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SNR of MP3, Split from Topic ID #96702
benski
post Aug 28 2012, 18:37
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QUOTE (jensend @ Aug 27 2012, 11:45) *
The LP noise floor is rather high- maybe -70dB under very good conditions. 12-bit sampling (RMS noise floor of -72dB) would be sufficient for LP use as long as your levels are right (peak signal above -6dB). (12-bit sampling was used for DV but hasn't seen any other widespread use).


The SNR of an MP3 is around 25-30dB. Does this mean that I only need to decode them to 6-bit PCM to capture all the details?

I am curious if anyone has ever done a detailed analysis of an LP's SNR in different frequency bands.

This post has been edited by benski: Aug 28 2012, 19:07
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pdq
post Aug 28 2012, 20:20
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QUOTE (benski @ Aug 28 2012, 13:37) *
The SNR of an MP3 is around 25-30dB.

I don't know why you say that. The dynamic range of mp3 is obviously much greater than 30 dB.

I would be more inclined to describe mp3's deviation from the original as distortion rather than noise, because it is quite capable of rendering very quiet passages with very little added noise.
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hlloyge
post Dec 30 2012, 11:15
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I don't get it. If I encode 1 kHz at 0 dB to mp3, will that mean I will get noise at -30 dB?
I find that hard to believe. Is there any sample that I can use to measure noise level in some program like Audacity, for example?
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saratoga
post Dec 30 2012, 19:02
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QUOTE (hlloyge @ Dec 30 2012, 05:15) *
I find that hard to believe.


That 5 or 6 bits worth of information has a lot more quantization noise then 15 or 16? What about that is unexpected?
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Neuron
post Dec 30 2012, 21:47
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QUOTE (saratoga @ Dec 30 2012, 19:02) *
QUOTE (hlloyge @ Dec 30 2012, 05:15) *
I find that hard to believe.


That 5 or 6 bits worth of information has a lot more quantization noise then 15 or 16? What about that is unexpected?


Yes, but MP3 is not 5 or 6 bit PCM. It is a 32 bit floating point file that contains lossily compressed information which is stored in a non-PCM way. Some guy on this forum once said that mp3 files vs. wav files are like vector graphics files vs bitmap files. By the type of reasoning some people think here, MIDI files would be around perhaps 0.001 bit.

A file with 2.9 bits of resolution would divide the waveform into only cca 7.4 steps and distort the sound wave into a near-squarewave shape. It would be unlistenable even with the most clever dithering and noise shaping in the universe unless you sampled it at megahertz rates, which mp3 definitely does not do.

And LPs can definitely not be stored in just 12 bits. SNR does not tell the whole picture. Quantization error is not "noise", it is ugly disharmonic distortion. Dither prevents that, but it increases dynamic range only, the SNR is actually worse. A dithered 12 bit recording would get rid of most of the distortion, but it would have more noise than an LP. The reason why tapes and LPs are sampled at 16 or 24 bits when digitalizing is that the noise floor of a tape or an LP is very different from digital quantization "noise". You can actually hear signals below the noise level on analog mediums while on undithered digital the signal just drops off.

A CD is definitely superior to LP in noise level, however this does not apply to low resolution digital recordings. A cheap 1980s tape boombox and an 8-bit file might have a similiar SNR, but the tape boombox will sound better as the noise on it is just hiss while 8-bit file will have an ugly, swishy noisy distortion sound in the quiet parts. That is because quantization noise is not static or white noise, it is just how our ears percieve a "steppier" waveform. I am no analog fanboy, but digital quantization error definitely sounds worse than analog noise. That is the same reason why an overdriven tube/transistor analog distortion pedal creates cool rock/metal guitar sounds while digital clipping creates horrible sound. Try digitally clipping a guitar recording in Audacity, you are not going to get a cool rock sound, you are gonna get some seriously horrible sound. Both phenomena are caused by clipping, but forms of clipping are very different from each other just like forms of noise.
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