Audio Backends - minimodem

Overview

minimodem supports multiple audio backend systems for maximum portability across different operating systems and configurations:

ALSA - Advanced Linux Sound Architecture (Linux)
PulseAudio - Cross-platform sound server (Linux, BSD, others)
sndio - Simple audio API (OpenBSD, other BSD systems)
sndfile - Audio file I/O (all platforms)

The build system automatically configures available backends based on your system’s libraries.

Backend Selection

System Default

When no backend is specified, minimodem uses the system default:

minimodem --rx 1200
minimodem --tx 1200

The default backend priority (from src/simpleaudio.c lines 83-93):

PulseAudio (if available)
ALSA (if PulseAudio not available)
sndio (if neither above available)

The system default is determined at compile time based on which libraries are available when minimodem is built.

Explicit Backend Selection

You can force a specific backend using command-line options:

# Use ALSA
minimodem --rx --alsa 1200

# Use PulseAudio (default, explicit)
minimodem --rx 1200

# Use sndio
minimodem --rx --sndio 1200

# Use file I/O
minimodem --rx --file recording.wav 1200

ALSA Backend

Overview

ALSA (Advanced Linux Sound Architecture) provides low-latency audio access on Linux systems.

Command-Line Option

--alsa[=device]
-A[device]

Device Specification

Default device:

minimodem --rx --alsa 1200
# Uses "default" ALSA device

Specific hardware device:

minimodem --rx --alsa=hw:0,0 1200
minimodem --rx --alsa=plughw:1,0 1200

Card and device numbers:

minimodem --rx --alsa=0,0 1200
# Automatically expands to plughw:0,0

minimodem --rx --alsa=1 1200
# Automatically expands to plughw:1,0

The plughw: prefix enables ALSA’s plugin layer for sample rate conversion and format adaptation. Use hw: for direct hardware access (may fail if sample rate doesn’t match).

Implementation Details

From src/simpleaudio-alsa.c lines 117-127:

char *be_device;
if ( ! backend_device ) {
    be_device = "default";
} else {
    if ( strchr(backend_device, ':') )
        snprintf(be_device, 32, "%s", backend_device);
    else if ( strchr(backend_device, ',') )
        snprintf(be_device, 32, "plughw:%s", backend_device);
    else
        snprintf(be_device, 32, "plughw:%s,0", backend_device);
}

Examples

List ALSA Devices

aplay -l
# Output:
card 0: PCH [HDA Intel PCH], device 0: ALC295 Analog [ALC295 Analog]
card 1: Device [USB Audio Device], device 0: USB Audio [USB Audio]

Use USB Audio Device

minimodem --rx --alsa=plughw:1,0 1200

Use Default Device

minimodem --tx --alsa 1200

Configuration

ALSA parameters (from src/simpleaudio-alsa.c line 152):

Format: S16 or FLOAT
Access: Interleaved
Channels: Typically 1 (mono)
Sample rate: Default 48000 Hz (configurable with --samplerate)
Latency: 100ms buffer
Soft resample: Enabled

If minimodem fails to open an ALSA device, check that:

The device exists (aplay -l)
No other application is using the device exclusively
You have permission to access the audio device

PulseAudio Backend

Overview

PulseAudio is a sound server that provides network-transparent audio and automatic device management.

Automatic Operation

PulseAudio is the default backend on most modern Linux systems:

minimodem --rx 1200
# Automatically uses PulseAudio if available

Implementation Details

From src/simpleaudio-pulse.c:

Uses pa_simple API for straightforward audio I/O
Automatically selects default source (recording) or sink (playback)
Handles sample format conversion automatically

Configuration

Buffer attributes (lines 116-127):

pa_buffer_attr attr = {
    .maxlength = (uint32_t)-1,
    .tlength = (uint32_t)-1,
    .prebuf = (uint32_t)-1,
    .minreq = (uint32_t)-1,
    .fragsize = (uint32_t)-1,
};

attr.fragsize = 0;  /* lowest possible capture latency */
attr.tlength = 0;   /* lowest possible playback latency */

Advantages

Automatic device management
Network audio support
Per-application volume control
Better integration with desktop environments

Examples

# Receive with PulseAudio
minimodem --rx 300

# Transmit with PulseAudio
echo "Test" | minimodem --tx 1200

Use pavucontrol (PulseAudio Volume Control) to monitor minimodem’s audio streams in real-time and adjust input/output devices on the fly.

sndio Backend

Overview

sndio is a small, simple audio and MIDI framework used primarily on OpenBSD and other BSD systems.

Command-Line Option

--sndio[=device]
-s[device]

Device Specification

Default device:

minimodem --rx --sndio 1200
# Uses SIO_DEVANY (system default)

Specific device:

minimodem --rx --sndio=rsnd/0 1200
minimodem --tx --sndio=rsnd/1 1200

Implementation Details

From src/simpleaudio-sndio.c lines 84-111:

const char *be_device;
if ( ! backend_device )
    be_device = SIO_DEVANY;
else
    be_device = backend_device;

hdl = sio_open(
    be_device,
    sa_stream_direction == SA_STREAM_RECORD ? SIO_REC : SIO_PLAY,
    0 /* nbio_flag */);

Supported Formats

S16: 16-bit signed integer (supported)
FLOAT: Not currently supported in sndio backend

The sndio backend does not support --float-samples option. Use S16 format (the default) instead.

Configuration

From lines 90-103:

par.bits = 16;
par.sig = 1;           /* signed */
par.le = SIO_LE_NATIVE; /* native endianness */
par.bps = SIO_BPS(par.bits);
par.rate = rate;
par.xrun = SIO_IGNORE;  /* ignore overruns/underruns */

File I/O Backend (sndfile)

Overview

The sndfile backend enables reading from and writing to audio files instead of live audio devices.

Command-Line Option

--file <filename>
-f <filename>

Supported Formats

From src/simpleaudio-sndfile.c lines 111-141, supported file formats include:

WAV - Microsoft Wave
AIFF - Apple Audio Interchange File Format
FLAC - Free Lossless Audio Codec
OGG - Ogg Vorbis
AU - Sun/NeXT audio
RAW - Raw PCM data
And many more…

The format is automatically detected from the file extension.

Examples

Receive from Audio File

minimodem --rx --file recording.wav 1200
minimodem --rx -f signal.flac 300

Transmit to Audio File

echo "Hello" | minimodem --tx --file output.wav 1200
echo "Test" | minimodem --tx -f data.flac 300

Use with Pipes

# Decode from stdin
sox -t alsa default -t wav - | minimodem --rx --file - 1200

# Encode to stdout
echo "Test" | minimodem --tx --file - 1200 | aplay

File I/O is perfect for:

Offline decoding and analysis
Creating test signals
Debugging communication issues
Archiving FSK transmissions

Implementation Details

Automatic format detection (lines 144-157):

static unsigned int
sndfile_format_from_path( const char *path )
{
    const char *p = strrchr(path, '.');
    if ( p )
        p++;
    else
        p = path;

    for ( sfmt=sndfile_formats; sfmt->str; sfmt++ )
        if ( strcasecmp(sfmt->str,p) == 0 )
            return sfmt->major_format;
    return SF_FORMAT_WAV; /* default */
}

Sample Format Selection

minimodem supports two sample formats:

S16 (16-bit Integer)

Default format:

minimodem --rx 1200
# Uses 16-bit signed integer samples

Pros: Lower memory usage, faster processing
Cons: Slightly lower precision
Typical use: Most applications, real-time decoding

FLOAT (32-bit Float)

Enable with --float-samples:

minimodem --tx --float-samples --file output.wav 1200

Pros: Higher precision, better for analysis
Cons: More memory, slightly slower
Typical use: File generation, quality-critical applications
Required: For RX mode (automatically enabled)

Receive mode automatically uses floating-point samples internally for FFT processing, regardless of the --float-samples option.

Sample Rate Configuration

Set sample rate with --samplerate (-R):

minimodem --rx --samplerate 48000 1200  # Default
minimodem --rx --samplerate 96000 1200  # High quality
minimodem --rx --samplerate 24000 1200  # Low bandwidth

Higher sample rates increase CPU usage and memory requirements. Use 48000 Hz unless you have specific needs:

96000 Hz: Very high baud rates or wide frequency spans
24000 Hz: Embedded systems or low-power devices

Build Configuration

Checking Available Backends

When building from source, configure displays enabled backends:

./configure
# Output:
option summary:
    alsa           yes (1)
    pulseaudio     yes (1)
    sndfile        yes (1)
    sndio          no (0)

Disabling Backends

From configure.ac lines 19-65:

# Build without ALSA
./configure --without-alsa

# Build without PulseAudio
./configure --without-pulseaudio

# Build without sndio
./configure --without-sndio

# Build without sndfile
./configure --without-sndfile

At least one backend must be enabled. If all system audio backends (ALSA, PulseAudio, sndio) are disabled, you must enable sndfile for file-only operation.

Troubleshooting

Backend Not Available

Error: This build of minimodem was configured without alsa support Solution: Rebuild minimodem with the required backend:

./configure --with-alsa
make clean && make
sudo make install

Device Busy

Error: Cannot create ALSA stream: Device or resource busy Solutions:

Close other applications using the audio device
Try PulseAudio backend instead: minimodem --rx 1200
Check for exclusive access: fuser -v /dev/snd/*

No Sound Output

Problem: Minimodem runs but no audio is transmitted Solutions:

Check mixer settings: alsamixer
Verify device selection: --alsa=plughw:0,0
Test audio path: speaker-test -t sine
Increase volume: Use system volume controls

Poor Quality or Dropouts

Problem: Audio has glitches, clicks, or missing frames Solutions:

Increase buffer size (not directly configurable in minimodem)
Close CPU-intensive applications
Use ALSA directly instead of PulseAudio
Reduce sample rate if very high

Best Practices

Use PulseAudio for desktop systems (easiest, most compatible)
Use ALSA directly for low-latency embedded applications
Use sndfile for offline processing and testing
Match sample rates between TX and RX when possible
Test audio path before attempting FSK communication

Performance Comparison

Backend	Latency	CPU Usage	Compatibility
ALSA	Low	Low	Linux only
PulseAudio	Medium	Medium	Cross-platform
sndio	Low	Low	BSD focus
sndfile	N/A	Low	All platforms

Advanced Examples

Record and Decode Pipeline

# Record audio and decode simultaneously
minimodem --rx --alsa=hw:1,0 1200 | tee output.txt

Multi-Channel Recording

# Capture from specific channel
minimodem --rx --alsa=plughw:0,0 1200

Network Audio Streaming

With PulseAudio network audio:

# On receiver
export PULSE_SERVER=tcp:receiver.local
minimodem --rx 1200

# On transmitter
export PULSE_SERVER=tcp:transmitter.local
echo "Data" | minimodem --tx 1200

Batch File Processing

# Decode multiple files
for file in *.wav; do
    echo "Processing $file"
    minimodem --rx -f "$file" 1200 > "${file%.wav}.txt"
done

Get Started

Core Concepts

Usage Guides

Protocols

Advanced

​Overview

​Backend Selection

​System Default

​Explicit Backend Selection

​ALSA Backend

​Overview

​Command-Line Option

​Device Specification

​Implementation Details

​Examples

​List ALSA Devices

​Use USB Audio Device

​Use Default Device

​Configuration

​PulseAudio Backend

​Overview

​Automatic Operation

​Implementation Details

​Configuration

​Advantages

​Examples

​sndio Backend

​Overview

​Command-Line Option

​Device Specification

​Implementation Details

​Supported Formats

​Configuration

​File I/O Backend (sndfile)

​Overview

​Command-Line Option

​Supported Formats

​Examples

​Receive from Audio File

​Transmit to Audio File

​Use with Pipes

​Implementation Details

​Sample Format Selection

​S16 (16-bit Integer)

​FLOAT (32-bit Float)

​Sample Rate Configuration

​Build Configuration

​Checking Available Backends

​Disabling Backends

​Troubleshooting

​Backend Not Available

​Device Busy

​No Sound Output

​Poor Quality or Dropouts

​Best Practices

​Performance Comparison

​Advanced Examples

​Record and Decode Pipeline

​Multi-Channel Recording

​Network Audio Streaming

​Batch File Processing

​See Also

Build docs developers (and LLMs) love

Overview

Backend Selection

System Default

Explicit Backend Selection

ALSA Backend

Overview

Command-Line Option

Device Specification

Implementation Details

Examples

List ALSA Devices

Use USB Audio Device

Use Default Device

Configuration

PulseAudio Backend

Overview

Automatic Operation

Implementation Details

Configuration

Advantages

Examples

sndio Backend

Overview

Command-Line Option

Device Specification

Implementation Details

Supported Formats

Configuration

File I/O Backend (sndfile)

Overview

Command-Line Option

Supported Formats

Examples

Receive from Audio File

Transmit to Audio File

Use with Pipes

Implementation Details

Sample Format Selection

S16 (16-bit Integer)

FLOAT (32-bit Float)

Sample Rate Configuration

Build Configuration

Checking Available Backends

Disabling Backends

Troubleshooting

Backend Not Available

Device Busy

No Sound Output

Poor Quality or Dropouts

Best Practices

Performance Comparison

Advanced Examples

Record and Decode Pipeline

Multi-Channel Recording

Network Audio Streaming

Batch File Processing

See Also