[linux-dvb] networked digital tuner project

Wolfgang Wegner wolfgang at leila.ping.de
Sat Jun 11 23:39:51 CEST 2005

Hi Brian,
On Sat, Jun 11, 2005 at 01:29:15PM -0700, Brian Kuschak wrote:
> Examples?  Remember that most tuner modules use i2c
> for all control (tuning, checking lock status, LNB
> control, Diseqc, etc).  Considering the time required
> for 100khz I2C messaging, the additional time
> contributed by transport over 100mbps Ethernet on a
> LAN is negligible.

I think it is quite different depending on which tuner
and modulation scheme you use. (Older) DVB-S frontends
required a continuous zig-zag-scan to keep the lock, while
some more modern (DVB-T) frontends require firmware to
be uploaded before they do work. Of course, all these are
I2C transfers, and so it can be done like you say, the
only thing is that I would feel more comfortable with
this being handled locally. ;-)

> What kind of timestamp would you like to see?  IIRC,
> RTP uses the 27mhz clock from the STC.  We could
> append a local 32-bit timestamp as well to each packet
> as it arrives.  Would this suffice?

Sorry for bringing this up. :-) Actually, it is not a
different time stamp (the 27MHZ STC in the RTP header
is perfect), but if you want to synchronize at the other
end of the LAN you have varying packet delays, making
"locking" a software-PLL to this difficult. What we did
was implement another header field carrying the accumulated
delay of a previous packet, which of course had to be updated
on each routing hop in between.
First we should try to get anything over the net, then we
can improve it. ;-)

> Regards,
> Brian


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