Sox

SoX − Sound eXchange, the Swiss Army knife of audio manipulation

SYNOPSIS

  sox [global-options] [ format-options] infile1
      [[format-options] infile2] . .. [format-options] outfile
      [effect [effect-options]] ...
 play [global-options] [ format-options] infile1
      [[format-options] infile2] . .. [format-options]
      [effect [effect-options]] ...
  rec [global-options] [ format-options] outfile
      [effect [effect-options]] ...

DESCRIPTION

Introduction

SoX reads and writes audio files in most popular formats and can optionally apply effects to them. It can combine multiple input sources, synthesise audio, and, on many s ystems, act as a general purpose audio player or a multi-track audio recorder. It a lso has limited ability to split the input into multiple output files.

All SoX functionality is available using just the sox command. To s implify playing and recording audio, if SoX is invoked a s play, t he output file is automatically set to be the default sound device, and if invoked a s rec, t he default sound device is used as an input source. Additionally, t he soxi(1) command provides a convenient way to just query audio file header information.

The heart of SoX is a library called libSoX. Those interested in extending SoX or using it in other programs should refer to the libSoX manual page: libsox(3).

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

The overall SoX processing chain can be summarised as follows:

Input(s) → Combiner → Effects → Output(s)

Note however, that on the SoX command line, the positions of the Output(s) and the Effects are swapped w.r.t. the logical flow j ust shown. Note also that whilst options pertaining to files are placed before their respective file name, the opposite is true for effects. To s how h ow this works in practice, here is a selection of examples of how S oX might be used. The simple

sox recital.au recital.wav

translates an audio file in Sun AU f ormat to a Microsoft WAV fi le, whilst sox recital.au −b 16 recital.wav channels 1 rate 16k fade 3 norm performs the same format translation, but also applies four effects (down-mix to one channel, sample rate change, fade-in, nomalize), and stores the result at a bit-depth of 16.

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

converts ‘raw’ (a.k.a. ‘headerless’) audio to a self-describing file format,

sox slow.aiff fixed.aiff speed 1.027

adjusts audio speed,

sox short.wav long.wav longer.wav

concatenates two a udio files, and

sox −m music.mp3 voice.wav mixed.flac

mixes together two a udio files.

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

plays a collection of audio files whilst applying a bass boosting effect,

play −n −c1 synth sin %−12 sin %−9 sin %−5 sin %−2 fade h 0.1 1 0.1

plays a synthesised ‘A m inor seventh’ chord with a pipe-organ s ound,

rec −c 2 radio.aiff trim 0 30:00

records half an hour of stereo audio, and

play −q take1.aiff & rec −M take1.aiff take1−dub.aiff

(with POSIX shell and where supported by hardware) records a new t rack in a multi-track recording. Finally,

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

records a stream of audio such as LP/cassette and splits in to multiple audio files at points with 2 seconds of silence. Also, it does not start recording until it detects audio is playing and stops after it sees 10 minutes of silence.

N.B. The above is just an overview of SoX’s c apabilities; detailed explanations of how to u se all SoX parameters, file formats, and effects can be found below in t his manual, in soxformat(7), and in soxi(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 fol- lows. ‘raw’ or ‘headerless’ formats do not contain this information, so the audio characteristics of these must be described on the SoX command line or inferred from those of the input file. The following four characteristics are used to describe the format of audio data such that it can be processed with SoX:

sample rate

The sample rate in samples per second (‘Hertz’ or ‘Hz’). Digital telephony traditionally uses a sample rate of 8000 Hz (8 kHz), though these days, 16 and even 3 2 k Hz are becoming more common. Audio Compact Discs use 44100 Hz (44.1 k Hz). Digital Audio Tape and many computer systems use 48 kHz. Professional audio systems often use 96 kHz.

sample size

The number of bits used to store each sample. To day, 1 6-bit is commonly used. 8-bit was popular in the early days of computer audio. 24-bit is used in the professional audio arena. Other sizes are also used.

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 the stored audio data takes up less space (i.e. disk space or transmission bandwidth) than the other format parameters and the number of samples would imply. 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. ‘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 over a u nit of time. It can depend on all of the above and is typically denoted as a number of kilo-bits 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. 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 supports applying Replay Gain information (for certain input file formats only; currently, at l east FLAC and Ogg Vorbis), but not generating it. Note that by default, SoX copies input file comments to output files that support comments, so output files may contain Replay Gain information if some was present in the input file. In this case, if anything other than a simple format conversion was performed then the output file Replay Gain information is likely to be incorrect and so should be recalculated using a tool that supports this (not SoX

The soxi(1) command can be used to display information from audio file headers.

Determining & Setting The File Format

There are several mechanisms available for SoX to use to determine or set the format characteristics of an audio file. Depending on the circumstances, individual characteristics may be determined or set using dif- ferent mechanisms.

To d etermine the format of an input file, SoX will use, in order of precedence and as given or a vailable: 1. Command-line format options. 2. The contents of the file header. 3. The filename extension. To s et the output file format, SoX will use, in order of precedence and as given or a vailable: 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 a ll files, SoX will exit with an error if the file type cannot be determined. Command-line format options may need to be added or changed to resolve t he problem.

Playing & Recording Audio

The play and rec commands are provided so that basic playing and recording is as simple as

play existing-file.wav

and

rec new-file.wav

These two c ommands are functionally equivalent to

sox existing-file.wav −d

and

sox −d new-file.wav

Of course, further options and effects (as described below) can be added to the commands in either form.

• * *

Some systems provide more than one type of (SoX-compatible) audio driver, e.g. ALSA & OSS, or SUNAU & AO. S ystems can also have more than one audio device (a.k.a. ‘sound card’). If more than one audio driver h as been built-in to SoX, and the default selected by SoX when recording or playing is not the one that is wanted, then the AUDIODRIVER environment variable can be used to override the default. For e xample (on many s ystems):

set AUDIODRIVER=oss
play ...

The AUDIODEV environment variable can be used to override the default audio device, e.g.

set AUDIODEV=/dev/dsp2
play ...
sox ... −t oss

or

set AUDIODEV=hw:soundwave,1,2
play ...
sox ... −t alsa

Note that the way of setting environment variables varies from system to system—for some specific exam- ples, see ‘SOX_OPTS’ below.

When playing a file with a sample rate that is not supported by the audio output device, SoX will automati- cally invoke the rate effect to perform the necessary sample rate conversion. For compatibility with old hardware, the default rate quality level i s s et to ‘low’. This can be changed by explicitly specifying the rate effect with a different quality level, e.g.

play ... rate −m

or by using the −−play−rate−arg option (see below).

• * *

On some systems, SoX allows audio playback volume to be adjusted whilst using play. W here supported, this is achieved by t apping the ‘v’ & ‘V’ keys d uring playback.

To h elp with setting a suitable recording level, SoX includes a peak-level m eter which can be invoked (before making the actual recording) as follows:

rec −n

The recording level s hould be adjusted (using the system-provided mixer program, not SoX) so that the meter is at most occasionally full scale, and never ‘ in the red’ (an exclamation mark is shown). See also −S below.

Accuracy

Many fi le formats that compress audio discard some of the audio signal information whilst doing so. Con- verting to such a format and then converting back again will not produce an exact copy of t he original audio. This is the case for many f ormats used in telephony ( e.g. A-law, GSM) where low s ignal bandwidth is more important than high audio fidelity, a nd for many f ormats used in portable music players (e.g. MP3, Vo rbis) where adequate fidelity can be retained even w ith the large compression ratios that are needed to make p ortable 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 c losely the original audio signal can be 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 for- mat is not less than in the source format. E.g. converting from an 8-bit PCM format to a 16-bit PCM for- mat is lossless but converting from an 8-bit PCM format to (8-bit) A-law i sn’t. N.B. SoX converts all audio files to an internal uncompressed format before performing any a udio process- ing. This means that manipulating a file that is stored in a lossy format can cause further losses in audio fidelity. E .g. with

sox long.mp3 short.mp3 trim 10

SoX first decompresses the input MP3 file, then applies the trim effect, and finally creates the output MP3 file by re-compressing the audio—with a possible reduction in fidelity above that which occurred when the input file was created. Hence, if what is ultimately desired is lossily compressed audio, it is highly recom- mended to perform all audio processing using lossless file formats and then convert to the lossy format only at the final stage.

N.B. Applying multiple effects with a single SoX invocation will, in general, produce more accurate results than those produced using multiple SoX invocations.

Dithering

Dithering is a technique used to maximise the dynamic range of audio stored at a particular bit-depth. Any distortion introduced by quantisation 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.

Specifically, by d efault, SoX automatically adds TPDF dither when the output bit-depth is less than 24 and any of t he 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 effective bit-depth within the internal processing chain

For e xample, adjusting volume with vol 0 .25 requires two a dditional 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, then SoX’s i nternal rep- resentation will utilise 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 option  may  be  given t o

override automatic dithering. To i nv oke dithering manually (e.g. to select a noise-shaping curve), see 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 could occur, as y ou might expect, when using the vol or gain effects to increase the audio volume. Clipping could also occur with many o ther effects, when converting one format to another, a nd ev en w hen simply playing the audio.

Playing an audio file often involves resampling, and processing by analogue components can introduce a small DC offset and/or amplification, all of which can produce distortion if the audio signal level w as ini- tially too close to the clipping point.

For t hese reasons, it is usual to make s ure that an audio file’s s ignal level h as some ‘headroom’, i.e. it does not exceed a particular level b elow t he maximum possible level f or the given r epresentation. Some stan- dards 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 C Ds, MP3s, etc. are now m astered at levels above 0dBFS i.e. the audio is clipped as delivered. SoX’s stat and stats effects can assist in determining the signal level in an a udio file. The gain or vol effect can be used to prevent clipping, e.g.

sox dull.wav bright.wav gain −6 treble +6

guarantees that the treble boost will not clip.

If clipping occurs at any p oint during processing, SoX will display a warning message to that effect. See also −G and the gain and norm effects.

Input File Combining

SoX’s input combiner can be configured (see OPTIONS below) to combine multiple files using any of the following methods: ‘concatenate’, ‘sequence’, ‘mix’, ‘mix-power’, ‘merge’, or ‘multiply’. The default method is ‘sequence’ for play, a nd ‘concatenate’ for rec and sox. For a ll methods other than ‘sequence’, multiple input files must have the same sampling rate. If necessary, separate SoX invocations can be used to make s ampling rate adjustments prior to combining. If the ‘concatenate’ combining method is selected (usually, t his will be by default) then the input files must also have the same number of channels. The audio from each input will be concatenated in the order given to form the output file.

The ‘sequence’ combining method is selected automatically for play. I t i s s imilar to ‘concatenate’ in that the audio from each input file is sent serially to the output file. However, here the output file may be closed and reopened at the corresponding 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.

If either the ‘mix’ or ‘mix-power’ combining method is selected then two or m ore input files must be given and will be mixed together to form the output file. The number of channels in each input file need not be the same, but SoX will issue a warning if they a re not and some channels in the output file will not contain audio from every input file. A m ixed audio file cannot be un-mixed without reference to the original input files.

If the ‘merge’ combining method is selected then two or m ore input files must be given a nd will be merged together to form the output file. The number of channels in each input file need not be the same. A merged audio file comprises all of the channels from all of the input files. Un-merging is possible using multiple invocations of SoX with the remix effect. For example, two m ono files could be merged to form one stereo file. 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 are considered to contain all zero.

When combining input files, SoX applies any s pecified effects (including, for example, the vol volume adjustment effect) after the audio has been combined. However, it is o ften useful to be able to set the vol- ume of (i.e. ‘balance’) the inputs individually, b efore combining takes place. For a ll combining methods, input file volume adjustments can be made manually using the −v option (below) which can be given f or one or more input files. If it is given f or only some of the input files then the others receive no v olume adjustment. In some circumstances, automatic volume adjustments may be applied (see below).

The −V option (below) can be used to show t he input file volume adjustments that have been selected (either manually or automatically).

There are some special considerations that need to made when mixing input files:

Unlike t he other methods, ‘mix’ combining has the potential to cause clipping in the combiner if no balancing is performed. In this case, if manual volume adjustments are not given, SoX will try to ensure that clipping does not occur by automatically adjusting the volume (amplitude) of each input signal by a factor of ¹/n, w here n is the number of input files. If this results in audio that is too quiet or otherwise unbalanced then the input file volumes can be set manually as described above. U sing the norm effect on the mix is another alternative.

If mixed audio seems loud enough at some points but too quiet in others then dynamic range compression should be applied to correct this—see the comp and effect.

With the ‘mix-power’ combine method, the mixed volume is approximately equal to that of one of the input signals. This is achieved b y 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 a nd the resultant distortion is generally imperceptible.

Output Files

SoX’s d efault behaviour is to take o ne or more input files and write them to a single output file. This behaviour can be changed by specifying the pseudo-effect ‘newfile’ within the effects list. SoX will then enter multiple output mode.

In multiple output mode, a new fi le is created when the effects prior to the ‘newfile’ indicate they a re done. The effects chain listed after ‘newfile’ is then started up and its output is saved to t he new fi le. In multiple output mode, a unique number will automatically be appended to the end of all filenames. If the filename has an extension then the number is inserted before the extension. This behaviour can be custom- ized by placing a %n anywhere in the filename where the number should be substituted. An optional num- ber can be placed after the % to indicate a minimum fixed width for the number.

Multiple output mode is not very useful unless an effect that will stop the effects chain early is specified before the ‘newfile’. If end of file is reached before the effects chain stops itself then no new fi le will be cre- ated as it would be empty.

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

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

Stopping SoX

Usually SoX will complete its processing and exit automatically once it has read all available audio data from the input files.

If desired, it can be terminated earlier by sending an interrupt signal to the process (usually by pressing the

keyboard interrupt key which is normally Ctrl-C). This is a natural requirement in some circumstances, e.g. when using SoX to make a r ecording. Note that when using SoX to play multiple files, Ctrl-C behaves slightly differently: pressing it once causes SoX to skip to the next file; pressing it twice in quick succession causes SoX to exit. Another option to stop processing early is to use an effect that has a time period or sample count to determine the stopping point. The trim effect is an example of this. Once all effects chains have stopped then SoX will also stop.

FILENAMES

Filenames can be simple file names, absolute or relative path names, or URLs (input files only). Note that URL support requires that wget(1) is available. Note: Giving SoX an input or output filename that is the same as a SoX effect-name will not work since SoX will treat it as an effect specification. The only work-around to this is to avoid such filenames. This is generally not difficult since most audio filenames have a filename ‘extension’, whilst effect-names do not.

Special Filenames

The following special filenames may be used in certain circumstances in place of a normal filename on the command line:

− SoX can be used in simple pipeline operations by using the special filename ‘−’ which, if used as an input filename, will cause SoX will read audio data from ‘standard input’ (stdin), and which, if used as the output filename, will cause SoX will send audio data to ‘standard output’ (stdout). Note that 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] . .."

This can be used in place of an input filename to specify the the given p rogram’s s tandard output(stdout) be used as an input file. Unlike − (above), this can be used for several inputs to one SoX command. For example, if ‘genw’ generates mono WAV formatted signals to its standard output, then the following command makes a stereo file from two g enerated signals: sox −M "|genw −−imd −" "|genw −−thd −" out.wav For h eaderless (raw) audio, −t (and perhaps other format options) will need to be given, preceding the input command.

"wildcard-filename"

Specifies that filename ‘globbing’ (wild-card matching) should be performed by SoX instead of by the shell. This allows a single set of file options to be applied to a group of files. For e xample, if the current directory contains three ‘vox’ files, file1.vox, file2.vox, and file3.vox, then

play −−rate 6k *.vox

will be expanded by the ‘shell’ (in most environments) to

play −−rate 6k file1.vox file2.vox file3.vox

which will treat only the first vox file as having a sample rate of 6k. With

play −−rate 6k "*.vox"

the given s ample rate option will be applied to all three vox files.

−p, −−sox−pipe

This can be used in place of an output filename to specify that the SoX command should be used as in input pipe to another SoX command. For e xample, the command:

play "|sox −n −p synth 2" "|sox −n −p synth 2 tremolo 10" stat

plays two ‘ files’ in succession, 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 or play (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. Note that here, ‘null file’ refers to a SoX-specific mechanism and is not related to any o perating-system mechanism with a similar name. Using a null file to input audio 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 to output audio amounts to discarding the audio and is useful mainly with effects that produce information about the audio instead of affecting it (such as noiseprof or stat). The sampling rate associated with a null file is by default 48 kHz, but, as with a normal file, this can be overridden if desired using command-line format options (see below).

Supported File & Audio Device Types

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

OPTIONS

Global Options

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

The SOX_OPTS environment variable can be used to provide alternative default values for SoX’s global options. For example:

SOX_OPTS="−−buffer 20000 −−play−rate−arg −hs −−temp /mnt/temp"

Note that setting SOX_OPTS can potentially create unwanted changes in the behaviour of scripts or other programs that invoke SoX. SOX_OPTS might best be used for things (such as in the given example) that reflect the environment in which SoX is being run. Enabling options such as −−no−clobber as default might be handled better using a shell alias since a shell alias will not affect operation in scripts etc.

One way to ensure that a script cannot be affected by SOX_OPTS is to clear SOX_OPTS at the start of thescript, but this of course loses the benefit of SOX_OPTS carrying some system-wide default options. An alternative approach is to explicitly invoke SoX with default option values, e.g.

SOX_OPTS="−V −−no-clobber"
...
sox −V2 −−clobber $input $output ...

Note that the way to set environment variables varies from system to system. Here are some examples: Unix bash: export SOX_OPTS="−V −−no-clobber" Unix csh:

setenv SOX_OPTS "−V −−no-clobber"

MS-DOS/MS-Windows:

set SOX_OPTS=−V −−no-clobber

MS-Windows GUI: via Control Panel : System : Advanced : Environment Variables Mac OS X GUI: Refer to Apple’s Technical Q&A QA1067 document.

−−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). Be aware that large values for −−buffer will cause SoX to be become slow to r espond to requests to terminate or to skip the current input file.

−−clobber

Don’t prompt before overwriting an existing file with the same name as that given f or the output file. This is the default behaviour.

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

Select the input file combining method; for some of these, short options are available: −m selects ‘mix’, −M selects ‘merge’, and −T selects ‘multiply’.

See Input File Combining above for a description of the different combining methods.

−D, −−no−dither

Disable automatic dither—see ‘Dithering’ above. A n e xample of why t his might occasionally be useful is 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, then, strictly speaking, no dither is needed if converting the file back to 16 bit. See also the stats effect for how to determine the actual bit depth of the audio within a file.

−−effects−file FILENAME

Use FILENAME 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 e ffects specified on the command line to be discarded. −G, −−guard

Automatically invoke the gain effect to guard against clipping. E.g. sox −G infile −b 16 outfile rate 44100 dither −s is shorthand for

sox infile −b 16 outfile gain −h rate 44100 gain −rh dither −s

See also −V, − −norm, and the gain effect.

−h, −−help

Show version number and usage information.

−−help−effect NAME

Show usage information on the specified effect. The name all can be used to show u sage on all effects.

−−help−format NAME

Show i nformation about the specified file format. The name all can be used to show i nformation on all formats.

−−i, −−info

Only if given as t he first parameter to sox, behave as soxi(1).

−m | −M

Equivalent to −−combine mix and −−combine merge, respectively.

−−magic

If SoX has been built with the optional ‘libmagic’ library then this option can be given to enable its use in helping to detect audio file types.

−−multi−threaded | −−single−threaded

By default, SoX is ‘single threaded’. If the −−multi−threaded option is given however then SoX will process audio channels for most multi-channel effects in parallel on hyper-threading/multi- core architectures. This may reduce processing time, though sometimes it may be necessary to use this option in conjunction with a larger buffer size than is the default to gain any benefit from multi-threaded processing (e.g. 131072; see −−buffer above).

−−no−clobber

Prompt before overwriting an existing file with the same name as that given f or the output file. N.B. Unintentionally overwriting a file is easier than you might think, for example, if you accidentally enter

sox file1 file2 effect1 effect2 ...

when what you really meant was

play file1 file2 effect1 effect2 ...

then, without this option, file2 will be overwritten. Hence, using this option is recommended.

SOX_OPTS (above), a ‘shell’ alias, script, or batch file may be an appropriate way of permanently enabling it. −−norm[=dB-level] Automatically invoke the gain effect to guard against clipping and to normalise the audio. E.g. sox −−norm infile −b 16 outfile rate 44100 dither −s is shorthand for sox infile −b 16 outfile gain −h rate 44100 gain −nh dither −s Optionally, t he audio can be normalized to a given l ev el ( usually) below 0 d BFS: sox −−norm=−3 infile outfile See also −V, − G, and the gain effect. −−play−rate−arg A RG Selects a quality option to be used when the ‘rate’ effect is automatically invoked w hilst playing audio. This option is typically set via the SOX_OPTS environment variable (see above). −−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 configura- tion of many of t he transfer-function based effects. For the first given e ffect that supports the selected plotting program, SoX will output commands to plot the effect’s t ransfer function, and then exit without actually processing any a udio. E.g. sox −−plot octave input-file −n highpass 1320 > highpass.plt octave highpass.plt −q, −−no−show−progress Run in quiet mode when SoX wouldn’t o therwise do so. This is the opposite of the −S option. −R Run in ‘repeatable’ mode. When this option is given, where applicable, SoX will embed a fixed time-stamp in the output file (e.g. AIFF) a nd will ‘seed’ pseudo random number generators (e.g. dither) w ith a fixed number, t hus 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 and rec, album for play where (at least) the first two i nput files are tagged with the same Artist and Album names, and track for play otherwise. −S, −−show−progress Display input file format/header information, and processing progress as input file(s) percentage complete, elapsed time, and remaining time (if known; shown in brackets), and the number of samples written to the output file. Also shown is a peak-level m eter, a nd an indication if clipping has occurred. The peak-level m eter shows up to two c hannels and is calibrated for digital audio as follows (right channel shown): dB FSD Display dB FSD Display −25 − −11 ==== −23 −9 ====−= −21 =− −7 ===== −19 == −5 =====− −17 ==− −3 ====== −15 === −1 =====! −13 ===− A t hree-second peak-held value of headroom in dBs will be shown to the right of the meter if this is below 6 dB. This option is enabled by default when using SoX to play or record audio

SoX(1) Sound eXchange SoX(1) −T Equivalent to −−combine multiply. −−temp DIRECTORY Specify that any t emporary files should be created in the given DIRECTORY . T his 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 v ersion number and exit.

−V[level]

Set verbosity. T his is particularly useful for seeing how a ny automatic effects have been invoked by SoX. SoX displays messages on the console (stderr) according to the following verbosity levels: 0 No messages are shown at all; use the exit status to determine if an error has occurred. 1 Only error messages are shown. These are generated if SoX cannot complete the requested commands. 2 Warning messages are also shown. These are generated if SoX can complete the requested commands, but not exactly according to the requested command parameters, or if clipping occurs. 3 Descriptions of SoX’s p rocessing phases are also shown. Useful for seeing exactly how SoX is processing your audio. 4 and above Messages to help with debugging SoX are also shown. By default, the verbosity level i s s et to 2 (shows errors and warnings). Each occurrence of the −V option increases the verbosity level by 1 . Alternatively, t he verbosity level c an be set to an absolute number by specifying it immediately after the −V, e .g. −V0 sets it to 0.

Input File Options

These options apply only to input files and may precede only input filenames on the command line.

−−ignore−length

Override an (incorrect) audio length given in an a udio file’s h eader. If t his option is given then SoX will keep 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 FA C TOR. T his 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 t hen in addition to the volume adjustment, the audio signal will be inverted. See also the norm, vol, a nd gain effects, and see Input File Balancing above. Input & Output File Format Options These options apply to the input or output file whose name they i mmediately 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 to that of the input file.

-b BITS, −−bits BITS

The number of bits (a.k.a. bit-depth or sometimes word-length) in each encoded sample. Not applicable to complex e ncodings such as MP3 or GSM. Not necessary with encodings that have a fixed number of bits, e.g. A/μ-law, ADPCM. For an i nput 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 e xample sox −r 16k −e signed −b 8 input.raw output.wa

converts a particular ‘raw’ file to a self-describing ‘WAV ’ fi le. For an o utput file, this option can be used (perhaps along with −e) to s et the output encoding size. By default (i.e. if this option is not given), the output encoding size will (providing it is supported by the output file type) be set to the input encoding size. For e xample sox input.cdda −b 24 output.wav converts raw CD d igital audio (16-bit, signed-integer) to a 24-bit (signed-integer) ‘WAV ’ fi le.

−c CHANNELS, −−channels CHANNELS

The number of audio channels in the audio file. This can be any n umber greater than zero. For an i nput 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 m ay be useful to use this option with a ‘head- ered’ file, in order to override the (presumably incorrect) value in the header—note that this is only supported with certain file types. Examples: sox −r 48k −e float −b 32 −c 2 input.raw output.wav converts a particular ‘raw’ file to a self-describing ‘WAV ’ fi le. play −c 1 music.wav interprets the file data as belonging to a single channel regardless of what is indicated in the file header. N ote that if the file does in fact have two channels, this will result in the file playing at half speed. For an o utput file, this option provides a shorthand for specifying that the channels effect should be invoked in o rder to change (if necessary) the number of channels in the audio signal to the num- ber given. For example, the following two c ommands are equivalent: sox input.wav −c 1 output.wav bass −b 24 sox input.wav output.wav bass −b 24 channels 1 though the second form is more flexible as it allows the effects to be ordered arbitrarily.

−e ENCODING, −−encoding ENCODING

The audio encoding type. Sometimes needed with file-types that support more than one encoding type. For example, with raw, WAV , or AU ( but n ot, for example, with MP3 or FLAC). The avail- able encoding types are as follows: signed-integer PCM data stored as signed (‘two’s c omplement’) integers. Commonly used with a 16 or 24 −bit encoding size. A v alue of 0 represents minimum signal power. unsigned-integer PCM data stored as unsigned integers. Commonly used with an 8-bit encoding size. A value of 0 represents maximum signal power. floating-point PCM data stored as IEEE 753 single precision (32-bit) or double precision (64-bit) float- ing-point (‘real’) numbers. A v alue of 0 represents minimum signal power. a-law International telephony s tandard for logarithmic encoding to 8 bits per sample. It has a precision equivalent to roughly 13-bit PCM and is sometimes encoded with reversed bit- ordering (see the −X option).

u-law, m u-law

North American telephony s tandard for logarithmic encoding to 8 bits per sample. A.k.a. μ-law. It h as a precision equivalent to roughly 14-bit PCM and is sometimes encoded with reversed bit-ordering (see the −X option).

oki-adpcm

OKI (a.k.a. VOX, D ialogic, or Intel) 4-bit ADPCM; it has a precision equivalent to roughly 12-bit PCM. ADPCM is a form of audio compression that has a good compro- mise between audio quality and encoding/decoding speed