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gsm_option(3) |
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gsm_option — customizing the GSM 06.10 implementation
#include "gsm.h"
int
gsm_option(handle, option, valueP);
gsm handle;
int option;
int * valueP;
The gsm library is an implementation of the final draft GSM 06.10 standard for full-rate speech transcoding, a lossy speech compression algorithm.
The gsm_option() function can be used to set and query various options or flags that are not needed for regular GSM 06.10 encoding or decoding, but might be of interest in special cases.
The second argument to gsm_option specifies what parameter should be changed or queried. The third argument is either a null pointer, in which case the current value of that parameter is returned; or it is a pointer to an integer containing the value you want to set, in which case the previous value will be returned.
The following options are defined:
GSM_OPT_VERBOSE
Verbosity level.
This option is only supported if the library was compiled
with debugging turned on, and may be used by developers of
compression algorithms to aid debugging.
The verbosity level can be changed at any time during
encoding or decoding.
GSM_OPT_FAST
Faster compression algorithm.
This implementation offers a not strictly
standard-compliant, but faster compression algorithm that is
compatible with the regular method and does not noticably
degrade audio quality.
The value passed to
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gsm_option(handle, GSM_OPT_FAST, & value) |
functions as a boolean flag; if
it is zero, the regular algorithm will be used, if not, the
faster version will be used.
The availability of this option depends on the hardware
used; if it is not available, gsm_option will return -1 on
an attempt to set or query it.
This option can be set any time during encoding or
decoding.
GSM_OPT_LTP_CUT
Enable, disable, or query the LTP cut-off optimization.
During encoding, the search for the long-term correlation
lag forms the bottleneck of the algorithm. The ltp-cut
option enables an approximation that disregards most of the
samples for purposes of finding that correlation, and hence
speeds up the encoding at a noticable loss in quality.
The value passed to
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gsm_option(handle, GSM_OPT_LTP_CUT, & value) |
turns the optimization on if
nonzero, and off if zero.
This option can be set any time during encoding or decoding;
it will only affect the encoding pass, not the decoding.
GSM_OPT_WAV49
WAV-style byte ordering.
A WAV file of type #49 contains GSM 06.10-encoded frames.
Unfortunately, the framing and code ordering of the WAV
version are incompatible with the native ones of this GSM
06.10 library. The GSM_OPT_WAV49 option turns on a different
packing algorithm that produces alternating frames of 32 and
33 bytes (or makes it consume alternating frames of 33 and
32 bytes, note the opposite order of the two numbers) which,
when concatenated, can be used in the body of a WAV #49
frame. It is up to the user program to write a WAV header,
if any; neither the library itself nor the toast program
produce complete WAV files.
The value passed to
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gsm_option(handle, GSM_OPT_WAV49, & value) |
functions as a boolean flag; if
it is zero, the library’s native framing algorithm
will be used, if nonzero, WAV-type packing is in effect.
This option should be used before any frames are encoded.
Whether or not it is supported at all depends on a
compile-time switch, WAV49. Both option and compile time
switch are new to the library as of patchlevel 9, and are
considerably less tested than the well-worn rest of the it.
Thanks to Jeff Chilton for the detective work and first free
implementation of this version of the GSM 06.10
encoding.
GSM_OPT_FRAME_CHAIN
Query or set the chaining byte.
Between the two frames of a WAV-style encoding, the GSM
06.10 library must keep track of one half-byte that is
technically part of the first frame, but will be written as
the first four bits of the second. This half-byte are the
lowest four bits of the value returned by, and optionally
set by,
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gsm_option(handle, GSM_OPT_FRAME_CHAIN, & value) |
This option can be queried and set at any time.
GSM_OPT_FRAME_INDEX
Query or set the current frame’s index in a
format’s alternating list of frames.
The WAV #49 framing uses two alternating types of frames.
Which type the next GSM-coded frame belongs to can be
queried, or, when decoding, announced, using
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gsm_option(handle, GSM_OPT_FRAME_INDEX, & value) |
For WAV-style framing, the
value should be 0 or 1; the first frame of an encoding has
an index of 0. At library initialization, the index is set
to zero.
The frame index can be queried and set at any time. Used in
combination with the GSM_OPT_FRAME_CHAIN, option, it
can be used to position on arbitrary GSM frames within a
format like WAV #49 (not accounting for the lost internal
GSM state).
gsm_option() returns -1 if an option is not supported, the previous value of the option otherwise.
Please direct bug reports to jutta@cs.tu-berlin.de and cabo@cs.tu-berlin.de.
toast(1), gsm(3), gsm_explode(3), gsm_print(3)
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gsm_option(3) | |