Real-time audio configuration

Configuring Csound for real-time audio input and output

Csound supports real-time audio through various backend drivers. Proper configuration of audio drivers, buffer sizes, and sample rates is essential for low-latency performance.

Audio output devices

Default audio device

csound -odac myfile.csd

The -odac flag selects the default audio output device using the default audio driver.

Specific audio device

Specify a device by name or number:

csound -odac2 myfile.csd          # Use device number 2
csound -odac:hw:1,0 myfile.csd    # Use specific ALSA device
csound -odac:Built-in myfile.csd  # Use device by name

Audio input devices

Default audio input

csound -iadc myfile.csd

Specific input device

csound -iadc2 myfile.csd          # Use device number 2
csound -iadc:hw:1,0 myfile.csd    # Specific ALSA device

Simultaneous input and output

csound -iadc -odac myfile.csd     # Full duplex
csound -iadc2 -odac3 myfile.csd   # Different devices

Audio drivers

Csound supports multiple audio backend drivers. Select a driver using the RTAUDIO environment variable or the -+rtaudio flag.

Available drivers

• PortAudio (portaudio or pa) - Cross-platform (default on most systems)
• JACK (jack) - Linux/macOS low-latency audio
• ALSA (alsa) - Linux native ALSA
• CoreAudio (auhal) - macOS native
• PulseAudio (pulse) - Linux PulseAudio
• WASAPI (wasapi) - Windows native
• ASIO (asio) - Windows low-latency (requires ASIO drivers)

Selecting a driver

csound -+rtaudio=jack -odac myfile.csd
csound -+rtaudio=alsa -odac myfile.csd
csound -+rtaudio=portaudio -odac myfile.csd

Or set the environment variable:

export RTAUDIO=jack
csound -odac myfile.csd

PortAudio

Default on most platforms. Good cross-platform compatibility:

csound -+rtaudio=portaudio -odac myfile.csd

JACK (Linux/macOS)

JACK provides professional low-latency audio routing:

jackd -d alsa -r 48000 -p 256 -n 2  # Start JACK daemon
csound -+rtaudio=jack -odac myfile.csd

JACK-specific options:

csound -+rtaudio=jack -odac:system:playback_ myfile.csd

Csound appears as a JACK client and can be connected to other JACK applications.

ALSA (Linux)

Direct ALSA access for lower latency:

csound -+rtaudio=alsa -odac:hw:0,0 myfile.csd
csound -+rtaudio=alsa -odac:plughw:1,0 myfile.csd  # With conversion

List ALSA devices:

aplay -l    # List playback devices
arecord -l  # List capture devices

PulseAudio (Linux)

Use PulseAudio backend:

csound -+rtaudio=pulse -odac myfile.csd

CoreAudio (macOS)

Native macOS audio:

csound -+rtaudio=auhal -odac myfile.csd

WASAPI (Windows)

Windows native low-latency:

csound -+rtaudio=wasapi -odac myfile.csd

ASIO (Windows)

For professional audio interfaces with ASIO drivers:

csound -+rtaudio=asio -odac myfile.csd

Listing available devices

List all available audio devices:

csound --devices              # List all audio devices
csound --devices=out          # List output devices only
csound --devices=in           # List input devices only

With a specific driver:

csound -+rtaudio=jack --devices
csound -+rtaudio=alsa --devices

Example output:

audio output devices:
    0: dac0 (Realtek ALC887-VD Analog)
    1: dac1 (HDA Intel PCH Front Headphone)
    2: dac2 (HD Pro Webcam C920 Analog Stereo)
audio input devices:
    0: adc0 (Realtek ALC887-VD Analog)
    1: adc1 (HD Pro Webcam C920 Analog Stereo)

Buffer configuration

Buffer sizes directly affect latency and CPU usage.

Software buffer size

The -b flag sets the software buffer in sample frames:

csound -odac -b 256 myfile.csd
csound -odac --iobufsamps=512 myfile.csd

This is the number of sample frames per control period (ksmps in the orchestra).

Hardware buffer size

The -B flag sets the hardware buffer:

csound -odac -B 1024 myfile.csd
csound -odac --hardwarebufsamps=2048 myfile.csd

This is the buffer size communicated to the audio driver.

Recommended buffer sizes

1. Low latency (5-12 ms)
2. csound -odac -b 128 -B 512 myfile.csd
   csound -odac -b 256 -B 1024 myfile.csd
   

3. For live performance, MIDI control, or interactive applications. Requires powerful CPU.
4. Standard latency (12-25 ms)
5. csound -odac -b 512 -B 2048 myfile.csd
   csound -odac -b 1024 -B 4096 myfile.csd
   

6. Good balance between latency and CPU efficiency.
7. High latency (25-100 ms)
8. csound -odac -b 2048 -B 8192 myfile.csd
   csound -odac -b 4096 -B 16384 myfile.csd
   

9. For CPU-intensive processing where latency is not critical.

Buffer relationship

Typically, hardware buffer should be 2-4x the software buffer:

csound -odac -b 256 -B 1024 myfile.csd   # B = 4 * b
csound -odac -b 512 -B 2048 myfile.csd   # B = 4 * b

Sample rate configuration

Override sample rate

csound -odac -r 44100 myfile.csd
csound -odac --sample-rate=48000 myfile.csd
csound -odac -r 96000 myfile.csd

Use system sample rate

csound --use-system-sr -odac myfile.csd

This queries and uses the audio device’s native sample rate.

Get system sample rate

csound --get-system-sr -odac

Prints the system sample rate and exits.

Control rate configuration

The control rate (kr) determines how often k-rate variables update:

csound -odac -k 4410 myfile.csd          # kr = 4410 Hz
csound -odac --control-rate=8820 myfile.csd

Alternatively, set ksmps in the orchestra:

sr = 44100
ksmps = 10     ; kr = sr / ksmps = 4410

Or override from command line:

csound -odac --ksmps=16 myfile.csd

Channel configuration

Number of output channels

csound -odac --nchnls=2 myfile.csd       # Stereo
csound -odac --nchnls=4 myfile.csd       # Quadraphonic
csound -odac --nchnls=8 myfile.csd       # 7.1 surround

Number of input channels

csound -iadc -odac --nchnls_i=2 myfile.csd    # Stereo input
csound -iadc -odac --nchnls_i=4 myfile.csd    # 4-channel input

Common real-time configurations

Live performance with MIDI

csound -M0 -odac -b 256 -B 1024 -d -m0 performance.csd

• -M0 = MIDI input from default device
• -odac = Audio output
• -b 256 -B 1024 = Low latency buffers
• -d = Suppress displays for better performance
• -m0 = Suppress messages

Audio input processing

csound -iadc -odac -b 512 -B 2048 effects.csd

Multi-channel output

csound -odac --nchnls=8 -b 512 -B 2048 surround.csd

JACK integration

csound -+rtaudio=jack -odac -iadc -b 256 myfile.csd

Low-latency ALSA

csound -+rtaudio=alsa -odac:hw:1,0 -b 128 -B 512 myfile.csd

Troubleshooting

Audio dropouts or glitches

• Increase buffer sizes: -b 1024 -B 4096
• Reduce number of instruments or processing
• Use -d to suppress displays
• Enable real-time scheduling (Linux): --sched
• Reduce message level: -m0

High latency

• Decrease buffer sizes: -b 256 -B 1024
• Use JACK or ALSA instead of PulseAudio (Linux)
• Use ASIO instead of MME (Windows)
• Enable real-time scheduling

Device not found

List devices and use exact name:

csound --devices
csound -odac:"Exact Device Name" myfile.csd

Sample rate mismatch

Match Csound’s sample rate to device:

csound --get-system-sr -odac    # Check device rate
csound -odac -r 48000 myfile.csd  # Set matching rate

Permission denied (Linux)

Add user to audio group:

sudo usermod -aG audio $USER

Real-time scheduling (Linux)

For lowest latency, enable real-time priority:

csound --sched -odac -b 128 -B 512 -d myfile.csd

Requirements:

• Must suppress displays (-d)
• Must use real-time audio (-odac or -iadc)
• May require system permissions

Set specific priority:

csound --sched=90 -odac -d myfile.csd

Configure system limits in /etc/security/limits.conf:

@audio   -  rtprio     95
@audio   -  memlock    unlimited