Audio Devices(such as CDs) output spectrum measurement Using REW and a Soundcard

icbcodc

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Hello John,

How's it going these days?

Today I'm thinking of CD ouput spectrum measurement using REW and a soundcard.

You know the flat spectrum of a audio source(such a CD device) in a car audio system is important, but I need to know the actural spectrum of it.

I know that REW can do a calibration to the soundcard with a loopback conncetion.

Can I use also a connection below
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Or a connection like this below:

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And in REW, should I use "making a measurement "->"SPL" the get the spectrum of the CD?

Or I just play a pink noise sound track in the CD and run RTA function in REW, then get the spectrum of the CD output? If this method is better , I can also use SMAART to get a more accurate spetrum curve.

Thank you in advance.
 
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John Mulcahy

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It depends what you plan to do with the result. In most cases the main characteristic of interest is the transfer function, how the device changes the magnitude and phase of signals passing through it. REW's SPL measurement generates a transfer function view, where the magnitude is shown at the replay level. A meaningful phase trace depends on picking a suitable reference time, either by getting REW to estimate and remove any delays in the measurement or using a loopback connection or other timing reference.

Sometimes the actual spectrum of the output is of interest, such as when looking for unwanted content like distortion products or hum. In that case the RTA would be used, with the excitation depending on what you are looking for.
 

icbcodc

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It depends what you plan to do with the result. In most cases the main characteristic of interest is the transfer function, how the device changes the magnitude and phase of signals passing through it. REW's SPL measurement generates a transfer function view, where the magnitude is shown at the replay level. A meaningful phase trace depends on picking a suitable reference time, either by getting REW to estimate and remove any delays in the measurement or using a loopback connection or other timing reference.

Sometimes the actual spectrum of the output is of interest, such as when looking for unwanted content like distortion products or hum. In that case the RTA would be used, with the excitation depending on what you are looking for.
Thank you ,John.

What I focus in this thread is whether the magnitude of signals is changed by the CD device. Because I can fix it in the DSP amplifier device connected behind the CD using EQ funtion.

The CD sends full range signals from 20hz to 20khz, so for one channel , I don't think the phase of signals is very important for me, maybe it's important for 2 channels (left and right)?

BTW, if I use REW to send the signal to the CD, the route is AUX of the CD, maybe it's different from the other way of playing CD disks? I'm not sure.
 

icbcodc

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In most cases the main characteristic of interest is the transfer function, how the device changes the magnitude and phase of signals passing through it. REW's SPL measurement generates a transfer function view, where the magnitude is shown at the replay level.
Hello John
I made measurement tonight using REW with a loopback for timing ref. I attached the measurement file and screenshots.

Something strange happened, I had to set the level of LINE IN to 0, then I could get the right calibration.

Can you find any setting wrong in the picture below?

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John Mulcahy

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The soundblaster panel shows signals that are mixed to the output, so you do need to set Line In to zero there otherwise you get a feedback loop. You can attach mdat files rather than images for easier review, but all looks OK. There is a little more ripple than I'd expect in that CD player response though. RT60 and Clarity are not meaningful when measuring electronics.
 

icbcodc

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The soundblaster panel shows signals that are mixed to the output, so you do need to set Line In to zero there otherwise you get a feedback loop. You can attach mdat files rather than images for easier review, but all looks OK. There is a little more ripple than I'd expect in that CD player response though. RT60 and Clarity are not meaningful when measuring electronics.
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Does this setting mean signals mixed to the output?

I attached the mdat file,but I don't know why it's not there.

I used the old circuit lines for speaker impedance measurement, just shorted the risistor.

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John Mulcahy

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Yes, Midi/CD is also mixed in, but usually nothing is being sent to that channel so it isn't so important to set it to zero.
 

icbcodc

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Sorry, I just want to know whether the information measurement in SPL Distortion and Impluse tabls of REW is useful to judge the quality of this KENWOOD CD.
 

John Mulcahy

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Yes, though you are seeing the combination of the AUX input and the output. You could use file playback and put a file on a CD to test the output only.
 

icbcodc

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Yes, though you are seeing the combination of the AUX input and the output. You could use file playback and put a file on a CD to test the output only.
That's a cool function in REW. Actually, AUX is not preferred for me.

This CD can play 24bit 96khz format files, so the sound card should also support 24bit 96khz? or the file should be saved as 16bit 44khz for better compatibility?

Should I save the sweep file like this cause the sound car only support 16bit 48khz.

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John Mulcahy

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You are capturing analog signals with the soundcard, so that shouldn't affect the file format. Pick the format according to what the player supports and what formats you want to see played, bearing in mind the card's limitations in capturing them. Use a higher signal level, closer to 0 dBFS, for best signal to noise ratio. Start the sweep at zero Hz for a full range capture and end it at half the sampling rate, there is no need or benefit in limiting it to 20 Hz .. 20 kHz.
 

icbcodc

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Thank you,I got it, I recreated the sweep file using the settings below

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This is a better method for CD player measurement. I will try it later.
 

icbcodc

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You are capturing analog signals with the soundcard, so that shouldn't affect the file format. Pick the format according to what the player supports and what formats you want to see played, bearing in mind the card's limitations in capturing them. Use a higher signal level, closer to 0 dBFS, for best signal to noise ratio. Start the sweep at zero Hz for a full range capture and end it at half the sampling rate, there is no need or benefit in limiting it to 20 Hz .. 20 kHz.
Hello John,

I tried to use the sweep file playing mode in REW to make a measurement,but REW can not detect the acoustic timing signal from the CD device via channel right.

The RCA connection is simple, CD channel right(red socket with "R" on it) is connected to Sound card line in channel right(red socket) .

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The calibration result looks fine.

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and I also tried the mute the MIDI/CD output, but I can't not get the right result when checking level,cause level to low.

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I have to mute the LINE IN as shown below to get the right result. This sound card looks strange.

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John Mulcahy

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For measuring with file, start the measurement, then, when REW says "waiting for timing reference" play back the file on the CD player.

On the soundcard I believe MIDI/CD and Line In should both be muted.
 

icbcodc

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For measuring with file, start the measurement, then, when REW says "waiting for timing reference" play back the file on the CD player.

On the soundcard I believe MIDI/CD and Line In should both be muted.
Yes, I did it like that, but the REW couldn't get the timing reference signal from the CD which was playing the SWEEP FILE.

For "Check level", If I mute them all the level is too low.

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Calibration could not run properly.

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John Mulcahy

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You may need to make an input selection on the soundcard control panel, some of the Creative cards had a selection for mic or line input. The -60 dB levels are typical of mic inputs.
 

icbcodc

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You may need to make an input selection on the soundcard control panel, some of the Creative cards had a selection for mic or line input. The -60 dB levels are typical of mic inputs.
There is no Mic settings on Creative Blaster control panel, now I'm using the 2/2.1 speaker as shown below.
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Even I changed the setting to earphone, the level was also too low.

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In the record panel of win7 sound card setting, no special thing and no useful settings.

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John Mulcahy

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Creative cards vary in their interfaces, but the settings generally need to be made in the Creative control panel. For example, on Audigy 2 selecting the Line In is done by selecting "Analog Mix" in the Record panel of the Basic tab of the Creative Surround Mixer then going to the Source panel and muting all the sources except for Line In. On the SB Live USB 24 the Surround Mixer had a mic/line selector above the mic In/Line In level control. I can't read the labels on your creative control panel screenshots and never used that card, sorry. Note that CMSS and any other effects must be turned off.
 

icbcodc

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Creative cards vary in their interfaces, but the settings generally need to be made in the Creative control panel. For example, on Audigy 2 selecting the Line In is done by selecting "Analog Mix" in the Record panel of the Basic tab of the Creative Surround Mixer then going to the Source panel and muting all the sources except for Line In. On the SB Live USB 24 the Surround Mixer had a mic/line selector above the mic In/Line In level control. I can't read the labels on your creative control panel screenshots and never used that card, sorry. Note that CMSS and any other effects must be turned off.
Sorry, I tried again and get several measurement results from playing the sweep file on the CD. I think the setting the sound card is strange but it's working.

This one is created using 24bit 96khz wave file played by the CD without mic/speaker cal on the measurement menu.

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This one is created using 16bit 48khz wave in the CD with mic/speaker cal (sound card cal file was loaded).

Should I use set the cal on the measurement panel?

mdat file attached:
 

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John Mulcahy

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Measurements look fine. Using the soundcard cal with file playback can be tricky, since it includes the effects of the soundcard input and output and you aren't using the soundcard output so there will be some amount of over-compensation.
 

icbcodc

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Measurements look fine. Using the soundcard cal with file playback can be tricky, since it includes the effects of the soundcard input and output and you aren't using the soundcard output so there will be some amount of over-compensation.
Thank you so much for your help.

I think I got to know the procedure.

Should I keep the calibration setting in REW preference before I measure the CD using sweep file?
 
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