Hi, Wolfgang Wegner wrote: > Hi, > > On Sat, Sep 29, 2007 at 04:18:31PM +0400, Manu Abraham wrote: >> Johannes Stezenbach wrote: >>>> 1) LNB drift >>> - LNBs have a constant error plus a temperature drift >>> (e.g. +/-1MHz error, +/-3Mhz drift for a temperature range >>> of -40 ... +60 °C -- cheap no name equipment usually worse) >> This is the old LNB, the one's we use are generally based on PLL's have some 15 - 20k drift > > hmm.... I am not sure how old our LNBs (at work) are, but there and with > my LNBs at home (around 2 years old) I have measured offsets of around > 1-2 MHz. > >>> - sw zig-zag is by no means stv0299 specific and is used by >>> (almost?) all DVB-S demod drivers >> >> hmm, i didn't mean swzigzag, but as you see from that discussion, >> it was the drift that i am looking at > > Is it really a difference? I am not sure if I understand it correctly, > but IMHO this is or could be handled as the same issue. > > For low symbol rates, the problem is that you can not get the frontend > locked to the transponder if the offset (probably being caused by drift > or simply constant frequency offset because of the cheap crystal in > frontend PLL and/or LNB) is too high. You can only overcome this by > zig-zag-scan. > > >> My point being to have zigzag specific to the demod, since each device of the >> devices which implements zigzag does it in a different way. >> The computation being different > > Why is the computation different? I did not look into it yet, but at > the moment I can only see stepsize and transponder bandwidth (symbol rate > in case of DVB-S) as the parameters the algorithm depends on. > A quick generalized comparison. (looked at 2 devices from 2 vendors) devices from STM does Fs / 2 zigzag, where Fs is variable. Intel uses Foff = (err1 * mclk / 65536 + err2 * srate / 262144) Although point to be noted is that dvb_frontend is not doing Fs/2 zigzag either. It does "some" zigzag. > [...] >>> IIRC Andrew de Quincey spent significant time optimizing the >>> zig-zag code and the parameters for various frontends. >>> >> I do remember the time where he spent so much time optimizing the >> swzigzag for the STV0299. > > It seems my thinking is too easy above, I will have to look into the > code to get enlightenment, I hope. :-) > >> You have any cases of broadcasters doing it in a non-standard way ? >> ie inverted transmission ? > > I do not remember the transponders, but there are some - not on > Astra/Hotbird, though. Maybe I can give an example on monday. > Thanks, Manu _______________________________________________ linux-dvb mailing list linux-dvb@xxxxxxxxxxx http://www.linuxtv.org/cgi-bin/mailman/listinfo/linux-dvb
