24 lines
1.2 KiB
Markdown
24 lines
1.2 KiB
Markdown
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# iOS broadcast-audio ring format
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The ReplayKit extension and host app exchange audio through `broadcast_audio.ring` in the
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`group.me.iamtalon.voicecat` App Group. Version 1 is a 64-byte little-endian header followed by
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96,000 signed 16-bit PCM samples (one second of stereo at 48 kHz).
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| Offset | Type | Meaning |
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|---:|---|---|
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| 0 | `uint32` | Magic `0x56434252` (`VCBR`) |
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| 4 | `uint32` | Version, currently `1` |
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| 8 | `uint32` | Channel count; active writers use `2` |
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| 12 | `uint32` | Sample rate; active writers use `48000` |
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| 16 | `uint32` | Active flag (`0` or `1`) |
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| 20 | 4 bytes | Reserved, zero |
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| 24 | `uint64` | Monotonic producer/write sample index |
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| 32 | `uint64` | Monotonic consumer/read sample index |
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| 40 | 24 bytes | Reserved, zero |
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The payload starts at byte 64 and contains interleaved little-endian `int16` PCM. The aligned
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indices are single-producer/single-consumer counters; producer and consumer publish their owned
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index with a release barrier and read the other index with an acquire barrier. A writer drops a
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whole input chunk when it cannot fit. Changing this layout requires a new version and compatible
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readers; offsets in version 1 must never be repurposed.
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