dmG726Encode - implements ITU (International Telecommunication Union)
Recommendation G.726 ADPCM compression for input of 64 kbit/s A-law, mulaw,
or linear data and output of compressed bit stream of 40, 32 ,24, or
16 kbit/s. G.726 replaced G.721 and G.723.
DMstatus dmG726Encode(DMG726encoder handle,
unsigned char *obuf
handle DMG726encoder structure, created by dmG726EncoderCreate(3dm),
specifies the signal processing parameters.
ibuf pointer to input sample data buffer, an array of short (16
bits) for linear input or unsigned char for A-law or mu-law
input. For linear input, samples are assumed to be two's
complement. Data buffer must be a multiple (the same multiple
as that of output bitstrem buffer) of 4, 8, 2, or 8 for
operations at 16, 24, 32, or 40 kbit/s respectively. This
assumes the sampling rate of 8 kHz.
obuf pointer to output compressed data buffer, an array of bytes.
The data format is unsigned char (8-bit). The output bitstrem
buffer consists of a multiple of 1, 3, 1, or 5 bytes, for
operations at 16, 24, 32, or 40 kbit/s respectively.
number of samples to be processed. it must always be a multiple
of 4, 8, 2, or 8 for operations at 16, 24, 32, or 40 kbit/s
Returns DM_FAILURE or DM_SUCCESS.
dmG726Encode(3dm) implements ITU G726 compression. G.726 compression
compresses one input sample to 2, 3, 4, or 5 bits for 16, 24, 32, or 40
kbit/s operations respectively. 16, 24, 32, or 40 kbit/s operation is
indicated by setting parameter DM_AUDIO_BITRATE to DM_G726_16_KBITS,
DM_G726_24_KBITS, DM_G726_32_KBITS, or DM_G726_40_KBITS using DMparams
and passed to dmG726EncoderSetParams(3dm). Input format is indicated by
setting DM_G726_NATIVE_FORMAT to DM_G726_ULAW, DM_G726_ALAW, or
DM_G726_LINEAR_PCM, respectively for inputting of mu-law, A-law, or
linear PCM data, using DMparams and passed to
dmG726EncoderSetParams(3dm). If input format is DM_G726_LINEAR_PCM, the
PCM data type will be short. Otherwise, the PCM data type will be
unsigned char. Therefore, the inbuf is void in the function prototyping.
G726 compression uses a linear prediction scheme. This means that the
compressed bits, which carrying compressed information, are interrelated
or interdependent. If a application calls afSeekFrame(3dm) to an offset
into the compressed bit stream file and then starts reading bits in the
middle of the bit stream and decompress them with dmG726Decode(3dm), the
interdependency of the bits at the cutting 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
There is no way to exactly recreate the decoded data in the middle of the
bit stream file without going all the way back to the beginning.
However, with a "preroll" value to read ahead a portion of the compressed
file, application can compensate for the amplitude problem.
G.726 algorithm was developed for sampling rate of 8 kHz, but could be
used from 8 kHz to 48 kHz.
dmG726EncoderDestroy(3dm), dmG726Decode(3dm), ITU (International
Telecommunication Union) Recommendation G.726.
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