What the FEC?

[Jim Lesurf]

I'm posting this here in case anyone else has come across the same
puzzle...

In particular, I'd be interested to know how the settings other people get
from scan compare. Is Angus TX unique?


On 08 Oct, no...@audiomisc.co.uk wrote:
> In article <2u6dnXaoprE5OOzN...@brightview.co.uk>, > Andy
> Burns <usenet....@adslpipe.co.uk> wrote:
> > Jim Lesurf wrote:

> > > Eyeballing the details I found that for Angus one of the FEC
> > > settings is FEC_AUTO for every SD station, but for Durris they are
> > > all FEC_1_2.

> > Does FEC_AUTO even *exist* in the DVB-T spec? I assumed it was just a
> > V4L concept for cards that didn't need to be told what FEC to use, did
> > you use the same "seed" data for all scans?

> Here are the 'raw'/'seed' files I use:

> # UK, Angus
> # T freq bw fec_hi fec_lo mod transmission-mode guard-interval hierarchy
> T 786000000 8MHz 2/3 NONE QAM64 8k 1/32 NONE
> T 730000000 8MHz 2/3 NONE QAM64 8k 1/32 NONE
> T 762000000 8MHz 2/3 NONE QAM256 32k 1/128 NONE
> T 738000000 8MHz 3/4 NONE QAM64 8k 1/32 NONE
> T 770000000 8MHz 3/4 NONE QAM64 8k 1/32 NONE
> T 794000000 8MHz 3/4 NONE QAM64 8k 1/32 NONE

> # UK, Durris
> # T freq bw fec_hi fec_lo mod transmission-mode guard-interval hierarchy
> T 530000000 8MHz 2/3 NONE QAM64 8k 1/32 NONE
> T 506000000 8MHz 2/3 NONE QAM64 8k 1/32 NONE
> T 482000000 8MHz 2/3 NONE QAM256 32k 1/128 NONE
> T 490000000 8MHz 3/4 NONE QAM64 8k 1/32 NONE
> T 514000000 8MHz 3/4 NONE QAM64 8k 1/32 NONE
> T 538000000 8MHz 3/4 NONE QAM64 8k 1/32 NONE

> I can't see any significant difference beyond having changed the
> frequencies for the relevant muxs. However the results differ in two
> ways.

> A) The Angus file generated by scan includes all the SD channels from
> *Durris* as well as all the Durris channels. So everything is duplicated
> with lines for both the Durris and Angus muxes - *except* that the
> Durris HD mux isn't included.

> B) The Angus file has 'FEC_AUTO' in the places where the Durris file has
> 'FEC_1_2'.


> Here is the first line of each output channel list generated by scan:
>
> Angus-
>
> BBC ONE Scot:786000000:INVERSION_AUTO:BANDWIDTH_8_MHZ:FEC_2_3:FEC_AUTO:QAM_64:TRANSMISSION_MODE_8K:GUARD_INTERVAL_1_32:HIERARCHY_NONE:101:102:4220

> Durris-

> BBC ONE Scot:530000000:INVERSION_AUTO:BANDWIDTH_8_MHZ:FEC_2_3:FEC_1_2:QAM_64:TRANSMISSION_MODE_8K:GUARD_INTERVAL_1_32:HIERARCHY_NONE:101:102:4220

> That said, both lists have 'FEC_AUTO' for the HD channels/mux.

> I noticed (A) some time ago and it made me wonder if something odd was
> going on with the Angus TX. Noticing (B) yesterday seems to confirm this.

> FWIW I asked someone at the BBC about this a while ago. But they didn't
> know. I suspect it may be down to the 'outsourced' side of things. A
> complication being that this is for TXs where the BBC and ITV have their
> 'Scottish' arrangements.

> When I started I made the innocent assumption that what I'd get from
> each TX mux in a region would be the same across the region. But I'm
> coming to doubt that.... which also makes me wonder if there are other
> differences of greater significance. e.g. maybe material is being
> reprocessed and rem-muxed more that we are aware!

> If I knew who was responsibile I'd ask them. But so far I haven't been
> able to track down who might be doing this. It may simply be that Angus
> has been set up incorrectly, or there may be a specific reason for these
> differences. How do we find out?

> Slainte,

> Jim

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[Andy Furniss]

The second FEC listed is actually unused for us as it's the low priority
one and HIERARCHY_NONE is set.

I get FEC_AUTO for HD mux I can see scan code does this when none is
specified, and the HD mux doesn't seem to appear in the standard NIT pid.

I see FEC_1_2 in my scan file this seems to be because that's what's in
the NIT.

dvbsnoop -n 0 -s ts -tssubdecode -nph -if
/path/to/a/short/mux-recording.ts 0x10

....

DVB-DescriptorTag: 90 (0x5a) [= terrestrial_delivery_system_descriptor]
descriptor_length: 11 (0x0b)
Center frequency: 0x04720d04 (= 745833.000 kHz)
Bandwidth: 0 (0x00) [= 8 MHz]
priority: 1 (0x01) [= HP (high priority) or
Non-hierarch.]
Time_Slicing_indicator: 1 (0x01) [= Time Slicing is
not used.)]
MPE-FEC_indicator: 1 (0x01) [= MPE-FEC is not used.)]
reserved_1: 3 (0x03)
Constellation: 2 (0x02) [= 64-QAM]
Hierarchy information: 0 (0x00) [= non-hierarchical
(native interleaver)]
Code_rate_HP_stream: 1 (0x01) [= 2/3]
Code_rate_LP_stream: 0 (0x00) [= 1/2]
Guard_interval: 0 (0x00) [= 1/32]
Transmission_mode: 1 (0x01) [= 8k mode]
Other_frequency_flag: 1 (0x01)
reserved_2: 4294967295 (0xffffffff)

...

I wonder if you get FEC_AUTO when scan reads the NONE from your initial
tuning file, but 1_2 when it's from the NIT.

Maybe you could do a scan with initial just one line -

T 762000000 8MHz 2/3 NONE QAM256 32k 1/128 NONE

and see if all SD show as FEC_1_2.