[linux-dvb] Low symbol rates on FF DVB cards

Georg Acher acher at in.tum.de
Sat Dec 3 20:35:51 CET 2005


On Sat, Dec 03, 2005 at 08:16:53PM +0100, Ralph Metzler wrote:
 
> Can you tell me which part of the calculations in the old code is
> wrong? I have a card with such a tuner here and with the old code
> there are some tuning problems. Sometimes it tunes right away, 
> sometimes it takes a very long time.

The original code in tua6100_set_tv_freq (this was in stv0299.c before all
the rearrangement on the tuner/PLL drivers started, don't know where it is
now):

        R = 64;
        A = 64;
        P = 64;  //32

        M = (freq * R) / 4;             /* in Mhz */
        N = (M - A * 1000) / (P * 1000);

This one is much better:

	int nfreq=freq/125;
        R=64;
        P=64;
        A=(2*nfreq)&(R-1);  // 1/16MHz Steps
        N=nfreq/(4*8);  // 64*1/16=4 MHz Steps

Additionally, I looked at the output of the switchable bandwidth filter with
a spectrum analyzer. It looks like the filter should be better used for
symbolrates >8Ms/s instead of the 4Ms/s mentioned in the SU1278 data sheet.

-- 
         Georg Acher, acher at in.tum.de         
         http://www.lrr.in.tum.de/~acher
         "Oh no, not again !" The bowl of petunias          



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