notes.txt

mediastreamer2是开源的网络传输媒体流的库

Notes on status of CoreAudio Implementation of PortAudio

Document Last Updated December 9, 2005

There are currently two implementations of PortAudio for Mac Core Audio.

The original is in pa_mac_core_old.c, and the newer, default implementation
is in pa_mac_core.c.
Only pa_mac_core.c is currently developed and supported as it uses apple's
current core audio technology. To select use the old implementation, replace
pa_mac_core.c with pa_mac_core_old.c (eg. "cp pa_mac_core_auhal.c
pa_mac_core.c"), then run configure and make as usual.

----------------------------------------

Notes on Original implementation:

by Phil Burk and Darren Gibbs

Last updated March 20, 2002

WHAT WORKS

Output with very low latency, <10 msec.
Half duplex input or output.
Full duplex on the same CoreAudio device.
The paFLoat32, paInt16, paInt8, paUInt8 sample formats.
Pa_GetCPULoad()
Pa_StreamTime()

KNOWN BUGS OR LIMITATIONS

We do not yet support simultaneous input and output on different 
devices. Note that some CoreAudio devices like the Roland UH30 look 
like one device but are actually two different CoreAudio devices. The 
Built-In audio is typically one CoreAudio device.

Mono doesn't work.

DEVICE MAPPING

CoreAudio devices can support both input and output. But the sample 
rates supported may be different. So we have map one or two PortAudio 
device to each CoreAudio device depending on whether it supports 
input, output or both.

When we query devices, we first get a list of CoreAudio devices. Then 
we scan the list and add a PortAudio device for each CoreAudio device 
that supports input. Then we make a scan for output devices.

-------------------------------------------

Notes on Newer/Default AUHAL implementation:

by Bjorn Roche

Last Updated December 9, 2005

Principle of Operation:

This implementation uses AUHAL for audio I/O. To some extent, it also
operates at the "HAL" Layer, though this behavior can be limited by
platform specific flags (see pa_mac_core.h for details). The default
settings should be reasonable: they don't change the SR of the device and
don't cause interruptions if other devices are using the device.

Major Software Elements Used: Apple's HAL AUs provide output SR
conversion transparently, however, only on output, so this
implementation uses AudioConverters to convert the sample rate on input.
A PortAudio ring buffer is used to buffer input when sample rate
conversion is required or when separate audio units are used for duplex
IO. Finally, a PortAudio buffer processor is used to convert formats and
provide additional buffers if needed. Internally, interleaved floating
point data streams are used exclusively - the audio unit converts from
the audio hardware's native format to interleaved float PCM and
PortAudio's Buffer processor is used for conversion to user formats.

Simplex Input: Simplex input uses a single callback. If sample rate
conversion is required, a ring buffer and AudioConverter are used as
well.

Simplex output: Simplex output uses a single callback. No ring buffer or
audio converter is used because AUHAL does its own output SR conversion.

Duplex, one device (no SR conversion): When one device is used, a single
callback is used. This achieves very low latency.

Duplex, separate devices or SR conversion: When SR conversion is
required, data must be buffered before it is converted and data is not
always available at the same times on input and output, so SR conversion
requires the same treatment as separate devices. The input callback
reads data and puts it in the ring buffer. The output callback reads the
data off the ring buffer, into an audio converter and finally to the
buffer processor.

Platform Specific Options:

By using the flags in pa_mac_core.h, the user may specify several options.
For example, the user can specify the sample-rate conversion quality, and
the extent to which PA will attempt to "play nice" and to what extent it
will interrupt other apps to improve performance. For example, if 44100 Hz
sample rate is requested but the device is set at 48000 Hz, PA can either
change the device for optimal playback ("Pro" mode), which may interrupt
other programs playing back audio, or simple use a sample-rate coversion,
which allows for friendlier sharing of the device ("Play Nice" mode).


Known issues:

- Latency: Latency settings are ignored in most cases. Exceptions are when
doing I/O between different devices and as a hint for selecting a realtively
low or relatively high latency in conjunction with
paHostFramesPerBufferUnspecified. Latency settings are always automatically
bound to "safe" values, however, so setting extreme values here should not be
an issue.

- Buffer Size: paHostFramesPerBufferUnspecified and specific host buffer sizes
are supported. paHostFramesPerBufferUnspecified works best in "pro" mode,
where the buffer size and sample rate of the audio device is most likely
to match the expected values.

- Timing info. It reports on stream time, but I'm probably doing something
wrong since patest_sine_time often reports negative latency numbers.

- xrun detection: The only xrun detection performed is when reading
and writing the ring buffer. There is probably more that can be done.

- abort/stop issues: stopping a stream is always a complete operation,
but latency should be low enough to make the lack of a separate abort
unnecessary. Apple clarifies its AudioOutputUnitStop() call here:
http://lists.apple.com/archives/coreaudio-api/2005/Dec/msg00055.html

- blocking interface: Not implemented.

- multichannel: It has been tested successfully on multichannel hardware
from MOTU: traveler and 896HD.

- sample rate conversion quality: By default, SR conversion is the maximum
available. This can be tweaked using flags pa_mac_core.h. Note that the AU
render quyality property is used to set the sample rat conversion quality
as "documented" here:
http://lists.apple.com/archives/coreaudio-api/2004/Jan/msg00141.html

- x86: I haven't tested it on an x86 Mac myself, but users have reported 
being able to comiple and run it.