afGetCompressionParams man page on IRIX

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afGetCompression(3dm)					 afGetCompression(3dm)

NAME
     afGetCompression, afGetCompressionParams - get the compression type and
     parameters for an audio track from an AFfilehandle structure

SYNOPSIS
     #include <dmedia/audiofile.h>

     int afGetCompression(AFfilehandle file, int trackid)

     int afGetCompressionParams(AFfilehandle file, int trackid,
		      int *compression, AUpvlist pvlist, int numitems)

PARAMETER
     file	   expects an AFfilehandle structure, previously created by a
		   call to afOpenFile(3dm).

     trackid	   is an integer which identifies an audio track in the file.
		   Since all currently supported file formats allow at most
		   one audio track per file, the value AF_DEFAULT_TRACK should
		   always be used for this argument for now.

     compression   is a pointer to an integer which will be filled in with the
		   symbolic constant indicating which compression scheme used
		   for the specified audio track.

     pvlist	   is an AUpvlist structure previously created via a call to
		   AUpvnew(3dm), initialized with parameters and parameter
		   types using AUpvsetparam(3dm) and AUpvsetvaltype(3dm).  It
		   will be filled with parameter values related to the
		   compression scheme of the specified audio track.

     numitems is the number of valid entries in the pvlist.

RETURN VALUE
     afGetCompression() returns a positive symbolic integer constant which
     describes the type of audio compression used for the specified audio
     track, or -1 on failure.

     afGetCompressionParams() returns the same symbolic integer in
     compression.  Additionally, the codec-specific parameters of pvlist are
     scanned up to numitems and the values are filled in.

DESCRIPTION
     Both afGetCompression() and afGetCompressionParams() are obsolete, having
     been replaced by afGetFormatParams(3dm), which uses the newer
     DMparams(3dm) parameter lists.  These older routines remain for backwards
     compatibility.

     afGetCompression() returns the type of audio compression used for an
     audio track.  Valid compression identifiers include:

									Page 1

afGetCompression(3dm)					 afGetCompression(3dm)

     AF_COMPRESSION_NONE	      no compression

     AF_COMPRESSION_G711_ULAW	      64kbps CCITT G.711 ulaw encoding, 8 kHz
				      16-bit

     AF_COMPRESSION_G711_ALAW	      64 kbps CCITT G.711 alaw encoding, 8kHz
				      16-bit

     AF_COMPRESSION_G722	      64 kbps CCITT G.722 ADPCM, 16 kHz 16-bit

     AF_COMPRESSION_G726	      16, 24, 32, or 40 kbps CCITT G.726
				      ADPCM, 8 kHz 16-bit

     AF_COMPRESSION_G728	      16 kbps CCITT G.728 ADPCM, 8 kHz 16-bit

     AF_COMPRESSION_GSM		      13 kbps European GSM 06.10 encoding, 8
				      kHz 16-bit

     AF_COMPRESSION_DVI_AUDIO	      32 kbps DVI ADPCM, 8 kHz 16-bit

     AF_COMPRESSION_FS1016	      48, 72, or 96 kbps US Federal Standard
				      1016 CELP, 8 kHz, 16-bit

     AF_COMPRESSION_MPEG1	      ISO/MPEG-1 Layers I and II.

     AF_COMPRESSION_AWARE_MULTIRATE   Aware, Inc's MultiRate I near-lossless
				      or lossless algorithm.

     AF_COMPRESSION_APPLE_ACE3	      not supported on SGI systems

     AF_COMPRESSION_APPLE_ACE8	      not supported on SGI systems

     AF_COMPRESSION_APPLE_MAC3	      not supported on SGI systems

     AF_COMPRESSION_APPLE_MAC6	      not supported on SGI systems

     The four Apple compression algorithms listed above are Apple-proprietary,
     and are not supported by the SGI Audio File Library.

     afGetCompressionParams() returns, via the pvlist argument, a list of
     compression parameters and their values associated with the current
     compression scheme.

     The parameters will be a subset of the following.	Parameter type is
     AU_PVTYPE_LONG unless otherwise noted:

     AF_MPEG_PARAM_LAYER
	  indicates MPEG layer:

	  AF_MPEG_LAYER_I

									Page 2

afGetCompression(3dm)					 afGetCompression(3dm)

	  AF_MPEG_LAYER_II (default)

     AF_MPEG_PARAM_CHANNEL_POLICY / AF_AWARE_PARAM_CHANNEL_POLICY
	  indicates how multiple channels are treated:

	  AF_MPEG_STEREO / AF_AWARE_STEREO
	       indicates that the channels are part of a single multi-channel
	       signal.	This includes 4-channel, etc.;

	  AF_MPEG_JOINT_STEREO (default)
	       indicates that the algorithm may have attempted to exploit
	       redundancy between channels for greater coding gain; Not valid
	       for MultiRate.

	  AF_MPEG_INDEPENDENT / AF_AWARE_INDEPENDENT
	       indicates that the separate channels are unrelated and were
	       processed separately, such as multi-lingual sound tracks.

	  Note that all of these settings only have meaning for a multi
	  channel signal.

     AF_MPEG_PARAM_BITRATE_TARGET (not for Aware MultiRate)
	  indicates the bitrate for all channels of compressed data, in bits
	  per second.  Note that for some schemes such as MPEG's maxrate (not
	  implemented) this is treated as an upper limit, whereas for MPEG's
	  fixrate, this is strictly achieved as a constant rate.  The
	  following is a list of valid bitrates for MPEG.

	  Layer 1:
	       32000, 64000, 96000, 128000, 160000, 192000, 224000, 256000,
	       288000, 320000, 352000, 384000, 416000, and 448000.

	  Layer 2:
	       32000, 48000, 56000, 64000, 80000, 96000, 112000, 128000,
	       160000, 192000, 224000, 256000, 320000, and 384000.

     Default value is 192 kbps / channel for layer 1 and 128 kbps / channel
     for layer 2.

     AF_MPEG_PARAM_BITRATE_POLICY / AF_AWARE_PARAM_BITRATE_POLICY
	  The mode of bit assignment:  The valid values depend on the
	  algorithm, as indicated below.

	  AF_MPEG_PARAM_BITRATE_POLICY (SGI MPEG):

	       AF_MPEG_FIXED_RATE (default)
		    Fixed bitrate per second.
		    The Compression ratio is determined by value of
		    AF_MPEG_PARAM_BITRATE_TARGET.

									Page 3

afGetCompression(3dm)					 afGetCompression(3dm)

	       AF_MPEG_CONST_QUAL
		    Bitrate is driven by the psychoacoustic model.  Enough
		    bits are assigned so that a constant Mask to Noise ratio
		    is attained.  See AF_MPEG_PARAM_CONST_QUAL_NMR.

	  AF_AWARE_PARAM_BITRATE_POLICY (Aware MultiRate):

	       AF_AWARE_CONST_QUAL
		    Enough bits are assigned so that the signal is 90+ dB
		    above the quantization noise.
		    Compression ratios are typically between 2.5:1 and 4:1.

	       AF_AWARE_LOSSLESS
		    Enough bits are assigned to provide for perfect
		    reconstruction.
		    Compression ratios are typically between 2:1 and 3:1.

     AF_MPEG_PARAM_CONST_QUAL_NMR (not for Aware MultiRate)
	  For SGI MPEG with AF_AWARE_CONST_QUAL, indicates the constant
	  quality mode noise-to-mask ratio in dB.  Zero indicates a theoretic
	  psychoacoustic imperceptible compression.  Positive values indicate
	  more compression and audible noise.  Negative values indicate less
	  compression and less perceptible noise.  The type of value of this
	  parameter is AU_PVTYPE_DOUBLE.

IMPORTANT NOTE
     Because this routine has been obsoleted, it only returns compression
     parameters associated with MPEG1 and Aware, Inc. MultiRate compression
     schemes.  All other compression schemes' parameters must be retrieved via
     afGetCompressionParams(3dm).

CAVEATS
     In future releases, new compression methods will be supported, so the
     same warnings that apply to afGetSampleFormat(3dm) (see the CAVEATS
     section of afGetSampleFormat(3dm) for these important notes) apply here.
     Because the library transparently decompresses input data and compresses
     output data regardless of the form of compression in the file, this may
     not seem like an issue.  All currently defined codecs convert the
     compressed data to and from AF_SAMPFMT_TWOSCOMP, 16-bit data. However,
     this is by no means guaranteed for future compression types.  So when
     reading or writing a file (even an AIFF-C file) containing compressed
     data, a program must call afGetSampleFormat(3dm) to get the "native"
     sample format of the codec, and check that the program is able to
     read/write using this sample format.  The "native" sample format of a
     codec is defined as the sample format of the data produced on
     decompression or expected for compression.

     All currently-released codecs in the Audio File Library will not change
     their native sample format, if the documentation currently specifies what
     that codec's native sample format is.  So, for example, if a programmer
     knows that the file format is AF_COMPRESSION_G711_ULAW, then the
     programmer can assume that the native format for that codec is

									Page 4

afGetCompression(3dm)					 afGetCompression(3dm)

     AF_SAMPFMT_TWOSCOMP, 16 because this was stated in the documentation.
     This is done to provide backwards compatibility.  However, it is not
     guaranteed that all future supported codecs will generate 16-bit signed
     integer data natively, nor is it guaranteed that all future codecs will
     have a single native sample format (some may be configurable or may vary
     depending on what kind of data was originally compressed).	 In every case
     though, the programmer can call afGetSampleFormat(3dm) to retrieve the
     sample format for the current codec on the file corresponding to the
     given AFfilehandle.

     There is a means by which an application can request the transparent
     conversion of whatever the native sample format of the codec is to the
     user's desired sample format.  See afSetVirtualSampleFormat(3dm) for a
     detailed explanation.  The library offers a runtime query of all of the
     supported compression methods, their textual names, their compression
     ratios (if available), and any other info that may be useful in making a
     runtime choice of codec.

SEE ALSO
     afOpenFile(3dm), afReadFrames(3dm), afInitCompression(3dm),
     afInitCompressionParams(3dm), aware(5), AUpvnew(3dm)

									Page 5

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