=== SIGNAL PATH (fixes audible/measurable on HW) ===
[CRITICAL] stereo: fix 38kHz subcarrier 1-sample phase offset
Encode() called Sample() before capturing Phase(), so the 38kHz
subcarrier used phase_{n+1} while pilot used phase_n — a constant
60° phase error at 228kHz, degrading stereo separation to ~50%.
Now captures phase BEFORE tick; stores lastPhase for coherent
RDS carrier derivation.
[HIGH] rds: phase-lock 57kHz carrier to pilot via StereoEncoder
RDS encoder had its own free-running 57kHz oscillator instead of
deriving from 3×pilot. Two independent float64 oscillators drift
apart over hours. Added NextSampleWithCarrier(carrier float64),
generator now passes stereoEncoder.RDSCarrier() (sin(3·pilotPhase))
so all subcarriers share one phase reference. NextSample() remains
as backward-compat fallback with internal carrier.
[MEDIUM] dsp/fmmod: fix phase wrapping for negative phase
Wrap condition only triggered on phase > pi. Negative phase from
negative-biased composite could grow unbounded, losing float64
precision after extended runtime. Now wraps on |phase| > pi.
[MEDIUM] dsp/oscillator: phase wrapping handles both directions
Same pattern as fmmod — only wrapped positive overflow. Now wraps
on phase >= 1 OR phase < 0 for defensive robustness.
=== DAUERLAUF / SBC STABILITY ===
[HIGH] offline/generator: eliminate per-chunk allocation (912KB @ 2.28MHz)
GenerateFrame() allocated a new []IQSample every call. At Pluto
rate (2.28MHz, 50ms chunks) that's 114k*8=912KB * 20/sec = 18MB/s
garbage, causing GC pauses and hardware underruns on Raspi.
Now pre-allocates frameBuf, reuses across calls. Grow-only policy,
never shrinks. Safe because driver.Write() is blocking.
[MEDIUM] output/file: batch-write entire frame in one syscall
Was writing 8 bytes per sample = 114k syscalls per chunk on Pluto.
Now serializes into a reusable byte buffer, single file.Write().
Orders of magnitude faster on Raspi SD-card I/O.
[MEDIUM] app/engine: add error backoff in TX loop
run() tight-looped at 100% CPU on persistent driver errors
(hardware disconnect, USB reset). Now backs off by chunkDuration
per error using select with ctx.Done() for clean cancellation.
[MEDIUM] app/engine: replace time.Sleep shutdown with sync.WaitGroup
Stop() used time.Sleep(2*chunkDuration) hoping run() would exit.
Race condition if hardware Write() stalls. Now uses wg.Wait() for
deterministic goroutine join before Flush/Stop.
=== CORRECTNESS / HYGIENE ===
[LOW] rds/normalize: filter to ASCII, prevent mid-rune truncation
normalizePS truncated at 8 bytes, not 8 characters. UTF-8 input
(e.g. umlauts) could split mid-rune producing corrupt RDS bitstream.
Now replaces non-ASCII with space before truncation.
[LOW] offline/generator: increment frame sequence counter
Sequence was hardcoded to 1 on every frame, useless for debugging.
Now increments per GenerateFrame() call.
[LOW] control: document that config PATCH doesn't reach running engine
Added TODO noting that POST /config updates server's copy only;
running Engine/Generator holds its own snapshot.
[COSMETIC] dsp/preemphasis: remove dead fields y1, a1
PreEmphasis.y1 and .a1 were never read in Process(). Removed from
struct, constructor, and Reset(). Fixed misleading doc comment on
PreEmphasizedSource (claims audio-rate, actually composite-rate).
Files changed:
internal/stereo/encoder.go - phase coherence fix
internal/rds/encoder.go - pilot-locked carrier API
internal/rds/normalize.go - ASCII safety
internal/offline/generator.go - buffer reuse + sequence + doc
internal/dsp/fmmod.go - bidirectional phase wrap
internal/dsp/oscillator.go - bidirectional phase wrap
internal/dsp/preemphasis.go - dead field removal
internal/output/file.go - batch write
internal/app/engine.go - WaitGroup + backoff
internal/control/control.go - TODO config propagation
Jan Svabenik