Need help understanding RMAA results of CD player

[percy]

Hi, I was messing around with RMAA trying to see what are the results of playing an audio CD from the cd drive v/s playing the wav file of those tracks from the harddisk. The results are interesting (atleast to me) and I'd like to get your thoughts on these, especially the differences and get some answers as to what could cause such behaviour. Although i have measured many devices with rmaa I am not sure how to interpret some things about these results and if anyone can share some thoughts, that would be great!


Basically a wav file is played and rmaa records it and spits out the results. What I did was play that wav file off the hdd first and then repeat the test by playing the wav file (burnt to an audio cd) from the cd drive.

I also threw in one more variable. I repeated the same set of tests (cd v/s hdd) with and without the SSRC resampler in foobar. i.e. first using the soundcard's resampler and then using foobar's resampler(assuming the soundcard does not resample the already resampled 48khz stream).

Sound card is Turtle Beach Santa Cruz and the drive is a LiteOn LTR-48246S. CD Drive is used via IDE.


Now lets see what we got. In all images, the white curve is the SoundCard resampler and the green line is the FooBar resampler.
Lets start with the frequency response. Don't pay attention to the sudden rolloff, the Y-axis (db) scale is very small.

First the frequency response with the CD Drive:




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and then the frequency response with the Hard Drive:





As you can see both are identical, but notice how it ripples after 1Khz whenever the soundcard's resampler is used. With Foobar's resampler its flat.

So what does this ripple indicate ??



Now lets move on to THD. First with the CD Drive:




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and then with the harddrive:




Notice how in the harddrive chart the spike is sharp and clean and the next harmonic spike is at 2Khz, but in the cd chart it widens up at the bottom and then it bounces on its way to 2Khz. What causes this ? Why is it different ?

Also, see the humps in the chart near 20Khz ? What are these humps ? and why are they different between the soundcard resampler and foobar resampler curves ?


Finally, the IMD charts. For CD Drive:-




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and for Harddrive:-




Exact same behaviour as the THD charts. The same broad spike and the 20Khz humps. Like I mentioned before I am not sure how to interpret these results. Can anyone please help me find answers to my questions ?

Thanks!

[percy]

If this may help stir any interest -

I did eventually find out that the test results are getting skewed (lobes/bounce around the main singal spike) when the CD drive has to spin up before playing the track.
If the track is played while the CD is already spinning (i.e. the drive did not spin down yet) then the results are fine.

So why would that cause the bad results ? Is it somethig to do with power supply sag (doubt it) or some timing/synchronizing issue in the way rmaa expects the input and output signals to be in sync (lag because of the spin up time) ?

I'm still interested in knowing what your throughts are about those humps/peaks near 20Khz and why the frequency response has ripple when using the soundcard resampler.

Thanks.

[stolbovoy]

Looks like you actually measured your sound card. Results are identical, except "IMD" charts. I not familiar of method used by RMAA and can't interpret "IMD" results.
My experience from daily listening properly clocked 1) HDD -> soundcard -> DAC vs 2) transport -> DAC is that difference is very subtle, I'm not sure that it even exists (1 seems to have slightly more resolution )