[linux-dvb] new DVB-T initial tuning for fr-nantes

BOUWSMA Barry freebeer.bouwsma at gmail.com
Thu May 20 13:27:48 CEST 2010


On śro (środa) 19.maj (maj) 2010, 20:50:00, Damien Bally wrote:

> There's also a new transponder called L8 today on channel 56 (QAM16) for
> local TV, so the fr-Nantes should look like this :
> 
> # Nantes - France
> # T freq bw fec_hi fec_lo mod transmission-mode guard-interval hierarchy

> T 754000000 8MHz AUTO NONE QAM16 8k AUTO NONE

One thing that would be good to do -- for someone who is in the
area served by a transmitter -- rather than use «AUTO» for the
FEC and Guard Interval values above, would be to perform a NIT
scan on the appropriate frequencies.

This should give as a result the actual transmitter frequency
and other parameters in use, and perhaps related ones from other
sites.

These values are configured by the broadcaster and are therefore
not guaranteed to be 100% accurate, but it is more likely that
they know the values they are using -- these values can be used
to locate other transmitter frequencies, as is the well-known
case for DVB-S satellite NIT tables.

I'll give an example using one satellite transponder, because this
machine I am using does not presently have a DVB-T (TNT) receiver
attached that works...

beer at herbert:/tmp/ntp-dev-4.2.5p183/ntpdate$ /home/beer/bin/dvbstream   -s 27500  -p v  -f 12525  -I 2  -D 4   -n 2 -o:/tmp/NIT.ts   16

(small file is written with NIT data)

beer at herbert:/tmp/ntp-dev-4.2.5p183/ntpdate$ dvbsnoop -s ts -tssubdecode -if /tmp/NIT.ts    | less

Here the data I see is specific to this satellite, but you should
see similar data for the particular terrestrial channel you tune
in your case...

                  Frequency: 19211520 (=  12.52500 GHz)
                  Orbital_position: 565 (=  23.5)
                  West_East_flag: 1 (0x01)  [= EAST]
                  Polarisation: 1 (0x01)  [= linear - vertical]
                  Kind: 0 (0x00)  [= DVB-S]
                  fixed ('00'): 0 (0x00)
                  Modulation_type: 1 (0x01)  [= QPSK]
                  Symbol_rate: 2576384 (=  27.5000)
                  FEC_inner: 3 (0x03)  [= 3/4 conv. code rate]

and
                  Frequency: 19227904 (=  12.56500 GHz)
                  Orbital_position: 565 (=  23.5)
                  West_East_flag: 1 (0x01)  [= EAST]
                  Polarisation: 0 (0x00)  [= linear - horizontal]
                  Kind: 0 (0x00)  [= DVB-S]
                  fixed ('00'): 0 (0x00)
                  Modulation_type: 1 (0x01)  [= QPSK]
                  Symbol_rate: 2576384 (=  27.5000)
                  FEC_inner: 2 (0x02)  [= 2/3 conv. code rate]

and
                  Frequency: 18323200 (=  11.79700 GHz)
                  Orbital_position: 565 (=  23.5)
                  West_East_flag: 1 (0x01)  [= EAST]
                  Polarisation: 0 (0x00)  [= linear - horizontal]
                  Kind: 0 (0x00)  [= DVB-S]
                  fixed ('00'): 0 (0x00)
                  Modulation_type: 1 (0x01)  [= QPSK]
                  Symbol_rate: 2576384 (=  27.5000)
                  FEC_inner: 3 (0x03)  [= 3/4 conv. code rate]

and
                  Frequency: 18372096 (=  11.85600 GHz)
                  Orbital_position: 565 (=  23.5)
                  West_East_flag: 1 (0x01)  [= EAST]
                  Polarisation: 0 (0x00)  [= linear - horizontal]
                  Kind: 0 (0x00)  [= DVB-S]
                  fixed ('00'): 0 (0x00)
                  Modulation_type: 1 (0x01)  [= QPSK]
                  Symbol_rate: 2576384 (=  27.5000)
                  FEC_inner: 4 (0x04)  [= 5/6 conv. code rate]

and some more transponders used by this provider (CS Link).


I know that at least one of my DVB-T receivers will tune into
the services on a particular channel even when I feed it the
wrong values, but apparently not all devices are capable of
this.

In the event a particular receiver requires «AUTO» and will
not work even with the ideally-correct values as given from
PID 16, then this should probably be brought to the attention
of a responsible developer to be fixed, rather than avoiding
the inclusion of the wrong values in the frequency lists.
Similarly a 167kHz frequency offset when such is not used
by the actual transmitter.

After all, the proper 1/32 (if correct) Guard Interval value
will tell me that use is made of a MFN, while 1/4 tells me
that there may be a large area covered by multiple transmitter
sites on the same frequency.  Or in the case of Paris, 1/8
tells me there is a smaller SFN configuration.

Also, the FEC rate together with the modulation 16/64-QAM
will tell me how much the expected combined data rate available
for all services will be, as well as how far removed from the
transmitter one can receive a signal at a given transmitter
power, and what sort of antenna effort will be necessary for a 
strong clean signal towards the fringes of reception.

Perhaps not very important for a normal user, but good things
to know when troubleshooting or providing recommendations for
reception, from afar -- the fact that I could receive the
analogue scrambled Canal+ is no guarantee I can now receive
any of the present TNT services (I can't).


merci
barry bouwsma



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