Record multiple channels from one multiplex

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Revision as of 05:54, 23 November 2006 by Renne (talk | contribs) (Design Phase)

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DVB transmissions are multiplexed Transport Streams containing several data streams (video, audio, epg, eit, ...) identified by PIDs. When you select a TV/Radio program or data service (like EPG, Teletext, IPv4, ...) the necessary PIDs are remuxed into a new data stream which can be handled by your application. In case of a media player one video PID and one or several audio PIDs are muxed into a TS- or PES-container (there can also be timing information like PCR).

The DVB-standard specifies the MPEG2 codec for video and audio. Alternatively AC3 can be used for audio.

As of the high bandwith of DVB multiplexes (up to 90 MBit/s) there is usually more than one TV- or radio-program in a multiplex. In contrast to the Windoze BDA driver, the LinuxDVB kernel driver can also deliver the complete Transport Stream of a multiplex.

By remuxing the TS into more than one new stream other TV applications like VDR can record several TV programs in a DVB multiplex at the same time using only one tuner for the multiplex.

Multiplex recording also allows overlapping of sequent recordings of the same program. That avoids the annoying problem with the end of a broadcast being in the next recording or the beginning of a broadcast being at the end of a previous recording.

This currently can't be done in MythTV.

Pledge

A pledge to pay a developer who implements this feature has started.

Countries with multiplexed DVB/ATSC transmissions

Africa

Antarctica

Asia

Australia

  • ABC and ABC2, with ABC2 repeating much of the ABC content later in the week (great if you run out of tuners)
  • SBS with SBS world News
  • Digital 44 (Sydney only) - datacasting of news/tab/expo/christian tv/parliament

Europe

  • Astra 19.2° and EutelSat 13.0° DVB-S (up to 20 radio or 8 TV programs per transponder/multiplex)

Finland

  • All TV stations on DVB-C/-T (up to 6 TV channels per multiplex)

Germany

  • All TV stations on DVB-C/-T (4 TV programs per VHF/UHF multiplex)

UK

The UK is made up of 6 multiplexes. As you can see below, with only 6 tuners you could have quite an incredible MythTV recording platform.

Mux 1 Mux 2 Mux A Mux B Mux C Mux D
BBC ONE ITV1 abc1 301 E4+1 Film4
BBC TWO ITV2 bid tv 302 / Community Sky Sports Ideal World
BBC THREE ITV3 Five 305 Sky Sports News ITV Play
BBC NEWS 24 / CBBC CITV Five Life BBC FOUR / CBeebies Sky Three The HITS
Channel 4 Five US BBC News Multiscreen UKTV History / smileTV TMF
E4 price-drop tv BBC Parliment UKTV Bright Ideas / f tn
More 4 QVC
Quiz Call Teachers' TV

In addition, there are a large number of radio stations.

The full list can be found here: [1]

Feel free to reformat or move the above data if you feel it's too big or not relevant for this page.

North America

Digital cable transmissions are multiplexed. The standard encoding appears to be QAM-256 for unencrypted channels. Usually there are also lots of audio only channels multiplexed into the stream, so each frequency can have a number of streams. Maximum for one frequency with Rogers cable in the Toronto area appears to be 14 streams on one frequency...a mixture of unencrypted video+audio, audio and encrypted HD.

Rogers has a total of 424 streams!

An example dvbscan output:

Unique name:frequency:modulation: video id: audio id: serviceid (pid)

  1. dumping lists (424 services)

[076e]:513000000:QAM_256:26:27:1902 [0773]:513000000:QAM_256:29:30:1907 [0774]:513000000:QAM_256:18:19:1908 [0776]:513000000:QAM_256:32:33:1910 [0775]:513000000:QAM_256:37:40:1909 [0778]:513000000:QAM_256:51:52:1912 [0779]:513000000:QAM_256:53:56:1913 [07b3]:519000000:QAM_256:21:24:1971 [07b5]:519000000:QAM_256:41:42:1973 [07b6]:519000000:QAM_256:43:44:1974 [07b7]:519000000:QAM_256:46:47:1975 [07b2]:519000000:QAM_256:49:50:1970

South America

Discussions

Please read these discussions, and the Trac ticket thoroughly before starting a new discussion about the subject on the mailing list.

If someone has time, it would be great if the ideas in these threads and the ticket could be summarised?


Documentation


Software Engineering

Analysis Phase

Current state:

  MythTV does not support multiplex reception

Goal:

  Multiplex reception

Design Phase

FAQ:

Q: How to get several recordings from one multiplex?
A: Grab the complete Transport Stream from a DVB/ATSC device into a ringbuffer and demux it into several MPEG streams.

Q: Does it make sense to store the TS on harddisk instead of using a ringbuffer?
A: No, unless you can afford 40 GByte diskspace per recording hour (DVB-S multiplexes can have bitrates up to 90 MBit/s).

Q: Is it possible to record encrypted and decode the recording offline/later?
A: In theory, yes. Practically NOT.

  1. The decoding is done by CAM modules with smartcards.
  2. The CAM modules use a extended PCMCIA interface with special wiring.
  3. You need a kernel driver for the CAMs.
  4. Also consider the high load for the PCI/PCMCIA interface.
  5. Legal issues in most countries.

Q: How to prevent MythTV-backend crashes by broken Transport Streams?
A:

  1. Validate Transport Stream by defining preconditions in the TS remuxer
  2. Fork a transport stream demuxer process for each recording.
  3. Make sure to handle process signals correctly (no zombies)!

Q: How to handle multiplexes in the recording scheduler?
A: Use source type (DVB-C/-S/-T), frequency (DVB-C/-S/-T) and polarisation (DVB-S) to identify programs on the same multiplex.

Q: How to adapt the recording scheduler for multiplex recordings?
A: No answer, yet ...

Implementation Phase

Far, far away ...

Testing Phase

Testing by SVN-Users ;-)