different "it should be done THIS way" threads on the MiniDSP forum can only serve to cloud the issue and would likely reduce confidence that such a plugin would have a worthwhile user base.

I hope this turns out to be a worthwhile opportunity for both the MiniDSP devs and the DIY community. The board seems to offer an extremely attractive package and would likely be a very good fit for this application. 

-Todd


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## highly (Jan 30, 2007)

Kinda surprised to see over 100 views and nobody has anything to say about this. Given the large number of threads here lately about rear fill I expected a little more of a response to an inexpensive, intelligent rear fill option for the DIY community!

The engineers are looking over early feasibility studies of the processing chain I offered and have identified some overhead issues. My proposed chain looked like this:










But due to processing limitations, this streamlined chain was proposed:










This provides most but not all of the capabilities of the original chain concept. Specifically it lacks the ability for EQ on a per-driver basis. I do not know how imperative this is in a rear-fill application. Since the signal will be bandpassed, however, we won't need 31 bands of EQ. If we go with 200-3KHz, 12 bands should do it. I posted asking if this might allow us to have EQ on both channels and still meet the processing goals. I'm awaiting a response at this point. 

Either way, it looks like this could be just what the doctor ordered for an intelligent rear fill setup on a very reasonable budget.

-Todd


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## lycan (Dec 20, 2009)

highly said:


> Kinda surprised to see over 100 views and nobody has anything to say about this. Given the large number of threads here lately about rear fill I expected a little more of a response to an inexpensive, intelligent rear fill option for the DIY community!
> 
> The engineers are looking over early feasibility studies of the processing chain I offered and have identified some overhead issues. My proposed chain looked like this:
> 
> ...


dude i absolutely love it!

Plenty of EQ for rear-fill, ability to do some "differential delay" between the two rear channels. My only comment is that you might not need final inversion between the two channels, since you have the differential delay.

In more detail, we often think (L-R) for one rear channel and (R-L) for the other. These are, of course, really the same signal ... but the inversion provides some degree of "decorrelation". The other easy way to get "decorrelation" between two channels, that are really based on a single difference signal, is by using different delays between them. What you have described allows BOTH ... which is great .. but we may want to turn off the final inversion, as we play with the differential delays. Make sense?

As a side note, another way to create decorrelation from a "mono" signal (like our difference signal) is to perform a fixed 90 degree phase shift. But let's not ask the minidsp guys to build us a Hilbert Transformer just yet


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## highly (Jan 30, 2007)

lycan said:


> dude i absolutely love it!


Precisely what I was hoping for!
Thanks for chiming in. The Lycan Seal of Approval is greatly appreciated as I am going on theory (and not practical real-world experience) in my requests to them. Knowing that I am on track with that helps keep me confident that I am not wasting their time!

Thanks again!
-Todd


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## MarkZ (Dec 5, 2005)

Looking good. I wonder how much more difficult it would be for them to provide a center channel option too? Change the L-R to L+R and you're in business. As I said elsewhere, there's a very simple low overhead (and free) VST plugin from Voxengo that does exactly that -- L+R on one output and L-R on another output.

Which makes me wonder if the MiniDSP folks could introduce VST support, or if that would undermine their business strategy...

Anyway, I don't want to muddy up your thread.

Edit: speaking of, the VST plugin is here. If any of you want to actually know what an L-R signal _sounds_ like, to get a better appreciation for how it can be useful for ambient fill, you can load up that VST plugin through a VST host and play some music. Both winamp and foobar2000 have VST host plugins. [I've only used the foobar2000 one -- foo_dsp_vst I think it's called]


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## highly (Jan 30, 2007)

I am sure everyone involved wished it was that simple! I seriously doubt that a VST plugin holds anything in common with the plugins this runs besides the name 'plugin'. 

I don't know what your background may or may not be here (embedded development), so please take this as informational and not in any way belittling (things can come across all wrong online!). The MiniDSP's 'plugin' is (simplified) an assembled firmware that defines the operating parameters of the silicon DSP onboard, provides an operating system for the onboard MCU, and provides an interface to the computer via the MCUs USB bus to access the MCU/DSP operating parameters. Though the VST and DSP may have similar algorithms for actually performing the signal processing, the DSP code would be very low level and in the silicon's 'language'; nothing like what would run an an X86 architecture. 

Admittedly, I do not know the specific architectural framework for the DSP, but I have completed many (homebrew) projects with Atmel AVR MCUs in Assembly and have a reasonable grasp of the general complexities involved. I'm by no means anything LIKE an expert!

From my discussion thus far, I would think that a center channel with its associated crossover, equalization, and time alignment would severely overtax the available resources on the DSP. A L+R signal is electrically simple to create, and a current plugin running on a dedicated MiniDSP would meet all of your requirements for center channel processing if L+R was all you were after. Just not likely to exist in the same plugin as Rear Fill.

-Todd


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## MarkZ (Dec 5, 2005)

Honestly, I hadn't been following the MiniDSP threads that closely. I wasn't sure what it was doing. Thanks for the explanation.


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## mosconiac (Nov 12, 2009)

I would find this a convenient way to derive the 2-octave signal (80-320Hz) Lycan presented in this thread (to enhance soundstage width):

http://www.diymobileaudio.com/forum/diy-mobile-audio-sq-forum/74088-midbass-arrays-revisited.html

In that case, I would hope the frequency range of the bandpass would accomodate. Also, the EQ requirements would be minimal.

THANK YOU FOR TAKING ON THIS EFFORT! It was brilliant to engage the miniDSP folks.


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## pionkej (Feb 29, 2008)

I had actually mentioned doing this in my build log I just started with the MiniDSP and didn't know a thread had been started about it. Maybe I am wrong here, but is any of this programming needed really?

What I had planned to do if I bought one of these to get l-r, was flip the phase on one output channel, wire my speakers bridged as Amp+/L+ L-/R+ R-/Amp- (this example is for a 4 channel amp that inverts one input signal when bridged). Then just use the T/A to delay and the crossover to bandpass. I think the 31 band EQ is per output pair but the PEQ is per channel so it could be used to shape the sound from each speaker. I was planning on using the other two outputs to power my tweeters as they normally would have been actively off my P01.

It would be great if they already had a pre-made solution, but if it doesn't come through, would the scenario above work as well? Just curious because I am still on fence about picking one up for that purpose in a couple months, and if my logic is wrong, then I won't be getting one unless they make a "rear fill" version.

Thanks.


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## highly (Jan 30, 2007)

pionkej said:


> What I had planned to do if I bought one of these to get l-r, was flip the phase on one output channel, wire my speakers bridged as Amp+/L+ L-/R+ R-/Amp- (this example is for a 4 channel amp that inverts one input signal when bridged). Then just use the T/A to delay and the crossover to bandpass. I think the 31 band EQ is per output pair but the PEQ is per channel so it could be used to shape the sound from each speaker. I was planning on using the other two outputs to power my tweeters as they normally would have been actively off my P01.
> 
> Thanks.


Well, I think the T/A of the current miniDSP only goes to 7.5ms (not certain, but believe I saw this somewhere). I also thought that you needed balanced line levels to create the L-R or use an inverting OpAmp and summing circuit with RCA line levels. This solution would take a regular stereo pair as input and provide the L-R output, 20+ ms of overall delay, EQ and then individual T/A for each driver into your current amplifier channels. It could be done other ways, but that 20ms of delay is not insignificant.



mosconiac said:


> I would find this a convenient way to derive the 2-octave signal (80-320Hz)...


I have seen nothing to indicate that the bandpass would limit your choice of frequency range in any way, and in that use I can see where a full 31-band EQ might cover more bases than my proposed limited 12-band for the rear fill functionality. I haven't read that thread recently; does it require exceptional delay values or a derived L-R signal in its implementation?

-Todd


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## MarkZ (Dec 5, 2005)

You could do L-R with dual voice coils, but I wouldn't recommend it.


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## pionkej (Feb 29, 2008)

highly said:


> Well, I think the T/A of the current miniDSP only goes to 7.5ms (not certain, but believe I saw this somewhere). I also thought that you needed balanced line levels to create the L-R or use an inverting OpAmp and summing circuit with RCA line levels. This solution would take a regular stereo pair as input and provide the L-R output, 20+ ms of overall delay, EQ and then individual T/A for each driver into your current amplifier channels. It could be done other ways, but that 20ms of delay is not insignificant.


I didn't know about the T/A limitations. I must have missed that. But I think I am spot on with my other assessment. If you have an amp that has a "bridge switch/button" you actually only have to wire the speakers how I mentioned above and not flilp phase on the processor AFAIK.

Here are a few comments from the Hey Werewolf thread that made me think this:



chad said:


> You combine L in phase with R out of phase, So you are adding (L)+(-R)... Mmm Kay? Now anything that is L+R, or panned dead center will be canceled out because in the summing it will net 0. If something is panned, say 1 dB in one direction then the result of the summing will cause it to have a 1 dB level over the CMRR of the receiving device. Anything panned hard right or left will result in full output.
> 
> But that's not the secret to the sauce. True Reverb is NOT L+R, it may get there in the space-time continuum but most of the time it's all over the place, because of this nearly all reverb in a recording will be reproduced by the front AND rear speakers in this application, anything ambient, that is allowed to float to "widen the stage" will go to the rears.
> 
> ...





Ge0 said:


> Someone with a conventional 2 channel STEREO system try this... No multi-channel Dolby Digital hocus pocus at this time. At first it might be easier to do this at home than in your car if you have that luxury. This is something I tried as a teenager with mom and dads stereo but never really understood. Now that I think about it, this may apply to this thread.
> 
> Keep both rear fill channels electrically in phase. Disconnect the negative leads of each speaker at the amp. Tie the negative leads together. The positive leads should still be wired to the amp as normal. Play music through the rears now. What do you hear? Vocals damn near cancel out but ambience is left? You should hear a close proximity of. Voices might have a bit of echo left and sound far off in the distance.
> 
> ...


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## highly (Jan 30, 2007)

Right. I misread your original post and thought you were trying to do it at the other end of the signal chain. I agree with your point there. 

Looks like the current 4-way, PEQ2 and 2.1 plugins do 7.5ms from a quick look at the .pdf files. The Mixer doesn't do any. It might still be possible though if you have another source of system delay that will give you the other 12.5ms, but I think that's more than what most processors do. Maybe a guitar stomp box if you can get it to L-R at the line level, but that won't work with the L-R at the speaker level out the amp wiring solution. By then the MiniDSP plugin solution looks pretty darned good. 

Not saying it can't be done; it certainly can. This just seems an awfully convenient solution. Even moreso if you can get by with 20watts on the rear fill channels and use their digital input amplifier. 

My thought is to use the MiniDSP and their 20W amplifier with the Hybrid L3's currently on my front stage. Replace the L3s with the L4s for front midrange duty and put the L3 in my C-pillars facing forward (on a MKIV GTi the C-pillar is H U G E). Add acoustic treatments over the rear hatch window and side windows between the B and C pillars (for competition) and it's a perfect environment for 'intentional reflections' from the back of the car.

That's the thought, anyway 

-Todd

Edit: A closer look at their amplifier and I think I'll just go with one of the many low power external amplifiers that are available. Better suited to the problem, environment, and power availability.


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## pionkej (Feb 29, 2008)

Yeah, I just totally missed the 7.5ms max T/A currently available on them. It trashes the idea right now. I have a P01 with 15ms of delay available (so I could get over 20ms), but I would have to use it on my h/u output which would affect both channels output on the MiniDSP. This would screw up my plan to route the two non-l-r channels on the 4 channel board back to my amp to control my FRONT tweeters.

Fingers crossed then that you get this completed then. If not, maybe you can explain it can be done manually like I mentioned above, but we would need 25ms of delay available. Maybe they could at least get that set up and we can do the rest through phase inversion.

Note that I was planning to use the little 20wx6 amp that came with the P01 for the rear fill and some Morel CDM88's in the rear c-pillar, so we had similiar ideas as far as setup goes.


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## thehatedguy (May 4, 2007)

Can I configure something similar in my Rane?


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## lycan (Dec 20, 2009)

thehatedguy said:


> Can I configure something similar in my Rane?


get busy with DragNet, spanky ... then you tell us !!!


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## SSSnake (Mar 8, 2007)

Todd,

Looks good! If I had stumbled across these mini-DSPs before building a carputer they would be in the car right now. The delayed and bandpassed difference for the rear channel is an excellent idea. I like the functionality shown in the flow diagram.


Mark,

Help me out. The page you linked mentions a mid-side pair. I guess that I am missing the L+R L-R functionality.

Also, if you are interested in 5.1 processing in a VST look here V.I Stereo to 5.1 Converter VST Plugin Suite

Jeff,

Winslow's new nickname "Drag Net Spanky". I can see the t shirt already.


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## MarkZ (Dec 5, 2005)

SSSnake said:


> Mark,
> 
> Help me out. The page you linked mentions a mid-side pair. I guess that I am missing the L+R L-R functionality.
> 
> Also, if you are interested in 5.1 processing in a VST look here V.I Stereo to 5.1 Converter VST Plugin Suite


M/S is supposedly computed by them as Mid=L+R, Side=L-R. The MSED plugin has two outputs for this. All of their newer plugins allow you to do processing in the M/S domain, so if you have extra channels you can theoretically incorporate L+R or L-R by muting either mid or side (whichever you don't want) before they're recombined.

People using VST hosts like console can do it much easier, by summing. L-R would be the sum of L and polarity-inverted R. I don't use a VST host...I use a wrapper in the player, so I don't have this option.


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## Hernan (Jul 9, 2006)

I've post this some weeks ago. 
Please, can anybody answer if this could work?

Using preamp to get L-R signal
I have a JL A 4300 Amp.

It has 4 inputs and 2 preouts.

There are two output modes for the preouts, 1&2 (copy channels 1&2) and "SUM" where all 4 channels are summed.

From the manual:
Note: Any signal delay between the front and rear
channels can result in a cancellation of signals
within certain frequency ranges when using the
“Sum” position. If you experience a loss of bass
output in the “Sum” position, compared to the
“1&2” position, you are likely dealing with a
delayed signal in either the front or rear outputs
of the source unit.

What should happend if instead of feeding the 1/2 and 3/4 with Front and Rear, we use LEFT and RIGHT?

My thoughts are that the channels should be summed, we should get plain mono. BUT... what if we flip one channel 180 degrees? The Sum now should cancel any conmon info between L & R.

Do we have a trick to get a L-R signal for the rear channels?
I know, both channels will share the same "mono" L-R info but the center channel info should be removed.


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## thehatedguy (May 4, 2007)

Just use one of the outputs- like left positive and right negative from the amp to the corresponding terminals on those speakers. Then connect the other two terminals together, which in this case would be left negative and right positive. Want to flip polarity, just change speaker wires at the amp.

I did this on my factory amplified system in my IS300 .


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## Hernan (Jul 9, 2006)

thehatedguy said:


> Just use one of the outputs- like left positive and right negative from the amp to the corresponding terminals on those speakers. Then connect the other two terminals together, which in this case would be left negative and right positive. Want to flip polarity, just change speaker wires at the amp.
> 
> I did this on my factory amplified system in my IS300 .


Won't the amp invert one channel when bridged?

I have one little problem. I'm using all the stereo outputs of the 701... and all are bandpassed. Where I can get a stereo signal? HU?

I have a spare 2 amp channels. No problem here.

I use delayed rear fill but from the center channel. In fact it works well, but I want to get the L-R sinal...


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## thehatedguy (May 4, 2007)

It's not bridged.


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## highly (Jan 30, 2007)

Hernan said:


> I have one little problem. I'm using all the stereo outputs of the 701... and all are bandpassed. Where I can get a stereo signal?


I'm planning to create the extra stereo pair from inside my H701 when I roll the OpAmps and replace the caps. The signal's there, I just need to make it available via RCAs. I'm feeding the H701 digitally but also want the other audio sources to have fill capability, so that's the only way for it to become available.

Check the RCA outputs on the headunit. I have a 205 and hooking up the H701 causes the HU RCAs to drive hot and into clipping at all times. I don't know if that's a design feature, but it is certainly annoying.


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## highly (Jan 30, 2007)

Wow.
For most decks that would only work when listening to CDs. I know my 205's digital out is only active when the CD/DVD player is active. iPod and radio get sent over the AInet link to the 701. I think that would be a better place to grab your digital signal, and it would not take a ton of work to get it conditioned to send optically to the DSP. I'd have to look to see if it was available after the volume control IC in digital format (can't remember if it was an analog side or digital side volume controller). Let me know if schematics for the 701 would come in handy as I've got them in .pdf. The DSP will take digital in with an addon board.

If you use the 701 here are a couple of setup tips: using the "rear" as your midrange on a 3-way setup provides you the ability to EQ through the crossover frequencies on all 3 drivers. Front1 gets the tweeters and Front2 gets the midbass, as they share one 31-band EQ and Rear gets its own 31 band EQ. Allows for correction of peaks in the crossover region and beyond to the driver that's causing the peak. Enabling "rear fill" also allows F1, F2, and R outputs an additional 9dB of gain before the amplifiers to help with noise floor. That gain does not effect the sub outs. 

I suppose planar drivers could work for your rear fill since they are bandpassed and at a much lower dB level. That would be interesting to hear!
Let us know how it goes if it ever makes it past the dream phase!
-Todd


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## jsun_g (Jun 12, 2008)

Sub'd. Following this with interest but little time on hand.

Any updates on whether the developers intend to follow through with this? Have a link to the MiniDsp forum discussion that you could post?


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## highly (Jan 30, 2007)

jsun_g said:


> Sub'd. Following this with interest but little time on hand.
> 
> Any updates on whether the developers intend to follow through with this? Have a link to the MiniDsp forum discussion that you could post?


The first thread linked to their website, this one links straight to the thread in their forum:

Clicky


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## Patrick Bateman (Sep 11, 2006)

highly said:


> This thread is a result of the discussion going on *here* about the use of intelligent rear fill to increase stage width and provide ambiance. I've posted in the _Space: the Final Frontier_ thread and linked back to this thread in order to minimize threadjacking the original thread.
> 
> The *MiniDSP* is a small form factor standalone DSP module with a $99 price point. It has two analog inputs and four analog outputs, and is configured through a $10 plugin that defines its functionality. I've posted a request to the MiniDSP forum about a plugin for intelligent rear fill, and was thrilled to see the interest it immediately generated from the developers.
> 
> ...


I think I'd be interested in one of these. I actually bought the Rockford unit on Ebay, but the seller didn't deliver.


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## highly (Jan 30, 2007)

Looks like good news!!

MiniDSP - Rear Fill Plugin - MiniDSP



> Todd,
> 
> It's indeed looking quite good so far. If everything going as planned, we should be releasing that new plug-in in the next few days.
> 
> DevTeam


Happy Memorial Day!
-Todd


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## Thaid and Bound (May 15, 2010)

highly said:


> Looks like good news!!
> 
> MiniDSP - Rear Fill Plugin - MiniDSP
> 
> ...


Well done Todd. Your contribution to the betterment of car audio shall be duly noted


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## highly (Jan 30, 2007)

Thaid and Bound said:


> Well done Todd. Your contribution to the betterment of car audio shall be duly noted



Thanks. I'm still rather surprised they had interest in pursuing it. Glad they did!
Now it's time for me to get the darned DSP board so I can make this WORK!
-Todd


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## highly (Jan 30, 2007)

Patrick Bateman said:


> I think I'd be interested in one of these. I actually bought the Rockford unit on Ebay, but the seller didn't deliver.


I've actually got an old Audiocontrol ESP sitting around, but don't have the remote knob for it and didn't have a way to add the delay in there, otherwise I'd have seriously considered that solution. I like this one better, though, and it was minimal work on my part to make it happen. Big bonus in my book!

-T


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## jsun_g (Jun 12, 2008)

Awesome! Glad that there's a company out there that listens and thanks to your initiative.

I was going to play around with my Alesis Nanoverb for rear fill processing but now have another something to consider


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## Thaid and Bound (May 15, 2010)

The guys at MiniDSP have now uploaded a datasheet for the new plugin, here:

http://www.minidsp.com/images/documents/Product Brief-Rear-Center channel plug-in.pdf


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## MaXaZoR (Apr 1, 2007)

Wow!!, I'll be picking one up if that's the case, hope they do a center channel DSP as well.


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## lycan (Dec 20, 2009)

nice work guys


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## MarkZ (Dec 5, 2005)

btw guys, I just implemented it this week in my car using my carPC (bandlimited, delayed>Haas, L-R) and it's a pretty neat effect. Definitely worth trying out. A little more subtle than I expected. Lycan and the others were exactly right that it doesn't pull the stage back one bit. So far, my best attempt at describing it is that it widens the stage considerably, but leaves the vocals untouched. 

It's so subtle that on many tracks, especially where vocals are prominent, I'm hard-pressed to hear any difference at all between muting the rear fill and not muting it. Then on other tracks, or instrumental passages, things like that, it's much more noticeable.


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## mosconiac (Nov 12, 2009)

Congrats on the datasheet from MiniDSP. 

Thanks for sharing your experiences, Mark.


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## lycan (Dec 20, 2009)

One thing to remember guys, there's a lot of "knobs" to turn here, and it's MUCH easier to get _wrong_, than it is to get _right_. You have to bandlimit, attenuate, and delay the signal appropriately. If what you're after is a "cabin-filling sound", or good tunes for the backseat passengers, then this is NOT the rear-fill for you.

Just like Mark wrote ... the effect is probably optimized when it's _subtle_, and not _very_ noticeable. All we're doing is creating a larger "sense of space". That's why we start with the L-R "ambient" signal (largely absent of vocals). You'll know it's about right when the front stage is _enhanced_, rather than _degraded_.


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## MarkZ (Dec 5, 2005)

Another thing I noticed... by default, my carPC plugin gives me L-R on one side and R-L on the other side. I flipped the phase so that both were giving me L-R and it seemed a lot better.

Also, just to remind you guys that carPCs rule and miniDSPs drool , the plugin I use also allows me to dial in as much L into the rear signal as I want, so I can have:

rear left = (L-R) + a*L
rear right = (L-R) + a*R

_And_ I can delay the (L-R) while not delaying the a*L, which would allow me to use the rear speakers for both the ambient fill and "cabin filling", as lycan put it. 

Currently I have a=0. I only bring it up as a possibility (and to show off).


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## lycan (Dec 20, 2009)

MarkZ said:


> Another thing I noticed... by default, my carPC plugin gives me L-R on one side and R-L on the other side. I flipped the phase so that both were giving me L-R and it seemed a lot better.
> 
> Also, just to remind you guys that carPCs rule and miniDSPs drool , the plugin I use also allows me to dial in as much L into the rear signal as I want, so I can have:
> 
> ...


well since you opened the door ... 

you're in the best possible position to try the _composite_ rear-fill signal that i mentioned in the "midbass arrays revisited" thread  The signal has two separate components :

1. significantly delayed (Haas) & bandlimited L-R for increased ambience (the purpose of this, and other, threads)
2. undelayed midbass width enhancement : a little "-R" mixed in with L, and "-L" mixed in with R. The idea is to use rear shelf drivers for midbass augmentation, time-aligned with front midbass drivers, but mixing-in a little inverted "other" channel to widen the cone of confusion from the rear shelf  This is _separate_ from ambience enhancement : here, the only delay will be a few milliseconds to time-align with front drivers, and the bandwidth will be midbass _only_ ... not midrange.

Again ... two separate functions for rear-shelf drivers : ambience enhancement, and midbass enhancement (with electronically "widened" rear-shelf midbass drivers). But these two functions can be added electronically, and used to control a single pair of rear-shelf drivers 

Maybe it's what you're already doing? If it all works, we have a new request for the miniDSP doods


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## lycan (Dec 20, 2009)

... i can elaborate on the "composite" rear-shelf signal (later tonight) with more details, if anyone is interested. We can call it "lycan's composite"


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## MarkZ (Dec 5, 2005)

I didn't read that midbass thread that closely. What's the purpose of this midbass enhancement?


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## jsun_g (Jun 12, 2008)

Mark, where are your rear fill drivers mounted & firing direction?



MarkZ said:


> btw guys, I just implemented it this week in my car using my carPC (bandlimited, delayed>Haas, L-R) and it's a pretty neat effect. Definitely worth trying out. A little more subtle than I expected. Lycan and the others were exactly right that it doesn't pull the stage back one bit. So far, my best attempt at describing it is that it widens the stage considerably, but leaves the vocals untouched.
> 
> It's so subtle that on many tracks, especially where vocals are prominent, I'm hard-pressed to hear any difference at all between muting the rear fill and not muting it. Then on other tracks, or instrumental passages, things like that, it's much more noticeable.


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## lycan (Dec 20, 2009)

MarkZ said:


> I didn't read that midbass thread that closely. What's the purpose of this midbass enhancement?


The purpose is to WIDEN their apparent location 

I think it's suitable for rear-shelf midbass drivers, to "align" them with "wider-placed" door or kickpanel midbass drivers.

EDIT :

here's the thought process :

- i want to augment my door (or kick) mounted midbass drivers, to help manage "midbass suckout" or other magnitude response anomalies in the midbass (say, between 80~320Hz)
- midbass drivers on the rear shelf, though, are right behind my head ... and will therefore localize like midbass drivers right in front of my head (same cone of confusion)
- therefore, without any additional signal processing, these additional midbass drivers will tend to narrow or collapse the stage, even if they help the combined magnitude response
- electronic enhancement in the midbass ... especially in the midbass ... can widen the apparent stage


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## MarkZ (Dec 5, 2005)

lycan: Ah, I see what you're getting at. My car won't be a very good guinea pig, as I don't actually _have_ midbass drivers. My midranges are highpassed in the 200Hz neighborhood. I deal with a dip at the 200Hz area by overlapping the crossover points and using a high Q (>0.7) filter.

jsun: Presently they're in the rear doors, just because I already had speakers there. If I decide to make this permanent, then I'll probably replace them with the same midranges I'm using for my front stage and move them to the rear deck.


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## lycan (Dec 20, 2009)

For clarification, here's how i break it down. Let's stand back, for perspective 

There are _several_ "interesting" signals that you can send to the speakers on the rear shelf. Let's examine each one in isolation first :

*1. Full range, undelayed, left (L) and right (R).*

This is what most people consider to be "rear fill". Variable attenuation is provided by the fader knob. It _will not_, typically, enhance your front stage ... it _will_, probably, pull the stage back. It will "fill the cabin with sound", and will provide some music for back-seat passengers  But it's not particularly interesting to me 

*2. Full range, delayed, left (L) and right (R).*

The delay that is added in this signal is in accordance with Haas ... on the order of ~20msec. Enough delay, so that the ear really hears the rears as separate from the front stage, but not so much delay that it sounds like artificial reverb. Problem here is, that it's a bit "unnatural" ... all frequencies and signals are still present, including vocals, and that's not really what ambience sounds like.

*3. Bandlimited, delayed, (L-R) difference signal.*

The difference signal is used as the basis for ambience. Vocals, typically common to both L & R, are removed by the subtraction. Bandlimiting is employed to further enhance the sense of ambience: extreme high frequencies are attenuated, as they would be if the sound were really coming from farther away (and bouncing off real walls, far away). The goal here is an increased "sense of space". Properly done, it _enhances_ the front stage. This is what this thread has been about, and this is what the miniDSP guys have given us  Yes ... when we go to a live concert, we face the stage. But we _don't_ typically face the stage from a phone booth. The idea here is to _enhance_ the "stage facing illusion", by increasing the sense of space.

*4. Undelayed L and R midbass.*

This category is NOT about ambience. This category is about augmenting the front midbass drivers. The only delay added, will be a couple milliseconds, perhaps, for time-aligning front & rear midbass drivers. The goal is to "smooth" the magnitude response of the midbass region (say, 80~320Hz) in a vehicle, by using a simple "array" with sources at different locations in the vehicle. One problem, however, is that extra midbass drivers on the rear shelf may collapse the stage ... it's like adding an extra pair of midbass drivers directly in front of the listener's heads. These additional drivers will "combine" with the far left & right drivers, creating a single ITD that's smaller (alternatively, creating a single cone of confusion that's shallower) ... the result being a narrower stage.

*5. Undelayed midbass, with width enhancement.*

This category is like the one above. It's NOT about ambience, nor is it about "filling the cabin with sound" (it also won't do much for rear-seat passengers). We just want to enhance the above category, with an electronic technique for increasing "virtual width", so that our rear-shelf midbass drivers won't narrow the stage. The idea is to mix-in a little bit of the opposite channel, with inverted polarity, according to this relationship:

*L(new) = (1+a)*L(old) - a*R(old)*
*R(new) = (1+a)*R(old) - a*L(old)*

where *L(new)* and *R(new)* are the newly-created left & right signals, sent to the left & right rear shelf speakers. The gains are chosen so that a mono, or centered, midbass signal will _not_ vary as we turn the "width control knob *a*"  Of course, an overall gain control on these signals would also be desirable, to vary the overall midbass augmentation. The signals would also have to be bandlimited to the midbass (approx 80~320Hz), and enough time delay would be needed to time-align with front midbass drivers.

There are probably other categories of interesting signals that can be sent to rear-shelf speakers  And of course, COMBINATIONS are possible ... any of these can be electronically added to any other, to form "composite" rear-shelf signals.

I'm a bit partial to : #3 plus #5


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## thehatedguy (May 4, 2007)

How would you do both 3 and 5?


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## lycan (Dec 20, 2009)

thehatedguy said:


> How would you do both 3 and 5?


1. create #3 (use miniDSP)
2. create #5 (maybe analog circuit, maybe next-gen miniDSP)
3. electronically add the above
4. send the composite, added signal to rear speaker amps


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## thehatedguy (May 4, 2007)

Breaking my heart...


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## lycan (Dec 20, 2009)

thehatedguy said:


> Breaking my heart...


... or get busy with DragNet spanky! In that Rane gear, you've got some of the most powerful DSP in audio 

make some diagrams, post 'em up !!


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## MarkZ (Dec 5, 2005)

CarPC folks:

1) use Voxengo's MSED (free) VST plugin to transform L & R -> (L+R) & (L-R)
2) filter each with your favorite crossover VST
3) delay each with your favorite delay VST

Alternatively:
(this would do #3 and #4)
1) Use Voxengo's Sound Delay to apply individual delay to (L+R) and (L-R) [extracted in the routing options]
2) Use Voxengo's GlissEQ to apply filtering to (L+R) and (L-R) [again, extracted in the routing options]

If you want to do #5, you can use foobar2000 as a player and use the matrix mixer plugin to set the "a" value in lycan's equations.


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## Robin W. (Feb 18, 2010)

It's too bad that it doesn't look like you can run Center AND Rear off the same board with the plugin, that would be a nice setup if 3 boards gave you 4 way active stereo + center and rear processing. ~$350 Shipped, that would give me exactly what I need in the G35, but a 4 board/2 plugin combo is still a bargain.


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## thehatedguy (May 4, 2007)

It's how to do #5 that I can't figure out.


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## lycan (Dec 20, 2009)

the equations from #5 above can be re-written as :

*L(new) = L + a*(L-R)*
*R(new) = R + a*(R-L)*

in other words, you can widen the virtual location of drivers operating in the _midbass_ (not midrange, not treble) by "mixing-in", or adding, some amount (controlled by parameter "*a*") of our friend, the stereo _difference_ signal 

But remember, signal #5 is _not_ about adding ambience, nor is it about filling the cabin with sound. The frequencies need to be constrained to the midbass (with appropriate crossovers), and only delayed a small amount for time-alignment with the wider-placed, front midbass drivers.


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## highly (Jan 30, 2007)

A little birdie says the plugin should be available tomorrow 

-Todd


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## lycan (Dec 20, 2009)

cajunner said:


> this seems to be what a SRS or Circle Surround attempts to do, as a circuit available in some stereos.


For a detailed description of what SRS does and how it works, look here :

http://www.diymobileaudio.com/forum/diy-mobile-audio-sq-forum/74088-midbass-arrays-revisited.html

yes, you _will_ have to read a few pages


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## highly (Jan 30, 2007)

The MiniDSP Center Channel/ Rear Fill plugin is released! WooHoo!!!
Just waiting on the hardware to arrive and time to actually IMPLEMENT this! As I type pretty much everything necessary is on its way, including a replacement set of C Pillars to hold a pair of Hybrid Audio L3's. I am projecting 2-3 weeks to have this all in the car and hopefully working!

-Todd


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## pionkej (Feb 29, 2008)

highly said:


> The MiniDSP Center Channel/ Rear Fill plugin is released! WooHoo!!!
> Just waiting on the hardware to arrive and time to actually IMPLEMENT this! As I type pretty much everything necessary is on its way, including a replacement set of C Pillars to hold a pair of Hybrid Audio L3's. I am projecting 2-3 weeks to have this all in the car and hopefully working!
> 
> -Todd


Awesome! I am seriously considering this setup but a couple months behind where you are in my build and budget. I can't wait to hear what you think about this once you get it installed.


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## mitchyz250f (May 14, 2005)

Ok I know this is sad but I can't find the 'rear fill plug in' on their website. I saw it once before but can't find it now. I can even find where they announce the relase of it. 
Help?


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