INC-0004: The Stand Went Dark Mid-Side, Because the Turntable Was Heard on One Channel and That One Went Silent

Summary

The turntable reaches the sound card through the Tape 1 output of an Onkyo A-8170, an integrated amplifier from the mid-1980s, whose aged tape selector contacts can drop a channel. Partway through a side, the left channel went silent. npagent records one channel, and what it records is the left, so it heard twenty seconds of silence, decided the record had stopped, and the stand went to standby while the record played on.

Nothing reported a fault, because by every rule in place there was none: the capture ran, the recognizer answered, and silence on the channel listened to is what a stopped record sounds like. Turning the amplifier’s Tape 1 selector back and forth brought the channel back.

Impact

From 01:34:53, the stand showed standby’s warm white in place of Eagles Live’s colors, for the rest of side four and through the investigation that followed. The screens and the console showed nothing playing on the turntable. iTunes, Spotify and every device were unaffected.

Detection

The owner, who saw the stand go back to standby before or during “Take It Easy” with the record still playing. Nothing else detected it: the console’s “Song detector” line stayed green throughout, because it says the sound card is heard, not that music is on both of its channels.

Timeline

Symptoms

1. The stand went to standby mid-side, with the record audibly playing.
2. The console showed the turntable stopped, and its “Song detector” green.
3. A reboot of the Pi changed nothing.
4. The sound card played the record on another computer.

Investigation

• A cap on the recognizer, the first suspicion: it is an endpoint nobody offered (ADR-0021) and could have stopped answering. Overturned by npagent’s own counts, limited=0 failed=0 every minute, and by the code: a recognizer that fails only leaves a track unnamed (services/nowplaying/internal/driver/turntable/turntable.go:177–192); it never makes the turntable stop.
• The side ending while nobody was there to flip it. Overturned by the owner: the stop came before the last track.
• npagent dying. Overturned by the log: the “stopped” line is its own, from the process that had run all evening, and it came back after the reboot no better.
• The wrong card, or a muted capture. Overturned on the Pi: NP_TURNTABLE_DEVICE=plughw:CODEC names the only capture device, and the card has no capture control to mute.
• The level, per channel, which settled it: two channels in, one silent. The record played on another computer because a system playing both channels plays the right one too. In the other room the amplifier feeds a 5.1 system, where one channel missing is not obvious.

Root cause

Two faults combined:

1. The amplifier’s Tape 1 path dropped its left channel. The Onkyo A-8170 is from the mid-1980s, and its tape selector contacts and tape jacks have aged: the Tape 1 selector is known to need working to carry both channels. It is never heard: listening is from phono, and Tape 1 out feeds only the sound card.
2. The turntable driver listens to one channel. It records mono through ALSA’s plug conversion (arecord -c 1, services/nowplaying/internal/driver/turntable/capture.go:25–26), and the evidence is that the one channel taken is the left: silence on it alone gave the driver silence. Twenty seconds below the silence level is a stopped record (ADR-0026, follow.go), and so the stand went to standby with music on the other channel.

Recovery

The amplifier’s Tape 1 selector was turned through its positions several times and set back to phono, which restored both channels’ contacts; the Pi read both again (29%|26%), and npagent was restarted with them. The contacts are only worked, not cleaned, and can fail again.

Contributing factors

• Nothing measures the channels apart, so a one-sided input looks exactly like silence, and nothing could say otherwise.
• The console’s “Song detector” was green, as ADR-0034 defines it: the driver was there. It does not say what it hears.
• The fault is on an output nobody listens to, so the owner had no way to hear it, and the other room’s 5.1 system hid it.

Corrective actions

Preventive actions

Follow-ups

None.