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 noticeably 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 noticeable 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 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
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,
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
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).