88 lines
2.2 KiB
Markdown
88 lines
2.2 KiB
Markdown
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# Alarm Audio Streaming Protocol
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## Overview
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The Raspberry Pi streams alarm audio to the ESP32-S3 over a dedicated WebSocket connection on **port 8766**. The protocol uses mixed text and binary frames — text for control messages, binary for raw PCM data.
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## Connection
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| Parameter | Value |
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|-----------|-------|
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| Port | 8766 |
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| Transport | WebSocket |
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| Direction | Pi (server) → ESP32 (client) |
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The ESP32 connects and stays connected. The server initiates alarm playback when needed.
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## Message Sequence
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```
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Pi ESP32
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│ │
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│◄──── WS connect ──────────│
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│ │
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│ (idle until alarm fires) │
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│ │
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├─ alarm_start (text) ──────►│ → open codec
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├─ PCM chunk (binary) ──────►│ → queue + play
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├─ PCM chunk (binary) ──────►│
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│ ... │
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├─ alarm_stop (text) ────────►│ → drain + close codec
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│ │
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```
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## Control Messages (Text Frames)
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### alarm_start
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Sent before the first PCM chunk. The ESP32 uses these parameters to configure the DAC.
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```json
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{
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"type": "alarm_start",
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"sample_rate": 24000,
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"channels": 2,
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"bits": 16
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}
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```
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### alarm_stop
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Sent after the last PCM chunk.
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```json
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{
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"type": "alarm_stop"
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}
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```
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## PCM Data (Binary Frames)
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- **Format:** Raw signed 16-bit little-endian, interleaved stereo (L, R, L, R, ...)
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- **Chunk size:** 4096 bytes (1024 stereo samples at 16-bit)
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- **Pacing:** Chunks sent at ~90% real-time speed to maintain buffer without overflow
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- **Byte order:** Little-endian (native for both Pi and ESP32)
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At 24kHz / 2ch / 16-bit:
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- Bytes per second: 96,000
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- Chunk duration: ~42.7 ms
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- Send interval: ~38.4 ms (90% pacing)
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## ESP32 Buffering
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- FreeRTOS queue: 10 slots of pointer-to-chunk
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- Total buffer capacity: ~427 ms of audio
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- Overflow policy: drop chunk and log warning
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## Future: Alarm Scheduling
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Not yet implemented. Planned JSON format for alarm configuration:
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```json
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{
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"alarm_time": "070000",
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"alarm_days": ["Mon", "Tue", "Wed"],
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"alarm_audio": "path/to/file.wav"
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}
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```
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