Compressed sound, what format to use?
I suspect you're not resetting the parameters at the start of each block. See http://wiki.multimedia.cx/index.php?title=Microsoft_IMA_ADPCM for how it does that for MS ADPCM.
Author
I suspect you're not resetting the parameters at the start of each block. See http://wiki.multimed...osoft_IMA_ADPCM for how it does that for MS ADPCM.
That sounds about right! Thanks for your reply! Although .. I'm having a bit of troubling getting my head around this.. Does this mean I should do the decoding in chunks? Right now I'm applying the decoding algorithm to the entire data:
[source]int decode(int16* dst, const uint8* src, uint srcSize)[/source]
where srcSize is the size of the encoded data in bits and my predictedValue and stepIndex continuously follow through to the next nibble-iteration. Do I need to instead do this per block (block align in fmt-chunk?)?
perhaps something like this:
[source]int decode(int16* dst, const uint8* src, int srcOffset, uint srcSize)[/source]
where srcOffset is the current offset into the source data?
Thanks in advance!
That sounds about right to me. Note that the MS-ADPCM format stores the predictedValue and stepIndex at the start of each block. I suspect your data will be similar.
You should be able to work out how much extra header data there is - if there was no block header the compression ratio would be exactly 4:1.
You should be able to work out how much extra header data there is - if there was no block header the compression ratio would be exactly 4:1.
Author
The audio I'm working with comes from .wav-files which I presume means I should follow this: http://wiki.multimed...osoft_IMA_ADPCM
I'm a bit confused on the "This field reveals the size of a block of IMA-encoded data." and that it then says "an individual chunk of data begins with the following preamble:". I'm guessing they both refer to the same thing?
So, from what I've interpreted I should do something like this:
Does that look correct? If it does I'll have to post some code because I can't get it bloody right!
I'm a bit confused on the "This field reveals the size of a block of IMA-encoded data." and that it then says "an individual chunk of data begins with the following preamble:". I'm guessing they both refer to the same thing?
So, from what I've interpreted I should do something like this:
foreach(adpcm_block in raw_audio_data)
{
var predictedValue = bytes 0 - 1 of adpcm_block
var stepIndex = byte2 of adpcm_block
// ignore byte 3 of adpcm_block
foreach(4bit nibble in adpcm_block) // this would start on byte 4 (the fifth byte) in the block?
{
decompress(nibble)
}
}Does that look correct? If it does I'll have to post some code because I can't get it bloody right!
I don't think block and chunk are the same thing, if you follow the link to the WAVEFORMATEX structure details and then onto the MSDN page you'll find the following;
Block alignment, in bytes. The block alignment is the minimum atomic unit of data for the wFormatTag format type. If wFormatTag is WAVE_FORMAT_PCM, nBlockAlign must equal (nChannels × wBitsPerSample) / 8. For non-PCM formats, this member must be computed according to the manufacturer's specification of the format tag.
[/quote]
So for a 16bit stereo sample you get (2 x 16) / 8 or 4 bytes per sample.
A 'chunk' on the other hand looks to be header + audio data, which will of course be larger than 4 bytes ;)
Author
Hey phantom and thanks for your reply!
I don't think block and chunk are the same thing, if you follow the link to the WAVEFORMATEX structure details and then onto the MSDN page you'll find the following;
Block alignment, in bytes. The block alignment is the minimum atomic unit of data for the wFormatTag format type. If wFormatTag is WAVE_FORMAT_PCM, nBlockAlign must equal (nChannels × wBitsPerSample) / 8. For non-PCM formats, this member must be computed according to the manufacturer's specification of the format tag.
So for a 16bit stereo sample you get (2 x 16) / 8 or 4 bytes per sample.
A 'chunk' on the other hand looks to be header + audio data, which will of course be larger than 4 bytes ;)
[/quote]
That's exactly what caused my confusion in the first place! Pulling apart wav-files I've been extracting various chunks (look here), which makes the following
If the IMA data is monaural, an individual chunk of data begins with the following preamble:
bytes 0-1: initial predictor (in little-endian format)
byte 2: initial index
byte 3: unknown, usually 0 and is probably reserved
[/quote]
make no sense to me, at least if they are referring to the header+data-chunks. But I would think they're actually referring to the blocks - on the other hand I heard/read/can't remember somewhere that IMA ADPCM cannot be decompressed per-block which would contradict each block having a header, that could've been in a dream though.. CONFUSED!
Edit: Uh, I'm running so many parallell possible solutions to this little problem - I'll go back to the one I was working on when posting the images above and apply my new knowledge. This thread can be put on ice for now.
This topic is closed to new replies.
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