184 lines
5.5 KiB
Groff
184 lines
5.5 KiB
Groff
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.\"
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.\" Copyright 1992-1995 by Jutta Degener and Carsten Bormann, Technische
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.\" Universitaet Berlin. See the accompanying file "COPYRIGHT" for
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.\" details. THERE IS ABSOLUTELY NO WARRANTY FOR THIS SOFTWARE.
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.\"
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.PU
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.TH GSM_OPTION 3
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.SH NAME
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gsm_option \(em customizing the GSM 06.10 implementation
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.SH SYNOPSIS
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#include "gsm.h"
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.PP
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int gsm_option(handle, option, valueP);
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.br
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gsm handle;
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.br
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int option;
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.br
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int * valueP;
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.SH "DESCRIPTION"
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The gsm library is an implementation of the final draft GSM 06.10
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standard for full-rate speech transcoding, a lossy
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speech compression algorithm.
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.PP
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The gsm_option() function can be used to set and query various
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options or flags that are not needed for regular GSM 06.10 encoding
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or decoding, but might be of interest in special cases.
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.PP
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The second argument to gsm_option specifies what parameter
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should be changed or queried.
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The third argument is either a null pointer, in which case
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the current value of that parameter is returned;
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or it is a pointer to an integer containing the value
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you want to set, in which case the previous value will
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be returned.
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.PP
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The following options are defined:
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.PP
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.I GSM_OPT_VERBOSE
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Verbosity level.
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.br
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.in+5
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This option is only supported if the library was compiled
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with debugging turned on, and may be used by developers of
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compression algorithms to aid debugging.
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.br
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The verbosity level can be changed at any time during encoding or decoding.
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.in-5
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.sp
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.PP
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.I GSM_OPT_FAST
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Faster compression algorithm.
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.br
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.in+5
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This implementation offers a not strictly standard-compliant, but
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faster compression algorithm that is compatible with the regular
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method and does not noticably degrade audio quality.
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.br
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The value passed to
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.br
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.nf
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gsm_option(handle, GSM_OPT_FAST, & value)
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.fi
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.br
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functions as a boolean flag; if it is zero, the regular algorithm
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will be used, if not, the faster version will be used.
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.br
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The availability of this option depends on the hardware used;
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if it is not available, gsm_option will return -1 on an attempt
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to set or query it.
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.br
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This option can be set any time during encoding or decoding.
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.in-5
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.ne 5
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.sp
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.PP
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.I GSM_OPT_LTP_CUT
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Enable, disable, or query the LTP cut-off optimization.
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.br
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.in+5
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During encoding, the search for the long-term correlation
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lag forms the bottleneck of the algorithm.
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The ltp-cut option enables an approximation that disregards most
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of the samples for purposes of finding that correlation,
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and hence speeds up the encoding at a noticable loss in quality.
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.br
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The value passed to
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.br
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.nf
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gsm_option(handle, GSM_OPT_LTP_CUT, & value)
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.fi
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.br
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turns the optimization on if nonzero, and off if zero.
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.br
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This option can be set any time during encoding
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or decoding; it will only affect the encoding pass, not
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the decoding.
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.sp
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.PP
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.I GSM_OPT_WAV49
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WAV-style byte ordering.
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.br
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.in+5
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A WAV file of type #49 contains GSM 06.10-encoded frames.
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Unfortunately, the framing and code ordering of the WAV version
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are incompatible with the native ones of this GSM 06.10 library.
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The GSM_OPT_WAV49 option turns on a different packing
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algorithm that produces alternating frames of 32 and 33 bytes
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(or makes it consume alternating frames of 33 and 32 bytes, note
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the opposite order of the two numbers) which, when concatenated,
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can be used in the body of a WAV #49 frame.
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It is up to the user program to write a WAV header, if any;
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neither the library itself nor the toast program produce
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complete WAV files.
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.br
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The value passed to
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.br
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.nf
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gsm_option(handle, GSM_OPT_WAV49, & value)
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.fi
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.br
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functions as a boolean flag; if it is zero, the library's native
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framing algorithm will be used, if nonzero, WAV-type packing is in effect.
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.br
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This option should be used before any frames are encoded.
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Whether or not it is supported at all depends on a
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compile-time switch, WAV49.
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Both option and compile time switch are new to the library
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as of patchlevel 9, and are considerably less tested than the
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well-worn rest of the it.
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.br
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Thanks to Jeff Chilton for the detective work and first free
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implementation of this version of the GSM 06.10 encoding.
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.sp
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.PP
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.I GSM_OPT_FRAME_CHAIN
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Query or set the chaining byte.
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.br
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.in+5
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Between the two frames of a WAV-style encoding, the GSM 06.10 library
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must keep track of one half-byte that is technically part of the first
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frame, but will be written as the first four bits of the second.
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This half-byte are the lowest four bits of the value returned by,
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and optionally set by,
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.br
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.nf
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gsm_option(handle, GSM_OPT_FRAME_CHAIN, & value)
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.fi
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.br
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This option can be queried and set at any time.
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.sp
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.PP
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.I GSM_OPT_FRAME_INDEX
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Query or set the current frame's index in a format's
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alternating list of frames.
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.br
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.in+5
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The WAV #49 framing uses two alternating types of frames.
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Which type the next GSM-coded frame belongs to can be queried, or,
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when decoding, announced, using
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.br
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.nf
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gsm_option(handle, GSM_OPT_FRAME_INDEX, & value)
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.fi
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.br
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For WAV-style framing, the value should be 0 or 1; the first frame
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of an encoding has an index of 0.
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At library initialization, the index is set to zero.
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.br
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The frame index can be queried and set at any time.
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Used in combination with the
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.IR GSM_OPT_FRAME_CHAIN ,
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option, it can be used to position on arbitrary GSM frames
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within a format like WAV #49 (not accounting for the lost
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internal GSM state).
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.in-5
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.SH "RETURN VALUE"
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gsm_option() returns -1 if an option is not supported, the
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previous value of the option otherwise.
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.SH BUGS
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Please direct bug reports to jutta@cs.tu-berlin.de and cabo@cs.tu-berlin.de.
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.SH "SEE ALSO"
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toast(1), gsm(3), gsm_explode(3), gsm_print(3)
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