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afInitCompression(3dm)					afInitCompression(3dm)

NAME
     afInitCompression, afInitCompressionParams, afAware - configure the audio
     compression type and parameters in an AFfilesetup structure for an audio
     track

SYNOPSIS
     #include <dmedia/audiofile.h>

     void afInitCompression(AFfilesetup setup, int track, int compression)

     void afInitCompressionParams(AFfilesetup setup, int track,
			    int compression, AUpvlist pvlist, int numitems)

PARAMETER
     setup	   is an AFfilesetup structure, previously created by a call
		   to afNewFileSetup(3dm).

     track	   is a positive integer which identifies an audio track in a
		   file configuration.

		   Since all currently supported file formats contain one
		   audio track per file, the constant value AF_DEFAULT_TRACK
		   should always be used here.

     compression   is a positive integer symbolic constant which indicates the
		   type of audio compression for the audio track.

     pvlist	   is an AUpvlist structure, previously created by a call to
		   AUpvnew(3dm), initialized with parameters, parameter types,
		   and parameter values using AUpvsetparam(3dm),
		   AUpvsetvaltype(3dm), and AUpvsetval(3dm).

     numitems	   is the number of valid entries in the pvlist as set via
		   AUpvnew(3dm).

DESCRIPTION
     afInitCompression()  and afInitCompressionParams() allows you to
     configure audio tracks for compressed data.  afInitCompression() allows a
     one-parameter compression scheme to set up a default codec configuration
     with a set of default compression parameters.  afInitCompressionParams()
     allows not only the choice of codec, but also for a list of codec
     specific compression parameters to be set.

     afInitCompressionParams() is now obsolete, having been replaced by
     afInitFormatParams(3dm).  It exists only for backwards compatibility.
     The only compression schemes which have parameters handled via
     afInitCompressionParams() are MPEG1 and the Aware, Inc. MultiRate. All
     other compression schemes which have parameters must be handled via
     DMparams(3dm) using afInitFormatParams(3dm).

									Page 1

afInitCompression(3dm)					afInitCompression(3dm)

     Fifteen values for compression may currently be passed to
     afInitCompression() or afInitCompressionParams()

     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.  Equivalent to
				AF_COMPRESSION_DEFAULT_MPEG_II.

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

     AF_COMPRESSION_DEFAULT_MPEG_I
				ISO/MPEG-1-audio layer I, joint-stereo, fixed
				rate at 192 Kbps/channel

     AF_COMPRESSION_DEFAULT_MPEG_II
				ISO/MPEG-1-audio layer II, joint-stereo, fixed
				rate at 128 Kbps/channel

     AF_COMPRESSION_AWARE_DEFAULT_MULTIRATE
				Aware, Inc.  MultiRate I operating in near-
				lossless mode.

     AF_COMPRESSION_AWARE_DEFAULT_LOSSLESS
				Aware, Inc.  MultiRate I operating in lossless
				mode.

     For afInitCompressionParams(), the parameters and values passed in pvlist
     should be taken from the following.  Parameter type is AU_PVTYPE_LONG
     unless otherwise noted.

									Page 2

afInitCompression(3dm)					afInitCompression(3dm)

     AF_MPEG_PARAM_LAYER
	  selects which MPEG layer:

	  AF_MPEG_LAYER_I

	  AF_MPEG_LAYER_II (default)

     AF_MPEG_PARAM_CHANNEL_POLICY / AF_AWARE_PARAM_CHANNEL_POLICY
	  chooses how multiple channels should be 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 attempt 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 should
	       be processed separately, such as multi-lingual sound tracks.

	  Note that all of these settings are equivalent for single channel
	  input.

     AF_MPEG_PARAM_BITRATE_TARGET (not for Aware MultiRate)
	  specifies the desired 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
	  Provides for different modes of bit assignment:  The valid values
	  depend on the algorithm, as indicated below.

	  AF_MPEG_PARAM_BITRATE_POLICY (SGI MPEG):

									Page 3

afInitCompression(3dm)					afInitCompression(3dm)

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

	       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, sets the constant quality
	  mode noise-to-mask ratio in dB.  Zero yields a theoretic
	  psychoacoustic imperceptible compression.  Positive values provide
	  more compression and noise becomes audible.  Negative values cause
	  less compression and less perceptible noise.	The type of value of
	  this parameter is AU_PVTYPE_DOUBLE.

     All compression encoding is handled automatically by afWriteFrames(3dm).
     An application program only needs to work with linear PCM data.

     AIFF files do not support compression, so the configuration parameter
     compression will be ignored by afOpenFile(3dm), and the value
     AF_COMPRESSION_NONE will automatically be associated with the audio data.

VALID INPUT AUDIO DATA FORMATS
     Many compression schemes have restrictions on sampling rates and channel
     counts:

     Sampling rates for MPEG must be 32 kHz, 44.1 kHz, or 48 kHz.

     Sampling rates for MultiRate must be 8 kHz, 11.025 kHz, 16 kHz, 22.05
     kHz, 32 kHz, 44.1 kHz, or 48 kHz.

     Both MPEG and MultiRate must have 1- or 2-channel input.

     G728 and GSM compression must always be 8 kHz sampling rate.

									Page 4

afInitCompression(3dm)					afInitCompression(3dm)

     G726, G728, and GSM compression must have 1-channel input.

CAVEATS
     The Audio File Library will continue to support additional file formats
     and compression types.  Be aware of this.	Not all file formats are able
     to store audio data with all of the supported compression types.  For
     example, AIFF-C can store compressed data whereas AIFF can't.  See
     afGetCompression(3dm) for more information.

SEE ALSO
     afNewFileSetup(3dm), afOpenFile(3dm), afWriteFrames(3dm),
     afGetCompression(3dm), AwareIntro(3dm), afInitFormatParams(3dm)

									Page 5

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