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
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
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
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
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
gsm_option(handle, GSM_OPT_WAV49, & value)
functions as a boolean flag; if it is zero, the librarys 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,
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 frames index in a formats
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
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).