5. Timing

Hosts such as Ableton Live process tracks on parallel threads in no guaranteed order. The bus therefore has a fixed latency of one block: a reader sees what a writer produced for the reader's previous block. The result does not depend on which thread ran first, so a set renders the same way every time, including offline export.

5.1 Blocks

Every block of every endpoint is framed by begin/end and carries:

• num_samples (1..4096). Hosts with larger blocks MUST split them; peers then split the same way.
• transport_sample_pos: the host's transport position of the first sample.
• playing: the transport runs (also during offline export).
• time_ns: a monotonic, system-wide clock (macOS CLOCK_UPTIME_RAW, Linux CLOCK_MONOTONIC, Windows QueryPerformanceCounter) or a host supplied equivalent.

5.2 Writing

For each output, once per block, the writer fills slot blocks_written mod 4 under its seqlock with the block's stamps and samples, sets block_index = blocks_written, then increments blocks_written (release). Writing again within the same block rewrites the same slot. Outputs not written during a block publish zeros at block end.

5.3 Selecting the block to read

Let W be blocks_written of the source output and prev the reader's previous block. Candidates are blocks W−1, W−2, W−3 (newest first); a candidate is only valid if its seqlock reads consistently and its block_index equals its number.

1. Position rule. If the reader's previous and current blocks are both playing, choose the newest playing candidate whose transport_sample_pos == prev.transport_sample_pos.
2. Time rule. Otherwise, or if rule 1 finds nothing, choose the newest candidate with time_ns + guard <= current.time_ns, where guard = 0.5 × num_samples / sample_rate (1 ms if the rate is unknown). This picks a block from an earlier processing cycle.
3. Hold. If neither rule finds a block, the reader repeats the last sample it read from that connection (0 at first).

Rule 1 is exact and deterministic, also when loops repeat positions and when export runs faster than real time. Rule 2 covers a stopped transport and peers that just appeared.

If the chosen block is shorter than the reader's block, the remainder holds the block's last sample.

5.4 Consequences

• Modulation arrives one block late. At 256 samples and 48 kHz that is 5.3 ms: inaudible for modulation, unsuitable for phase-critical audio.
• A self-patched endpoint (output into its own input) works and also has one block of latency.
• Live's delay compensation does not know about the bus.

These rules are what Phase 0 of the project measures in real hosts; they may be refined before 1.0.