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.

[Neuron]

I did an Audacity convert to mono + match both volumes using Amplify + invert one of the track and Mix and render way of subtracting one track from the other. I got a "noise signal", peaking only 11.3 dB below the top. The weird thing is, the quieter parts of the song (not silence, only 9-10 dB below maximum) had "noise" as quiet as -45 dB while the louder parts went up to 11.3 dB. This is not all that is strange, because the "noise" left by the subtraction is actually a recognisable signal of the music, although very noisy (I will post a sample in the uploads section). By contrast, the dithered 8-bit signal is very audibly noisy, but it leaves a normal noise signal at -35.3 dB (which is a bit weird as well as 8-bit sound should have a 48 dB SNR and dithered 42 dB as I used only 1-bits of triangular dither, plus, the original signal is slightly present in the noise as well). The undithered 8-bit signal produces normal random noise at -48.2 dB as expected. Both dithered and undithered 8-bit signal have obvious, strong noise in them, unlike the ADPCM files.

So, how to make sense of this? And why every difference signal except for the non-dithered 8-bit one has recognisable music it it?

[extrabigmehdi]

I got a "noise signal", peaking only 11.3 dB below the top.

Measuring RMS is more representative of loudness than peak.

Well, neither 4-bit ADPCM or 8-bit PCM are really "bad".

I guess , that higher bit depth is only useful, if you want to reach higher dynamic range.

[Neuron]

I got a "noise signal", peaking only 11.3 dB below the top.

Measuring RMS is more representative of loudness than peak.

Well, neither 4-bit ADPCM or 8-bit PCM are really "bad".

I guess , that higher bit depth is only useful, if you want to reach higher dynamic range.

Well, bit depth of 8-bit PCM or 4-bit ADPCM has quite obvious noise. It is not annoying, but it is also clearly not CD quality. Also, as I learned here, 4-bit ADPCM actually has a 96 dB dynamic range. (because it is mapped to 16 bit). The SNR and dynamic range are the same in undithered PCM, but not in dithered PCM or any flavour of ADPCM. If I understood correctly, this is why you can hear the signal below the noise level on dithered PCM, ADPCM and mp3, kind of like on cassette tapes. The "noise floor" of undithered PCM is more like the event horizon of a black hole - you cannot have a signal under the noise level, it is forever lost.

8-bit PCM, unlike ADPCM, has a lower dynamic range (48 dB), but it is not the same kind of dynamic range reduction as in clipped and overcompressed awful "loudness war" sound. Clipping destroys the most significant bits of the recording (it cuts the dynamic range "from the top", making waveforms have square wave shapes on their peaks), while normal bit rate reduction removes the least significant bits, decreasing dynamic range "from the bottom" (dithering is more complicated through). 8-bit PCM will not dynamically compress the sound at all, but it will distort fadeouts and quiet parts and this is why dithering is used - it reduces the SNR but increases the dynamic range.

This post has been edited by Neuron: Jan 30 2013, 21:16