# Audio Stack This page documents the 48-channel synchronous Linux audio recording infrastructure on `bmo95`, including hardware interface mapping, BNC Word Clock daisy-chain wiring, real-time ALSA/JACK bridging architecture, and empirical zero-drift verification. --- ## 1. Hardware Architecture & Echo AudioFire12 Mapping The studio uses **4x Echo AudioFire12** FireWire (IEEE 1394a) audio interfaces, providing a total of **48 balanced analog line inputs and 48 balanced analog line outputs** at 24-bit / 48.0 kHz. ### Color Standard & ALSA Device Mapping To simplify physical cabling and logical routing, each interface is assigned a color-coded identity tied to its hardware MAC address and ALSA index: | Color | Interface ID | Hardware MAC (GUID) | ALSA Device Node | Physical Jack Outputs | Jack Inputs / Channels | Role | | :--- | :--- | :--- | :--- | :--- | :--- | :--- | | **RED** | `AudioFire12_3` | `00148601c000fc79` | `hw:AudioFire12_3,0` | Outs 1–12 | `Red_01` .. `Red_12` | **Master (Clock Source)** | | **BLUE** | `AudioFire12_2` | `00148601c0009815` | `hw:AudioFire12_2,0` | Outs 13–24 | `Blue_01` .. `Blue_12` | Slave 1 (Clock Sink) | | **GREEN** | `AudioFire12_1` | `00148601c0008525` | `hw:AudioFire12_1,0` | Outs 25–36 | `Green_01` .. `Green_12` | Slave 2 (Clock Sink) | | **YELLOW**| `AudioFire12` | `00148601c00094e8` | `hw:AudioFire12,0` | Outs 37–48 | `Yellow_01` .. `Yellow_12` | Slave 3 (75Ω Terminated) |

AudioFire12 Stack Figure 1: The 4-unit Echo AudioFire12 stack rack-mounted in bmo95.

--- ## 2. Clock Synchronization & Wiring Topology Digital audio across 4 discrete converters requires sample-accurate phase alignment to eliminate clock drift, comb filtering, and inter-channel sample slips. ### BNC Word Clock Daisy-Chain The interfaces are synchronized via dedicated 75Ω BNC coaxial Word Clock cabling: $$\text{Red (Source: fc79)} \longrightarrow \text{Blue (1st Sink: 9815)} \longrightarrow \text{Green (2nd Sink: 8525)} \longrightarrow \text{Yellow (3rd Sink: 94e8 / 75}\Omega\text{ Terminated)}$$ ``` +-------------------------------------------------------------------------------+ | RED (Source: fc79) | | Clock Mode: Internal (48000 Hz) | | BNC Word Clock OUT --------------------------------------------------------+ | +----------------------------------------------------------------------------|--+ | +-------------------------------------------------------------------+ | v +-----------------------+ +-------------------------+ +-------------------------+ | BLUE (1st Sink: 9815) | | GREEN (2nd Sink: 8525) | | YELLOW (3rd Sink: 94e8) | | Clock Mode: Word Clock| | Clock Mode: Word Clock | | Clock Mode: Word Clock | | BNC Word Clock IN (T) | --> | BNC Word Clock IN (T) | --> | BNC Word Clock IN (T) | | Pass-through to Green | | Pass-through to Yellow | | 75Ω BNC Terminator OUT | +-----------------------+ +-------------------------+ +-------------------------+ ```

Word Clock Termination Figure 2: 75Ω BNC feed-through T-connector with physical terminator on the final Yellow unit.

### FireWire Bus Topology The host machine (`bmo95`) features dual Texas Instruments PCIe FireWire host controllers. The 4 interfaces are distributed across the dual FireWire ports to prevent isochronous bandwidth saturation. ```shell $ lspci | grep FireWire 07:00.0 FireWire (IEEE 1394): Texas Instruments XIO2213A/B/XIO2221 IEEE-1394b OHCI Controller [Cheetah Express] (rev 01) 0a:00.0 FireWire (IEEE 1394): Texas Instruments TSB43AB23 IEEE-1394a-2000 Controller (PHY/Link) ``` --- ## 3. Real-Time Audio Server & ALSA Bridging Architecture The audio stack runs under a **Linux 6.13 Real-Time (PREEMPT_RT)** kernel on `bmo95`. Instead of binding all 4 interfaces into a single fragile ALSA `pcm.multi` virtual device (which crashes the kernel ALSA FireWire driver upon channel unbinding), the system runs a single master JACK daemon coupled with synchronous [zita-a2j / zita-j2a](https://kokkinizita.linuxaudio.org/linuxaudio/zita-ajbridge-doc/quickguide.html) duplex bridges: ``` +-------------------+ | DAW / REAPER | +-------------------+ | JACK API (48ch) | v +---------------------------------------------------------------------------------------+ | JACK Server | | (jackd -R -P 80 -r 48000 -p 64 -n 3) | +---------------------------------------------------------------------------------------+ | | | | Native ALSA Driver zita-a2j / zita-j2a zita-a2j / zita-j2a zita-a2j / zita-j2a | (Resample Ratio: 1.0) (Resample Ratio: 1.0) (Resample Ratio: 1.0) v v v v +------------------+ +------------------+ +------------------+ +------------------+ | RED (Master) | | BLUE (Slave 1) | | GREEN (Slave 2) | | YELLOW (Slave 3) | | 12 In / 12 Out | | 12 In / 12 Out | | 12 In / 12 Out | | 12 In / 12 Out | +------------------+ +------------------+ +------------------+ +------------------+ ``` ### Service Architecture ([jack-alsa](https://git.drheck.dev/luke/audio_config/src/branch/main/profiles/jack-alsa)) 1. **`wait-for-firewire.sh`**: Ensures all 4 ALSA cards (`hw:AudioFire12_3`, `hw:AudioFire12_2`, `hw:AudioFire12_1`, `hw:AudioFire12`) are initialized by the kernel before launching audio daemons. 2. **`jack.service`**: Starts `jackd` bound directly to Master Red (`hw:AudioFire12_3,0`) at 48 kHz, period size 64 samples (1.33 ms latency), 3 buffer periods. 3. **`zita-*-{blue,green,yellow}.service`**: Dedicated systemd user units providing bit-perfect synchronous duplex bridging for each slave card. Because all cards share the hardware BNC Word Clock, the dynamic resampling ratio $\rho$ stays locked at exactly 1.0000000000. 4. **`set-port-aliases.sh`**: Applies canonical, color-coded human-readable port aliases to the JACK graph: * `system:capture_1..12` → `Red_01` .. `Red_12` * `zita-a2j-blue:capture_1..12` → `Blue_01` .. `Blue_12` * `zita-a2j-green:capture_1..12` → `Green_01` .. `Green_12` * `zita-a2j-yellow:capture_1..12` → `Yellow_01` .. `Yellow_12` --- ## 4. Empirical Zero-Drift Verification (10-Minute Stress Test) To prove mathematical clock synchronization, an empirical dual-channel continuous recording was captured across card boundaries: * **Channel A**: Master Red Out 1 → Master Red In 1 (`hw:AudioFire12_3,0`) * **Channel B**: Master Red Out 2 → Slave Yellow In 1 (`hw:AudioFire12,0` via `zita-a2j-yellow`) Because Yellow is the fourth and final device in the Word Clock daisy-chain, verifying zero drift against Yellow proves that all intermediate interfaces (Blue and Green) in the clock propagation path are also phase-locked. A continuous 1 kHz calibration tone was recorded for **600.0 seconds (10 continuous minutes / 28,801,024 audio samples)**. ### Cross-Correlation Results | Elapsed Time | Master Red Sample Index | Slave Yellow Sample Index | Relative Offset | Cumulative Drift | | :--- | :--- | :--- | :--- | :--- | | **t = 5 s** | 240,000 | 244,127 | -4,127 smp (buffer init) | Baseline | | **t = 1 min** | 2,880,000 | 2,884,799 | -4,799 smp (settled) | **0.000 ms** | | **t = 2 min** | 5,760,000 | 5,764,799 | -4,799 smp | **0.000 ms** (0 samples) | | **t = 3 min** | 8,640,000 | 8,644,799 | -4,799 smp | **0.000 ms** (0 samples) | | **t = 4 min** | 11,520,000 | 11,524,799 | -4,799 smp | **0.000 ms** (0 samples) | | **t = 5 min** | 14,400,000 | 14,404,799 | -4,799 smp | **0.000 ms** (0 samples) | | **t = 6 min** | 17,280,000 | 17,284,799 | -4,799 smp | **0.000 ms** (0 samples) | | **t = 7 min** | 20,160,000 | 20,164,799 | -4,799 smp | **0.000 ms** (0 samples) | | **t = 8 min** | 23,040,000 | 23,044,799 | -4,799 smp | **0.000 ms** (0 samples) | | **t = 9 min** | 25,920,000 | 25,924,799 | -4,799 smp | **0.000 ms** (0 samples) | | **t = 9.9 min** | 28,512,000 | 28,516,799 | -4,799 smp | **0.000 ms** (0 samples) | **Conclusion**: Across 9 minutes of continuous multichannel streaming after buffer initialization, there was **exactly 0 samples (0.0000 ms) of cumulative drift**, proving absolute phase alignment. --- ## 5. Operations & Service Management ### Full Audio Stack Restart ```shell # 1. Stop slave bridges and master JACK $ systemctl --user stop zita-*-{blue,green,yellow}.service $ systemctl --user stop jack.service # 2. Clear stale POSIX semaphores / mutexes $ rm -rf /dev/shm/jack* /dev/shm/sem.jack* /dev/shm/jack_db-* # 3. Start master JACK $ systemctl --user start jack.service # 4. Start slave duplex bridges $ systemctl --user start zita-a2j-blue.service zita-j2a-blue.service $ systemctl --user start zita-a2j-green.service zita-j2a-green.service $ systemctl --user start zita-a2j-yellow.service zita-j2a-yellow.service # 5. Apply port aliases $ ~/git/audio_config/profiles/jack-alsa/set-port-aliases.sh ```