[mythtv-users] One PCI DVB-T card, 2 items in lspci

[Simon Hobson]

Mike Perkins <mikep at randomtraveller.org.uk> wrote:

>> Thanks Mike. In afraid I don't really understand the difference. What's the
>> difference between the whole transport stream and a tuned mpeg stream of
>> one subchannel in practical terms?
>> 
> A Transport Stream is a whole bunch of subchannels multiplexed together. These can consist of TV, Radio, EPG and other streams of data or content.
> 
> You can either take the whole Transport Stream and save it for later processing, use your software (ie Myth) to select which one or more streams are of interest to you and save them (multirec), or let the tuner do the selecting for you, in which case the filtered stream comes out the .2 port.

To expand on that, take an example : http://www.ukfree.tv/transmitters/tv/Winter_Hill
On on transmitted frequency (UHF Ch50, 706MHz) there is a multiplex that contains a plethora of logical channels :
1 BBC One North West, 2 BBC Two England, 7 BBC Three, 9 BBC Four, 120 CBBC, 121 CBeebies, 130 BBC News, 131 BBC Parliament, 601 BBC Red Button 1,  200 BBC Red Button, 700 BBC Radio 1, 701 BBC Radio 1Xtra, 702 BBC Radio 2, 703 BBC Radio 3, 704 BBC Radio 4, 705 BBC Radio 5 Live, 706 BBC Radio 5 Live Sports Extra, 707 BBC Radio 6 Music, 708 BBC Radio 4 Extra, 709 BBC Asian Network, 710 BBC World Service, 719 BBC Radio Manchester (BBC local radio)
Plus, as pointed out, other stuff that's important be we don't tend to see.

So when you want to watch (say) BBC1, the tuner needs to tune to 706MHz - at which point it'll receive a huge data stream (around 16Mbits/sec IIRC) which contains all these logical channels. To actually get the BBC1 program, we need to pick the packets out of the stream that represent this single program, and discard the rest.
Many (most ?, all ?) tuners have the ability to do this in the hardware - in which case we'll just get a single stream of variable bitrate (typically several Mbit/s for a BBC channel) - or we can have the tuner just send the whole 16Mbit/s and let the software do it.

It's a bit like ordinary networking where we can have data coming from different places (mail from your mail server, web pages from various web servers, etc) - and the packets just get queued up to be sent in turn down one bit of wire. At our end, the networking stack picks the packets apart and delivers the data to the right programs.

Takes a bit of getting used to for people used to the old analogue days when one UHF radio channel carried one program. Mind you, it also doesn't help that someone decided to call the bits of spectrum "channels" and also we call the different streams of output from a broadcaster using the same word "channel" - so now we have a situation where we use "channel" for 3 different things, the UHF frequency, the logical number that identifies a program stream (in this case 1), and the program stream itself ("BBC One") !
It does lead to confusion, and sometimes it needs careful wording to avoid confusion.