Re: Gain difference from Mac OS 9 to Mac OS X
Re: Gain difference from Mac OS 9 to Mac OS X
- Subject: Re: Gain difference from Mac OS 9 to Mac OS X
- From: Jeff Moore <email@hidden>
- Date: Mon, 24 Jun 2002 13:12:20 -0700
On Monday, June 24, 2002, at 11:38 AM, Andrew Taylor wrote:
We have a speech recognition application.
We use standard Sound Manager calls to open a device, get/set the
input channel gain and collect the input sound.
Input characteristics are quite simple: 16 bit integer, 16 khz, mono.
On Mac OS 9, we can set the gain so that USB microphones deliver
enough volume to do the processing we need to do.
On ALL versions of Mac OS X, this same method does not work. We can't
set the gain high enough to collect the same sound input levels.
What has changed in Sound Manager to deliver this different behavior
and what can I do about it?
On Mac OS 9, the depending on which hardware you have, the driver would
do more or less work to make all the audio devices appear the same. On
Mac OS X, we've taken the opposite tack. We want the drivers to
represent their hardware as it is. Fundamentally, this is one of the
things that has changed between the two OS's.
That said, the input gain selectors in the Sound Input Manager have
full access to the hardware volume range. The curve may be slightly
different (meaning that the same value will generate different volume
levels on the two OS's), but the same amount of gain is in the
hardware. The hardware certainly hasn't changed between the two OS's,
so you may have been relying on some additional work that the driver on
9 was doing on your behalf (and without your knowledge or any warning).
--
Jeff Moore
Core Audio
Apple
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