Ticket #9777 (new Bug Report - General)
Opened 2 years ago
Last modified 10 months ago
Channel scanner stores centre freq instead of offset for DVB-T
|Reported by:||david_a <baroque@…>||Owned by:||danielk|
|Component:||MythTV - Channel Scanner||Version:||0.24-fixes|
The channel scanner (DVB-T full scan, in Australia) is storing centre frequencies instead of offset frequencies, for stations using the offset. Consequently some tuner cards cannot lock during normal operation of mythTV, resulting in empty recordings and failure to lock in LiveTV.
Several of the DVB-T transmitters in Australia use their offset frequency, 125kHz from the nominal centre frequency (evidently to avoid interference between them and adjacent services such as legacy analog).
The workaround is to manually edit the transports, or re-scan by individual frequency if necessary, however until a user is aware of this issue, they would be wondering (as I was) why their tuner fails to work with some stations.
I investigated the scanning process using different models of tuner, and found the scanner behaviour was the same, i.e. it consistently stores the centre frequency even for the stations using the offset. Therefore I believe the problem lies in the scanner logic, rather than its interaction with the hardware or drivers.
Since some tuners, with enough timeout, will find the signal while scanning either the centre or offset frequency, it can be deduced that storing the frequency sent to the tuner (or with strongest signal reading) may not be reliable. Instead, the scanner I believe should wait for receipt of the Network Information Table, which contains the correct transmitter information, as shown below in the following grep output from scanner log file (for city of Melbourne).
$ grep -i terrestrialdelivery scanlog TerrestrialDeliverySystemDescriptor: Frequency: 177500000 TerrestrialDeliverySystemDescriptor: Frequency: 191620000 TerrestrialDeliverySystemDescriptor: Frequency: 219500000 TerrestrialDeliverySystemDescriptor: Frequency: 226500000 TerrestrialDeliverySystemDescriptor: Frequency: 536625000 TerrestrialDeliverySystemDescriptor: Frequency: 557625000
3 of the 6 transmitters use their offset frequency (of which it is noticeable that one indicates a 120kHz offset as opposed to nominal 125kHz, a minor discrepancy).
Perhaps the scanner code is already *supposed* to be using the information from the NIT for each transport, however in practice it is not doing so, as evidenced by centre frequencies being stored for all stations as shown in following display of dtv_multiplex table:
mysql> select networkid,frequency from dtv_multiplex; +-----------+-----------+ | networkid | frequency | +-----------+-----------+ | 4115 | 177500000 | | 4114 | 191500000 | | 4116 | 219500000 | | 4112 | 226500000 | | 12802 | 536500000 | | 12832 | 557500000 | +-----------+-----------+
Being aware that DVB frequency offsets apply also in other countries (where different conventions may apply to the content of the NIT), the use of the frequency information from the NIT would of course require a sanity check to test whether the information is present, and whether it is within reasonable tolerance of a tabulated centre or offset frequency for the relevant country.
In Australia at least, using the NIT information may offer a solution to this DVB-T scanning problem.
I am attaching the scanner log produced with mythtv-setup -v channelscan,siparser,record
[Also as a footnote I will mention for anyone looking at the channel allocation tables published by the national authority at acma.gov.au, that for the VHF band below channel 10, there is a difference of one between the mythtv scanner channel number and the official channel number, due to the presence of channel "9A" in the official allocation. This however only affects the channel numbering in logs written by the scanner, and is of no consequence to the operation of the scanner, since it has the correct frequency table beginning at 177.5 MHz and with 7 MHz spacing.]
comment:1 Changed 22 months ago by aklinbail@…
comment:2 Changed 22 months ago by aklinbail@…
comment:3 Changed 22 months ago by david_a <baroque@…>
comment:4 Changed 22 months ago by aklinbail@…
comment:5 Changed 22 months ago by david_a <baroque@…>
comment:6 Changed 21 months ago by david_a <baroque@…>
comment:7 Changed 14 months ago by stuarta
- Cc stuarta added