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:
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...
Originally Posted by Chad
Remember a TRS [tip ring sleeve] connector in pro audio it not stereo, itís 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 ľ inch stereo is on a headphone. Conversely woul will very rarely see a balanced 1/8Ē connection itís almost always 2 channel audio. So yeah, the ľ inch is mono, but itís TRS to facilitate a balanced connection thatís void of phantom power and can be hit with a +4 input.
Here's a few pictures of what my cable looks like:
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.
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.
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: