linux-kernel - Re: Scanning for TV channels over DVB-T and DVB-T2

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Message-ID: <778d08be-b606-018a-c2bc-164fbbc33615@free.fr>
Date:   Wed, 10 Jun 2020 17:22:05 +0200
From:   Marc Gonzalez <marc.w.gonzalez@...e.fr>
To:     linux-media <linux-media@...r.kernel.org>,
        Sean Young <sean@...s.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Jan Pieter van Woerkom <jp@...w.nl>,
        Brad Love <brad@...tdimension.cc>,
        Antti Palosaari <crope@....fi>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: Scanning for TV channels over DVB-T and DVB-T2

On 09/06/2020 16:31, Marc Gonzalez wrote:

> On 08/06/2020 17:30, Marc Gonzalez wrote:
> 
>> Suppose we know that several channels are transmitted on a given frequency
>> by terrestrial antenna. However, we don't know if the signal is "encoded"
>> (not sure this is the right term) in DVB-T or DVB-T2 modulation.
>>
>> Do we have to scan the frequency /twice/
>> first with DTV_DELIVERY_SYSTEM = SYS_DVBT
>>  then with DTV_DELIVERY_SYSTEM = SYS_DVBT2 ?
>>
>> Or is there some optimization where both modulations are handled
>> in a single step?
>>
>> Or maybe it depends on the tuner driver?
> 
> Digging a bit deeper...
> 
> My system sports a Silicon Labs Si2168 DVB-T2/T/C demodulator
> 
> The data sheet states:
> 
> "For DVB-T2 and T2-Lite:
> 	DVB-T2 versus DVB-T automatic detection"
> 
> "In DVB-T and DVB-T2 reception, parameters necessary for device synchronization
> are broadcast in the transmission parameters (respectively TPS and P1 symbol).
> When these transmission parameters are found and decoded, the demodulator is
> automatically and accordingly configured to achieve full synchronization."
> 
> 
> This HW is supported upstream by drivers/media/dvb-frontends/si2168.c
> https://elixir.bootlin.com/linux/latest/source/drivers/media/dvb-frontends/si2168.c
> 
> I have a nagging feeling because si2168_set_frontend() appears to
> behave differently for SYS_DVBT and for SYS_DVBT2...
> 
> https://elixir.bootlin.com/linux/latest/source/drivers/media/dvb-frontends/si2168.c#L250
> https://elixir.bootlin.com/linux/latest/source/drivers/media/dvb-frontends/si2168.c#L297
> https://elixir.bootlin.com/linux/latest/source/drivers/media/dvb-frontends/si2168.c#L345
> 
> Maybe setting SYS_DVBT2 would also work for DVB-T transmissions?
> (TODO: test that)
> 
> Any insight would be greatly appreciated.

FTR, on IRC, Brad pointed out this patch of his:
https://patchwork.kernel.org/patch/10744999/