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SNR of MP3, Split from Topic ID #96702
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 Aug 28 2012, 20:27
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QUOTE (benski @ Aug 28 2012, 19:37) *
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.


Huh?
That would mean that I couldn't be able to listen to classical music in mp3 format without some background hiss and noise always present.
Or did you mean something else?
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saratoga
post Aug 28 2012, 20:29
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QUOTE (pdq @ Aug 28 2012, 15:20) *
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.


Dynamic range isn't the same as SNR. However, I remember testing it at about 25dB for my tracks IIRC.

QUOTE (pdq @ Aug 28 2012, 15:20) *
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.


Yes, I think his point is that SNR is not entirely meaningful if the noise isn't spectrally or temporally flat, or else it is correlated with the input signal.
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Arnold B. Kruege...
post Aug 29 2012, 12:41
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QUOTE (benski @ Aug 28 2012, 13:37) *
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.



In which alternative universe? At what ludicrously low bit rate?

SNR is a measurable quantity. I've measured it in high-bit rate MP3s. The exact number escapes me but I seem to recall some number north of 80 dB. It was so high that I questioned the use of bandwidth to maintain it.
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2Bdecided
post Aug 29 2012, 17:45
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QUOTE (Arnold B. Krueger @ Aug 29 2012, 12:41) *
QUOTE (benski @ Aug 28 2012, 13:37) *
The SNR of an MP3 is around 25-30dB.


In which alternative universe? At what ludicrously low bit rate?

SNR is a measurable quantity. I've measured it in high-bit rate MP3s. The exact number escapes me but I seem to recall some number north of 80 dB. It was so high that I questioned the use of bandwidth to maintain it.
The dynamic range of mp3 is insane. The format itself has several hundred dB of dynamic range, and some 24-bit decoders give the expected 140dB.

But the "THD+N" (if you want to call it that) of mp3 with a complex test signal is often about 25dB below the test signal.

Cheers,
David.
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Alexey Lukin
post Aug 29 2012, 21:32
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QUOTE (benski @ Aug 28 2012, 13:37) *
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.

You are right about the SNR of mp3 files: it is around 20 dB at 128 kbps and around 30 dB at 256 kbps. However the noise of mp3 is relative to the signal level, while the noise of PCM quantization is constant. So, mp3 files can indeed decode to a wide dynamic range, even beyond capabilities of the 16-bit PCM format. It all depends on the signal. In some sense, mp3 is similar to a 32-bit float format: its noise floor is ~150 dB below the signal level.

The noise floor of a LP record is approximately constant in time, but it is not white. So, the flat-dither 12-bit PCM format may not be sufficient to capture the LP signal.
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Alexey Lukin
post Aug 29 2012, 21:37
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QUOTE (Arnold B. Krueger @ Aug 29 2012, 07:41) *
SNR is a measurable quantity. I've measured it in high-bit rate MP3s. The exact number escapes me but I seem to recall some number north of 80 dB. It was so high that I questioned the use of bandwidth to maintain it.

Arnie, you might have measured the lowest possible noise in mp3. It can indeed drop below 100 dB. But the coding distortion ("noise") of mp3 is proportional to the signal amplitude and complexity. For most typical music it is really only 2030 dB below the signal level.
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Arnold B. Kruege...
post Aug 30 2012, 12:48
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QUOTE (Alexey Lukin @ Aug 29 2012, 16:37) *
QUOTE (Arnold B. Krueger @ Aug 29 2012, 07:41) *
SNR is a measurable quantity. I've measured it in high-bit rate MP3s. The exact number escapes me but I seem to recall some number north of 80 dB. It was so high that I questioned the use of bandwidth to maintain it.

Arnie, you might have measured the lowest possible noise in mp3. It can indeed drop below 100 dB. But the coding distortion ("noise") of mp3 is proportional to the signal amplitude and complexity. For most typical music it is really only 2030 dB below the signal level.


I took a closer look at the actual situation with music. I took a music file and added two narrow notches about a decade apart using CEP 2.1's FFT filter. I found that the notches that I created were limited to about 60-65 dB which seems sufficient to give a reliable indication about the issue at hand, even though it is shy of the 80 dB I had been talking about.

This was a back-of-the envelope exploration, not a thesis project! ;-)

I encoded the notched file @ 320K with CEP 2.1's built-in encoder, converted back to .wav for analysis and found that while the bottoms of the notches lost some of their squareness, the depth of the notches did not change that much. Still around 60 dB down. That seems to be more like my number.

I again encoded the file this time @ 128K with CEP 2.1's built-in encoder, converted back to .wav for analysis and found that the bottoms squareness were in the 20 dB range. That seems to be more like your number.

I think these rough results are representative, as far as they go.
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Alexey Lukin
post Aug 30 2012, 15:14
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A simpler test is a subtraction test: subtract the mp3 waveform from the original waveform and measure the difference. Your test is probably valid too, but the notch should be narrow and you should rate the noise to the original signal level.
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Arnold B. Kruege...
post Aug 30 2012, 15:22
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QUOTE (Alexey Lukin @ Aug 30 2012, 10:14) *
A simpler test is a subtraction test: subtract the mp3 waveform from the original waveform and measure the difference.


In general tests based on subtracting samples are prone to being misleading because there are too many influences that are only weakly relevant that affect their results. Many have no or very limited bearing on audibility, such as a ten sample time shift which can cause a misleadingly large difference signal.

QUOTE
Your test is probably valid too, but the notch should be narrow and you should rate the noise to the original signal level.


Please read my post again as I addressed both issues. My notches were as narrow as I could make them with the tool used, and the notch depth being given in dB, inherently related the noise level in the notch to the original signal level.
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greynol
post Aug 30 2012, 15:33
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QUOTE (Arnold B. Krueger @ Aug 30 2012, 07:22) *
In general tests based on subtracting samples are prone to being misleading because there are too many influences that are only weakly relevant that affect their results. Many have no or very limited bearing on audibility, such as a ten sample time shift which can cause a misleadingly large difference signal.

This doesn't invalidate the legitimacy of the numbers you were reluctant to accept. Moreover, this was the underlying point of those presenting them.


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Alexey Lukin
post Aug 30 2012, 16:51
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QUOTE (Arnold B. Krueger @ Aug 30 2012, 10:22) *
In general tests based on subtracting samples are prone to being misleading because there are too many influences that are only weakly relevant that affect their results. Many have no or very limited bearing on audibility, such as a ten sample time shift which can cause a misleadingly large difference signal.

Of course, it's up to the tester to ensure that the subtraction test is correct. Please be assured that my numbers reflect the actual quantization noise of mp3, not "other influences". Mp3 is basically a waveform coder, so you can easily align the decoded waveform with the original waveform. And there's little correlation between the original waveform and the residual signal.


QUOTE (Arnold B. Krueger @ Aug 30 2012, 10:22) *
Please read my post again as I addressed both issues. My notches were as narrow as I could make them with the tool used, and the notch depth being given in dB, inherently related the noise level in the notch to the original signal level.

Right, I agree on this. However there are other problems with your test. Mp3 noise does not necessarily increase the signal level in specific frequency ranges, it can also decrease it, esp. in the notches! So, I would not rely on a measurement of the notch depth for noise level estimation, as in some cases the depth can actually increase, but the noise would still be present around the notch.
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Alexey Lukin
post Aug 30 2012, 17:24
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Here is an illustration. Although notches are not very narrow here, I hope the point about the notch depth is made clear.


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pdq
post Aug 30 2012, 17:48
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I'm a little confused as to what I am seeing. Was the original signal white noise at -50 dB with two notches?
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Alexey Lukin
post Aug 30 2012, 17:58
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Yes. The power spectral density was approx. 50 dB/10 Hz, the overall signal power was 20 dB.
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Arnold B. Kruege...
post Aug 30 2012, 17:59
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QUOTE (pdq @ Aug 30 2012, 12:48) *
I'm a little confused as to what I am seeing. Was the original signal white noise at -50 dB with two notches?


Kinda sorta. The original signal appears to have spectral characteristics that are stationary and have a very smooth, broad frequency distribution, such as we would see with a random noise signal.

Note that the signal was not at -50 dB. It is a signal that filled the FFT bins to the -50 dB point which is also dependent on the number of bins. The signal could have easily been just a few dB below FS.

When we are analyzing the performance of perceptual coders, the well-known and highly significant statistical differences between random noise and music make any results obtained that way very questionable. The example provided tends to support the previous statements of its author.

I used a recording of real-world acoustic music for my evaluations.

Also, I performed my evaluation at two significantly different bit rates, and compared the differences.

This post has been edited by Arnold B. Krueger: Aug 30 2012, 18:00
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Alexey Lukin
post Aug 30 2012, 18:05
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You'd get the same results on music too, provided that notches have similar widths and positions to my white noise example. I used white noise as a simple example of the fact that mp3 quantization may either reduce or increase the apparent signal level in certain frequency bands.
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Arnold B. Kruege...
post Aug 30 2012, 18:18
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QUOTE (Alexey Lukin @ Aug 30 2012, 13:05) *
You'd get the same results on music too, provided that notches have similar widths and positions to my white noise example. I used white noise as a simple example of the fact that mp3 quantization may either reduce or increase the apparent signal level in certain frequency bands.



That's just it Alexey, I didn't get the same results on music for the reason I gave. Using noise signals to evaluate the technical performance of MP3 encoders is a well-known path to pessimistic results.
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Alexey Lukin
post Aug 30 2012, 18:33
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But you didn't use the notches that I used. Anyway, here's a similar result on real music:

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Arnold B. Kruege...
post Aug 30 2012, 19:19
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QUOTE (Alexey Lukin @ Aug 30 2012, 13:33) *
But you didn't use the notches that I used. Anyway, here's a similar result on real music:




No, I used notches around 400 Hz and 2 KHz, cause that is where the music usually is.

You did get the MP3 encoder to deepen the notch which probably happened because the encoder figured out that your relatively high notch band was masked and was didn't need to be budgeted any bandwidth.

My bands, focused into the realms where the ear is pretty sensitive, went the other way, degree depending on the amount of available data bandwidth.

This post has been edited by Arnold B. Krueger: Aug 30 2012, 19:19
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Alexey Lukin
post Aug 30 2012, 20:36
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When you put notches at lower frequencies, what you see after the encoding is mostly a subband leakage (or aliasing), not a quantization noise.
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Alexey Lukin
post Aug 30 2012, 20:42
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I still think that the best way to measure mp3 noise is to do a subtraction test.
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pdq
post Aug 31 2012, 14:52
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QUOTE (benski @ Aug 28 2012, 13:37) *
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?

So, after all of the back and forth of how to measure the SNR of an mp3, has this question been answered?
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dhromed
post Aug 31 2012, 15:28
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It's probably going to be really noisy, even if nothing is subjectively drowned in the noise.
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Alexey Lukin
post Aug 31 2012, 19:31
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QUOTE (pdq @ Aug 31 2012, 09:52) *
QUOTE (benski @ Aug 28 2012, 13:37) *
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?

So, after all of the back and forth of how to measure the SNR of an mp3, has this question been answered?

The answer is obvious: no, 6-bit PCM is not sufficient.
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