Hi,
Manu Abraham wrote:
> Oliver Endriss wrote:
> ...
> > Ok, some calculations according your formula
> >
> >>>>> BW = (1 + RO) * SR/2 + 5) * 1.3
> >
> > 45 MSPS:
> > BW = ((1 + 0.35) * 45/2 + 5) * 1.3 = 46
> >
> > -> cutoff 36 MHz (maximum value supported)
> >
> > 27 MSPS:
> > BW = ((1 + 0.35) * 27/2 + 5) * 1.3 = 30,2
> >
> > -> cutoff 31 MHz
> >
> > 22 MSPS:
> > BW = ((1 + 0.35) * 22/2 + 5) * 1.3 = 25,8
> >
> > -> cutoff 26 MHz
> >
> > Are these calculations correct, or did I miss something here?
>
>
> It looks fine, just round it off to the next integer. ie always round it
> up, rather than rounding it down. For the cutoff at 36MHz, it is fine as
> well, since at the last step, you will not need an offset, since it
> would be the last step in the spectrum.
> ...
> > Afaics a simple pre-calculated lookup table with 32 entries should do
> > the job. At least for the cut-off frequency.
>
> That's possible, since you need only 32 precomputed entries, rather than
> continuous values. That would be much better too, without any runtime
> overheads. Just the table needs to be done nice.
Now I found some time to come back to this issue,
I prepared a small patch to set the cutoff according to Manu's formula.
The calculation is simple enough for integer arithmetic, so it is not
worth to prepare a lookup-table.
@ldvb:
Please test and report whether it works for you.
CU
Oliver
--
----------------------------------------------------------------
VDR Remote Plugin 0.4.0: http://www.escape-edv.de/endriss/vdr/
----------------------------------------------------------------
diff -r 8f1c4ba078eb linux/drivers/media/dvb/frontends/tda826x.c
--- a/linux/drivers/media/dvb/frontends/tda826x.c Wed Mar 26 17:41:21 2008 +0100
+++ b/linux/drivers/media/dvb/frontends/tda826x.c Thu Apr 10 22:17:48 2008 +0200
@@ -76,12 +76,23 @@ static int tda826x_set_params(struct dvb
struct tda826x_priv *priv = fe->tuner_priv;
int ret;
u32 div;
+ u32 ksyms;
+ u32 bandwidth;
u8 buf [11];
struct i2c_msg msg = { .addr = priv->i2c_address, .flags = 0, .buf = buf, .len = 11 };
dprintk("%s:\n", __FUNCTION__);
div = (params->frequency + (1000-1)) / 1000;
+
+ /* BW = (1 + RO) * SR/2 + 5) * 1.3 [SR in MSPS, BW in MHz] */
+ /* with R0 = 0.35 and some transformations: */
+ ksyms = params->u.qpsk.symbol_rate / 1000;
+ bandwidth = (878 * ksyms + 6500000) / 1000000 + 1;
+ if (bandwidth < 5)
+ bandwidth = 5;
+ else if (bandwidth > 36)
+ bandwidth = 36;
buf[0] = 0x00; // subaddress
buf[1] = 0x09; // powerdown RSSI + the magic value 1
@@ -90,7 +101,7 @@ static int tda826x_set_params(struct dvb
buf[2] = (1<<5) | 0x0b; // 1Mhz + 0.45 VCO
buf[3] = div >> 7;
buf[4] = div << 1;
- buf[5] = 0x77; // baseband cut-off 19 MHz
+ buf[5] = ((bandwidth - 5) << 3) | 7; /* baseband cut-off */
buf[6] = 0xfe; // baseband gain 9 db + no RF attenuation
buf[7] = 0x83; // charge pumps at high, tests off
buf[8] = 0x80; // recommended value 4 for AMPVCO + disable ports.
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