dmGSMEncode(3dm)

NAME
     dmGSMEncode - implements the European GSM 06.10 provisional standard for
     full-rate speech transcoding, prI-ETS 300 036, which uses RPE/LTP
     (regular pulse excitation/long term prediction) coding at 13 kbit/s.


SYNOPSIS
     #include <dmedia/dm_audioutil.h>

     DMstatus dmGSMEncode(DMGSMencoder handle,
                       short *ibuf, unsigned char *obuf, int numSamples)


PARAMETER
     handle    DMGSMencoder structure, created by dmGSMEncoderCreate(3dm),
               specifies the signal processing parameters.


     ibuf      pointer to input sample data buffer, an array of size of a
               multiple of 160.  The data format is short (16-bit).  The
               samples are assumed to be two's complement.   The sampling rate
               must be 8 kHz.


     obuf      pointer to output data buffer, an array of a multiple of 33
               bytes.  The data format is unsigned char (8-bit).


     numSamples
               number of sample in the input buffer to be processed.  the
               value of numSamples passed to the GSM decode/decode routines
               must always be a multiple of 160.  The compressed bitstrem
               buffer consists of a multiple of 33 bytes.


RETURNED VALUE
     Returns DM_FAILURE or DM_SUCCESS.


DESCRIPTION
     dmGSMEncode(3dm) implements GSM 06.10 compression.


     GSM 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 dmGSMDecode(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
     the beginning.
     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.


NOTE
     The compressed bitrate now is 13200 bits/s.  This is not the same as
     standard specification of 13000 bits/s because the first 4 bits of each
     33 byte are not useful information bits.  This bits arrangement was
     adopted using the original scheme by Jutta Degener (jutta@cs.tu-
     berlin.de) and Carsten Bormann (cabo@cs.tu-berlin.de), Communications and
     Operating Systems Research Group, TU Berlin for compatibility with
     typical UNIX applications.  And may be modify later.

     GSM algorithm was developed for sampling rate of 8 kHz.


SEE ALSO
     dmGSMEncoderCreate(3dm), dmGSMEncoderDestroy(3dm), dmGSMDecode(3dm), ETSI
     GSM 06.10 standard.