Typical signal to noise ratio of ADPCM Options

[Neuron]

So, I have decided to return to this forum (despite my little conflict with greynol) and I have kind of a weird question. I would like to know what is the rough SNR of a 4-bit ADPCM recording (IMA and Microsoft). For those curious why I am asking this, well, I like ripping music and sound effects from old games, and I would like know the technical side as well.

[extrabigmehdi]

Well, I used the normalize function of soundforge .
It says that the RMS for file "Eric_Brosius___02___Med_Sci_1___44_Khz_lossless.flac" (i.e 44khz, lossless) is -19db
Now for the error , I compute the difference between the previous file and ADCPM version, (i.e Eric_Brosius___02___Med_Sci_1___44_Khz_IMA_ADPCM.wav)
using the channel converter function of soundforge.
The normalize function says that the RMS of error is -38.2 db


So basically you have two number for 44hz version :
signal: -19 db
noise: -38.2 db


but sorry I don't know the final formula to compute the snr, especially if you consider that soundforge give negative values for RMS .

[Neuron]

Well, I used the normalize function of soundforge .
It says that the RMS for file "Eric_Brosius___02___Med_Sci_1___44_Khz_lossless.flac" (i.e 44khz, lossless) is -19db
Now for the error , I compute the difference between the previous file and ADCPM version, (i.e Eric_Brosius___02___Med_Sci_1___44_Khz_IMA_ADPCM.wav)
using the channel converter function of soundforge.
The normalize function says that the RMS of error is -38.2 db


So basically you have two number for 44hz version :
signal: -19 db
noise: -38.2 db


but sorry I don't know the final formula to compute the snr, especially if you consider that soundforge give negative values for RMS .

Thank you very much, but what does the first value mean? RMS of what? The second I understand is of noise but the first...
Can you do to the same comparision for the MS ADPCM file please? And perhaps verify it by doing the same with an 8-bit LPCM file? An 8-bit LPCM file should have a SNR of around 48 dB (42 dB with dithering). If the results will be too different something is off.

This post has been edited by Neuron: Jan 30 2013, 11:32

[extrabigmehdi]

Thank you very much, but what does the first value mean? RMS of what? The second I understand is of noise but the first...

Well to compute the SNR , you need to compare "loudness" of noise, to "loudness" of original signal.
So basically I compute how loud is the original lossless file (i.e RMS of file "Eric_Brosius___02___Med_Sci_1___44_Khz_lossless.flac").


Here are more measurements:
"Standard RMS":
signal: -19 db
noise IMA ADPCM: -38.2 db
noise Micosoft ADPCM: -38 db


So the IMA is slightly less noisy, but the difference measured is not significant.

But we don't hear all frequencies at same level, and soundforge offers the option to compute RMS
by taking account of "equal loudness curve".

"RMS using equal loudness curve option":
signal: -16.6db
noise IMA ADPCM: -33db
noise Micosoft ADPCM: -32.9

So the difference is even less significant while using "equal loudness contour" option.

And finally, a last experiment, I've computed the average of both ADCPM versions (IMA + Microsoft) ,
and measured the "standard RMS" of the noise level difference (compared to lossless) .
I find -39.6 db, which is better than both ADPCM version , separately.
So it "might" be interesting to combine two game rips.

This post has been edited by extrabigmehdi: Jan 30 2013, 13:57