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125 lines
4.8 KiB
Plaintext
125 lines
4.8 KiB
Plaintext
Notes on status of CoreAudio Implementation of PortAudio
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Document Last Updated December 9, 2005
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There are currently two implementations of PortAudio for Mac Core Audio.
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The original is in pa_mac_core_old.c, and the newer one is in pa_mac_core_auhal.c.
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Only pa_mac_core_auhal.c is currently developed and supported as it uses apple's
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current core audio technology. To select one of the implementations, copy the
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appropriate file to pa_mac_core.c, (eg. "cp pa_mac_core_auhal.c pa_mac_core.c")
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then run configure and make as usual.
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----------------------------------------
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Notes on Original/Default implementation:
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by Phil Burk and Darren Gibbs
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Last updated March 20, 2002
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WHAT WORKS
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Output with very low latency, <10 msec.
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Half duplex input or output.
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Full duplex on the same CoreAudio device.
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The paFLoat32, paInt16, paInt8, paUInt8 sample formats.
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Pa_GetCPULoad()
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Pa_StreamTime()
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KNOWN BUGS OR LIMITATIONS
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We do not yet support simultaneous input and output on different
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devices. Note that some CoreAudio devices like the Roland UH30 look
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like one device but are actually two different CoreAudio devices. The
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Built-In audio is typically one CoreAudio device.
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Mono doesn't work.
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DEVICE MAPPING
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CoreAudio devices can support both input and output. But the sample
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rates supported may be different. So we have map one or two PortAudio
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device to each CoreAudio device depending on whether it supports
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input, output or both.
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When we query devices, we first get a list of CoreAudio devices. Then
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we scan the list and add a PortAudio device for each CoreAudio device
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that supports input. Then we make a scan for output devices.
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-------------------------------------------
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Notes on Newer AUHAL implementation:
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by Bjorn Roche
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Last Updated December 9, 2005
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Principle of Operation:
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This implementation uses AUHAL for audio I/O. To some extent, it also
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operates at the "HAL" Layer, though this behavior can be limited by
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platform specific flags (see pa_mac_core.h for details). The default
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settings should be reasonable: they don't change the SR of the device and
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don't cause interruptions if other devices are using the device.
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Major Software Elements Used: Apple's HAL AUs provide output SR
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conversion transparently, however, only on output, so this
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implementation uses AudioConverters to convert the sample rate on input.
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A PortAudio ring buffer is used to buffer input when sample rate
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conversion is required or when separate audio units are used for duplex
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IO. Finally, a PortAudio buffer processor is used to convert formats and
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provide additional buffers if needed. Internally, interleaved floating
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point data streams are used exclusively - the audio unit converts from
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the audio hardware's native format to interleaved float PCM and
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PortAudio's Buffer processor is used for conversion to user formats.
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Simplex Input: Simplex input uses a single callback. If sample rate
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conversion is required, a ring buffer and AudioConverter are used as
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well.
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Simplex output: Simplex output uses a single callback. No ring buffer or
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audio converter is used because AUHAL does its own output SR conversion.
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Duplex, one device (no SR conversion): When one device is used, a single
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callback is used. This achieves very low latency.
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Duplex, separate devices or SR conversion: When SR conversion is
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required, data must be buffered before it is converted and data is not
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always available at the same times on input and output, so SR conversion
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requires the same treatment as separate devices. The input callback
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reads data and puts it in the ring buffer. The output callback reads the
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data off the ring buffer, into an audio converter and finally to the
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buffer processor.
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Known issues:
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- Latency: Latency settings are ignored except when doing I/O between different
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devices. Still, latency should be very low.
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- Timing info. It reports on stream time, but I'm probably doing something
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wrong since patest_sine_time often reports negative latency numbers.
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- xrun detection: The only xrun detection performed is when reading
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and writing the ring buffer. There is probably more that can be done.
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- abort/stop issues: stopping a stream is always a complete operation,
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but latency should be low enough to make the lack of a separate abort
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unnecessary. Apple clarifies its AudioOutputUnitStop() call here:
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http://lists.apple.com/archives/coreaudio-api/2005/Dec/msg00055.html
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- blocking interface: Not implemented.
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- multichannel: I don't have a multichannel IO box, so I didn't test it.
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It should work though ;) I haven't done anything with "channel maps"
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yet, so it's possible that more needs to be done.
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- sample rate conversion quality: the input audio converter is set to
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the highest quality. I don't do anything about the output converter, and
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I am not sure how AUHAL manages quality.
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- x86: I haven't tested it on an x86 Mac, but I tried to be aware of it, so
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it should work without too much trouble.
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