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replaygain, intersample overs, and reconstruction stages
SiriusB
post Nov 30 2007, 23:25
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My understanding of intersample overs is that they are waveform peaks occuring between samples, that aren't reconstructed to their 'true' amplitude by consumer gear because the samples on either side of the peak are too close to 0dBFS already. These intersample peaks can be as much as 6dB 'over' 0dBFS, or more commonly a few dB over. They 'happen' when a production is monitored using peak meters that only measure peak values of the samples, rather than the levels of the reconstructed waveforms. So if an engineer slams the sample peaks repeatedly up to near 0dB, as part of the loudness wars, the file will still show as 'legal' using the inadequate meter. It will be 'illegal' in terms of the level of the true waveform peak levels. Consumer gear and consumer software D/A stages don't 'know how' to reconstruct peaks that are >0dBFS, outputting distortion instead.

Assuming I've got this right (tell me if not), does lowering sample peak level a few dB in the digital domain -- e.g. by applying negative replaygain values -- prior to D/A reconstruction increase our chance of recovering the 'true' waveform at output? Or is that accurate peak shape lost forever?

Another way to phrase this is, how do consumer D/A stages normally reconstruct peaks that occur between samples, and why can't they do it when such peaks are over 0dBFS?

This post has been edited by SiriusB: Nov 30 2007, 23:38
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DVDdoug
post Dec 5 2007, 22:30
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I don't know what "pro gear" does to "reconstruct" the wave. However, I do know that the true intersample data is LOST! For all we know, there may have been an intersample dip rather than an intersample peak! (We do know that there is not a complex waveform between samples, because the signal has been low-pass filtered as required by Nyquist.)

A low-pass filter will often have a slight peak (1 to 3dB) just-before the cutoff frequency. The low-pass filter following the DAC could "reconstruct" a high-frequency peak greater than 0dBFS. However, the filter and following analog circuitry will generally have some headroom above 0dB, so this should not contrubute to clipping. (If the analog section is causing clipping, then digitally reducing the gain will help.)

QUOTE
I don't recall seeing 'new' curved peaks when I use Audition to look at the waveforms of loud tracks I've replaygained. Flat-topped peaks still look flat-topped.
When you "look" at the waveforms, you are looking at the digitized samples. So, there is no intersample data (or intersample reconstruction). You would have to observe the analog output with an oscilloscope to see these effects. And, ReplayGain does not change the shape of the wave. In fact, it doesn't change audio data at all! It simply changes the gain applied during playback.

This post has been edited by DVDdoug: Dec 5 2007, 23:03
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SiriusB
post Dec 5 2007, 23:03
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QUOTE (DVDdoug @ Dec 5 2007, 16:30) *
I don't know what "pro gear" does to "reconstruct" the wave. However, I do know that the true intersample data is lost! For all we know, there may have been an intersample dip rather than an intersample peak! (We do know that there is not a complex waveform between samples, because the signal has been low-pass filtered as required by Nyquist.)


'Reconstruct' is not bizarre terminology -- digital gear uses reconstruction filters. And my refefnce to 'pro gear' being able to reconstruct the correct intersample amplitude between two samples near or at 0dBFS, is straight from the papers of Lund or Aldrich.

QUOTE
A low-pass filter will often have a slight peak just-before the cutoff frequency. The low-pass filter following the DAC could "reconstruct" a high-frequency peak greater than 0dBFS. However, the filter and following analog circuitry will generally have some headroom above 0dB, so this should not contrubute to clipping.

QUOTE
I don't recall seeing 'new' curved peaks when I use Audition to look at the waveforms of loud tracks I've replaygained. Flat-topped peaks still look flat-topped.
When you "look" at the waveforms, you are looking at the digitized samples.


But in Audition at least, you aren't looking at a simple 'connect the samples with a straight line' representation. For example, a 44.1 kHz-sampled 15kHz sine wave in Audition definitely shows some peaks rising and falling between two samples. That's the software doing a visual representation of predicted output (a 'reconstruction' if you will). I was just curious how it knows to show that part of the curve a peak, rather than say, a flat line between the two samples, or even a dip.

My recollection was that flat intersample regions stayed flat when viewed after downward digital amplification (eg after applying replaygain values to the file to 'see' what it would look like). But no longer trusting my 'recollection', I just did this experiment myself in Audition -- first I normalized such a region of a 15 kHz sine wav (ripped from a home audio setup CD with test tones) so that peak sample value is 0dBFS. Now the above-mentioned intersample 'peak' is flat. Saved the file. Reopened it. Reduced level so samples peak at -3 dB instead. Et voila, the original rounded intersample peak is back. So I'm a believer that it can work.


QUOTE
So, there is no intersample data (or intersample reconstruction). You would have to observe the analog output with an oscilloscope to see these effects. And, ReplayGain does not change the shape of the wave. In fact, it doesn't change audio data at all! It simply changes the gain applied during playback.



I already knew what replaygain does and does not do in this regard, thanks. I know the software itself merely calculates a digital amplification value, and that playback software merely applies this to output, not altering the actual file itself -- unless you tell it to (e.g. an option for foobar2000 file saving) in order to create a 'permanently' re-levelled version.

This post has been edited by SiriusB: Dec 5 2007, 23:59
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greynol
post Dec 5 2007, 23:35
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QUOTE (SiriusB @ Dec 5 2007, 14:03) *
I normalized such a region of a 15 kHz sine wav (ripped from a home audio setup CD with test tones) so that peak sample value is 0dBFS. Now the above-mentioned intersample 'peak' is flat. Saved the file. Reopened it. Reduced level so samples peak at -3 dB instead. Et voila, the original rounded intersample peak is back. So I'm a believer.
So we're good here.

Now what's left is your question about consumer gear. Did the documents at t.c. electronic linked by mixminus1 provide any information that you found helpful?


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Posts in this topic
- SiriusB   replaygain, intersample overs, and reconstruction stages   Nov 30 2007, 23:25
- - greynol   QUOTE (SiriusB @ Nov 30 2007, 14:25) Assu...   Nov 30 2007, 23:54
- - mixminus1   TC Electronic has some very good papers on various...   Dec 1 2007, 00:39
- - Kees de Visser   QUOTE (SiriusB @ Dec 1 2007, 00:25) Anoth...   Dec 1 2007, 08:25
- - SebastianG   QUOTE (SiriusB @ Nov 30 2007, 23:25) Thes...   Dec 2 2007, 10:02
- - SiriusB   Check the papers by Aldrich and by Lund. 6dB is...   Dec 3 2007, 06:53
- - SebastianG   Oh, it's even worse! Theoretically, you ca...   Dec 3 2007, 17:45
|- - knutinh   QUOTE (SebastianG @ Dec 3 2007, 17:45) Oh...   Dec 7 2007, 23:12
|- - SebastianG   QUOTE (knutinh @ Dec 7 2007, 23:12) QUOTE...   Dec 8 2007, 09:42
- - SiriusB   My questions are ultimately about reconstruction. ...   Dec 5 2007, 19:20
|- - greynol   QUOTE (SiriusB @ Dec 5 2007, 10:20) And d...   Dec 5 2007, 19:43
|- - SiriusB   QUOTE (greynol @ Dec 5 2007, 13:43) QUOTE...   Dec 5 2007, 21:44
- - greynol   If you reduce the level of the data before it hits...   Dec 5 2007, 22:07
- - DVDdoug   I don't know what "pro gear" does to...   Dec 5 2007, 22:30
|- - greynol   QUOTE (DVDdoug @ Dec 5 2007, 13:30) I don...   Dec 5 2007, 22:57
|- - SiriusB   QUOTE (DVDdoug @ Dec 5 2007, 16:30) I don...   Dec 5 2007, 23:03
|- - greynol   QUOTE (SiriusB @ Dec 5 2007, 14:03) I nor...   Dec 5 2007, 23:35
|- - SiriusB   QUOTE (greynol @ Dec 5 2007, 17:35) QUOTE...   Dec 5 2007, 23:57
|- - Vitecs   Interesting subject. My 2 cents: QUOTE (SiriusB ...   Dec 7 2007, 15:58
- - greynol   I'd really have to see screenshots of what you...   Dec 6 2007, 00:04
- - SiriusB   That's the sort of commercial CD example I...   Dec 7 2007, 17:54
- - knutinh   1. Nyquist was right. In theory, a waveform of ban...   Dec 7 2007, 22:54
|- - benski   QUOTE (knutinh @ Dec 7 2007, 16:54) 5. Co...   Dec 7 2007, 23:05
|- - greynol   QUOTE (benski @ Dec 7 2007, 14:05) Your s...   Dec 8 2007, 22:33
- - greynol   It's quite easy to show a sync pulse in CEP/Au...   Dec 7 2007, 23:06


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