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Pathological example of a intersample peak, 11dB, discussion.
Rescator
post Jan 10 2013, 04:36
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Pathological example of a intersample peak that was artificially created:

~0dB peak, ~20dBFS RMS (squarewave), +10.87dB intersample peak, 44.1KHz, 32bit float.

http://www.hydrogenaudio.org/forums/index....showtopic=98752

Please keep any discussion of the test sample in this tread, rather than where it's simply "stored".


The problem:
If oversampled the true peak is reveal to be almost +11dB.
A DAC would need 11dB headroom (or alternatively ~12dB which equals 2 bits) to handle this wav correctly.

The solution?:
A "quick fix" for a 24bit (or float) audio chain, would be to reduce the volume by 12dB somewhere.
Volume loss can be later compensated by simply increasing the analog volume (the user turning the knob a little higher).

*** The rest is somewhat opinionated. ***


Thoughts:
As such the "bottom 3 bits" of audio could be considered waste-able, 2 bits to handle pathological intersample peaks, + 1 bit due to quantization/noisefloor/dither.
A "24bit" DAC would have no issues, 21bits to use is a lot. Likewise a "20bit" DAC would still have 17bits to use.
Ideally the 11 (or 12) dB volume reduction would be done by the DAC just before the reconstruction stage.

Issues?:
For a 12dB headroom DAC one would need to crank up the playback volume, so such a DAC would sound more quiet than most other DACs.
Noisefloor of the amplifier and other parts of the equipment/audio chain is also an issue.
But even "cheap" gear has around -80dBFS to -100dBFS noisefloor.
Also considering that a normal living room can easily have a +50dB noisefloor, so loosing out on the 12dB or so of the quietest audio is not an issue.

So if taking CD audio as an example, a 12dB adjustment would cause the content in the -96dBFS to -84dBFS range to be lost.
The loss can be avoided by simply passing the 16bit audio as 24bit or 32bit float instead.
Under Windows Vista and Windows 7 and Windows 8 all audio is changed to 32bit so this is a non-issue.

How to avoid intersample peaks on gear without the needed headroom?:
On Windows you can simply make sure that you never raise the volume (in Windows) above -12dBFS (~45% volume),
and instead use the analog volume knob (if there is one on your system or gear) instead.

11dB really?
Yep! Then again this is a pathological example.
"Normally" the intersample peak is within 1dB of the digital peak, and in some rare cases up to 2 to 3dB higher.
If you make/master music, then the final mix/pressing master/encoding/exporting should have 2 or 3dB headroom.
So as long as no peaks go above 3dB you should be pretty darn safe from causing any clicks or distortion for the end user.

The example here is a pure spike, and humans tend not to like to listen to pops, clicks, static, test tones, or similar.
So encountering anything like this "in the wild" is very rare.

Is it really that bad?:
Please remember that intersample peaks do not damage equipment, at least I've never heard or read about such happening, and the CD was invented like ages ago.
So if this was a practical issue we'd have heard about it along time ago as equipment got fried etc. And we'd have had a solution years ago as well.

The only thing it does is damage the audio quality, that is if you actually can hear/notice it at all. You are more likely to hear crackling/distortion from overly compressed music.
And ironically it is that type of overly compressed music that has the most intersample peaks that go above 0dBFS.
Solution? Stop compressing the hell out of music. Use 20dB or more headroom and intersample peaks will most likely never be an issue.



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"Normality exist in the minds of others, not mine!" - Rescator
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knutinh
post Feb 4 2013, 09:20
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Since inter-sample overshoot is a problem for the analog stage of a DAC, what would happen at the corresponding stage in an ADC, assuming the same analog/digital waveform? Would it clip (producing different digital samples, implying that the samples can only be greated digitally), or would it just pick non-clipped samples? I guess that depends on if the ADC is essentially a text-book passive analog filter hooked up to a point-sampler, or if it is a multirate (digitally filtered using fixed-point arithmetics) design.

It seems that inter-sample over values are quoted with great accuracy and confidence, even though the exact reconstruction filter is not specified. Are you using an accurate approximation of the ideal sinc filter when discussing this? I guess that a different filter (e.g. lower bandwidth, non-linear phase) could produce fairly different results.

-h
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2Bdecided
post Feb 4 2013, 13:08
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QUOTE (knutinh @ Feb 4 2013, 08:20) *
Since inter-sample overshoot is a problem for the analog stage of a DAC...
It's also a problem for the digital section, i.e. the over sampling + reconstruction filter.
QUOTE
...what would happen at the corresponding stage in an ADC, assuming the same analog/digital waveform?
No sane person digitally samples at levels near clipping - they leave sufficient headroom. Insane people who push the levels like that will probably get clipping, either due to the analogue electronics, the digital processing (oversampling ADC and digital anti-alias filter), or the fact that the peak happens to occur on-sample rather than between samples.

The concern is almost completely with audio that has been processed after the ADC to increase the apparent loudness.

QUOTE
It seems that inter-sample over values are quoted with great accuracy and confidence, even though the exact reconstruction filter is not specified.
The EBU R128 definition is pretty strict, though it doesn't necessarily give the absolute highest possible true peak.

Cheers,
David.
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knutinh
post Mar 4 2013, 13:58
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QUOTE (2Bdecided @ Feb 4 2013, 13:08) *
QUOTE (knutinh @ Feb 4 2013, 08:20) *

It seems that inter-sample over values are quoted with great accuracy and confidence, even though the exact reconstruction filter is not specified.
The EBU R128 definition is pretty strict, though it doesn't necessarily give the absolute highest possible true peak.

Cheers,
David.


I vaguely remember something about "phase scrabling" peaks in radio transmission - i.e. messing with the phase so as to minimize peaks while keeping the average levels (or, effectively maximizing the average levels with minimal audible distortion).

Could this be done in a DAC/SRC application? If complexity/delay was of no concern, one could choose between a set of prototype filters that sounded equally good, selecting the filter that minimized intersample overs? Is not this a neater (although certainly overkill) solution than throwing away a few dB of SNR for all material?

Or, one could have a two-path filtering, switching to a cruder interpolation in those few segments where intersample overs are an issue (linear interpolation?)

-k
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Posts in this topic
- Rescator   Pathological example of a intersample peak, 11dB, discussion.   Jan 10 2013, 04:36
- - Rescator   http://forums.digitalspy.co.uk/showthread....25#po...   Jan 10 2013, 07:51
- - Kees de Visser   At a first glance it seems that this test signal i...   Jan 10 2013, 07:57
- - lvqcl   http://www.hydrogenaudio.org/forums/index....st...   Jan 10 2013, 15:52
- - 2Bdecided   While we're quoting past debates, I remember o...   Jan 10 2013, 16:06
- - John_Siau   When designing Benchmark's new DAC2 HGC D/A co...   Jan 10 2013, 16:10
|- - saratoga   QUOTE (John_Siau @ Jan 10 2013, 10:10) I ...   Jan 10 2013, 17:07
|- - Rescator   QUOTE (John_Siau @ Jan 10 2013, 16:10) A ...   Jan 11 2013, 11:27
|- - Banned   QUOTE (Rescator @ Jan 11 2013, 11:27) it ...   Jan 12 2013, 12:24
|- - Rescator   QUOTE (Banned @ Jan 12 2013, 12:24) QUOTE...   Jan 12 2013, 13:14
|- - Banned   QUOTE (Rescator @ Jan 12 2013, 13:14) Nic...   Jan 12 2013, 15:54
|- - Rescator   QUOTE (Banned @ Jan 12 2013, 15:54) Examp...   Jan 12 2013, 18:14
|- - Banned   QUOTE (Rescator @ Jan 12 2013, 18:14) QUO...   Jan 12 2013, 20:52
|- - Rescator   QUOTE (Banned @ Jan 12 2013, 20:52) QUOTE...   Jan 12 2013, 22:53
|- - 2Bdecided   QUOTE (Rescator @ Jan 12 2013, 21:53) To ...   Jan 14 2013, 12:47
- - bennetng   I've seen +4.8dB intersample peak in a song, b...   Jan 10 2013, 18:19
- - 2Bdecided   I don't see how on-sample values above 0dB FS ...   Jan 10 2013, 19:56
- - bennetng   I just generated another synthetic example wavefor...   Jan 10 2013, 20:26
|- - Rescator   QUOTE (bennetng @ Jan 10 2013, 20:26) I j...   Jan 12 2013, 13:22
|- - bennetng   QUOTE (Rescator @ Jan 12 2013, 20:22) QUO...   Jan 12 2013, 16:36
- - bandpass   $ sox InterSamplePeak.wav -n gain -11.9 stats...   Jan 11 2013, 13:13
- - bandpass   Here's how to do the EBU upsampling/filtering ...   Jan 12 2013, 23:17
|- - Rescator   QUOTE (bandpass @ Jan 12 2013, 23:17) Her...   Jan 12 2013, 23:22
- - Rescator   bandpass, 2bdecided, saratoga and John Siau will h...   Jan 14 2013, 00:51
|- - 2Bdecided   QUOTE (Rescator @ Jan 13 2013, 23:51) ban...   Jan 14 2013, 13:00
||- - Rescator   QUOTE (2Bdecided @ Jan 14 2013, 13:00) Ar...   Jan 14 2013, 16:49
||- - bug80   QUOTE (Rescator @ Jan 14 2013, 16:49) QUO...   Feb 4 2013, 12:45
|- - John_Siau   QUOTE (Rescator @ Jan 13 2013, 18:51) ban...   Jan 14 2013, 18:22
- - bandpass   Using LibreOffice (free and available on Windows):   Jan 14 2013, 17:06
- - 2Bdecided   Yep, that's exactly it. (I just used Excel). ...   Jan 14 2013, 17:20
|- - Rescator   (@bandpass Darn you, stop teaching me new tricks. ...   Jan 14 2013, 20:29
|- - bandpass   QUOTE (Rescator @ Jan 14 2013, 19:29) if ...   Jan 15 2013, 09:57
- - John_Siau   The "+11 dB" test signal (that started t...   Jan 14 2013, 18:40
|- - Rescator   QUOTE (John_Siau @ Jan 14 2013, 18:40) Th...   Jan 14 2013, 19:37
- - Wombat   After reading about that topic a bit i found that ...   Jan 14 2013, 22:31
|- - Alexey Lukin   QUOTE (Wombat @ Jan 14 2013, 17:31) After...   Feb 4 2013, 19:49
- - knutinh   Since inter-sample overshoot is a problem for the ...   Feb 4 2013, 09:20
|- - 2Bdecided   QUOTE (knutinh @ Feb 4 2013, 08:20) Since...   Feb 4 2013, 13:08
|- - knutinh   QUOTE (2Bdecided @ Feb 4 2013, 13:08) QUO...   Mar 4 2013, 13:58
|- - John_Siau   QUOTE (knutinh @ Mar 4 2013, 07:58) I vag...   Mar 4 2013, 17:51
|- - 2Bdecided   QUOTE (knutinh @ Mar 4 2013, 12:58) I vag...   Mar 4 2013, 18:21
- - John_Siau   QUOTE (2Bdecided @ Feb 4 2013, 07:08) QUO...   Feb 4 2013, 18:08


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