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 files.
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.
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.
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 > 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
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.