man/man1/audioconvert.1

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 AUDIOCONVERT 1 "Feb 8, 2020"
 NAME
audioconvert - convert audio file formats
 SYNOPSIS
audioconvert [-pF] [-f outfmt] [-o outfile]
 [ [-i infmt] [file]...] ...


 DESCRIPTION
audioconvert converts audio data between a set of supported audio
encodings and file formats. It can be used to compress and decompress audio
data, to add audio file headers to raw audio data files, and to convert between
standard data encodings, such as u-law and linear PCM.

If no filenames are present, audioconvert reads the data from the
standard input stream and writes an audio file to the standard output.
Otherwise, input files are processed in order, concatenated, and written to the
output file.

Input files are expected to contain audio file headers that identify the audio
data format. If the audio data does not contain a recognizable header, the
format must be specified with the -i option, using the rate,
encoding, and channels keywords to identify the input data format.

The output file format is derived by updating the format of the first input
file with the format options in the -f specification. If -p is not
specified, all subsequent input files are converted to this resulting format
and concatenated together. The output file will contain an audio file header,
unless format=raw is specified in the output format options.

Input files may be converted in place by using the -p option. When
-p is in effect, the format of each input file is modified according to
the -f option to determine the output format. The existing files are then
overwritten with the converted data.

The file(1) command decodes and prints the audio data format of Sun audio
files.
 OPTIONS
The following options are supported:
-p

In Place: The input files are individually converted to the format
specified by the -f option and rewritten. If a target file is a symbolic
link, the underlying file will be rewritten. The -o option may not be
specified with -p.


-F

Force: This option forces audioconvert to ignore any file header
for input files whose format is specified by the -i option. If -F
is not specified, audioconvert ignores the -i option for input
files that contain valid audio file headers.


-f outfmt

Output Format: This option is used to specify the file format and data
encoding of the output file. Defaults for unspecified fields are derived from
the input file format. Valid keywords and values are listed in the next
section.


-o outfile

Output File: All input files are concatenated, converted to the output
format, and written to the named output file. If -o and -p are not
specified, the concatenated output is written to the standard output. The
-p option may not be specified with -o.


-i infmt

Input Format: This option is used to specify the data encoding of raw
input files. Ordinarily, the input data format is derived from the audio file
header. This option is required when converting audio data that is not preceded
by a valid audio file header. If -i is specified for an input file that
contains an audio file header, the input format string will be ignored, unless
-F is present. The format specification syntax is the same as the
-f output file format.
Multiple input formats may be specified. An input format describes all input
files following that specification, until a new input format is specified.


file

File Specification: The named audio files are concatenated, converted to
the output format, and written out. If no file name is present, or if the
special file name `-' is specified, audio data is read from the standard
input.


-?

Help: Prints a command line usage message.


 "Format Specification"
The syntax for the input and output format specification is:

keyword=value[,keyword=value \|.\|.\|.\|]

with no intervening whitespace. Unambiguous values may be used without the
preceding keyword=.
rate

The audio sampling rate is specified in samples per second. If a number is
followed by the letter k, it is multiplied by 1000 (for example, 44.1k =
44100). Standard of the commonly used sample rates are: 8k, 16k, 32k, 44.1k,
and 48k.


channels

The number of interleaved channels is specified as an integer. The words
mono and stereo may also be used to specify one and two channel
data, respectively.


encoding

This option specifies the digital audio data representation. Encodings
determine precision implicitly (ulaw implies 8-bit precision) or
explicitly as part of the name (for example, linear16). Valid encoding
values are:
ulaw

CCITT G.711 u-law encoding. This is an 8-bit format primarily used for
telephone quality speech.


alaw

CCITT G.711 A-law encoding. This is an 8-bit format primarily used for
telephone quality speech in Europe.


linear8,

linear16,

linear32

Linear Pulse Code Modulation (PCM) encoding. The name identifies the number of
bits of precision. linear16 is typically used for high quality audio
data.


pcm

Same as linear16.


g721

CCITT G.721 compression format. This encoding uses Adaptive Delta Pulse
Code Modulation (ADPCM) with 4-bit precision. It is primarily used for
compressing u-law voice data (achieving a 2:1 compression ratio).


g723

CCITT G.723 compression format. This encoding uses Adaptive Delta Pulse
Code Modulation (ADPCM) with 3-bit precision. It is primarily used for
compressing u-law voice data (achieving an 8:3 compression ratio). The audio
quality is similar to G.721, but may result in lower quality when used
for non-speech data.


The following encoding values are also accepted as shorthand to set the sample
rate, channels, and encoding:
voice

Equivalent to encoding=ulaw,rate=8k,channels=mono.


cd

Equivalent to encoding=linear16,rate=44.1k,channels=stereo.


dat

Equivalent to encoding=linear16,rate=48k,channels=stereo.


format

This option specifies the audio file format. Valid formats are:
sun

Sun compatible file format (the default).


raw

Use this format when reading or writing raw audio data (with no audio header),
or in conjunction with an offset to import a foreign audio file format.


offset

(-i only) Specifies a byte offset to locate the start of the audio
data. This option may be used to import audio data that contains an
unrecognized file header.


 USAGE
See largefile(5) for the description of the behavior of
audioconvert when encountering files greater than or equal to 2 Gbyte
(2^31 bytes).
 EXAMPLES
Example 1 Recording and compressing voice data before storing it

Record voice data and compress it before storing it to a file:

example% audiorecord | audioconvert -f g721 > mydata.au


Example 2 Concatenating two audio files

Concatenate two Sun format audio files, regardless of their data format, and
output an 8-bit ulaw, 16 kHz, mono file:

example% audioconvert -f ulaw,rate=16k,mono -o outfile.au infile1 infile2


Example 3 Converting a directory to Sun format

Convert a directory containing raw voice data files, in place, to Sun format
(adds a file header to each file):

example% audioconvert -p -i voice -f sun *.au


 SEE ALSO
audioplay(1), audiorecord(1), file(1),
largefile(5)
 NOTES
The algorithm used for converting multi-channel data to mono is implemented by
simply summing the channels together. If the input data is perfectly in phase
(as would be the case if a mono file is converted to stereo and back to mono),
the resulting data may contain some distortion.