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Low latency codecs
knutinh
post Apr 27 2011, 07:35
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What are the choices for low latency codecs, and what is the fundamental information/perceptual trade-off?

I see that large block sizes are needed to do operations on narrow frequency bands, suitable for some masking stuff, but exploiting temporal correlation could be done against a historical reference (not introducing significant delay)?

-k
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NullC
post Apr 28 2011, 03:08
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QUOTE (knutinh @ Apr 26 2011, 22:35) *
What are the choices for low latency codecs, and what is the fundamental information/perceptual trade-off?
I see that large block sizes are needed to do operations on narrow frequency bands, suitable for some masking stuff, but exploiting temporal correlation could be done against a historical reference (not introducing significant delay)?


Masking is a very fuzzy thing. As Garf said, it's not masking that gets you, it's the fact that you lose coding gain.

You can do very narrow frequency domain operations from the temporal domain. It's all the same thing, after all. E.g. in Opus CELT mode we have a single backwards looking predictor, but it's only really useful for highly harmonic signals. For those signals it's helps a lot, otherwise it's not really useful.

Backwards-looking prediction of multiple components has the problem that they're not easily separable. For a signal with multiple strong inharmonic tones there would need to be multiple offsets. Consider the problem of a single video block which has separate components moving in different directions at once. You could try to separate the block into over-complete components and then predict those but the separation process is computationally hard and might not be possible to do well with low latency. (Or at least we found that short-time low latency sinusoidal coding didn't appear to work so well, though we perhaps didn't try hard enough this time around as it seemed out of the question computational cost wise)

Making more complicated prediction robust against loss is also quite tricky/impossible, and virtually all low latency applications need to deal with loss (if you can retransmit you probably don't need a low latency codec!)

There are some neat things that could be done if you don't care much about robustness... E.g. http://ieeexplore.ieee.org/xpl/freeabs_all...rnumber=5413930 But techniques that end up resulting in NxN matrix multiplies or approximations therein work a heck of a lot better (complexity wise) for separable 8x8 pixel blocks than they do for 240 sample audio frame.

This post has been edited by NullC: Apr 28 2011, 03:49
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knutinh
post Apr 28 2011, 10:04
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QUOTE (NullC @ Apr 28 2011, 04:08) *
Masking is a very fuzzy thing. As Garf said, it's not masking that gets you, it's the fact that you lose coding gain.

Is it possible to say something about how much of the compression is related to pure source-coding, and how much is relying on psycho-acoustically guided lossy coding?

FLAC can do 2:1 lossless encoding, while AAC can do 10:1 or whatever perceptually "as good as lossless" encoding. Can one assume that the first 50% of the AAC compression stems from source redundancy, while the remaining factor stems from (hopefully) irrelevancy?

-k
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Garf
post Apr 28 2011, 11:30
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QUOTE (knutinh @ Apr 28 2011, 11:04) *
Can one assume that the first 50% of the AAC compression stems from source redundancy, while the remaining factor stems from (hopefully) irrelevancy?


Yes, that looks correct. Another example: MPEG-4 SLS can act as a lossless AAC coder, and when it does, it achieves ratios comparable to classic lossless codecs.
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_mē_
post Apr 28 2011, 15:00
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QUOTE (Garf @ Apr 28 2011, 12:30) *
QUOTE (knutinh @ Apr 28 2011, 11:04) *
Can one assume that the first 50% of the AAC compression stems from source redundancy, while the remaining factor stems from (hopefully) irrelevancy?


Yes, that looks correct. Another example: MPEG-4 SLS can act as a lossless AAC coder, and when it does, it achieves ratios comparable to classic lossless codecs.


A bit OT, but is there any public SLS encoder available?
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Garf
post Apr 28 2011, 16:07
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QUOTE (_mē_ @ Apr 28 2011, 16:00) *
A bit OT, but is there any public SLS encoder available?


There is one in the MPEG reference sources. Those aren't free, of course.
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Posts in this topic
- knutinh   Low latency codecs   Apr 27 2011, 07:35
- - saratoga   QUOTE (knutinh @ Apr 27 2011, 02:35) but ...   Apr 27 2011, 07:51
|- - knutinh   QUOTE (saratoga @ Apr 27 2011, 08:51) Mos...   Apr 27 2011, 08:56
|- - saratoga   QUOTE (knutinh @ Apr 27 2011, 03:56) QUOT...   Apr 28 2011, 03:21
|- - knutinh   QUOTE (saratoga @ Apr 28 2011, 04:21) Try...   Apr 28 2011, 09:55
|- - Garf   QUOTE (knutinh @ Apr 28 2011, 10:55) This...   Apr 28 2011, 11:35
|- - knutinh   QUOTE (Garf @ Apr 28 2011, 12:35) QUOTE (...   Apr 28 2011, 15:07
|- - googlebot   QUOTE (knutinh @ Apr 28 2011, 15:07) 3. V...   Apr 28 2011, 16:10
||- - Garf   QUOTE (googlebot @ Apr 28 2011, 17:10) QU...   Apr 28 2011, 16:15
||- - googlebot   The OP asked for low delay codecs. Opus at 22 ms i...   Apr 28 2011, 17:28
|- - Garf   QUOTE (knutinh @ Apr 28 2011, 16:07) 3. V...   Apr 28 2011, 16:13
- - C.R.Helmrich   QUOTE (knutinh @ Apr 27 2011, 08:35) ...e...   Apr 27 2011, 12:06
- - Garf   In the case of AAC, both Main and LTP profile went...   Apr 27 2011, 13:10
- - NullC   QUOTE (knutinh @ Apr 26 2011, 22:35) What...   Apr 28 2011, 03:08
|- - knutinh   QUOTE (NullC @ Apr 28 2011, 04:08) Maskin...   Apr 28 2011, 10:04
||- - Garf   QUOTE (knutinh @ Apr 28 2011, 11:04) Can ...   Apr 28 2011, 11:30
||- - _mē_   QUOTE (Garf @ Apr 28 2011, 12:30) QUOTE (...   Apr 28 2011, 15:00
||- - Garf   QUOTE (_mē_ @ Apr 28 2011, 16:00) A bit O...   Apr 28 2011, 16:07
|- - knutinh   QUOTE (NullC @ Apr 28 2011, 04:08) Maskin...   Apr 30 2011, 18:58
|- - Garf   QUOTE (knutinh @ Apr 30 2011, 19:58) ...   May 1 2011, 17:09
- - 2Bdecided   NICAM. But it's hardly efficient! Cheer...   Apr 28 2011, 12:03


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