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Re: 0 latency?
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Re: 0 latency?


  • Subject: Re: 0 latency?
  • From: Jeff Moore <email@hidden>
  • Date: Tue, 04 Sep 2001 12:00:15 -0700

on 9/2/01 4:29 PM, Ian Ollmann <email@hidden> wrote:

> When I query the default hardware device for its audio latency, the result
> value is 0. Is that a bit of a brag, or does this mean something other
> than what I think it means?

The latency property of the device is the latency in the hardware totally
outside of the control of the software. A latency of 0 simply means that
there effectively isn't any additional hardware latency between writing to
the hardware's DMA buffer and it getting onto the wire.

Most dedicated audio devices connected via PCI will have 0 hardware latency.
Where you see numbers other than 0 is on other transports like USB and
FireWire, when the device is doing some processing on it's own, or where
there is some kind of hardware A/V synch (especially for cards that do RT
video effects).

This number is not the latency between your IO proc's output and the wire.
That number is roughly the sum of the hardware latency, the driver safety
offset (which is how close to the DMA head you can write safely), and the
buffer size.

--

Jeff Moore
Core Audio
Apple


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