ADR-0023: A Source’s Equipment Can Say It Is On, and a Source That Is On Holds the Stand


Status	Proposed — until the beacon has been timed on the switch it shares and a record has been held through its silences; see What would settle it
Date	2026-10-12
Scope	How EdS knows the turntable is switched on: a small device on the same switched outlet, whose connection to the broker is the whole of what it says. And what nowplayd does knowing it.
Extends	ADR-0008, whose rule now has one thing before it. ADR-0018: the console reads one more topic, and still only looks.
Related	ADR-0021, whose driver this is for and whose account of the wiring this corrects; ADR-0016 and ADR-0022, the other settings that treat named sources differently

Context

The turntable driver (ADR-0021) reports what it hears, and three things it cannot hear are in the way.

It cannot hear that the turntable is off. ADR-0021 has the agent Pi beside the amplifier, on its tape output. That is not where things are going. The turntable is upstairs, with a converter that turns it into S/PDIF, and the S/PDIF runs to the basement, where the Pi and its sound card are. Down there the digital feed is chosen between the media server and the turntable, and S/PDIF is there whether or not anything is playing on it. So what the driver hears is not proof of a record, and silence is not proof of none.

It cannot say that a record between its sides is still what is on. nowplayd picks a playing source over a paused one over a stopped one (ADR-0008). Lift the needle and the turntable is stopped, a track left paused in iTunes that afternoon outranks it, and the room is lit for an album nobody is listening to (CHG-0013 left this open, as “a question for the arbiter, and a decision”).

It cannot hold against another source that is playing. Two sources playing each win when their track changes.

One fact answers all three, and it is not in the sound: the turntable is switched on. It stands on a power strip that switches its other outlets with it, which is how the converter comes on. Anything else on that strip comes on with it too.

Options considered

Infer it from the audio. Nothing to add. But the audio is what cannot tell: the feed is always present, and is not always the turntable’s.

Tap something on the audio path - the converter’s lock light, the selector. It would be exact. It is also a wire into equipment that works, in the path the music takes, to learn what the power strip already knows.

A beacon that publishes the turntable source’s own presence, nowplaying/source/turntable/status. No change to nowplayd at all: a source that is offline already does not count. But that topic is the driver’s, and means its sound card is delivering samples (ADR-0006); two writers on one retained topic, each with a will, would say whichever spoke last. And it could only take the turntable away. It could not hold it.

The driver on the Pi reads the beacon and gates itself. The knowledge would stay with the turntable’s own code. But a driver subscribes to nothing but its commands, and the half of the problem that matters most - a silent turntable against a paused iTunes - is between sources, which no driver sees.

A beacon with a topic of its own, read by nowplayd, which is told which source it speaks for. One new device, one new thing before the arbiter’s rule.

Decision

A device on the turntable’s switched outlet is connected to the broker while the turntable is on, and says nothing else. nowplayd is told which source that switch belongs to, holds that source as current while the switch is on, and does not hear it while the switch is off.

The beacon

Its message is its connection. It publishes online, retained, to eds/beacon/<id>/status each time it connects, and its last will is offline, retained, on the same topic. Nothing runs when the power goes; the broker says it.
It knows nothing of turntables. The firmware (firmware/power-beacon) is the same for anything on a switch. Its name, turntable-power-01, and nowplayd’s setting are the only places that say what shares its outlet.
It asks for a keepalive of ten seconds, where the stand and the screen leave the library’s two minutes. A broker sends a will after one and a half keepalives, so the room learns the switch went off in about fifteen seconds, not three minutes.
It is a registered device like the others (ADR-0017), with its own account, which may publish its presence and its events (ADR-0005) and may read nothing. Its secrets are provisioned, never compiled in (ADR-0004). The WiFi, credentials, event log and partition table are copies of the stand’s, as the screen’s are.

nowplayd

NP_SOURCE_POWER names, for a source, its beacon: turntable=turntable-power-01. nowplayd subscribes to those beacons' topics and to no others.
While the switch is on, that source is current, before playing, paused and stopped are compared at all. A record between its sides is reported as what it is: the turntable, stopped, with no track.
While the switch is off, that source is not heard, whatever its driver reports.
A beacon that has not been heard from changes nothing. The source is arbitrated as it was before there was a beacon, the way a source is taken to be online until it says otherwise. So nowplayd can be given the setting before the beacon exists, and a broker account not yet granted the topic is a warning in the log and not a turntable that never plays.
The switch does not stand in for the driver. A source that is switched on and whose driver is offline, or has never reported, holds nothing.

The console

It shows each beacon it hears of beside the other parts, lit when the switch is on. Off is not shown as a fault: it is a turntable that is off.

Consequences

A side can be turned over without the room changing. Stopped with no track publishes no new cover, and the cover topics are left as they were, so the stand stays lit from the sleeve.
Until a record is named, the stand shows what it showed before. Switching the turntable on makes it current at once, but it has no track for the first half-minute, and nothing repaints the room for that.
While the turntable is on, the cover screen’s buttons do nothing. A command goes to the source that is current (ADR-0020), and the turntable takes none. iTunes cannot be paused from the screen with the turntable on. That is the price of “holds”, and it is paid knowingly.
iTunes playing with the turntable on is not shown. The switch is taken at its word.
nowplayd knows a kind of device now, where it knew only sources. It knows a topic and the word on it, and not what the device is.
A beacon is one more thing that can be wrong. If it loses WiFi while the turntable plays, the record stops being heard after about fifteen seconds, and the room goes to whatever else is current until it is back. Its events say how often: more mqtt.connected than system.boot is a connection that came and went.
A will needs a broker to send it. A beacon switched off while the broker is down leaves online retained until it next connects. The turntable would be held, silent, and the screen’s buttons idle, until the turntable is next switched on and off.
“About fifteen seconds” is the broker’s to keep. One and a half keepalives is when it may send the will, and it looks every few seconds: on the bench it took between twelve and nineteen.
The strip decides what “on” means. It switches on what the turntable draws. That has been reliable for the converter, by its owner’s account, and the beacon inherits it.

What would settle it

Shown on the bench, 2026-10-12: the firmware builds for the board to be used, an ESP32-S2 with 4 MB of flash, starts on it, and reports itself unprovisioned. nowplayd’s rule is tested as sequences of events, the arbiter’s own way. Provisioned, and restarted three times from its reset pin beside the access point, the board said online 3.0 to 3.1 seconds after starting (CHG-0015). That is a reset and not the strip switching on, and not where it will live. Held in reset to stand in for its power going, it read offline on the console after 12.3 and 18.6 seconds; let go, online after 7.6 and 3.1. nowplayd, deployed, named the switch as it started. With the turntable’s driver online and silent and a track paused in iTunes, the turntable was current while the beacon was on, and iTunes 15.5 seconds after it was held in reset.

Not yet shown:

The seconds from the switch going on to online on the bus.
The seconds from the switch going off to offline.
A day on the strip’s outlet without the connection coming and going.
A side turned over with iTunes left paused: the stand keeps the sleeve and nowplaying/current stays the turntable’s.
The beacon off: nowplaying/current goes back to what else there is. Shown with the board held in reset; the turntable’s own switch is the first two items.