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What's the meaning of "sample rate" in MP3/Vorbis/AAC?, Considering the audio was transformed
sheh
post Oct 7 2012, 03:44
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What's the meaning of "sample rate" in MP3/Vorbis/AAC? Wouldn't it be meaningless after transform, same as bit depth?

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pdq
post Oct 10 2012, 17:12
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So am I understanding correctly that 99.9% of decoding is independent of the original sample rate, and only at the very end where the wav header is filled in does it even matter?
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saratoga
post Oct 10 2012, 17:30
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QUOTE (benski @ Oct 10 2012, 11:16) *
QUOTE (lvqcl @ Oct 7 2012, 13:56) *
Winamp MP3 decoder still have Full/Half/Quarter quality setting


Yes, and it does this by not doing the calculations for all the bands, and only doing a 16 or 8pt iMDCT at the end. Remember that MP3 is cascade of 32 18-pt iMDCTs followed by a 32-pt iMDCT. This was done mainly to reuse hardware and software from Layer 2 decoders. From memory, I think there's a step in between that would prevent an implementation from simply using a 576 point MDCT.


Its actually a subband decomposition followed by an MDCT on each subband. So basically when you decode you do the iMDCT which gives you a bunch of MP2 style subbands. Then you use an inverse filterbank to put the subbands back into a single signal. The downsample by 2 trick works because of some symmetry in the filterbank that lets you throw away half of the samples in exchange for half as much work in the filterbank. Since the filterbank is much slower then the iMDCT, this was used on old systems to speed up decoding. Its not really useful though once you break ~50-60MHz CPUs.

QUOTE (pdq @ Oct 10 2012, 12:12) *
So am I understanding correctly that 99.9% of decoding is independent of the original sample rate, and only at the very end where the wav header is filled in does it even matter?


No, I would say the opposite. In a transform codec, entire lossy decoding process (everything after huffman decoding) depends on the sample rate.

Edit: For example, a lot of codecs don't even use the same MDCT at different sample rates (e.g. Vorbis, WMA).

This post has been edited by saratoga: Oct 10 2012, 17:32
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Posts in this topic
- sheh   What's the meaning of "sample rate" in MP3/Vorbis/AAC?   Oct 7 2012, 03:44
- - saratoga   No, the transform occurs on sampled data and so th...   Oct 7 2012, 05:05
|- - sheh   I guess the way I put it does seem like a retread....   Oct 9 2012, 05:21
|- - saratoga   QUOTE (sheh @ Oct 9 2012, 00:21) Benski s...   Oct 9 2012, 19:03
- - pdq   Still. since the data are stored as frequencies an...   Oct 7 2012, 14:23
- - [JAZ]   When we say that lossy streams don't have a st...   Oct 7 2012, 14:44
- - jensend   I'm not intimately acquainted with the details...   Oct 7 2012, 16:46
- - benski   fs is part of the MDCT equation. If you didn...   Oct 7 2012, 18:32
|- - sheh   Okay, so the original fs needs to be plugged in so...   Oct 8 2012, 18:33
- - lvqcl   Winamp MP3 decoder still have Full/Half/Quarter qu...   Oct 7 2012, 18:56
|- - benski   QUOTE (lvqcl @ Oct 7 2012, 13:56) Winamp ...   Oct 10 2012, 16:16
- - saratoga   Using a dct means you could easily drop the sample...   Oct 7 2012, 19:00
|- - saratoga   QUOTE (sheh @ Oct 8 2012, 13:33) Okay, so...   Oct 8 2012, 23:41
- - jensend   I still think you're wrong here. Once you...   Oct 7 2012, 19:57
|- - saratoga   QUOTE (jensend @ Oct 7 2012, 14:57) I sti...   Oct 7 2012, 20:16
- - Dynamic   I think one problem is that we're never doing ...   Oct 8 2012, 23:04
- - [JAZ]   Mmm.. the tranformation that (most) lossy codecs d...   Oct 9 2012, 19:03
|- - Dynamic   QUOTE ([JAZ] @ Oct 9 2012, 19:03)...   Oct 9 2012, 22:22
- - pdq   So am I understanding correctly that 99.9% of deco...   Oct 10 2012, 17:12
|- - saratoga   QUOTE (benski @ Oct 10 2012, 11:16) QUOTE...   Oct 10 2012, 17:30
- - [JAZ]   QUOTE (Dynamic @ Oct 9 2012, 23:22) As [J...   Oct 10 2012, 17:52
|- - saratoga   QUOTE ([JAZ] @ Oct 10 2012, 12:52...   Oct 10 2012, 18:20
- - [JAZ]   Winamp with the in-house decoder (i.e. not the fra...   Oct 10 2012, 18:41
|- - saratoga   QUOTE ([JAZ] @ Oct 10 2012, 13:41...   Oct 10 2012, 18:55
- - Dynamic   My recollection is that Winamp 1.7 running under W...   Oct 11 2012, 16:14
- - sheh   So sample rate matters. I suppose without being fa...   Oct 11 2012, 23:20


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