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SoX(1) Sound eXchange SoX(1)

NAME

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 manysystems, act as a general purpose audio

player or a multi-track audio recorder.Italso has limited ability to split the input into multiple output ﬁles.

All SoX functionality is available using just the sox command. Tosimplify playing and recording audio, if

SoX is invokedas play,the output ﬁle is automatically set to be the default sound device, and if invokedas

rec,the default sound device is used as an input source. Additionally,the 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,graphical audio editor,use 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 ﬂowjust shown. Note also that whilst options pertaining to ﬁles are placed before their

respective ﬁle name, the opposite is true for effects. Toshowhow this works in practice, here is a selection

of examples of howSoX might be used. The simple

sox recital.au recital.wav

translates an audio ﬁle in Sun AUformat to a Microsoft WAV ﬁle, 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 twoaudio ﬁles, and

sox −m music.mp3 voice.wav mixed.flac

mixes together twoaudio 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 ‘Aminor seventh’ chord with a pipe-organsound,

rec −c 2 radio.aiff trim 0 30:00

sox December 31, 2014 1

SoX(1) Sound eXchange SoX(1)

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 newtrack 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 isjust an overviewofSoX’scapabilities; detailed explanations of howtouse all SoX

parameters, ﬁle formats, and effects can be found belowinthis 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 pro-

cessed with SoX:

sample rate

The sample rate in samples per second (‘Hertz’ or ‘Hz’). Digital telephonytraditionally uses a

sample rate of 8000 Hz (8 kHz), though these days, 16 and even32kHz are becoming more com-

mon. Audio Compact Discs use 44100 Hz (44

1kHz). Digital Audio Tape and manycomputer

systems use 48 kHz. Professional audio systems often use 96 kHz.

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. 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 overaunit

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-lawtelephonysignal 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 allowtextual ‘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,atleast FLACand Ogg Vorbis), but

not generating it. Note that by default, SoX copies input ﬁle comments to output ﬁles that support com-

ments, 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).

sox December 31, 2014 2

SoX(1) Sound eXchange SoX(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 determine the format of an input ﬁle, SoX will use, in order of precedence and as givenoravailable:

1. Command-line format options.

2. The contents of the ﬁle header.

3. The ﬁlename extension.

To set the output ﬁle format, SoX will use, in order of precedence and as givenoravailable:

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.

Forall 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 resolvethe 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 twocommands 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.Systems can also have more than one audio device (a.k.a. ‘sound card’). If more than one

audio driverhas 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.

Forexample (on manysystems):

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

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 levelisset 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).

sox December 31, 2014 3

SoX(1) Sound eXchange SoX(1)

***

On some systems, SoX allows audio playback volume to be adjusted whilst using play.Where supported,

this is achievedbytapping the ‘v’ & ‘V’ keysduring playback.

To help with setting a suitable recording level, SoX includes a peak-levelmeter which can be invoked

(before making the actual recording) as follows:

rec −n

The recording levelshould 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

Manyﬁle 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 copyofthe original

audio. This is the case for manyformats used in telephony(e.g. A-law, GSM) where lowsignal bandwidth

is more important than high audio ﬁdelity,and for manyformats used in portable music players (e.g. MP3,

Vorbis) where adequate ﬁdelity can be retained evenwith the large compression ratios that are needed to

makeportable 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 howclosely 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-lawisn’t.

N.B. SoX converts all audio ﬁles to an internal uncompressed format before performing anyaudio 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,bydefault, SoX automatically adds TPDF dither when the output bit-depth is less than 24 and

anyofthe 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

Forexample, adjusting volume with vol0.25 requires twoadditional 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’sinternal 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.

sox December 31, 2014 4

SoX(1) Sound eXchange SoX(1)

Use the −V option to see what processing SoX has automatically added. The −D option may be givento

override automatic dithering. To inv 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,asyou might expect, when using the vol or gain effects to increase the audio

volume. Clipping could also occur with manyother effects, when converting one format to another,and

ev e nwhen 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 levelwas ini-

tially too close to the clipping point.

Forthese reasons, it is usual to makesure that an audio ﬁle’ssignal levelhas some ‘headroom’, i.e. it does

not exceed a particular levelbelowthe maximum possible levelfor the givenrepresentation. 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, manyCDs, MP3s, etc. are

nowmastered at levels above 0dBFS i.e. the audio is clipped as delivered.

SoX’s stat and stats effects can assist in determining the signal levelinanaudio 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 anypoint during processing, SoX will display a warning message to that effect.

See also −G and the gain and norm effects.

Input File Combining

SoX’sinput combiner can be conﬁgured (see OPTIONS below) to combine multiple ﬁles using anyofthe

following methods: ‘concatenate’, ‘sequence’, ‘mix’, ‘mix-power’, ‘merge’, or ‘multiply’. The default

method is ‘sequence’ for play,and ‘concatenate’ for rec and sox.

Forall methods other than ‘sequence’, multiple input ﬁles must have the same sampling rate. If necessary,

separate SoX invocations can be used to makesampling rate adjustments prior to combining.

If the ‘concatenate’ combining method is selected (usually,this 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.Itissimilar 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 twoormore 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 theyare not and some channels in the output ﬁle will not contain

audio from every input ﬁle. Amixed audio ﬁle cannot be un-mixed without reference to the original input

ﬁles.

If the ‘merge’ combining method is selected then twoormore input ﬁles must be givenand will be merged

together to form the output ﬁle. The number of channels in each input ﬁle need not be the same. Amerged

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, twomono 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.

sox December 31, 2014 5

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