Re: Host Time epoch
Re: Host Time epoch
- Subject: Re: Host Time epoch
- From: John Iversen <email@hidden>
- Date: Thu, 10 Feb 2005 12:21:03 -0800
Steve,
Sorry, I'm not aware of this issue, but it's of interest to me, and is
rather fundamental. Would you mind elaborating? What is the nature of the
non-monotonicity--I can't imagine you mean actual reversals in the host
time, I hope. Instead, do you mean that the rate of increase of the
HostTime is variable, so it drifts away from 'real time'? How large is the
effect in your experience?
To take a concrete example: Given that MIDITimeStamps are based on
'HostTime' (right?), are you saying that if midi events are received in at
exactly 1 sec intervals the increases in Host time (as seen in the
MIDITimeStamp of incoming packets) would not be identical (give or take
some small margin of error)? If that's really the case, I don't see how any
sequencer could possibly work reliably and not show tempo drift, so I may
not be getting the drift of what you're talking about here.
Thanks,
John
At 11:54 AM 2/10/2005, Steve Checkoway wrote:
On Feb 9, 2005, at 3:09 PM, Jeff Moore wrote:
The source of the time the HAL provides is the system bus clock. It is
accessed via the routines in <mach/mach_time.h>. This counter is reset at
boot time and increments monotonically at the system bus clock speed (I think).
I still have yet to get the time from AudioDeviceGetCurrentTime to
increase monotonically. I was told that this was fixed with the next
version of Mac OS X but I have now updated versions twice and it is still
an issue with 10.3.8.
Unless AudioGetCurrentHostTime uses an entirely different method of
computing times, I would not count on monotonicity.
- Steve
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