# Using TrueRTA and MobilePre to measure Frequency Response of headunits



## ErinH (Feb 14, 2007)

Note: This assumes you already know how to use TrueRTA to simply play and record signal. This also assumes you have a mobile pre. However, adapting this tutorial to work for those who do not shouldn't be too hard if you just think about what you're doing. 


Thanks to Chad for the wiring advice. 

Here's a basic tutorial on how to set up your laptop and mobilepre to measure frequency response from a headunit or any other source such as a DSP.
A little DIY work is done to make your own cable. However, it's very simple.

*Tools and supplies needed:*

Spare wire
Solder
Soldering Gun
RCA Connector
TRS STEREO*** 1/4" Connector
*** See Chad's quote below.

The above supplies shouldn't cost you more than $10 tops. Most of the stuff you probably already have laying around.
And FWIW, radio shack does sell solderless 1/4" and RCA connectors. At least, mind does. So, for people without a solidering iron, you have no excuse. 


*The Cable You Need to Make:*
The cable you're going to make should have both an RCA phono male plug, and also have leads for speaker wire. This will allow you to test both type of connections off a headunit should your headunit not have RCA outputs (ie: OEM headunit).

To do this, make sure that you jump over an extra cable either out of your RCA connection, or you run of wire.
In my case, I used a cat5 cable and at the 1/4" TRS Plug, I used 2 conductors for each connection. One of each then goes to each of the end connections. 
This should make more sense when you see the following...

The RCA and speaker lead connections are straightforward. Also, so is the 1/4" TRS as well.
Use the following link and go to #18 and it should be very clear just how to wire up a 1/4" TRS cable.
Sound System Interconnection



THIS IS IMPORTANT... and I've quoted Chad here because he can explain it better than I could. Make sure that you buy the STEREO 1/4" connector; NOT the mono connector. Explain away, Chad...


Chad said:


> Remember a TRS [tip ring sleeve] connector in pro audio it not stereo, it’s balanced with the tip being the non inverting [pin 2 of the XLR], the ring being inverting [pin 3 of the xlr] and the sleeve being ground [pin one]. This is done for patchbays to conserve space. The only time you will really see a ¼ inch stereo is on a headphone. Conversely woul will very rarely see a balanced 1/8” connection it’s almost always 2 channel audio. So yeah, the ¼ inch is mono, but it’s TRS to facilitate a balanced connection that’s void of phantom power and can be hit with a +4 input.


Now, you should be able to easily wire up the 1/4" connector. This end goes into your mobilepre's line level input so that you can then record the signal you sent to your headunit. 
Here's a few pictures of what my cable looks like:



















*Voltage Divider:*
This is purely up to you. I've still not located the specs on the MobilePre stating it's accepted input voltage, however it should be decent. Consider most headunits don't even put out higher than 4V MAX.
But, to be safe, you can easily make a voltage divider to cut down on the input voltage going into the MobilePre. 
Here's VERY simply directions on how to do this. 
Voltage Divider Calculator

Simply type in the voltage in, enter some resistance values (that you may have laying around, or that you can purchase from RS) and check the output voltage. Shoot for a low voltage. Say... around 4V (arbitrary). 
Then, wire up the resistors just as the diagram in the link shows and you're set. 


*Final Connection:*
The final connection should look like the diagram below. 

You want to have the mobilepre sending signal to your headunit's AUX input simply to make it easy on you. This allows you to control your signal into the headunit at the push of a button rather than the click of a CD player. HOWEVER, if you don't want to do this for whatever reason, then you don't have to. Just burn a cd with pink noise and that's good enough. 

The headunit's output will either be via speaker wires or an RCA. Use one of these with the cable you made. That's why you put both types on the end. :thumbsup:













*That's it!*

I think that should cover everything. Again, this is really simple:
1/4" TRS plug goes into the mobilepre's recording input, while the other end of this cable goes to your headunit's preout/speaker level outs. 
The AUX wire is optional, but I highly recommend it. 
The voltage divider is up to you, but I recommend it.

Shouldn't cost you more than $10 and you'll then be able to start measuring your headunit's output at various volumes and get results like so:












:thumbsup:


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## DanMan (Jul 18, 2008)

Thanks for taking the time and effort to post this.

I am really disappointed in myself that I have never been able to make good use of my "RTA for a $100".

I will get a MobilePre at some point and try this out for sure.


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## JayinMI (Oct 18, 2008)

OK. Just to make sure I understand, I leave the "S" unused on the end where the speaker level/RCA connectors are? I could see the floating "ground" being an issue since my laptop/MobilePre aren't sharing a ground with the car, so the reference could be off.


Also will this vary if I am testing a balanced differential (non-TRS) signal? The output of my radio is a little under 4V, balanced differential. I get like 3.69 across both terminals, but only like 1.85 if I measure from one terminal to ground.

(I'm sure this is more of a "Chad" question, but I'd like to make sure so I don't blow anything up, or get incorrect readings by doing something wrong.)

Jay


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## JayinMI (Oct 18, 2008)

Well, I answered my own question. I hit up RS and got the 1/4" stereo ends to make my cable. Used some extra 61801EZ I had from my RCA's. 
It worked fine, as far as I can tell.

My factory head unit doesn't have a "Loudness" option, but appears to have some form of it because it boosts bass and treble at lower volumes. The lower the volume, the more the boost.

I managed to find a setting that is reasonably flat through volume/Bass/Treble Settings.

I appear to have a significant roll off on the high end around 12.5k and on the low end around 50Hz...I imagine cabin gain will help with that and if I mount my tweeters near the windshield, I imagine the reflections may boost the highs some...we'll see.

Thanks for the tutorial, Erin. This was my first time using my MobilePreUSB...although I used it with my demo copy of FuzzMeasure on my Macbook.

Jay


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## ErinH (Feb 14, 2007)

Glad you got it figured out, Jay.

I was going to try to come back and answer your question when I had time to look into a bit. It's been a few months since I've looked at this stuff so I couldn't remember off the top of my head.

Post up graphs if you have time. I'd like to see what you're testing and how it looks.


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## JayinMI (Oct 18, 2008)

I posted them up in my build log, I didn't want to clutter up your tutorial.
I used Fuzzmeasure w/ 1/12 Octave smoothing.

Frequency Response Graphs

Jay


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## ErinH (Feb 14, 2007)

By all means, clutter away.

Maybe more folks will follow suit.


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## JayinMI (Oct 18, 2008)

From my build log:











^ This is with the volume at 15 (of 35) with Bass/Mid/Treble at 0.










^ This is with the volume at 30 (of 35) B/M/T at 0.

At this volume level, when I checked last I was well over 2V...but not sure where it starts clipping. Generally the stereo sounds pretty clean up over 25.










^ This is max volume, B/M/T at 0.

I'd bet it's clipping up here, but I'll check it with the Oscilloscope function in TrueRTA at some point.

I played around some more, and came up with what I feel will be a good point to leave it at...










That was 25 vol, B -3, M/T at 0....should give me a decent voltage signal, likely no clipping and a pretty flat response, except for the roll of on the high and low end. Looks like it starts around 12.5k on the high end and around 50Hz on the low end.

I'm hoping cabin gain will help cure some of the low end that's missing, and that mounting my tweets on the a pillars or near the windshield will boost the highs some...of course, I don't know how the BitOne.1 is going to deal with it...if it flattens it out, then I guess it won't be a huge issue anyway.

Finally, here's Vol 15, B -3, M/T at 0.










Notice the high and low got boosted when I turned the volume down. I don't see anyway to turn off the loudness effect...

I didn't include low volume graphs because they were all over the place...and obviously, I won't be listening to it with the volume that low. 

Jay


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## Phil Indeblanc (May 27, 2015)

This is usually preset in the HU to allow you to turn the volume up without distorting or damaging the speakers. Its the same I guess looking at it that you are turning the end frequency up in lower volume on inverse of turing those frequencies down as you raise the volume. 

Its not that those signals go up, its that those signals go to "NORMAL" vs, when volume ges up, the end frequencies are cut down.


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