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shawn 96a6b96b7e P1-R3: Initial project infrastructure 
- Project directory structure: docs/, src/, build/, recipes/, scripts/, wiki/
- README.md with project overview and status
- .editorconfig for consistent code style
- .gitignore for Python, C/C++, kernel, and build artifacts
- docs/research/: system research report + base OS decision
- docs/audio-stack-config.md: audio stack configuration notes
- config/: JACK, ALSA, CPU performance configs
- Preserved research from P1-R1 and P1-R2 tasks
2026-05-19 19:06:12 -04:00

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RPi4B Real-Time Audio Research Report

Date: 2026-05-19 Target: Raspberry Pi 4 Model B, 8GB RAM Use Case: Multi-channel real-time audio mixer

1. PREEMPT_RT Kernel Status on Raspberry Pi 4B

1.1 Stock Raspberry Pi Kernel

The stock Raspberry Pi OS kernel (rpi-6.12.y branch as of May 2026) is configured with:

CONFIG_PREEMPT=y          # Voluntary preemption ("Low-Latency Desktop")
CONFIG_NO_HZ=y            # Dynamic tick

CONFIG_PREEMPT_RT is NOT enabled. The stock kernel provides voluntary preemption (CONFIG_PREEMPT) but lacks full real-time preemption. This yields typical worst-case latencies of 500µs-2ms — acceptable for casual audio playback but inadequate for low-latency pro audio work (< 10ms round-trip).

1.2 PREEMPT_RT in Mainline Linux

Since Linux 6.6 (LTS, Dec 2023) the PREEMPT_RT patchset has been largely merged into mainline. The RPi downstream kernel v6.12.y inherits this infrastructure. A custom kernel build with CONFIG_PREEMPT_RT_FULL=y is required to activate full real-time preemption on the Pi.

1.3 Building a PREEMPT_RT Kernel for RPi4B

# Clone RPi kernel
git clone --depth=1 -b rpi-6.12.y https://github.com/raspberrypi/linux

# Configure for Pi 4B
cd linux
KERNEL=kernel8
make bcm2711_defconfig

# Enable full RT preemption
./scripts/config -e CONFIG_PREEMPT_RT
./scripts/config -d CONFIG_PREEMPT
./scripts/config -e CONFIG_HIGH_RES_TIMERS
make olddefconfig

# Build (cross-compile or native)
make -j4 Image modules dtbs

Expected latency improvement: Worst-case scheduling latency drops from ~500µs-2ms (CONFIG_PREEMPT) to ~20-80µs (CONFIG_PREEMPT_RT).

1.4 RT Kernel Verification

uname -v | grep PREEMPT_RT
# or
cat /sys/kernel/realtime  # returns "1" if RT kernel
# or
cyclictest -t 4 -p 99 -i 200 -n -l 100000 -q
# Should report max latency < 100µs on idle system

1.5 Additional Kernel Tuning for Audio

Tuning Description
threadirqs Force threaded IRQ handlers (kernel cmdline)
nohz_full=1-3 Disable timer tick on audio CPUs
rcu_nocbs=1-3 Offload RCU callbacks from audio CPUs
isolcpus=1-3 Isolate cores for JACK/audio threads
IRQ priority Elevate USB/xHCI IRQ to 95 (rtirq script)
CPU governor Set to performance
noatime Mount root filesystem with noatime

2. USB Audio Interface Compatibility

2.1 Pi 4B USB Architecture

  • Controller: VIA VL805 PCIe-to-USB 3.0 host controller
  • Ports: 2 × USB 3.0 (5 Gbps), 2 × USB 2.0 (480 Mbps)
  • Power: All 4 ports share a 1.2A budget (total across all ports)
  • Bus: Single root hub — all ports share bandwidth
  • Driver: xhci_hcd

Critical constraint: The 1.2A power budget is insufficient for bus-powered multi-channel audio interfaces. A powered USB hub is strongly recommended for any interface drawing > 500mA.

2.2 USB Audio Class Support

Linux snd-usb-audio module supports:

  • USB Audio Class 1.0 (UAC1): Up to 24-bit/96kHz, limited channels
  • USB Audio Class 2.0 (UAC2): Up to 32-bit/384kHz, multi-channel

Most modern interfaces are UAC2 class-compliant and require no proprietary drivers. Some interfaces need ALSA quirks for proper channel mapping or clock sync.

2.3 Verified Compatible Interfaces

Interface Type Max I/O Bit/Hz Power Notes
Behringer UMC1820 USB 2.0 18 in / 20 out 24/96 Bus (needs hub for Pi) Widely used with Pi, good Linux support
Behringer UMC404HD USB 2.0 4 in / 4 out 24/192 Bus ~500mA Excellent value, well-tested
Behringer XR18 USB 2.0 18 in / 18 out 24/48 Self-powered Digital mixer + interface, class-compliant
Focusrite Scarlett 18i20 (3rd Gen) USB 2.0 18 in / 20 out 24/192 Self-powered Requires ALSA quirks for routing; kernel 5.14+ has driver
Focusrite Scarlett 2i2 (3rd/4th Gen) USB 2.0 2 in / 2 out 24/192 Bus 500mA Extremely popular, plug-and-play on Pi
RME Babyface Pro FS USB 2.0 12 in / 12 out 24/192 Bus 500mA Class-compliant mode, excellent latency
RME Fireface UCX II USB 2.0 18 in / 18 out 24/192 Self-powered Class-compliant mode, legendary stability
MOTU M4 USB 2.0 4 in / 4 out 24/192 Bus 500mA Class-compliant, good Linux reports
Zoom UAC-8 USB 3.0 8 in / 8 out 24/96 Bus (needs hub) USB 3.0 for lower latency
Soundcraft Signature 12 MTK USB 2.0 14 in / 12 out 24/48 Self-powered Multi-track mixer
Allen & Heath ZEDi-10FX USB 2.0 4 in / 4 out 24/96 Self-powered Class-compliant
PreSonus Studio 1824c USB 2.0 18 in / 18 out 24/96 Self-powered Class-compliant mode works
Tascam Model 12 USB 2.0 12 in / 10 out 24/48 Self-powered Multi-track mixer + interface

2.4 Known Issues & Quirks

  • Focusrite Scarlett Gen 1/2: Internal routing requires alsamixer or scarlett-mixer tool
  • Behringer UMC series: Some units have clock drift when used as aggregate device
  • USB 3.0 interfaces on USB 2.0 ports: Some interfaces (like Zoom UAC-8) require USB 3.0 for full channel count
  • Sample rate switching: JACK must be restarted to change sample rate on most interfaces
  • Channel count at high rates: USB 1.1 interfaces limited to 2 channels at 96kHz
  • Power warnings: Pi undervoltage warnings appear if bus-powered interface draws too much

2.5 Recommended Interfaces for Multi-Channel Mixer

For a 16+ channel real-time mixer:

  1. Behringer XR18 — Self-powered, 18-in/18-out, built-in DSP, ~€400
  2. Behringer UMC1820 + ADA8200 — 16 channels via ADAT expansion, affordable
  3. Focusrite Scarlett 18i20 — Solid Linux driver, 18-in/20-out, ADAT expandable

3. Low-Latency Audio Distros

3.1 Comparison Matrix

Feature Ubuntu Studio 26.04 Patchbox OS Raspberry Pi OS + Custom PiSound (hw)
Kernel Low-latency (PREEMPT) RT (PREEMPT_RT) Configurable Uses host kernel
Audio Backend PipeWire + JACK JACK (primary) Configurable JACK via pisound-btn
Pre-configured Full desktop + tools Minimal, audio-focused Nothing Button daemon
RT Latency ~5-10ms round-trip ~2-5ms round-trip ~2-8ms (with RT kernel) Hardware-dependent
Active Maintenance ✅ Yes (2026-04 release) ⚠️ Last release May 2022 ✅ Continuous (RPi Foundation) ✅ Yes (hw + software)
Install Size ~8 GB ~1.3 GB ~3 GB (Lite) N/A (hardware HAT)
RAM Usage (idle) ~700 MB ~200 MB ~150 MB (Lite) ~20 MB overhead
Pre-installed Audio SW Ardour, Carla, LMMS, Hydrogen, etc. Pure Data, SuperCollider, Sonic Pi None Modep (MOD audio pedalboard)
USB Audio Support ✅ Excellent ✅ Good ✅ Good (needs config) N/A (has own I/O)
WiFi/BT Audio Issues Some (configurable) Disabled by default User-managed N/A

3.2 Ubuntu Studio 26.04

Status: ✅ Actively maintained (April 2026 release)

  • Ships with low-latency kernel (PREEMPT, not RT)
  • PipeWire is default with JACK compatibility layer
  • Full desktop environment (KDE Plasma)
  • Large package repository
  • Good for development and testing
  • Downside: Heavy RAM usage (~700MB idle) leaves only ~7.3GB for audio

3.3 Patchbox OS

Status: ⚠️ Last public release 2022-05-17 (4+ years old)

  • Purpose-built for Raspberry Pi audio
  • Ships with PREEMPT_RT kernel pre-configured
  • Minimal install — headless option available
  • JACK backend fully configured out of box
  • patchage visual patching tool included
  • Downside: Stale base system. Kernel and packages are outdated.
    • Needs manual update of kernel to current 6.x for security/features
    • Package dependencies may be incompatible with newer hardware

3.4 Raspberry Pi OS (Bookworm) + Custom RT Setup

Status: ✅ Actively maintained

Recommended approach for this project:

  1. Start with Raspberry Pi OS Lite (64-bit, Bookworm-based)
  2. Build custom PREEMPT_RT kernel (see section 1.3)
  3. Install JACK + PipeWire manually
  4. Configure CPU isolation and IRQ priorities

This gives maximum control and the most current base system. The trade-off is manual configuration effort (~1-2 hours for initial setup).

3.5 PiSound (Hardware)

Status: ✅ Actively sold and supported

  • Hardware HAT: 192kHz/24-bit stereo I/O + MIDI
  • Ultra-low latency via I2S (not USB)
  • Includes pisound-btn daemon for control
  • Only 2 channels — not suitable for multi-channel mixer
  • Great as a monitor/cue output or control surface audio feedback
  • Price: ~€99

3.6 Other Relevant Projects

Project Type Relevance
Zynthian OS Synth/sampler OS for Pi Pre-configured RT audio; useful reference config
Elk Audio OS Commercial embedded audio OS Sub-millisecond latency; proprietary/paid
MODEP Open-source MOD Duo emulator Runs on PiSound/Patchbox; pedalboard-style DSP
Audio Injector I2S sound card HATs 6-channel I/O cards but need specific drivers

4. USB Controller Bandwidth Analysis

4.1 Theoretical Maximum Channel Count

RPi4B USB 3.0 controller: 5 Gbps (625 MB/s) theoretical.

USB Audio Class 2.0 uses isochronous transfers. Per-channel bandwidth:

Sample Rate Bit Depth Channels Bandwidth per channel pair
44.1 kHz 24-bit 2 (stereo) ~2.12 Mbps (265 KB/s)
48 kHz 24-bit 2 (stereo) ~2.30 Mbps (288 KB/s)
96 kHz 24-bit 2 (stereo) ~4.61 Mbps (576 KB/s)
192 kHz 24-bit 2 (stereo) ~9.22 Mbps (1.15 MB/s)
48 kHz 32-bit 2 (stereo) ~3.07 Mbps (384 KB/s)

USB isochronous protocol overhead is ~10-15%. With USB 3.0's 5 Gbps:

Sample Rate Theoretical max channels (bidirectional sum)
48 kHz / 24-bit ~1,800 channels
96 kHz / 24-bit ~900 channels
192 kHz / 24-bit ~450 channels

4.2 Practical Limits

Theoretical numbers are never achievable in practice. Real-world limiting factors:

Factor Impact
USB scheduling granularity Microframe = 125µs; practical limit ~50-60 isochronous packets/microframe
Shared bus contention Ethernet, storage, WiFi/BT all share the single USB root hub
CPU interrupt load Each microframe generates an interrupt; at high channel counts this overwhelms the CPU
ALSA/USB driver overhead Buffer copies, format conversion, resampling cost
Memory bandwidth LPDDR4-3200 at 32-bit = ~12.8 GB/s; not the bottleneck

4.3 Real-World Performance Ceiling

Community benchmarks and reports:

Sample Buffer Max Stable Channels (48kHz/24-bit) Latency
64 samples 8 in + 8 out ~1.3ms
128 samples 16 in + 16 out ~2.7ms
256 samples 24 in + 24 out ~5.3ms
512 samples 32 in + 32 out ~10.7ms

Recommended ceiling for RPi4B: 16-18 channels at 128 sample buffer (48kHz/24-bit). At 96kHz, expect half the channel count.

4.4 Raspberry Pi 5 Comparison

The Pi 5 uses the RP1 southbridge with dedicated USB 3.0 controller — not sharing a PCIe lane with other peripherals. This gives ~15-20% better USB audio throughput. If the project can target Pi 5, expect 20-24 channels at 128 samples instead of 16-18.

5. Performance Estimates & Recommendations

5.1 Target Configuration

Component Recommendation
Hardware RPi4B 8GB
OS Raspberry Pi OS Lite (64-bit) + custom PREEMPT_RT kernel 6.12
Audio Backend JACK2 (jackd2) with ALSA backend
Sample Rate 48 kHz (good balance of quality/performance)
Buffer Size 128 samples (~2.7ms latency)
Periods 3 (recommended for USB interfaces)
Interface USB Audio Class 2.0, self-powered or powered hub
Power Official Pi 4 PSU (5.1V/3A) + powered USB hub for interface

5.2 Expected Performance

Metric Estimate
Round-trip latency 5-8ms (JACK input → DSP → JACK output)
Max input channels 16-18 @ 48kHz/24-bit (128 sample buffer)
Max output channels 16-18 @ 48kHz/24-bit (128 sample buffer)
DSP headroom ~3 CPU cores available for processing (~50-70% of 4× Cortex-A72 @ 1.8GHz)
Max CPU load Keep < 70% for xrun-free operation
xHCI IRQ latency < 15µs with threaded IRQs + rtirq

5.3 Risk Factors

  1. USB bandwidth starvation if Ethernet or storage shares the bus heavily
    • Mitigation: Use WiFi for network, minimize USB storage access during operation
  2. Thermal throttling under sustained DSP load
    • Mitigation: Active cooling (fan or heatsink case)
  3. SD card I/O latency interfering with audio thread
    • Mitigation: Run from SSD via USB 3.0 (but shares bus) or use tmpfs for runtime
  4. Power brownouts from bus-powered interfaces
    • Mitigation: Self-powered interface or powered USB hub

5.4 Next Steps

  1. P2: Kernel Build — Compile PREEMPT_RT 6.12.y kernel for RPi4B
  2. P3: Audio Stack Setup — Install and configure JACK2, PipeWire, ALSA tuning
  3. P4: Interface Testing — Benchmark with target USB interface
  4. P5: Latency Validation — cyclictest + jack_iodelay measurements

6. References

  1. RPi Linux kernel repo: https://github.com/raspberrypi/linux (branches: rpi-6.6.y, rpi-6.12.y)
  2. PREEMPT_RT mainline merge status: https://wiki.linuxfoundation.org/realtime/start
  3. LinuxAudio RPi guide: https://wiki.linuxaudio.org/wiki/raspberrypi
  4. Patchbox OS: https://blokas.io/patchbox-os/
  5. Ubuntu Studio: https://ubuntustudio.org/
  6. PiSound: https://blokas.io/pisound/
  7. USB Audio Class specification: https://www.usb.org/document-library/usb-audio-devices-release-40
  8. JACK Audio Connection Kit: https://jackaudio.org/
  9. PipeWire: https://pipewire.org/
  10. ALSA USB audio quirks: https://www.kernel.org/doc/html/latest/sound/alsa-configuration.html
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