You should not write a sound input component at this point, since its
interface is limited to the very old Sound Manager capabilities (sample
rates < 65 kHz, no greater than 2 channels, native 16-bit integer
samples instead of CoreAudio's higher precision 32-bit Float samples).
If the device you want to capture audio from does not have a CoreAudio
driver (kernel extension if you wish to make it available across all
processes, or as a HAL plugin in user space if you just want to support
single process IO). It's unclear to me from your e-mail whether the
device in question already has a CoreAudio driver or not (what is the
device?). If an audio device has a CoreAudio driver, it can be used
from applications that use the CoreAudio api's, or from apps that use
the Sound Manager/Sound Input Component API's, since the Sound Input
Component API's are a vestige of older Mac OS'es, and have been
implemented in OS X as sitting atop CoreAudio anyway.
Kelly Jacklin, another engineer here at Apple, answered a similar
question on the coreaudio mailing list about a month ago. Here's what
he said:
On Nov 23, 2004, at 1:01 PM, Tommy Schell wrote:
I have been told that I need to write a CoreAudio plugin that works
with the Audio HAL as an input and output to accomplish this task.
This is correct. I'll let someone on the CoreAudio team point you in
the right direction for any sample code, but you need to write either
an IOAudio device driver (i.e. kernel audio driver) or a HAL plug-in
(user-level audio driver) if you want to appear as an audio device to
Final Cut Pro, since it uses the CoreAudio device abstraction for
playback.
For capture (i.e. input) Final Cut Pro uses QuickTime, and while you
could currently get away with a sinp component, you would be
ill-advised to do so, since vending your device through CoreAudio will
get you compatibility, as well as future-proofing should a future
version of QuickTime use CoreAudio directly...
Is it possible to handle the audio as a thread within Quicktime
components, or does a plugin need to be a separate process located
in the plugins folder?
Plug-ins are not separate processes... Plug-ins are a code packaging
mechanism that allows your code to be discovered and loaded within a
host process. In this manner, a CoreAudio HAL plug-in is very similar
to a QuickTime component, in that it is _not_ global to the system, it
is instantiated within the host process.
Just write a proper IOAudio driver or HAL plug-in (your choice), and
you will show up as an audio interface to Final Cut Pro.
If a CoreAudio plugin needs to be a separate process, how do my
Quicktime components or Firewire threads communicate with and pass
data to and from the plugin?
It doesn't, and you shouldn't. Daemon processes and IPC is not the
right way to write a user-level audio device driver. Again, as a HAL
plug-in, you will be loaded within the host application's process
space (Final Cut Pro, in this case) and will run within that context.
Because your Quicktime components and Firewire threads are also in
this same process, you have many choices of how to share state and
rendezvous between these plug-ins/components.
Or, if you choose to write your driver as an IOAudio driver, you will
be hosted within the kernel, and can expose buffers into the user
client's address spaces as appropriate.
kelly jacklin
pro apps - final cut pro
apple computer, inc.
Hope that helps.
-Brad Ford
QuickTime Engineering
On Dec 22, 2004, at 1:37 AM, Nidhi Chadha wrote:
Hi All ,
What are functions performed by the sound input component in mac X ?
I am getting MPEG4 data stream from the digital camcorder thru USB
port .I am writing VDig component for the video data . What should I
do for playing audio captured from the camcorder ? The audio driver is
already available in mac X . Do I need to write sound input component
for my camcorder ?
Pls suggest
Thanks and regards
Nidhi
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