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Revision as of 02:07, 13 August 2006 by Groogle (talk | contribs) (Add information on "no audio" problem, tidy up format.)
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scan is one of the dvb-apps tools. It scans specific frequencies for programs that can be received and generates a channel list. Depending on where you get your dvb-utils package, the program to use is called scan or dvbscan. In the following examples, we'll use the name dvbscan. The First steps with a budget DVB card page shows examples using different names.

dvbscan does not have a man page. If you run it without parameters, it gives you an overview of the commands, which is usually only sufficient for experts.

dvbscan requires a channel information file to do its job. These files are supplied with the package, but the location where they are installed may vary. In the following examples, the directory is /usr/local/share/dvb/scan/dvb-t/; other installations also use /usr/share/doc/dvb-utils/eamples/scan/dvb-t/. In each case, the name of the file you're looking for is of the form cc-Ttttt, where cc is a two-letter country abbreviation, and Ttttt is the name of the location of the transmitter. So in Adelaide, Australia, you'd look for a file called au-Adelaide; in Oxford, UK (the example at First steps with a budget DVB card) you would use uk-Oxford.

Note also that there are other files with similar names:

  • /usr/share/doc/dvb-utils/eamples/dvb-t/au-Adelaide is a copy of /usr/share/doc/dvb-utils/eamples/scan/dvb-t/au-Adelaide.
  • /usr/local/share/dvb/zap/dvb-t/au-Adelaide is a canned version of the output of dvbscan. It may be out of date, so it's better to generate it from dvbscan, but if you can't get dvbscan to work, you might get further by using this file.

dvbscan outputs a lot of data. Here's some of it:

$ dvbscan /usr/local/share/dvb/scan/dvb-t/au-Adelaide 
scanning /usr/local/share/dvb/scan/dvb-t/au-Adelaide
using '/dev/dvb/adapter0/frontend0' and '/dev/dvb/adapter0/demux0'
initial transponder 226500000 1 3 9 3 1 1 0

This information tells you what hardware and channels dvbscan is talking to. There are often many lines of transponder information. The information shown here is wrapped over two lines to make it easier to read.

>>> tune to: 226500000:INVERSION_AUTO:BANDWIDTH_7_MHZ:FEC_3_4:FEC_AUTO:QAM_64:
0x0000 0x0250: pmt_pid 0x0102 ABC -- ABC HDTV (running)
0x0000 0x0251: pmt_pid 0x0100 ABC -- ABC TV Adelaide (running)
0x0000 0x0257: pmt_pid 0x0106 ABC -- ABC DiG Jazz (running)
Network Name 'ABC Adelaide'

This is the tuning information for the first transponder. Each transponder can transmit multiple programmes. The last one is a radio programme, which you can see from the output later on.

During scanning, you may see error messages like:

>>> tune to: 571500000:INVERSION_AUTO:BANDWIDTH_7_MHZ:FEC_2_3:FEC_2_3:QAM_64:
WARNING: >>> tuning failed!!!

This is not fatal. It's possible that the scan files contain information about stations that are not yet, or no longer, in service. Check the output at the end to see whether you have all the advertised programmes.

At the end, dvbscan outputs a list of channels corresponding to what it has found, for example:

TEN Digital:219500000:INVERSION_AUTO:BANDWIDTH_7_MHZ:FEC_3_4:FEC_1_2:QAM_64:

This is a "colon-separated" file. It consists of a number of fields delimited by colons. Here's the meaning of the three lines shown above:

channels.conf format
Field First programme Second programme Third programme
Programme name ABC HDTV ABC DiG Jazz TEN Digital
Frequency (Hz) 226500000 226500000 219500000
FEC name, HP FEC_3_4 FEC_3_4 FEC_3_4
FEC name, LP FEC_3_4 FEC_3_4 FEC_1_2
Constellation QAM_64 QAM_64 QAM_64
Video pid 2314 0 512
Audio pid 0 2318 650
Service ID 592 599 1617

These fields vary depending on the type of modulation; the ones shown are for QFDM (and, paradoxically, not for QAM). For other forms of modulation, the meanings of the fields differ. See the source code (and document it, please!) at util/scan/dump-zap.c.

Looking at the values, it's clear that ABC DiG Jazz is a radio programme, since it doesn't have a video pid. Less clear is why ABC HDTV does not have an audio pid. This is a real issue: this means that when tuning to these streams, no audio is output. We'll look at how to solve this problem below.

The frequencies shown here are in Hz. They correspond to 226.5 MHz for ABC and 219.5 MHz for TEN. Note that the first two programmes are the same frequency, so they come in the same MPEG stream. They're distinguished by the pids.

creating channels.conf

The data shown above is what tzap and friends want in their channels.conf file. tzap expects to find it in a file ~/.tzap/channels.conf. Create it like this:

$ cd ~
$ mkdir .tzap
$ dvbscan /usr/local/share/dvb/scan/dvb-t/au-Adelaide > .tzap/channels.conf

Presumably czap and szap require directories called ~/.czap and ~/.szap, but you should confirm this.

No sound with HDTV

scan does not alwyas recognize the audio streams, at least for DVB-T. In Australia, most channels broadcast HDTV with AC3 audio, which scan does not recognize when invoked as above. The result is the output shown above for ABC HDTV: no audio.

This is almost certainly a bug in scan. Until it is fixed, you can proceed in the following way:

  • Create the channels.conf file as shown above.
  • Run tzap to tune to the stream:
$ tzap -S "ABC HDTV" &

The -S ("silent") option tells tzap not to output its usual signal quality information; the & tells the shell to issue a new prompt when it has started tzap.

Next, run dvbscan with the -c (scan on currently tuned transponder only) option:

$ dvbscan -c
using '/dev/dvb/adapter0/frontend0' and '/dev/dvb/adapter0/demux0'
0x0000 0x0250: pmt_pid 0x0102 ABC -- ABC HDTV (running)
0x0000 0x0251: pmt_pid 0x0100 ABC -- ABC TV Adelaide (running)
dumping lists (6 services)
ABC HDTV         (0x0250) 01: PCR 0x0905 V 0x090a TT 0x090c AC3 0x090b
ABC TV Adelaide  (0x0251) 01: PCR 0x0080 V 0x0200 A 0x028a (eng) TT 0x0240 AC3 0x0294

This output was limited to the interesting information. The last two lines show the output for ABC HDTV and ABC TV Adelaide. ABC TV Adelaide has a video pid 0x200, an audio pid 0x28a, and an AC3 pid 0x294. The meanings of the other values still need to be documented.

By contrast, ABC HDTV has a video pid 0x90a, and AC3 pid 0x90b--and no audio pid. This is the problem. In the output from scan based on the transmitter data, the corresponding values are (leaving out the middle of the lines, which aren't of interest here):

ABC HDTV: ... :2314:0:592
ABC TV Adelaide: ... :512:650:593

As described above, the third-last value is the video pid, and the second-last value is the audio pid. In this output they're in decimal, while in the output of dvbscan -c they're in hexadecimal; but they correspond.

To work around the lack of audio pid in ABC HDTV, convert the value of the AC3 pid (0x90b) to decimal (2315) and edit the channels.conf file:

ABC HDTV: ... :2314:2315:592