Label AAC granules in the sample rate's timescale

[ustacode]

The bug

AudioEncoderRingBuffer.createOutputBuffer built each granule's timestamp offset like this:

let offsetTimeStamp = CMTime(
    value: CMTimeValue(workingIndex),          // a FRAME COUNT
    timescale: workingBufferPresentationTimeStamp.timescale   // the incoming PTS's scale
)

workingIndex counts frames, so the only timescale that converts it to a duration is the sample rate. Inheriting the incoming timestamp's scale is correct only when the producer stamps in sample time — which AVCaptureAudioDataOutput does, which is why this never surfaced.

A host-clock producer stamps at timescale 1e9. There, an offset of 1024 frames is 1.024 microseconds instead of 21.3 ms. Every granule carved from one appended buffer landed on essentially the same timestamp, then the timeline jumped by the whole buffer. MpegTsWriter drives the PCR from these labels and RtmpStream computes FLV deltas from them, so they reach the wire.

(CMTimeAdd does not rescue this: LCM(48000, 1e9) = 3e9 overflows Int32, so the sum keeps the host timescale.)

Also fixed

The final createOutputBuffer of an append re-arms latestPresentationTimeStamp before discovering there is nothing left to copy. That stale value then labelled the next granule with the previous buffer's timestamp. Reset on append — but only when outputIndex == 0, because a granule spanning two appends must keep the timestamp of the buffer that started it. That guard is why the third test exists.

Verification

Reverted the fix and re-ran: the new tests fail with exactly the predicted values.

✘ Each append labels its granule with its own timestamp
    (b.first?.seconds → 1.000001024) == 2.0
✘ Granules from one host-clock-stamped buffer are spaced by real time
    (abs(delta - step) → 0.021332309333332983) < 0.0001

40 tests in 10 suites pass with it. Scripts/check-containment.sh clean.

Upstream

This is vendored Moblin code and the bug is upstream's. Worth sending back — flagging rather than doing, since that is the repo owner's call.

🤖 Generated with Claude Code