[linux-dvb] Re: Summary (to Holger :-)

[Holger Waechtler <holger@convergence.de>]

Dennis Noordsij wrote:
> Hi,
> 
> Here is a summary of some of the discussion that went on over the weekend, 
> things that I'm curious about :)
> 
> * What is the difference in practice between zigzagging frequencies and having 
> the frequecy_bend code? 

dvb_bend_frequency() prevents that two frontends tune to exactly the 
same frequency. I have a configuration here where interferences between 
tuners cause both to lose the lock when the second one tunes to the 
common frequency.


> If I disable the frequency bend, in the case of dual 
> tuners tuning to the same transponder, doesn't the zigzag already introduce a 
> bend? Don't these functions overlap? Disabling frequency bending improves the 
> tuning speed for me (for HEAD).

zigzagging applies to all frequencies until a lock is detected. 
Frequency bending just moves the desired frequency on step (stepsize of 
the frequency divider) aside.


> * Since the zigzagging in the recovery thread is to compensate for LNB drift, 
> is it really useful for DVB-C applications? (Not sure about DVB-T). I doubt 
> the frequency drifts at all, and once you have figured out the *exact* 
> frequency (maybe with the help of the dvbsak library and scanner utility) 

Well, I wanted to remove it a while ago, but this caused many problems, 
including the +/-125 and +/-250kHz offset problem of the VES1820. The 
zigzag scan just makes the driver a bit more robust and hides incorrect 
parameters. I'm not sure if this is a good thing.


> wouldn't that always just work? Wouldn't it be faster to be really persistant 
> about locking to a certain frequency rather than hopping around looking for 
> an alternative? (admittedly, in REL-0-9-2 this was not an issue at all).

When you only have one frontend attempting to tune to a particular 
frequency and the frequency is correct, you should get an immediate 
lock. Only in cases when a second frontend tries to tune to the same 
frequency or the specified frequency is not correct bending and 
zigzagging will get active.


> * Where is the timeout specified for deciding wether or not there is a lock 
> and before trying a different frequency? I can see many (inserted) printk's 
> "trying freq xxxxx Hz", "trying freq yyyyy Hz" log messages, many times 
> without locking, and then suddently it will lock on freq xxxx which has 
> already been tried a dozen times before without success.

where have you seen these printk's?

Holger



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