#260 Digital audio (SyncTERM:A) — large latency on the ALSA output backend when a sound starts while other audio is playing (PulseAudio backend unaffected)

[Rob Swindell]

Environment

• SyncTERM current master (1.10a), Linux
• Audio Output Mode: ALSA (PulseAudio disabled in Setup)

Symptom

A digital sound triggered over the SyncTERM:A audio API is heard with a large delay when other audio is already playing (a background music loop, or an overlapping sound effect). A single sound triggered from silence is in sync. Likewise, a Flush on a channel takes a correspondingly long time to actually go silent — the already-buffered audio plays out first.

Reproducibility

• Consistent on the ALSA backend.
• Switching Audio Output Mode to PulseAudio makes the latency disappear — concurrent sounds play in sync; switching back to ALSA brings it back. (This is the clearest isolation: same content, same machine, only the output backend differs.)
• Observed via a BBS door that loops music on one channel and plays sound effects on other channels over the audio API — the effects lag only while the music is playing. Should reproduce with any two concurrent sounds on different channels.

Possibly relevant (observation, not a diagnosis)

In xpbeep.c, the ALSA snd_pcm_hw_params sequence sets access/format/rate/channels but does not call any buffer-size or period-size setter; the PulseAudio path sets buffer_attr.tlength explicitly. Flagging only in case it's pertinent.