Re: AGC in usb-audio devices
Re: AGC in usb-audio devices
- Subject: Re: AGC in usb-audio devices
- From: Jeff Moore <email@hidden>
- Date: Fri, 9 Nov 2007 09:47:37 -0800
On Nov 8, 2007, at 5:48 PM, Bob Stuller wrote:
Folks, Greetings!
Can anyone tell me what is the state of Automatic Gain Control (AGC)
in Core Audio?
Unchanged.
In Sound Manager we would routinely turn off AGC on all input
devices. Since I don't see any mention of AGC in the headers, I've
always assumed that Core Audio wasn't giving us that control over
the devices. And, because I assumed that all devices would default
to 'AGC Off', I thought that was a-okay.
Unbeknownst to many, much of the that hardware appeared to have AGC,
probably did not have it at all and was getting it emulated in
software. That was one of the major problems of the audio hardware
support on Classic Mac OS. You never really knew what you were dealing
with. One of the changes in philosophy that we made in the transition
to Mac OS X was that we decided that we would no longer support
emulating in software features that are lacking in the hardware. Thus,
if a given device had no volume controls, you would not see any volume
controls.
It turns out that not a lot of hardware really has AGC support so by
and large it disappeared when we took out the software emulation. This
in turn left little incentive to include support for it in the system
as a standard control. This is not a final answer as we reconsider
these sorts of situations all the time. I doubt we'd ever due software
AGC at the driver level, but adding a standard control to support
hardware that has it seems non-controversial. Feel free to file a
feature request in BugReporter.
So, now that we seem to see signs of AGC being on in certain USB
Audio devices,
Any such support is particular to that USB device.
I have a couple of related questions. First, is there a way to
examine the state of AGC on arbitrary (probably usb) input devices?
Is there a way to turn it off if it's on?
The answer to both questions is currently the same: only if the
manufacturer of the device provides such support through a HAL plug-in
or other means.
I'm betting that there are folks here who know a lot about this
subject - including whether there are usb audio devices that run by
default with some sort of agc turned on.
It wouldn't surprise me if there were USB devices that had AGC and had
it enabled by default. You'd need to talk to the manufacturer to know
for sure.
--
Jeff Moore
Core Audio
Apple
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