[pvrusb2] Re: Problem with 20060102A

[Mike Isely]

On Tue, 3 Jan 2006, Mike Isely wrote:

> On Tue, 3 Jan 2006, Ingo Flaschberger wrote:
>
>> Hello,
>> 
>> How long should switching "normally" need?
>> 
>> 
>> bye,
>> 	ingo flaschberger
>
> There are no time delays or other arbitrary pauses in the pvrusb2 driver 
> which might enforce some kind of minimal switching delay.  However I can't 
> speak for the support modules and given the nature of the hardware there will 
> certainly be some time delay.  For example, I2C transactions are proxied over 
> USB and happen single-file - no pipelining.  I2C is already pretty slow and a 
> channel change potentialy involves reconfiguring a lot of I2C controlled 
> stuff.  So that can cause a delay.  Also the video hardware itself requires 
> some time to lock onto the new channel - and then be noticed by the 
> corresponding modules.
>
> Finally I imagine that the mpeg2 encoder itself introduces latency as well. 
> In fact, if you run a normal TV right next to a pvrusb2-equipped PC and watch 
> the same channel, I've actually see a 1-2 second latency in the video.
>
> So it's hard to pin down what the maximum delay might be during a frequency 
> change.  A second or so is probably not entirely unreasonable. There's not 
> much that the driver can to do influence this, beyond not unnecessarily 
> reconfiguring too much hardware when the frequency is changed.
>
>  -Mike
>

Just to follow up, I did some empirical measurements tonight using 
mplayer.  While mplayer was streaming, I repeatedly used sysfs to change 
the frequency.  The time from changing sysfs to mplayer showing the new 
channel is roughly 1 second.  Streaming never stops; it's just a latency.

Changing channels with this driver in xawtv takes a little longer, but 
xawtv does a lot of steps to change a channel - including actually 
stopping and restarting streaming.  In that case it seems to be slightly 
over 1 second to maybe 1.5 seconds.

   -Mike

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