# Measuring frequency response with REW



## lashlee (Mar 31, 2014)

Let me start by saying I'm more of a car audio guy but I appreciate any kind of music reproduction! I just bought a new car and I'm stuck with the factory radio due to OEM integration. I would like to know what is best way to measure the output of the OEM radio? I'd like to know what parts I would need to verify that the output is in fact a full spectrum. 

Thanks in advance to anyone that can offer any help! :T


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## subterFUSE (May 10, 2014)

lashlee said:


> Let me start by saying I'm more of a car audio guy but I appreciate any kind of music reproduction! I just bought a new car and I'm stuck with the factory radio due to OEM integration. I would like to know what is best way to measure the output of the OEM radio? I'd like to know what parts I would need to verify that the output is in fact a full spectrum.
> 
> Thanks in advance to anyone that can offer any help! :T


I am also car audio guy, and the REW software has been used with great success by many car audio enthusiasts. The answer is yes, you can use REW to measure the OEM source response. I have done it personally in my own car.


First question is what kind of car do you have?
Second question is whether or not there is a preamp output from the factory head unit?

If your car has a preamp output from the head unit, then you could simply plug that output into your sound card and measure via REW. You would obviously need an Input on your car source, too. In my car, I have an RCA input adapter which I used.

However, if your car does not have a preamp output, you will need to use a Line Converter with the amplified outputs from your OEM amplifier.

I drive a new Audi which has the MMI system, yet I have done a complete stereo system upgrade while keeping the MMI as my head unit source. You simply need a DSP which can convert the amplified signal from the OEM system into a preamp signal. Audison BitOne, Arc Audio PS8, or Mosconi 6to8 are all examples of car audio DSP units with this capability.

In my car, I currently have the Audison BitOne. I recently used REW to test the frequency response of my OEM source. Here was the result when I ran the REW test sweep signal through the Audi MMI, and then through my DSP, and then back into REW. All DSP EQ and crossover features disabled.










That's a lot of factory EQ!


By contrast, here is the result when I tested by routing REW directly into my DSP via the AUX input, and then back.












I applied some Input EQ to the AUX Input and re-measured. Got this:












Anyway, I just ordered a new digital preamp called a MoBridge DA1. It is designed for cars that use the MOST fiber optic system, and it gives an Optical Digital preamp output. This will let me bypass the Bose amplifier in my car completely. I am hoping that all of that factory EQ is done at the Bose amp, in which case I should have a much flatter input response.


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## AudiocRaver (Jun 6, 2012)

One good way is to use Room EQ Wizard in Real Time Analyzer (RTA) mode while playing full-bandwidth pink noise through your system. You can download this file:

View attachment Pink Noise MP3.mp3


The file is one minute of stereo Pink Noise at -20 dB FS (its RMS level is the same as a -20 dB FS sine wave). It is not peak-limited. There are peaks that reach -6.5 dB, so watch out for clipping.

Burn the file to a CD and play it in your system _one channel at a time_ while measuring with REW in RTA mode and you will get a good picture of your system's frequency response. You may want to take snapshots of numerous points around the head position in the driver's seat and visually "average" them - unfortunately there is no way to average several RTA measurements together in REW. Another possibility is running the RTA mode with slow averaging while moving the mic through the LP area.

There are others around with far more experience in automotive sound then I have, hopefully they will pitch in with their ideas.

*EDIT:* The method given above by subterFUSE is a far better approach, if you can pull it off.


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## lashlee (Mar 31, 2014)

Thanks for the input. The car is a 2014 Honda Accord Hybrid, with the 7 speaker OEM setup. The output of the radio to the amp is differetial balanced so I can tap onto the signal there and send it to the laptop. I would like to know what is the best equipment to use to do that. Is it ok to just use an RCA to 1/8" adpater to the soundcard of the laptop or should I look into some sort of preamp? I've got a bunch of test CD's and planned on setting it up like AudiocRaver stated but I would rather use the approach subterFUSE listed. Someone on the Accord forum tested a 2014 Accord Sport with a 4 speaker setup but it showed a roll off below 80Hz on all channels. My car has (at least according to the schematic) six channels of output from the radio to the amp. I'm really trying to verify if my car is full range on all channels or if its internally crossed over on the front and rears and sub. 

As for the equipment going in the car I've already got an Arc PS8, PG Ti21600.5, Hertz Mille mid and tweeter and probably a JL TW5 sub (yet to be determined). I've got an AudioControl matrix plus 6 channel line driver on order as well since my car is supposed to have a very low output signal, but hopefully I won't need to install it.


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## subterFUSE (May 10, 2014)

lashlee said:


> Thanks for the input. The car is a 2014 Honda Accord Hybrid, with the 7 speaker OEM setup. The output of the radio to the amp is differetial balanced so I can tap onto the signal there and send it to the laptop. I would like to know what is the best equipment to use to do that. Is it ok to just use an RCA to 1/8" adpater to the soundcard of the laptop or should I look into some sort of preamp? I've got a bunch of test CD's and planned on setting it up like AudiocRaver stated but I would rather use the approach subterFUSE listed. Someone on the Accord forum tested a 2014 Accord Sport with a 4 speaker setup but it showed a roll off below 80Hz on all channels. My car has (at least according to the schematic) six channels of output from the radio to the amp. I'm really trying to verify if my car is full range on all channels or if its internally crossed over on the front and rears and sub.
> 
> As for the equipment going in the car I've already got an Arc PS8, PG Ti21600.5, Hertz Mille mid and tweeter and probably a JL TW5 sub (yet to be determined). I've got an AudioControl matrix plus 6 channel line driver on order as well since my car is supposed to have a very low output signal, but hopefully I won't need to install it.


I am about to put the Arc Audio PS8 into my car, and replace the BitOne. The PS8 is THE BEST car audio DSP product on the market. Absolutely no one else offers the 31 bands of fully adjustable parametric EQ per channel. That's unheard of in the car audio realm. , you can't even do that with miniDSP. The PS8 is the perfect car audio DSP to use with REW. You can use the Auto EQ filters in REW, and then copy the parameters directly into the PS8. So simple.


If your source offers a preamp output, which it seems you are saying it does, then you can connect that directly to your laptop soundcard and measure with REW. Don't use another preamp because that might color your signal and give you false results. The only question will be how you plug the output from your soundcard into your car's source unit? In my car, I used the Audi Music Interface with an RCA input. I had to order a special adapter from Audi for this.

You'll want to run a standard measurement sweep from 20Hz to 20kHz. Your ideal response will be a flat line like I posted above. However, this is unlikely with a factory radio. Most factory systems have some EQ applied to them. But don't worry if your car EQs the signal. You can use REW to create the Auto EQ filters to correct the car's response, and match to your House Curve all in one step.


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## lashlee (Mar 31, 2014)

Thanks! I'm super pumped about the PS8. I've got an Alpine H701 in my old competition car so I look forward to the adjustability of the Arc piece. My biggest hurdle will be getting familiar with REW. I've only played with it briefly but I'm trying to figure it out! 

As for the input, my car has a simple 1/8" jack and I figured I would just run a 1/8" to 1/8" cable from the laptop to the car. Or would it be better to just use the output of the radio and a pink noise disc? I'll also pick your brain when it comes to integrating the auto eq of REW into the PS8. I think a tutorial might be very helpful! (Hint hint!)


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## subterFUSE (May 10, 2014)

lashlee said:


> Thanks! I'm super pumped about the PS8. I've got an Alpine H701 in my old competition car so I look forward to the adjustability of the Arc piece. My biggest hurdle will be getting familiar with REW. I've only played with it briefly but I'm trying to figure it out!
> 
> As for the input, my car has a simple 1/8" jack and I figured I would just run a 1/8" to 1/8" cable from the laptop to the car. Or would it be better to just use the output of the radio and a pink noise disc? I'll also pick your brain when it comes to integrating the auto eq of REW into the PS8. I think a tutorial might be very helpful! (Hint hint!)


Yes, just run the 1/8" connection from REW to your car and measure with the standard measurement sweeps. Or you can use the REW noise generator and RTA.


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