[vdr] Re: Time jumps

[Wolfgang Fritz <wolfgang.fritz@gmx.net>]

Florian Steinel wrote:

> Sergei Haller wrote:
> 
>>does someone have more information on the transmitted time stamp?
>>
> Do you mean
> - PDC? ()
>    (transmit hours, minutes seconds)
>   (ETSI EN 300 231 V01.03.01)
> - Teletext - E.14 Real-time clock Page Header Packets
>   (transmit hours, minutes seconds)
>   (ETSI EN 300 706 V01.02.01)
> - SI TDT  UTC Time (Page 23 ff)
>    (ETSI EN 300 468 V01.04.01)
> 
>> - which format does the time stamp have? does it include microseconds
>> (I
>>   doubt it makes sense to transmit them because of the different
>>   lengths between the sat and the dish depending on the geographic
>>   location, but who knows?
>>
> As for the above standards , no microsecs.
> 
>> - how accurate is the time stamp within one transponder?
>>   does it jump back and forward?
>>  
>>
> It should not, but how knows?
> Someone with more knowledge should answere this.
> Maybe mod dvbsak and monitor some Transponder?!
> 
>>one possibility would be to do the following:
>>   if the system clock is off by at least XX seconds (what is a good
>>   value? 2? 10? 20?), then set the clock, causing a time jump on the
>>   system clock.
>>   if the diff is less than XX seconds, use adjtime.

You may estimate the systematic drift of your system clock by comparing 
it to a reliable external clock source (don't know if the satellite 
time is good enough) and then compensate the systematic drift with 
adjtimex (see man adjtimex). This way the system time should always be 
in the "adjtime range" and no clock jumps should happen.

I am doing this on a server by checking the time against an NTP server 
once a day. The systematic drift of my clock is abt 24 seconds/day (!!) 
and I am able to compensate it to less than 25ms/day.

Wolfgang
>>
>>with that, if one uses always the same transponder for time synching
>>and the transmitted time is not jumping, after some while the system
>>time should get more and more correct (like with ntp)
>>
>>c ya
>>        Sergei
>>  
>>
> 
> 
> 
> 




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