dmDVIAudioDecode - decode audio data compressed using the IMA
(Interactive Multimedia Association) Recommended ADPCM decompression,
based on Intel's DVI algorithm.
DMstatus dmDVIAudioDecode(DMDVIaudiodecoder handle,
unsigned char *ibuf, short *obuf,
handle DMDVIaudiodecoder structure, created by a previous call to
dmDVIAudioDecoderCreate(3dm). It contains the state associated
with the decoder.
ibuf A pointer to the compressed input data buffer. The data format
is unsigned char (8-bit).
obuf A pointer to the output sample data buffer. The data format is
short (16-bit). The samples are assumed to be two's
complement. Sampling rate may range from 8 kHz to 48 kHz, but
has to be the same as encoder.
nsamples The number of samples to be processed. For the 4:1 algorithm
(the only one currently supported), the value of nsamples
passed to the DVI encode/decode routines must always be a
multiple of 2.
Returns DM_FAILURE or DM_SUCCESS.
dmDVIAudioDecode(3dm) implements Intel, Inc.'s DVI IMA4:1 ADPCM
compression for monaural audio data.
DVI IMA ADPCM compression exists in two forms, 4 bits per sample (IMA4:1)
and 3 bits per sample (IMA3:1). The compression form may be retrieved
using dmDVIAudioDecoderGetParams(3dm) via the parameter
DM_DVI_AUDIO_BITS_PER_SAMPLE. Its possible values are:
DM_DVI_AUDIO_4BITS_PER_SAMPLE [Toc] [Back]
4:1 compression. This is the only form currently supported by the
SGI Digital Media libraries.
DM_DVI_AUDIO_3BITS_PER_SAMPLE [Toc] [Back]
3:1 compression. Currently not supported.
DVI AUDIO FILE BLOCK MODE [Toc] [Back]
DVI audio compression uses a linear prediction scheme. This means that
the compressed bits are interrelated or interdependent. If a application
seeks to an offset into the compressed bit stream file and then starts
reading at that point in the stream and decompresses it via
dmDVIAudioDecode(3dm), the interdependency of the bits at the start point
is lost. The application will initially get back data with a lower
amplitude than if he had read the same data back during a complete pass
over the file from the beginning.
In pure DVI ADPCM data, there is no way to accurately begin decoding data
in the middle of the bit stream file without starting the beginning. Two
solutions to this problem have been developed, one by Intel, Inc. and one
by Apple, Inc. Both involve breaking the compressed data up into blocks,
each with a small header encapsulating the information needed to allow
decoding to begin at any block boundry with no loss of data. WAVE(4) and
AIFF-C(4) format sound files using IMA DVI compression take advantage of
this block method, and if such a file is read using the Audio File
Library, accurate file seeks are possible.
The data contained in one of these files may also be decoded accurately
using dmACConvert(3dm) if it is configured properly.
CONFIGURING DMAC TO PARSE IN BLOCK MODE [Toc] [Back]
Three parameters are needed to configure the DMaudioconverter to allow
parsing of these block streams:
DM_AUDIO_CODEC_MAX_BYTES_PER_BLOCK [Toc] [Back]
Indicates the number of bytes of compressed data per block,
including the header. For AIFF-C(4) files, this value is always 34
* channel_count. For WAVE(4) files, this value varies. If
afOpenFile(3dm) is used to open a WAVE(4) file, the value of this
parameter can be retrieved directly using afGetFormatParams(3dm).
DM_AUDIO_CODEC_FRAMES_PER_BLOCK [Toc] [Back]
Indicates the number of frames of uncompressed data contained in
each block. For AIFF-C(4) files, this value is always 64. For
WAVE(4) files, this value varies. If afOpenFile(3dm) is used to
open a WAVE(4) file, the value of this parameter can be retrieved
directly using afGetFormatParams(3dm).
DM_DVI_AUDIO_BLOCK_TYPE [Toc] [Back]
This DVI-specific parameter specifies whether the data is compressed
using the Intel scheme, the Apple scheme, or no blocking at all
(i.e., "pure" DVI). Its possible values are:
DM_DVI_AUDIO_BLOCK_TYPE_NONE -- For stream use only.
DM_DVI_AUDIO_BLOCK_TYPE_APPLE -- For AIFF-C files.
DM_DVI_AUDIO_BLOCK_TYPE_INTEL -- For WAVE files.
Stereo audio streams and streams with block data cannot be parsed using
dmDVIAudioDecode() directly -- you must use dmACConvert(3dm). It is
highly recommended that the Audio File Library be used for all to- and
from-disk compression and decompression.
dmDVIAudioDecoderDestroy(3dm), dmDVIAudioEncode(3dm), afReadFrames(3dm),
IMA Recommended Practices for Enhancing Digital Audio Compatibility in
Multimedia Systems, rev 3.00.
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