Re: Built-in audio devices unexpected behavior
Re: Built-in audio devices unexpected behavior
- Subject: Re: Built-in audio devices unexpected behavior
- From: Jeff Moore <email@hidden>
- Date: Tue, 19 Aug 2008 08:40:06 -0700
On Aug 19, 2008, at 5:23 AM, Mikael Hakman wrote:
While performing some tests using built-in audio devices I observed
following behavior not consistent with my expectations.
•
Whenever my application sets virtual format on either input or
output device to standard Float32 PCM then physical format changes
to 16-bits even if I set it to 24-bits in AMS.
I'd consider this user error on your part.
When you set the virtual format, the device is free to map the virtual
format to whatever physical format it desires. If you want to set a
specific physical format, you should set that specific physical format
rather than setting the virtual format and leaving it up to the device.
•
According to HALLab and to returned kAudioDevicePropertyLatency/
kAudioStreamPropertyLatency properties, latency of these devices is
30 samples. Connecting output to input, outputting a single pulse,
and measuring the number of samples until the pulse appears on input
yields a total latency of about 900 samples when using 128 sample
buffer sizes. I would expect total latency of 128 + 30 + 30 = 188
samples.
Again. More user error on your part. You are leaving out the safety
offset in your calculation of throughput latency. You are also leaving
out the input buffering as well.
•
Whenever optical input signal ceases then input device changes its
input stream from digital to line-in despite optical connector still
being physically connected. I would expect reported input stream
still being digital but with no channels in it when there is no
valid digital signal. Output device does it right – when optical
jack is connected then the stream is digital, when analog jack is
connected then the stream is line-out.
Could you please comment on these observations.
The SPDIF in requires a clock to work. If it doesn't have one, it has
to switch to something that works and can give timing. The output has
no such problems since it is generating the clock.
--
Jeff Moore
Core Audio
Apple
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