Re: Logical lpcm format.
Re: Logical lpcm format.
- Subject: Re: Logical lpcm format.
- From: Jeff Moore <email@hidden>
- Date: Tue, 17 Feb 2004 12:43:02 -0800
The driver does the conversion from the physical format to the virtual
format. It can be bypassed by taking hog mode
(kAudioDevicePropertyHogMode) on the device and turning off mixing
(kAudioDevicePropertySupportsMixing). That said, not every device
supports hog mode or turning mixing off as well as devices that don't
support 16 bit samples and only provide 8 bit samples or 24 bit fully
packed samples or a host of other interesting integer sample formats.
So if you're writing a app for other people to use, you'll probably
need to deal with floats anyway.
Not to mention, the amount of CPU involved in converting from int to
float isn't really all that much. It's way down in the noise of most
profiling I've done, especially if the app is careful to disable the
streams it isn't using. Plus, floating point numbers have other virtues
that have nothing to do with quality. For instance, if you are doing
signal processing with integers, you are going to burn away the savings
from skipping the conversion to floating point on headroom management
and the other hassles of fixed point math.
But if you are still determined to forge ahead with native mode in the
HAL, then please make sure that your application restores the device to
the state it found it in when it quits.
On Feb 17, 2004, at 10:46 AM, Zachary Drew wrote:
The physical stream format of my line-in audio hardware is 16 bit
signed
int lpcm. The HAL converts this to 32 bit float lpcm. My question:
Does the HAL use the cpu to perform this conversion? I'm writing a very
performance-sensitive application which is trying to get 16 bit
unsigned
int samples from the line-in source and I would like to aviod needless
conversions. Is there a way to bypass this conversion and get 16 bit
signed int format from HAL? Is there any reason this shouldn't be done
(the captured samples will never be saved as audio data in my
application so quality is not an issue)?
Thanks,
-Zach
--
Jeff Moore
Core Audio
Apple
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