'Normalization' of PCM audio - subjectively benign? Options

[RockFan]

Hi All,

A thread back in March discussed whether normalization of WAV files is 'lossy', and I remember that no-one thought to point out that operative word is actually 'destructive'.

Digital processing of PCM audio such as normalization, compression (or expansion), equalisation, etc' is 'destructive', not 'lossy', although both mean 'irreversable'. It has to be said, this seems as much a matter of semantics or even philosophy as much as exact engineering terminology.

Anyway, this is of some interest to me, I'm using a 24-bit ADC to record from vinyl (and taking it pretty seriously as an archiving project - I get one shot at some discs, so I want to do it right), and as it happens there is insufficient gain on the line from the phono-amp I'm using to it to get close to 0dB with some discs, many peak at -6, -8, even -12 dB.

However, even -20 dB or more in 24 bits is greater than 16-bits of resolution. A 24-bit recording which peaks at, say, -16dB, can be 'normalized' to 0dB, and will *still* have information below digital silence in 16-bits, and ideally require dither on conversion.

Obviously I could use a preampifier in addition to the phono-stage to get around the level issues, but I'd rather keep to the 'minimal' signal path I'm using now.

What I'd like to ask people here is; in their experience, is normalization completely 'benign', sonically? Are the algoritms used in different applications much the same, or are some better than others?

R.

[Axon]

Just to put some hard numbers down on the signal-to-noise ratio of vinyl vs CD.

I took a recording of the Hi Fi News test record, with one of its +15db, 300hz tracks. (It was one of Andy's, actually.) I made a large (5 million point) FFT amplitude spectrum plot, and compared the 300hz peak amplitude with an eyeballed average noise amplitude around the peak. It comes out to be 80-84db. Now, you could probably eke another 3db out of this if your cart was able to track a +18db tone. Very few can. And this SNR number is compromised quite a bit by the speed variation of the table, which spreads the power of the test tone out by quite a bit. You might get 10-20db back in your SNR if you manage to factor that back in. Oh, and there's more background noise at 300hz due to rumble and hiss and what not, and the observed background noise at around 10khz is lower by about 16db, so let's just pretend that we're looking at the 10khz noise floor in this recording, rather than the 300hz noise floor. So for the sake of argument, let's call the measured vinyl dynamic range to be 123db. (84 db measured + 3db from not testing the complete headroom of vinyl + 20db expected from eliminating speed variation issues + 16db for a more ideal noise floor measurement)

Note that most people would call that a very, very generous figure.

In comparison: I made a wav file in Audacity composed of a 300hz tone of amplitude 1.0 I exported it to a 16 bit 44.1 wav with noise-shaped dither.I did another large FFT of the result. The peak-to-average ratio came out to.. drumroll... 151db. No, that is not a typo. A 16-bit WAV file is perfectly capable of 151db of dynamic range.

Conclusion: 16-bit recording has a noise density that is at least 28db lower than vinyl.

This post has been edited by Axon: Aug 30 2006, 18:40

[RockFan]

I have to say, as something of a vinyl die-hard, I've always viewed the 'dynamic range' attributed to CD to be very generous.

Typical distortion through a top-notch DAC is something like this;

-60dB - 0.22%
-70dB - 3%
-80dB - 8%
-90dB - 30+%
-100dB - distortion = signal

The nature of this distortion is little discussed, but it needs to be understood that it is not like the evenly distributed harmonic distortion (typically mostly 2nd, some 3rd a little 4th and so on) that predominates in the analogue domain - it is randomly distributed 'quantization noise' and is extremely obnoxious at levels over a fraction of a percent.

It passes 'unnoticed' because it is only affecting low level stuff like ambience and reverb.

If the entire signal were afflicted with this distortion at approaching 1% it would be rather unpleasent to listen to. At anything over 3% it would be unlistenable.

I am compelled to wonder, then, how it came to be that signals distorted to the the extent they are below -70dB are included in CD's 'dynamic range' numbers. More properly, they are actually it's 'signal to noise'.

Conversely, vinyl disc is damned for it's 'signal to noise' of <70dB, when in truth it has a 'dynamic range' considerably better than this.

Which is the better music carrier? You decide!

edit >> we could also discuss CD's notional 'bandwidth' or 'time-domain resolution'. Perhaps not.

This post has been edited by RockFan: Aug 30 2006, 22:37

[2Bdecided]

It's got nothing to do with "sounding realistic".

With correct dither, it's about noise - pure and simple.

If can be just four bits - it will sound perfectly "realistic", but with bucket loads of noise on top!

It has everything to do with 'realism'. 16/44 typically introduces several percent quantization distortion below -70dB (typically, played back through a good, linear DAC it reaches 6-10 % by -80dB, and more distortion than signal by -100dB).

I have to say, as something of a vinyl die-hard, I've always viewed the 'dynamic range' attributed to CD to be very generous.

Typical distortion through a top-notch DAC is something like this;

-60dB - 0.22%
-70dB - 3%
-80dB - 8%
-90dB - 30+%
-100dB - distortion = signal

It doesn't matter how many times you say this, or how many times some idiot audiofools say it on other boards, it doesn't make it true!

The truth is very simple...

Correct dither prevents quantisation distortion and replaces it with benign "uncorrelated" noise, below which the original signal is infinitely resolvable (as far as the noise can be cancelled / averaged away / "heard through")

From your quotes, I don't think you understand dither. There are some fairly bad explanations on the web, but the principle is simple: add "correct" random noise before the rounding or truncation stage. This makes the truncation a stochastic, rather than deterministic process. This means that, rather than always being rounded to the nearest 16-bit value (or truncated to the one below), a given sample value could be rounded up or down depending on the amplitude of the noise at that instant - and with "correct" noise, the probability of being rounded up vs the probability of being rounded down is directly proportional to the amplitude of the original sample value between the two nearest possible 16-bit sample values.

To put it simply in decimal, an original value of 2.9 has ten times more chance of being rounded to 3.0 that to 2.0, but it can go either way. Without dither, it would always end up as 3.0, and at the limit a sine wave always looks like a square wave. With dither, a sine wave looks like a weird noisy square-ish wave, but sounds like a sine wave plus "uncorrelated" noise. That is because, to have any appreciate of "frequency" (never mind the actual psychoacoustics of how human ears work) you have to look across time. Looking at the dithered output across time, it is a sine wave plus noise. You started with a sine wave, you added noise, this gave you a sine wave plus noise. Then you truncated, but you still have a sine wave plus noise!

(To the really smart people: I know I'm just explaining rectangular dither, while triangular is optimal. To the really really smart people: I know the noise isn't genuinely uncorrelated, but its uncorrelated to its second moment with triangular dither, which Lipshitz and Vanderkooy seem to regard as sufficient - who am I to argue?)

The nature of this distortion is little discussed, but it needs to be understood that it is not like the evenly distributed harmonic distortion (typically mostly 2nd, some 3rd a little 4th and so on) that predominates in the analogue domain - it is randomly distributed 'quantization noise' and is extremely obnoxious at levels over a fraction of a percent.

Without dither, quantisation noise is typically largely harmonic, with the caveat that, being generated in the sampled domain, it will alias above fs/2.

Here are some pictures from Cool Edit Pro.



Without dither, the -90dB FS sine wave has 50% harmonic distortion at 16-bits.

With dither, the harmonic distortion is absent, and there is broadband noise instead.

With noise shaped dither, the level of this noise in the most sensitive region of hearing is lowered, thus increasing the perceived signal to noise ratio.


My claim about 4-bit being distortion-free wasn't an idle boast. I've tried it.

I've even put a 6-bit example on line here...

http://mp3decoders.mp3-tech.org/24bit2.html

Scroll down to "To dither, or not to dither?" and have a listen.

What do you think?

(the noise doesn't sound quite right because I mp3 encoded the result to post it to the web - you can try the experiment yourself and listen to the pure linear PCM output using Cool Edit Pro/Audition)

edit >> we could also discuss CD's notional 'bandwidth' or 'time-domain resolution'. Perhaps not.

We have done so many times before. The threads are in the FAQ.

I won't say any more (like scream TOS 8) because I'm trying to be constructive, but I'm surprised we haven't had a moderator in here!

I hope this post is helpful.

Cheers,
David.

This post has been edited by 2Bdecided: Aug 31 2006, 13:13

[RockFan]

Typical distortion through a top-notch DAC is something like this;

-60dB - 0.22%
-70dB - 3%
-80dB - 8%
-90dB - 30+%
-100dB - distortion = signal

It doesn't matter how many times you say this, or how many times some idiot audiofools say it on other boards, it doesn't make it true!

What? Those numbers are typical of measured results from a good DAC. You do understand that? measurements at the output of a DAC, not digital-domain analysis?

You seem to be one these people that take exception to suggestions that there are any shortcomings in digital audio (and CD in particular) at all, that it's anything but flawless (just like Phillips said it was back in '83), even advocacy of analogue and vinyl disc, to the point of ready and childish name-calling like "audiofool".

If you're happy with CD, I'm not going to take it way from you (although Sony/Phillips might), unlike what befell those who wished to continue buying LPs 15 or so years ago.

The problem is, you see, a not inconsiderable number of people actually would like to be able to choose which format they buy their music on, including vinyl disc, and the obduracy of views like yours don't help very much.

Analogue does have particular virtues (along with it's intractable flaws, of course) in it's reproduction of music and CD is far from "perfect" , however often and loudly militant digiphiles insist otherwise.

And BTW matey, speaking of TOS, you'll probably find the use of insults, even if you think you're being clever and using them obliquely, is also a breech of them.

This post has been edited by RockFan: Aug 31 2006, 16:47