On Fri, 6 Jun 2014, nu774 wrote:
Maybe for the sake of bit-exact identical result with Apple's implementation?
They ships sample IMA4 encoder/decoder in AudioCodecSDK_ACPlugIn.zip .
It can be downloaded from Apple site, but you can browse them here (someone
has mirrored at github):
https://github.com/darcyliu/CocoaSampleCode/tree/master/AudioCodecSDK_ACPlugIn/Codecs/IMA4
As far as I can see from Apple source, they use 16bit predictor internally
but only writes MSB 9 bits to each frame.
The 9 bits predictor in the stream is picked only when internally predicted
value significantly differs from it (usually due to seeking or something).
Oh, thanks for pointing this out. I suspected it was to try to match
apple's decoder, but when I tried decoding using QuickTime and exporting
to a wav file, tiny_psnr found it to be quite different from our output
(to the point that it was hard to say if this actually made any difference
or not).
// Martin
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