USB Audio streaming start
USB Audio streaming start
- Subject: USB Audio streaming start
- From: Vyacheslav Matyushin <email@hidden>
- Date: Mon, 10 Oct 2011 07:06:15 -0700 (PDT)
Hi,
I'm developing an USB Audio driver and I'm stuck with streaming start.
For streaming to work without gaps I must have a number of USB transactions queued ahead,
so when a callback for completed transactions is called there will still be queued transactions to be sent over a bus.
Right now I have 4 buffers each containing 32 transactions (or 4 ms of data) for output stream.
When a transaction for a buffer is completed a callback is called which queues samples for output again.
As it takes 4 ms to process 32 transactions (or 32 micro frames) a callback is constantly called every 4 ms.
CoreAudio assumes that audio engine advances through it's buffers (increasing and wrapping an offset) at a constant rate.
So at the beginning of the streaming I can't send all 4 buffers with real samples
because I will have to take 16 ms of data from output stream buffers and advance audio engine's offset by 16 ms.
Next callbacks will advance the offset by only 4 ms each.
This way I will stream our first (sample_rate/10) output samples faster than tenth of a second.
This will produce a HAL error because I will wrap around audio engine buffers earlier than HAL expected.
Current implementation sends 4 empty buffers at the streaming start so I don't have to touch IOAudioEngine's offset.
When they are sent and their callbacks are called I finally start to output samples using them.
Unfortunately this means that I will have to send 16 ms of zero data before real samples.
If I decrease 4 buffers to 2 chances are higher that under increased system load
I will start to receive an error from IOUSBFamily complaining about an outdated number of queued transaction.
If anyone is aware of a better way to start streaming and keep USB busy I would be happy to know it.
Thanks,
Vyacheslav Matyushin.
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