audioconvert - convert audio file formats


audioconvert [-pF] [-f outfmt] [-o outfile]
[ [-i infmt] [file]...] ...


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


The following options are supported:

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.

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

-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 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=.

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.

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.

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:

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

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

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

Same as linear16.

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).

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:

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

Equivalent to

Equivalent to

This option specifies the audio file format. Valid formats

Sun compatible file format (the default).

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.

(-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.


See largefile(7) for the description of the behavior of audioconvert when
encountering files greater than or equal to 2 Gbyte (2^31 bytes).


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 >

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 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


audioplay(1), audiorecord(1), file(1), largefile(7)


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.

February 8, 2020 AUDIOCONVERT(1)