mixer

Audio mixing utility

The mixer utility mixes multiple sound files with independent control over start times, amplitudes, and channel routing. It supports time-varying gain curves and flexible output channel mapping.

Syntax

mixer [-flags] soundfile [-flags] soundfile ...

Flags apply to the next soundfile in the command line.

Global output options

Option	Description
-o <filename>	Output sound filename (default: test)
-A	Create AIFF format output
-W	Create WAV format output
-h	No header (raw output)

Output sample format

Option	Description
-c	8-bit signed char
-8	8-bit unsigned char
-a	A-law encoded
-u	μ-law encoded
-s	16-bit short int (default)
-l	32-bit long int
-f	32-bit float

Per-file mixing options

Option	Description
-S <samples>	Start sample number for next input file
-T <seconds>	Start time in seconds for next input file
-F <factor>	Amplitude scale factor for next input
-F <filename>	Time-varying gain curve file for next input

Channel routing

Option	Description
-1	Include channel 1 of next input
-2	Include channel 2 of next input
-3	Include channel 3 of next input
-4	Include channel 4 of next input
^ <n> <m>	Route input channel n to output channel m

Additional options

Option	Description
-R	Continuously rewrite header during writing
-H <n>	Heartbeat: 1=spinner, 2=dots, 3=numeric
-N	Ring bell when complete
-v	Verbose mode for debugging
-- <file>	Log output to file

Examples

Basic mix of two files

mixer -o mix.wav file1.wav file2.wav

Mix with start times

mixer -o output.wav file1.wav -T 2.0 file2.wav -T 5.0 file3.wav

Starts file2 at 2 seconds and file3 at 5 seconds.

Mix with gain control

mixer -o mix.wav -F 0.5 quiet.wav -F 1.5 loud.wav

Mix with sample-accurate timing

mixer -o output.wav file1.wav -S 44100 file2.wav -S 88200 file3.wav

Starts file2 at sample 44100 (1 sec at 44.1kHz) and file3 at sample 88200 (2 sec).

Time-varying gain curve

mixer -o mix.wav -F curve.txt audio.wav

With curve.txt containing:

0 0.0
1000 1.0
50000 0.5
100000 0.0

Format: sample_number gain_factor

Channel routing examples

Mono to stereo (left channel):

mixer -o stereo.wav -^1 1 mono.wav

Mono to stereo (right channel):

mixer -o stereo.wav -^1 2 mono.wav

Stereo swap:

mixer -o swapped.wav -^1 2 -^2 1 stereo.wav

Extract and route specific channels:

mixer -o out.wav -^2 1 -^4 2 multitrack.wav

Routes input channel 2 to output 1, input channel 4 to output 2.

Complex multi-file mix

mixer -o final.wav \
  -F 0.8 drums.wav \
  -T 0.5 -F 0.6 bass.wav \
  -T 1.0 -F 0.7 guitar.wav \
  -T 2.0 -F 1.0 -F vocal_curve.txt vocals.wav

Gain scaling

Fixed gain

-F <factor> multiplies all samples by the factor:

• 0.5 - Reduce by 6 dB
• 1.0 - No change (default)
• 2.0 - Increase by 6 dB
• 0.0 - Silence

Time-varying gain

-F <filename> uses a breakpoint file with format:

sample1 gain1
sample2 gain2
...

• Sample numbers are absolute positions in output file
• Gains are linearly interpolated between breakpoints
• Use srate × time to convert time to samples

Example: Fade in over 2 seconds at 44.1kHz:

0 0.0
88200 1.0

Output duration

Output file length is determined by:

• Latest ending input file
• Calculated as: max(start_time + duration) across all inputs

Channel configuration

Output channels determined by:

1. Maximum channel referenced in routing
2. If no routing specified, uses first input’s channel count

Default behavior:

• Mono inputs → sum to mono output
• Stereo inputs → sum to stereo output
• Mixed channel counts → expand to maximum needed

Clipping and overflow

mixer tracks samples exceeding output range:

mixer: 145 samples out of range

To avoid clipping:

1. Reduce input gains with -F
2. Use float output format (-f) for no clipping
3. Scale result with scale utility

Performance

Memory usage

Memory = output_samples × output_channels × 4 bytes

For 60 seconds stereo at 44.1kHz:

60 × 44100 × 2 × 4 = 21.168 MB

Processing time

Linear with:

• Total output duration
• Number of input files
• Number of channels

Gain curve file tips

Create fade curves

Fade in (0 to 1 over 2 seconds):

0 0.0
88200 1.0

Fade out (1 to 0 from 8 to 10 seconds):

352800 1.0
441000 0.0

Crossfade between two files:

• File 1: Fade out starting at overlap point
• File 2: Fade in starting at overlap point

Generate curves with scripts

Python script for exponential fade:

import math
srate = 44100
for i in range(0, srate * 2, 1000):
    gain = (i / (srate * 2)) ** 2  # Exponential curve
    print(f"{i} {gain}")

Common mixing scenarios

Music production

mixer -o song.wav \
  -F 0.8 drums.wav \
  -F 0.6 bass.wav \
  -F 0.7 guitar.wav \
  -T 8.0 -F 0.9 vocals.wav

Podcast production

mixer -o podcast.wav \
  -F 0.9 speech.wav \
  -T 0 -F 0.3 music_bed.wav \
  -T 120 -F 0.5 interlude.wav

Sound design

mixer -o scene.wav \
  -F ambience_curve.txt ambience.wav \
  -T 5.5 -F 2.0 explosion.wav \
  -T 5.7 -F 1.5 debris.wav \
  -T 7.0 -F 0.8 aftermath.wav

Limitations

• Maximum 32 input files
• All inputs must have same sample rate
• Channel routing limited to channels 1-4
• Output clipping not prevented (use float or scale down)

Related utilities

• scale - Scale amplitude of result
• sndinfo - Check file properties
• srconv - Convert sample rates before mixing