Introduction

SoX reads and writes audio files in most popular formats and can optionally apply effects to them. It can combine multiple input sources, synthesize audio and, on many systems, act as a general purpose audio player or as a multitrack audio recorder. It can also split the input into multiple output files.

All SoX functionality is available using just the sox_ng command. To simplify playing and recording audio, if SoX is invoked as play_ng, the output file is automatically set to be the default sound device and, if invoked as rec_ng, the default sound device is used as an input source. Additionally, the soxi_ng command provides a convenient way to query audio file header information.

The heart of SoX is a library called libsox_ng. Those interested in extending SoX or using it in other programs should refer to the libsox_ng manual page.

SoX is a command-line audio processing tool particularly suited to making quick, simple edits and to batch processing. If you need an interactive, graphical audio editor, use audacity(1).

The overall SoX processing chain can be summarized as follows:

On the SoX command line, the positions of the Output(s) and the Effects are swapped w.r.t. the logical flow just shown and, while options pertaining to files are placed before their respective file names, the opposite is true for effects. To show how this works in practice, here is a selection of examples of how SoX might be used. The simple

   sox_ng recital.au recital.wav

   sox_ng recital.au -b 16 recital.wav channels 1 rate 16k fade 3 norm

   sox_ng -r 16k -e signed -b 8 -c 1 voice-memo.raw voice-memo.wav

   sox_ng slow.aiff fixed.aiff speed 1.027

   sox_ng short.wav long.wav longer.wav

   sox_ng -m music.mp3 voice.wav mixed.flac

   play_ng "The Moonbeams/Greatest/*.ogg" bass +3

   play_ng -n -c1 synth sin %-12 sin %-9 sin %-5 sin %-2 fade h 0.1 1 0.1

   rec_ng -c 2 radio.aiff trim 0 30:00

   play_ng -q take1.aiff & rec -M take1.aiff take1-dub.aiff

   rec_ng -r 44100 -b 16 -e signed-integer -p \
	silence 1 0.50 0.1% 1 10:00 0.1% | \
	sox_ng -p song.ogg silence 1 0.50 0.1% 1 2.0 0.1% : \
	newfile : restart

The above is just an overview of SoX's capabilities. Detailed explanations of how to use all SoX parameters, file formats and effects can be found below in this manual, in soxformat_ng(7) and in soxi_ng(1).

File Format Types

SoX can work with `self-describing' and `raw' audio files. `Self-describing' formats (e.g. WAV, FLAC, MP3) have a header that completely describes the signal and encoding attributes of the audio data that follows. `raw' or `headerless' formats do not contain this information, so the audio characteristics of these must be described on the SoX command line, except for a few which can be inferred from the filename extension such as .gsm, always -c1 -r8000 -e gsm, and named raw formats like .f32, .s16 and .ul which give the encoding but not the sample rate or number of channels.

The following four characteristics are used to describe the format of audio data:

sample rate

The sample rate in samples per second (`Hertz' or `Hz'). Digital telephony traditionally uses a sample rate of 8000Hz (8kHz) though 16 and even 32kHz are becoming more common. Audio Compact Discs use 44100Hz (44.1kHz), Digital Audio Tape and many computer systems use 48kHz and professional audio systems often use 96kHz.

sample size

The number of bits used to store each sample. Today, 16-bit is commonly used, 8-bit was popular in the early days of computer audio and 24-bit is used in the professional audio arena.

data encoding

The way in which each audio sample is represented (or `encoded'). Some encodings have variants with different byte-orderings or bit-orderings, some compress the audio data so that it takes up less disk space or transmission bandwidth than uncompressed formats. Commonly-used encoding types include floating point, μ-law, ADPCM, signed-integer PCM, MP3 and FLAC.

channels

The number of audio channels contained in the file. One (`mono') and two (`stereo') are widely used and `surround sound' audio typically contains six or more channels.

The term `bit rate' is a measure of the amount of storage occupied by an encoded audio signal per unit of time. It can depend on all of the above and is typically denoted as a number of kilobits per second (kbps). An A-law telephony signal has a bit rate of 64 kbps, MP3-encoded stereo music typically has a bit rate of 128-196 kbps and FLAC-encoded stereo music typically has a bit rate of 550-760 kbps.

Most self-describing formats also allow textual `comments' to be embedded in the file that can be used to describe the audio in some way, e.g. for music, the title, the author, etc.

One important use of audio file comments is to convey `Replay Gain' information. SoX can apply Replay Gain automatically for formats that contain comments but does not generate it. By default, SoX copies comments from the first input file to output files that support comments, so output files may contain Replay Gain information which is incorrect. This can be fixed, when converting input files with --replay-gain enabled, by removing all comments using --comment "" or removing just the REPLAYGAIN comment with

soxi_ng -a in.au | grep -v REPLAYGAIN > comments
sox_ng --replay-gain=track in.au --comment-file comments out.au

Determining and setting the File Format

SoX uses several mechanisms to determine or set the format of an audio file. Depending on the circumstances, individual characteristics may be determined or set using different mechanisms.

To determine the format of an input file, SoX uses, in order of precedence and as given or available:

1.

Command-line format options,

2.

The contents of the file header,

3.

The filename extension.

To set the output file format, SoX uses, in order of precedence and as given or available:

1.

Command-line format options,

2.

The filename extension,

3.

The input file format characteristics or the closest that is supported by the output file type.

For all files, SoX exits with an error if the file type cannot be determined. Command-line format options may need to be added or changed to resolve the problem.

Playing & Recording Audio

The play_ng and rec_ng commands are provided so that basic playing and recording is as simple as

   play_ng existing-file.wav

   rec_ng new-file.wav

   sox_ng existing-file.wav -d

   sox_ng -d new-file.wav

When playing a file with a sample rate that is not supported by the audio output device, SoX automatically invokes the rate effect to perform the necessary sample rate conversion. For compatibility with old hardware, the default rate quality level is set to `low'. This can be changed by explicitly specifying the rate effect with a different quality level, e.g.

   play_ng ... rate -m

On some systems, SoX allows the audio playback volume to be adjusted while using play_ng. Where supported, this is achieved by tapping the `v' & `V' keys during playback. If there is a softvol effect in the chain, these keys will adjust that instead of the hardware mixer.

To help with setting a suitable recording level, SoX includes a peak level meter which can be invoked (before making the actual recording) as follows:

   rec_ng -n

Accuracy

Many file formats that compress audio discard some of the audio signal information. Converting to such a format and back again will not produce an exact copy of the original audio. This is the case for many formats used in telephony (e.g. A-law, GSM) where low signal bandwidth is more important than high audio fidelity and for formats used in portable music players (e.g. MP3, Ogg Vorbis) where adequate fidelity can be retained even with the large compression ratios that are needed to make portable players practical.

Formats that discard audio signal information are called `lossy'. Formats that do not are called `lossless'. The term `quality' is used as a measure of how closely the original audio signal is reproduced when using a lossy format.

Audio file conversion with SoX is lossless when it can be, i.e. when not using lossy compression, when not reducing the sampling rate or number of channels and when the number of bits used in the destination format is not less than in the source format. For example, converting from an 8-bit PCM format to a 16-bit PCM format is lossless but converting from an 8-bit PCM format to (8-bit) A-law isn't.

SoX converts all audio files to an internal, uncompressed 32-bit format before performing any audio processing. This means that manipulating a file that is stored in a lossy format can cause further losses in audio fidelity. E.g. with

   sox_ng long.mp3 short.mp3 trim 10

Dithering

Dithering is a technique used to maximize the dynamic range of audio stored at a particular bit-depth. Any distortion introduced by quantization is decorrelated by adding a small amount of white noise to the signal. In most cases, SoX can determine whether the selected processing requires dither and will add it during output formatting if appropriate.

By default, SoX automatically adds TPDF dither when the output bit-depth is less than 24 and any of the following are true:

• bit-depth reduction has been specified explicitly using a command-line option
• the output file format supports only bit-depths lower than that of the input file format
• an effect has increased the effective bit-depth within the internal processing chain

For example, adjusting the volume with vol 0.25 requires two additional bits in which to losslessly store its results (since 0.25 decimal equals 0.01 binary) so, if the input file bit-depth is 16, SoX's internal representation will use 18 bits after processing this volume change. In order to store the output at the same depth as the input, dithering is used to remove the additional bits.

Use the -V option to see what processing SoX has automatically added. The -D (--no-dither) option may be given to override automatic dithering. To invoke dithering manually (e.g. to select a noise-shaping curve) use the dither effect.

Clipping

Clipping is distortion that occurs when an audio signal level (or `volume') exceeds the range of the chosen representation. In most cases, clipping is undesirable and so should be corrected by adjusting the level prior to the point in the processing chain at which it occurs.

In SoX, clipping can happen when using the vol or gain effects to increase the audio volume. Clipping can also occur with many other effects, when converting one format to another and even when simply playing the audio.

Playing an audio file often involves resampling and processing by analog components and that can introduce a DC offset or amplification, all of which can produce distortion if the audio signal level was initially too close to the clipping point.

For these reasons, it is usual to make sure that an audio file's signal level has some `headroom', i.e. it does not exceed a particular level below the maximum possible level of the given representation. Some standards bodies recommend as much as 9dB headroom, but in most cases, 3dB (≈ 70% linear) is enough. Note that this wisdom seems to have been lost in modern music production; in fact, many CDs, MP3s, etc. are now mastered at levels above 0dBFS and the audio is clipped as delivered.

SoX's stat and stats effects can assist in determining the signal level of an audio file. The gain or vol effect can be used to prevent clipping, e.g.

   sox_ng dull.wav bright.wav gain -6 treble +6

If clipping occurs at any point during processing, SoX displays a warning message to that effect.

See the global -G (--guard) option and the gain and norm effects.

Input File Combining

SoX's input combiner can be configured with the --combine global option to combine multiple files using one of the following methods: concatenate, sequence, mix, mix-power, merge and multiply, with shorthands -m for --combine mix, -M for merge and -T for multiply.

The default is sequence for play_ng, and concatenate for sox_ng.

For methods other than sequence, multiple input files must have the same sampling rate. If necessary, separate SoX invocations can be used to make sampling rate adjustments prior to combining them. and, with concatenate, the input files must also have the same number of channels.

The sequence combining method is similar to concatenate in that the audio from each input file is sent serially to the output file but here, the output file may be closed and reopened at the transition between input files. This may be just what is needed when sending different types of audio to an output device but is not generally useful when the output is a normal file.

With the mix or mix-power combining methods, the number of channels in each input file need not be the same but SoX issues a warning if they are not and some channels in the output file will not contain audio from every input file. A mixed audio file cannot be unmixed without reference to the original input files.

If the merge combining method is selected the number of channels in each input file need not be the same and a merged audio file comprises all channels from all the input files and unmerging is possible using multiple invocations of SoX with the remix effect. For example, two mono files could be merged to form one stereo file and the first and second mono files would become the left and right channels of the stereo file.

The multiply combining method multiplies the sample values of corresponding channels treated as numbers in the interval -1 to +1. If the number of channels in the input files is not the same, the missing channels will contain silence.

When combining input files, SoX applies any specified effects (including, for example, the vol volume adjustment effect) after the audio has been combined. However, it is often useful to be able to set the volume of the inputs individually (i.e. `balance' them) before combining takes place. For all combining methods, input file volume adjustments can be made manually using the -v option, which can be given for one or more input files. If it is given for only some of the input files, the others receive no volume adjustment. In some circumstances, automatic volume adjustments may be applied. The global -V option can be used to show the input file volume adjustments that have been selected manually or automatically.

Some special considerations need to made when mixing input files:

Unlike the other methods, mix combining can cause clipping in the combiner if no balancing is performed. In this case, if manual volume adjustments are not given, SoX tries to ensure that clipping does not occur by automatically adjusting the volume (amplitude) of each input signal by a factor of ¹/n, where n is the number of input files. If this results in audio that is too quiet or otherwise unbalanced, the input file volumes can be set manually with -v. Using the norm effect on the mix is another alternative.

If mixed audio seems loud enough at some points but too quiet in others, dynamic range compression can be applied to correct this - see the compand effect.

With the mix-power combine method, the mixed volume is approximately equal to that of one of the input signals. This is achieved by balancing using a factor of ¹/√n instead of ¹/n. Note that this balancing factor does not guarantee that clipping will not occur but the number of clips will usually be low and the resulting distortion is usually imperceptible.

Output Files

SoX's default behaviour is to take one or more input files and write them to a single output file.

This behaviour can be changed by placing the newfile pseudo-effect within the effects list and SoX will enter multiple output mode.

In multiple output mode, a new file is created when the effects prior to the newfile indicate that they are done. The effects chain listed after newfile is then started up and its output is saved to the new file.

In multiple output mode, a unique number is appended automatically to all filenames and, if the filename has an extension, the number is inserted before the extension. This behaviour can be customized by placing %n in the filename where the number should be substituted. An optional number can be placed after the % to indicate a minimum width for the number with leading zeroes. %n defaults to two digits or, if no %n is included, to three digits before the filename extension.

Multiple output mode is not very useful unless an effect that stops the effects chain is specified before newfile. If the end of the file is reached before the effects chain stops, no new file is created as it would be empty.

The following is an example of splitting the first 60 seconds of an input file into two 30 second files and ignoring the rest.

   sox_ng song.wav ringtone%1n.wav trim 0 30 : newfile : trim 0 30

Stopping SoX

Usually, SoX completes its processing and exits automatically once it has read all audio data from the input files.

It can also be terminated earlier by sending it an interrupt signal, usually by pressing the keyboard interrupt key which is normally Ctrl-C. This is required It is needed when using SoX to make a recording.

When SoX is playing multiple files, Ctrl-C behaves slightly differently: pressing it once skips to the next file; pressing it twice in quick succession causes SoX to exit. However, when playing multiple files decoded with ffmpeg, this will kill all the pending file reads, resulting in a second or so of each - what each process had already written to its pipe.

Another way to stop processing early is to use an effect that has a time period or sample count; the trim effect is an example of this. Once all effects chains have stopped, SoX stops.

Special Filenames

The following filenames may be used in certain circumstances in place of a normal filename:

-

SoX can be used in simple pipeline operations by using the special filename `-' which, if used as an input filename, causes SoX to read audio data from the `standard input' (stdin) and, if used as the output filename, will cause SoX to send audio data to the `standard output' (stdout). When using this option for the output file, and sometimes when using it for an input file, the file type (see -t below) must also be given.

"|program [options] ..."

An initial `pipe' character specifies that the given command's standard output (stdout) should be used as an input file. Unlike the special filename -, this can be used for several inputs to one SoX command. For example, if a program genw generates a mono WAV signal on its standard output, the following command makes a stereo file from two generated signals:

   sox_ng -M "|genw --imd -" "|genw --thd -" out.wav

For headerless (raw) audio and some other formats, -t needs to be given before the input command.

"wildcard-filename"

Specifies that filename `globbing' (wildcard matching) should be performed by SoX instead of by the shell if the wildcard-filename contains the characters *, ? or characters ranges such as [A-Z]. This allows a single set of file options to be applied to a group of files. For example, if the current directory contains three files, file1.vox, file2.vox and file3.vox,

   play_ng --rate 6k *.vox

is expanded by the shell (in most environments) to

   play_ng --rate 6k file1.vox file2.vox file3.vox

which only treats the first `vox' file as having a sample rate of 6k. With

   play_ng --rate 6k "*.vox"

the given sample rate option is applied to all the files.

If you do not want SoX to glob the filenames, you can use the option --no-glob before each filename that should not be globbed, which is necessary if you need to process files whose names contain wildcard characters.

-p, --sox-pipe

This can be used in place of an output filename to specify that its output will be used as the input to another SoX command. For example, in the command:

   play_ng "|sox_ng -n -p synth 2" "|sox_ng -n -p synth 2 tremolo 10"

play_ng thinks it's playing two files in succession that come from pipes, but in fact they both come from other invocations of SoX, each with different effects.

-p is in fact an alias for `-t sox -'.

-d, --default-device

This can be used in place of an input or output filename to specify that the default audio device (if one has been built into SoX) is to be used. This is akin to invoking rec_ng or play_ng as described above.

-n, --null

This can be used in place of an input or output filename to specify that a `null file' is to be used. Here, `null file' refers to a SoX-specific mechanism and is not related to any operating system mechanism with some special name.

Using a null file as an input is equivalent to using a normal audio file that contains an infinite amount of silence and, as such, is not generally useful unless used with an effect that specifies a finite time length such as trim or synth.

Using a null file as an output amounts to discarding the audio and is mainly useful with effects that produce information about the audio instead of affecting it such as noiseprof, stat and spectrogram.

The sampling rate associated with a null file is by default 48kHz but, as with a normal file, this can be overridden using format options such as -r (see below).

Supported File and Audio Device Types

See soxformat_ng(7) for a list and description of the supported file formats and audio device drivers.

Global Options

These options can be specified on the command line at any point before the first effect name.

The SOX_OPTS environment variable can be used to provide alternative default values for SoX's global options. See ENVIRONMENT (below).

-A freq

Set the frequency of note A4 to freq instead of 440Hz.

--buffer bytes, --input-buffer bytes

Set the size in bytes of the buffers used for processing audio (default 8192). --buffer applies to input, effects and output processing; --input-buffer applies only to input processing, for which it overrides --buffer if both are given.

Large values for --buffer may cause SoX to be become slow to respond to requests to terminate or to skip to the next input file.

--clobber

Don't prompt before overwriting an existing file that has the same name as an output file. This is the default behaviour; to override it, use --no-clobber.

--combine concatenate|merge|mix|mix-power|multiply|sequence

Select the input file combining method. See Input File Combining above for a description of them.

-D, --no-dither

Disable automatic dither - see Dithering above. This may be useful to ensure that SoX produces the same output in successive runs or if a file has been converted from 16 to 24 bit with the intention of doing some processing on it, but in fact no processing is needed after all and the original 16 bit file has been lost, in which case no dither is needed when converting the file back to 16 bits. See the stats effect for how to determine the actual bit-depth of the audio within a file.

--effects-file filename

Read a file to obtain all effects and their arguments. The file is parsed as if the values were specified on the command line. A new line can be used in place of the special : marker to separate effect chains. For convenience, such markers at the end of the file are normally ignored; if you want to specify an empty last effects chain, use an explicit : by itself on the last line of the file. This option causes any effects specified on the command line to be discarded.

-G, --guard

Automatically invoke the gain effect to guard against clipping. E.g.

   sox_ng -G in.au -b 16 out.au rate 44100 dither -s

is shorthand for

   sox_ng in.au -b 16 out.au gain -h rate 44100 gain -rh dither -s

See -V, --norm, and the gain effect.

-h, --help

Show SoX's version number and usage information.

--help-effect name

Show usage information for the specified effect. The name all can be used to show it for all available effects.

--help-format name

Show information about the specified file format. The name all can be used to show information for all supported formats.

--i, --info

If given as the first parameter to sox_ng, behave as soxi_ng.

--interactive

Enable control of SoX by keyboard strokes, making

-k|--keymap key:effect.field[+|−|*|/|=]value

Pressing the specified key will change an effect's parameter, adding or subtracting value for linear changes, multiplying or dividing by value for logarithmic control or setting the parameter to that value. For example:

   play_ng -V -k D:dolbyb.gain+2 -k d:dolbyb.gain-2 in.wav dolbyb

lets you hear the results of Dolby B removal and adjust the Threshold Gain in steps of 2dB to find the best setting.

--keymap turns --interactive mode on and -V makes it display the new value each time it is changed.

When you have the same effect more than once, you can control just with something like effect2.field to affect only the second occurrence of effect in the effects chain and for effects with multiple stages, like echo.delay, the stage can be selected with something like effect.field1.

The manual entries for individual effects say which parameters can be adjusted in this way, --help-effect tells you which of an effect's parameters can be keymapped and

   sox_ng --help-effect all | grep Keymap

gives a complete list; note that hyphenated parameters like gain-out have a keymap of gain_out.

-m|-M

Equivalent to --combine mix and --combine merge respectively.

--magic

If SoX has been built with the optional `libmagic' library, this option enables its use in helping to detect audio file types.

--multi-threaded|--single-threaded

By default, SoX is `single threaded' but if the --multi-threaded option is given, SoX processes audio channels for most multichannel effects in parallel on hyperthreading and multicore processors.

A larger buffer size than the default may be needed to benefit more from multithreaded processing (e.g. 131072; see --buffer above) and setting OMP_WAIT_POLICY=PASSIVE in the environment before launching SoX avoids a defect in OpenMP that can make it inredibly slow if several invocations of SoX are running simultaneously.

--no-clobber

Prompt before overwriting an existing file with the same name as that given for the output file.

Unintentionally overwriting a file is easier than you might think, for example, if you accidentally enter

   sox_ng file1 file2 effect1 effect2 ...

when what you really meant was

   play_ng file1 file2 effect1 effect2 ...

then, without this option, file2 will be overwritten. Hence, using this option is recommended and can be set in the SOX_OPTS environment variable (see ENVIRONMENT below).

--norm[=dB-level]

Automatically invoke the gain effect to guard against clipping and to normalize the audio. E.g.

   sox_ng --norm in.au -b 16 out.au rate 44100 dither -s

is shorthand for

   sox_ng in.au -b 16 out.au gain -h rate 44100 gain -nh dither -s

Optionally, the audio can be normalized to a given level, usually below 0 dBFS:

   sox_ng --norm=-3 in.au out.au

See -V, -G and the gain effect.

--play-rate-arg arg

Selects a quality option to be used when the rate effect is invoked automatically when playing audio. This option is typically set via the SOX_OPTS environment variable (see ENVIRONMENT below) and its default value, when playing, is -l (low quality but fast). See the rate effect for other alternatives.

--plot gnuplot|octave|off

If not set to off (the default if --plot is not given), run in a mode that can be used in conjunction with the gnuplot program or the GNU Octave program to assist with the selection and configuration of many of the transfer function-based effects. For the first given effect that supports the selected plotting program, SoX outputs commands to plot the effect's transfer function and stops without actually processing any audio. E.g.

   sox_ng --plot octave input-file -n highpass 1320 > highpass.plt
   octave highpass.plt

-q, --no-show-progress

Run in quiet mode when SoX wouldn't otherwise do so. This is the opposite of the -S option. To suppress error and warning messages, see -V below.

-R

Run in `repeatable' mode. When this option is given, SoX embeds a time stamp in the output file if its format supports comments and will seed pseudo random number generators, as used by dither, with that number, ensuring that successive SoX invocations with the same inputs and the same parameters yield the same output.

--replay-gain track|album|off

Select whether or not to apply replay gain adjustment to input files. The default is off for sox_ng and rec_ng, album for play_ng when (at least) the first two input files are tagged with the same Artist and Album names and track for play_ng otherwise.

-S, --show-progress

Display input file format/header information, processing progress as a percentage of the input file(s), elapsed time, remaining time (if known, in brackets) and the number of samples written to the output file. Also shown is a peak level meter, and an indication of whether clipping has occurred. The peak level meter shows up to two channels and is calibrated for digital audio as follows:

dB FSD	Display		dB FSD	Display
-25	-		-11	====
-23	=		-9	====-
-21	=-		-7	=====
-19	==		-5	=====-
-17	==-		-3	======
-15	===		-1	=====!
-13	===-

A three-second peak-held value of the headroom in dBs is shown to the right of the meter if the headroom is less than 6dB.

This option is enabled by default when using SoX to play or record audio but can be disabled with -q.

-T

Equivalent to --combine multiply

--temp directory

Specify that any temporary files should be created in the given directory. This can be useful if there are permission or free space problems with the default location. In this case, using `--temp .' (to use the current directory) is often a good solution.

--version

Show SoX's version number and exit.

-V[level]

Set verbosity. This is particularly useful for seeing how any automatic effects have been invoked by SoX.

SoX displays messages on the console (stderr) according to the following verbosity levels:

Each occurrence of the -V option increases the verbosity level by 1. Alternatively, the verbosity level can be set to an absolute number by specifying it immediately after the -V, e.g. -V0 shuts it up.

Input File Options

These options apply to the first input filename that follows them on the command line.

--ignore-length

Override the audio length given in an audio file's header. If this option is given, SoX keeps reading audio until it reaches the end of the input file.

-v, --volume factor

Intended for use when combining multiple input files, this option adjusts the volume of the file that follows it on the command line by a factor of factor. This allows it to be `balanced' w.r.t. the other input files. This is a linear (amplitude) adjustment, so a number less than 1 decreases the volume and a number greater than 1 increases it. If a negative number is given then, in addition to the volume adjustment, the audio signal will be inverted.

See the norm, vol and gain effects, Input File Balancing above and the Special Filenames' section on wildcard-filenames.

Input & Output File Format Options

These options apply to the input or output file whose name they immediately precede on the command line and are used mainly when working with headerless file formats or when specifying a format for the output file that is different from that of the input file.

-b bits, --bits bits

Set the number of bits (a.k.a. bit-depth or word length) in each encoded sample. It is not applicable to complex encodings such as MP3 or GSM and not necessary with encodings that have a fixed number of bits such as A-law, μ-law and ADPCM.

For an input file, the most common use for this option is to inform SoX of the number of bits per sample in a `raw' (`headerless') audio file. For example

   sox_ng -r 16k -e signed -b 8 input.raw output.wav

converts a particular `raw' file to a self-describing `WAV' file.

For an output file, this option can be used to set the output encoding size. By default, the output encoding size is be set to the input encoding size, provided it is supported by the output file type. For example:

   sox_ng input.cdda -b 24 output.wav

converts raw CD digital audio (16-bit, signed-integer) to a 24-bit (signed-integer) `WAV' file.

-c CHANNELS, --channels CHANNELS

Sets the number of audio channels in the audio file. This can be any number greater than zero.

For an input file, the most common use for this option is to inform SoX of the number of channels in a `raw' (headerless) audio file. Occasionally, it may be useful to use this option with a headered file to override the (presumably incorrect) value in the header but this is only supported with certain file types. For example:

   sox_ng -r 48k -e float -b 32 -c 2 input.raw output.wav

converts a `raw' file to a self-describing `WAV' file.

   play_ng -c 1 music.wav

interprets the file data as belonging to a single channel regardless of what is indicated in the file header and if the file does in fact have two channels, it is played at half speed.

For an output file, this option provides a shorthand for specifying that the channels effect should be invoked in order to change (if necessary) the number of channels in the audio signal to the number given. For example, the following two commands are equivalent:

   sox_ng input.wav -c 1 output.wav bass -b 24
   sox_ng input.wav      output.wav bass -b 24 channels 1

though the second form is more flexible as it allows effects to be ordered arbitrarily.

-e ENCODING, --encoding ENCODING

Set the audio encoding type, sometimes needed with file types that support more than one encoding scheme such as raw, WAV or AU but not with MP3 or FLAC. The available encoding types are as follows:

 

Encoding names can be abbreviated where this would not be ambiguous; e.g. unsigned-integer can be given as un, but not u (ambiguous with u-law).

For an input file, the most common use for this option is to inform SoX of the encoding of a `raw' (`headerless') audio file (see the examples in -b and -c above).

For an output file, this option can be used (perhaps with -b) to set the output encoding. For example:

   sox_ng input.cdda -e float output1.wav
   sox_ng input.cdda -b 64 -e float output2.wav

converts a raw CD digital audio (16-bit signed integer) to floating point `WAV' files of single and double precision respectively.

If this option is not given, the output encoding will be the same as the input encoding, provided it is supported by the output file type.

--no-glob

Specifies that filename `globbing' (wildcard matching) should not be performed by SoX on the following filename. For example, if the current directory contains the two files `five-seconds.wav' and `five*.wav', then

   play_ng --no-glob "five*.wav"

can be used to play just the single file `five*.wav'.

-r, --rate rate[k]

Gives the sample rate in Hz (or kHz if followed by k) of the file.

For an input file, the most common use for this option is to inform SoX of the sample rate of a `raw' (`headerless') audio file (see the examples in -b and -c above). Occasionally it may be useful to use this option with a `headered' file to override the value in the header, though this is only supported with certain file types. For example, if audio was recorded with a sample rate of 48k from a source that played back a little too slowly, say 1.5%,

   sox_ng -r 48720 input.wav output.wav

would correct the speed by changing only the file header (but see the speed effect for the more usual solution to this problem).

For an output file, this option provides a shorthand for specifying that the rate effect should be invoked in order to change (if necessary) the sample rate of the audio signal to the given value. For example, the following two commands are equivalent:

   sox_ng input.wav -r 48k output.wav bass -b 24
   sox_ng input.wav        output.wav bass -b 24 rate 48k

though the second form is more flexible as it allows rate options to be given and allows the effects to be ordered arbitrarily.

-t, --type FILE-TYPE

Give the type of an audio file. For both input and output files, this option is commonly used to inform SoX of the type a `headerless' audio file where the actual/desired type cannot be determined from the filename extension. For example:

   another-command | sox_ng -t mp3 - output.wav
   sox_ng input.wav -t raw output.bin

It can also be used to override the type implied by an input filename extension but, if overriding with a type that has a header, SoX exit with an error message if such a header is not actually present.

There are also pseudo filetypes that tell SoX to use a specified format module that handles more than one type of audio file such as -t sndfile and -t ffmpeg or when a type of file can be handled by several different format modules, such as WAV files containing MP3-encoded data.

Furthermore, the file type can be used to select a particular audio device driver for recording and playing.

See soxformat_ng(7) for a list of supported file types.

-L, --endian little
-B, --endian big
-x, --endian swap

 

These options specify whether the byte order of the audio data is, respectively, `little endian', `big endian' or the opposite to that of the system on which SoX is being used. Endianness applies only to data encoded as floating point or as signed or unsigned integers of 16 or more bits. It is often necessary to specify one of these options for headerless files and sometimes necessary for (otherwise) self-describing files. A given endian-setting option may be ignored for an input file whose header contains a specific endianness identifier or for an output file that is actually an audio device.

N.B. Unlike other format characteristics, the endianness (byte, nibble, & bit ordering) of the input file is not automatically used for the output file. For example, when the following is run on a little-endian system:

   sox_ng -B audio.s16 trimmed.s16 trim 2

trimmed.s16 will be created as little-endian;

   sox_ng -B audio.s16 -B trimmed.s16 trim 2

must be used to preserve big-endianness in the output file.

The -V option can be used to check the selected orderings.

-N, --reverse-nibbles

Specifies that the nibble ordering of the samples (i.e. the 2 halves of a byte) should be reversed, which is sometimes useful with ADPCM-based formats.

See the N.B. in the section on -x above.

-X, --reverse-bits

Specifies that the bit ordering of the samples should be reversed, which is sometimes useful with a few (mostly headerless) formats.

See the N.B. in the section on -x above.

Output File Format Options

These options only apply to output files and may only precede an output filename on the command line.

--add-comment TEXT

Append a comment to the output file header (where applicable).

--comment TEXT

Specify the comment text to store in the output file header (where applicable).

SoX provides a default comment `Processed by SoX' if this option (or --comment-file) is not given. To specify that no comment should be stored in the output file, use --comment "" .

--comment-file FILENAME

Specify a file containing the comment text to store in the output file header (where applicable).

-C, --compression FACTOR

Set the compression factor for variably-compressed output file formats. If this option is not given then a default compression factor applies. The compression factor is interpreted differently for different compressed file formats; for details, see the description of the file formats that use this option in soxformat_ng(7)