dmG728Encode(3dm)					     dmG728Encode(3dm)

NAME    [Toc]    [Back]

     dmG728Encode - implements the ITU (International Telecommunication	Union)
     Recommendation G.728 compression (LD-CELP).

SYNOPSIS    [Toc]    [Back]

     #include <dmedia/dm_audioutil.h>

     DMstatus dmG728Encode(DMG728encoder handle,
		       short *ibuf, unsigned char *obuf, int nsamples)

PARAMETER    [Toc]    [Back]

     handle    DMG728encoder structure,	created	by dmG728EncoderCreate(3dm),
	       specifies the signal processing parameters.


     ibuf      pointer to input	sample data buffer. 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.  The data	format is unsigned
	       char (8-bit).  This bitstrem buffer must	consist	of a multiple
	       of 10 bytes.


     nsamples  number of samples in the	input buffer to	be processed. the
	       value of	nsamples passed	to the G.728 encode/decode routines
	       must always be a	multiple of 40.

RETURNED VALUE    [Toc]    [Back]

     Returns DM_FAILURE	or DM_SUCCESS.

DESCRIPTION    [Toc]    [Back]

     dmG728Encode(3dm) implements the ITU Recommendation G.728 compression.
     The uncompressed data sampling rate is 8000 Hz.  And the compressed
     bitrate is	16000 bits/s.


     ITU Recommendation	G.728 compression also 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 dmG728Decode(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.



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dmG728Encode(3dm)					     dmG728Encode(3dm)



     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.  We recommend
     2 second "preroll".

NOTE    [Toc]    [Back]

     G.728 algorithm was developed for sampling	rate of	8 kHz.

SEE ALSO    [Toc]    [Back]

     dmG728EncoderCreate(3dm), dmG728EncoderDestroy(3dm), dmG728Decode(3dm),
     ITU Recommendation	G.728.


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