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 con- venient 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 pro- grams 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

sox December 31, 2014 1

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 i nput combiner can be configured (see OPTIONS below) to combine multiple files using any of t he 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 m erged 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 num- bers in the interval −1 to +1). If the number of channels in the input files is not the same, the missing chan- nels 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 balanc- ing is performed. In this case, if manual volume adjustments are not given, SoX will try to ensure that clip- ping 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 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 b y b alancing using a factor of ¹/√n instead of ¹/n. N ote that this balancing factor does not guarantee that clipping will not occur, b ut 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 f ormatted 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).

−−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 d etermine 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 n ew line can be used in place of the special : marker to separate effect chains. For c onvenience, 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 v ersion number and usage information.

−−help−effect NAME

Show u sage 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 e nable 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 acci- dentally enter

sox file1 file2 effect1 effect2 ... when what you really meant was