Em 28-03-2011 14:20, [email protected] escreveu:
> Hi Mauro,
>
>> This patch adds support for both ISDB-T and DVB-C @6MHz used in
>> Brazil, and adds a new bit rate of 5.2170 MSymbol/s, found on QAM256
>> transmissions at some Brazilian cable operators.
>
> Good. :)
>
>> While here, fix compilation with kernels 2.6.39 and later, where the
>> old V4L1 API were removed (so, linux/videodev.h doesn't exist anymore).
>> This is needed to compile it on Fedora 15 beta.
>
> videodev.h should have never been in there. Was already reported and will be
> removed instead.
>
>> @@ -1985,6 +1986,10 @@
>> dvbc_symbolrate_min=dvbc_symbolrate_max=0;
>> break;
>> case FE_QAM:
>> + // 6MHz DVB-C uses lower symbol rates
>> + if (freq_step(channel, this_channellist) == 6000000) {
>> + dvbc_symbolrate_min=dvbc_symbolrate_max=17;
>> + }
>> break;
>> case FE_QPSK:
>> // channel means here: transponder,
>
> This one causes me headache, because this one has side-effects to all other
> DVB-C cases using 6MHz bandwidth.
> Are there *any cases* around, where some country may use DVB-C with
> symbolrates other than 5.217Mbit/s?
If you take a look at EN 300 429[1], The DVB-C roll-off factor (alpha) is
defined as 0.15.
[1] EN 300 429 V1.2.1 (1998-04), chapter 9, page 16, and table B.1
So, the amount of the needed bandwidth (e. g. the Nyquist cut-off frequency) is
given by:
Bw = Symbol_rate * (1 + 0.15)
E. g. the maximum symbol rate is given by:
Symbol_rate(6MHz) = 6000000/1.15 = 5217391.30434782608695652173
Symbol_rate(7MHz) = 7000000/1.15 = 6086956.52173913043478260869
Symbol_rate(8MHz) = 8000000/1.15 = 6956521.73913043478260869565
As you see, for Countries using 6MHz bandwidth, the maximum value is about
5.127 Mbauds.
With the current w_scan logic of assuming 6.9 or 6.875 Mbauds by default for
DVB-C,
w_scan will never find any channel, if the channel bandwidth fits into a 6MHz
(or 7MHz)
channel spacing.
So, the above change won't cause regressions, although it could be improved.
IMHO, a different logic could be used instead, if the user doesn't use the -S
parameter, like:
int dvbc_symbolrate[] = {
7000000,
6956500, /* Max Symbol rate for 8 MHz channel bandwidth
*/
6956000,
6952000,
6950000,
6900000,
6875000,
6811000,
6790000,
6250000,
6111000,
/* Weird: I would expect 6086 or 6086.5 here, as the max rate for 7MHz
spacing */
5900000, /* Require at least 7 MHz channel bandwidth */
5483000,
5217000, /* Max Symbol rate for 6 MHz channel bandwidth
*/
5156000,
5000000,
4000000,
3450000,
};
for (i = 0; i < ARRAY_SIZE(dvbc_symbolrate)) {
if (freq_step(channel, this_channellist) / 1.15 > dvbc_symbolrate[i])
next;
/* Scan channel */
...
}
Btw, this looks weird to me:
static int dvbc_symbolrate(int index)
{
switch(index) {
...
case 11: return 7000000;
...
While it might work, the amount of bandwidth is 8.050 MHz. Yet, as tuner
low-pass filter
will probably not attenuate too much the extra 50kHz, I don't doubt that
someone would
be tempted to use this rate, but probably some boards will have troubles with
that.
> I know that for Europe there are many cases where low symbolrates are used,
> even if higher would be possible.
>
> If there are any doubts, i would prefer a solution like this and add all
> countries know to use this srate:
>
> dvbc_symbolrate_min=dvbc_symbolrate_max=0;
> break;
> case FE_QAM:
> + // 6MHz DVB-C uses lower symbol rates
> + switch (this_channellist) {
> + case DVBC_BR:
> + dvbc_symbolrate_min=dvbc_symbolrate_max=17;
> + break;
> + default:;
> + }
> break;
Yeah, this would work for Brazil, provided that we won't find any other operator
here using a Symbol rate bellow the maximum allowed.
> case FE_QPSK:
> // channel means here: transponder,
>
>
> I need an valid answer which solution is better to accept this patch.
>
> cheers,
> Winfried
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