Audio Engine Tuning

Linux Audio Engine & Realtime Kernel Tuning

Target Systems: Linux (PipeWire 0.3+, JACK2, ALSA)
Key Goal: Low latency (≤ 5.8 ms roundtrip), zero xruns (buffer underruns), maximum DSP thread efficiency.

This document details the audio engine settings applied by ReaFull to REAPER and the recommended host-level configurations for professional Linux audio workstations.

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1. Overview of the ReaFull Audio Optimization Layer

Achieving reliable low-latency audio on Linux requires coordinating the Linux kernel, audio server (PipeWire/JACK/ALSA), and the DAW's internal threading engine.

ReaFull automates REAPER's internal audio engine parameters while respecting existing custom hardware routing.

graph LR
    subgraph Kernel ["Linux Realtime & System Subsystems"]
        RT_Limits["Realtime Limits (@audio rtprio 95)"]
        MLock["RAM Memlock (unlimited)"]
        Gov["CPU Scaling (performance)"]
    end

    subgraph AudioServer ["Audio Server Layer"]
        PW["PipeWire / JACK2 Engine"]
        ALSA["Direct ALSA Kernel Driver"]
    end

    subgraph REAPER ["REAPER Audio Engine (ReaFull Configured)"]
        Workthreads["workthreads = CPU Cores"]
        MLockAll["linux_mlockall = 1 (Lock buffers in RAM)"]
        RTPrio["alsa_rtprio / jack_rtprio = 90"]
        NoPM["linux_disable_pm = 1 (Disable throttling)"]
        SincHQ["r8brain & 768pt Sinc Resampling"]
    end

    Kernel --> AudioServer
    AudioServer --> REAPER

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2. Automated REAPER Audio Engine Parameters

When audio_tuning is deployed, ReaFull configures the following low-latency parameters inside reaper.ini:

2.1 Realtime Scheduling and Memory Locking

Key	Value	Purpose and Technical Rationale
linux_mlockall	1	Lock physical RAM: Prevents Linux kernel memory paging from swapping audio buffer pages to disk. Eliminates momentary timing jitter and audio dropouts.
alsa_rtprio	90	Realtime SCHED_FIFO Priority: Assigns high realtime priority (90) to the primary audio processing thread, above normal desktop applications.
jack_rtprio	90	Assigns realtime priority (90) for JACK/PipeWire client threads.
linux_disable_pm	1	Disable Power Management: Prevents CPU frequency downclocking or deep C-state sleep transitions during active DSP playback.
linux_auto_pasuspend	1	Automatically suspends conflicting legacy PulseAudio server instances when opening direct ALSA devices.
audio_closeifidle	0	Keeps audio hardware streams open when stopped, avoiding re-initialization clicks.

2.2 Thread Allocation and Anticipative DSP

Key	Value	Purpose and Technical Rationale
workthreads	CPU_COUNT	Automatically sets worker threads equal to physical/logical CPU cores for symmetric parallel track rendering.
afx	1	Enables REAPER's Anticipative FX processing engine for non-realtime tracks, freeing RT threads for live inputs.
afxb	200	Sets Anticipative FX render buffer size to 200 ms for maximum plugin stability on dense sessions.
afxrender	1	Enables anticipative processing on track and stem renders.

2.3 Studio High-Precision Resampling

Key	Value	Mode	Description
playresamplemode	5	r8brain / 512pt Sinc (HQ)	Studio-grade real-time sample rate conversion with minimum phase distortion.
projrenderresample	6	r8brain / 768pt Sinc (Extreme)	Ultra-clean mastering sample rate conversion for final stem and mix exports.

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3. Hardware Heuristic Auto-Detection Profiles

During installation, ReaFull checks aplay -l for known audio hardware to configure optimal sample rates, bit depths, and block sizes automatically:

3.1 Behringer UMC404HD 192k / UMC204HD (hw:U192k)

alsa_indev=hw:U192k
alsa_outdev=hw:U192k
linux_audio_bits=32
linux_audio_bsize=256
linux_audio_bufs=3
linux_audio_nch_in=4
linux_audio_nch_out=4
linux_audio_srate=48000
linux_audio_srateor=1

Roundtrip latency: ~5.3 ms at 48kHz / 256 samples.

3.2 Presonus AudioBox USB (hw:USB)

alsa_indev=hw:USB
alsa_outdev=hw:USB
linux_audio_bits=24
linux_audio_bsize=256
linux_audio_bufs=3
linux_audio_nch_in=2
linux_audio_nch_out=2
linux_audio_srate=48000
linux_audio_srateor=1

3.3 Generic / Custom Audio Interfaces

If existing audio settings are already defined, ReaFull preserves your custom interface names (alsa_indev, alsa_outdev, jack_launchcmd) while injecting the thread and realtime priority optimizations.

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4. Host OS System-Level Configuration (Recommended)

To allow REAPER to take full advantage of realtime priorities and memory locking, configure your Linux host permissions:

4.1 Realtime Group Limits (/etc/security/limits.d/audio.conf)

Ensure your user is in the audio or realtime group, and that the following limits are defined:

# /etc/security/limits.d/99-realtime-audio.conf
@audio   -  rtprio     95
@audio   -  memlock    unlimited
@audio   -  nice      -19

Add your user to the group:

sudo usermod -a -G audio $USER

4.2 PipeWire Low-Latency Buffer Locking

If using PipeWire (pipewire-pulse / pipewire-jack), enforce fixed quantum buffer sizes during DAW sessions:

# Force PipeWire buffer to 256 samples at 48kHz (~5.3ms latency)
pw-metadata -n settings 0 clock.force-quantum 256
pw-metadata -n settings 0 clock.force-rate 48000

# Reset to dynamic automatic mode when finished
pw-metadata -n settings 0 clock.force-quantum 0

4.3 CPU Frequency Scaling Governor

Ensure CPU cores do not throttle dynamically during audio sessions:

# Set CPU scaling governor to 'performance' across all cores
echo performance | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor