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Graph-level multirate capability audit (2026-11-02)

Source head: 0f616a440cecdbdaff6234be48f682eec1184587 (origin/main).

Decision

No graph-level implementation change. SignalGraph prepares one host sample rate for a compiled snapshot. A node may implement local oversampling or resampling, but the graph has no per-node rate domain, conversion edge, or second scheduler. Adding one without a named consumer would expand latency, real-time, and offline contracts together, so this slice records the boundary instead of adding graph-wide runtime.

Representative workflow attempted

The representative mixed-rate workflow is two serial custom nodes that would ideally run at independent rates, followed by an output node. The available workflow is SignalGraph::prepare(sample_rate, max_block_size) followed by ordinary connect edges and process(); every node receives the same prepared host rate and host partition size. The adjacent supported workflows are:

  • node-local oversampling/resampling inside a Processor or custom node;
  • sparse control-rate automation; and
  • dense audio-rate modulation (a parameter lane, not sample-rate conversion).

Rate-domain finding and controls

  • core/host/src/signal_graph_prepare.cpp stores one graph sample rate and propagates it into each process block.
  • core/host/include/pulp/host/custom_node_type.hpp gives custom preparation the graph rate and block size; latency metadata feeds the existing PDC plan.
  • core/host/include/pulp/host/signal_graph_connection.hpp has no conversion or rate-domain fields; audio_rate_modulation is only a dense parameter lane.
  • test/test_signal_graph_executor_parity.cpp is the positive control for dense modulation, including modulation through PDC.
  • test/test_signal_graph_offline_parity.cpp is the positive control for sidechain/PDC and block-partition parity.

The planted negative is a hypothetical second node domain (for example, 96 kHz alongside a 48 kHz host). No public API or current preparation path can produce that observation; seeing it would indicate an accidental graph-wide rate split and should fail review.

Realtime and portability limits

This is source and test-contract evidence, not a numerical qualification of each node-local resampler. It does not prove conversion quality, alias rejection, or cross-platform timing. A future graph rate-domain proposal must specify conversion quality and latency, buffer accounting, PDC, modulation clock semantics, allocation-free callback behavior, and online/offline parity fixtures on the supported platforms before runtime work begins.

Disposition

Keep this as a bounded no-change decision. Re-open only for a named graph-authored mixed-rate workflow, starting with an explicit conversion node and measured PDC contract rather than an implicit per-node rate field.