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[media] tda18271c2dd: Fix saw filter configuration for DVB-C @6MHz

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Currently, the driver assumes that all QAM carriers are spaced with
8MHz. This is wrong, and may decrease QoS on Countries like Brazil,
that have DVB-C carriers with 6MHz-spaced.

Fortunately, both ITU-T J-83 and EN 300 429 specifies a way to
associate the symbol rate with the bandwidth needed for it.

For ITU-T J-83 2007 annex A, the maximum symbol rate for 6 MHz is:
	6 MHz / 1.15 = 5217391 Bauds
For  ITU-T J-83 2007 annex C, the maximum symbol rate for 6 MHz is:
	6 MHz / 1.13 = 5309735 Bauds.

As this tuner is currently used only for DRX-K, and it is currently
hard-coded to annex A, I've opted to use the roll-off factor of 0.15,
instead of 0.13.

If we ever support annex C, the better would be to add a DVB S2API
call to allow changing between Annex A and C, and add the 0.13 roll-off
factor to it.

This code is currently being used on other frontends, so I think we
should later add a core function with this code, to warrant that
it will be properly implemented everywhere.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
This commit is contained in:
Mauro Carvalho Chehab 2011-07-28 15:49:43 -03:00
parent 86cf786c04
commit cf845297d5
1 changed files with 19 additions and 1 deletions
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20
drivers/media/dvb/frontends/tda18271c2dd.c
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@ -1123,6 +1123,21 @@ static int release(struct dvb_frontend *fe)
return 0;
}
/*
* As defined on EN 300 429 Annex A and on ITU-T J.83 annex A, the DVB-C
* roll-off factor is 0.15.
* According with the specs, the amount of the needed bandwith is given by:
* Bw = Symbol_rate * (1 + 0.15)
* As such, the maximum symbol rate supported by 6 MHz is
* max_symbol_rate = 6 MHz / 1.15 = 5217391 Bauds
*NOTE: For ITU-T J.83 Annex C, the roll-off factor is 0.13. So:
* max_symbol_rate = 6 MHz / 1.13 = 5309735 Baud
* That means that an adjustment is needed for Japan,
* but, as currently DRX-K is hardcoded to Annex A, let's stick
* with 0.15 roll-off factor.
*/
#define MAX_SYMBOL_RATE_6MHz 5217391
static int set_params(struct dvb_frontend *fe,
struct dvb_frontend_parameters *params)
{
@ -1146,7 +1161,10 @@ static int set_params(struct dvb_frontend *fe,
break;
}
else if (fe->ops.info.type == FE_QAM) {
Standard = HF_DVBC_8MHZ;
if (params->u.qam.symbol_rate <= MAX_SYMBOL_RATE_6MHz)
Standard = HF_DVBC_6MHZ;
else
Standard = HF_DVBC_8MHZ;
} else
return -EINVAL;
do {