For FreeView which appears to use Stat Muxing do they channels have a guaranteed amount of bandwidth? Here in the US our networks like to have around an average 12-16Mbps while most of SD subchannels ideally would like 4 Mbps but will settle for no less than 1 Mbps (this is all ATSC 1.0 and MPEG-2]. There are some stations thanks to the spectrum auction are carrying 2 1080i feeds + 2 SD feeds - the 1080is average around 8Mbps each and SD around 1Mbps each.
The UK and US OTA systems are based on two totally different concepts.
In the US each station has a 6MHz channel of its own - which it then carves up between channels. Each 6MHz ATSC 8VSB mux can carry 19.2Mbs of video and audio - and MPEG2 is the dominant codec for SD and HD. The system was originally designed to allow one 1080i or 720p HD service, to mirror the existing 6MHz NTSC-M SD analogue service a station was allocated, though the system allowed for a reduction of bitrate to allow for additional SD sub-channels (and as encoders have improved, more/HD subchannels became an option)
In the UK the BBC were given one 8MHz DVB-T mux (later a second was gifted), iTV and C4 shared a DVB-T mux etc. - so there isn't a direct mapping from analogue channels to digital muxes.
The 24MHz BBCA mux (aka PSB1) carries BBC One, BBC Two, CBBC, CBeebies/BBC Four, BBC News Channel, BBC Parliament, BBC Red Button video and BBC Radio services (plus BBC Alba and BBC Scotland in Scotland) These are all 16:9 SD MPEG2 services.
ITV/C4's mux (D3/4 aka PSB2) is similar.
The second BBC mux, (PSB3 aka BBCB) switched to DVB-T2 at 40.25Mbs and carries BBC One HD, BBC Two HD, CBBC HD, ITV HD, C4 HD, C5 HD - all 1080i h.264 (which is a lot more efficient than MPEG2). The BBC handles coding and mux for this mux - with ITV, C4 and C5 paying for carriage (but the BBC don't use this to make money)
Everything is statmuxed - with a pretty equitable system of everyone averaging roughly the same bitrate of 4.5Mbs with a peak of 16.5Mbs
is useful to see how European DVB services operate.
You'll see Germany is using DVB-T2 but with h.265/HEVC and 1080p50 (not 1080i25) and with even lower average bitrates. The US system is using a 20+ year old codec - until ATSC 3.0 hits that is (if it does)
What is transmitted on BBC One and ITV during opts?
Transmitted - i.e. what is sent from the transmitters to people at home? On BBC One and ITV SD you get your local regional news. On BBC One HD you will get a slate telling you to switch to BBC One, followed by a BBC promotional barker reel. ITV HD carries some regions (but I think you may not get the correct region on ITV HD and have to switch to SD to guarantee this)
If you mean distributed to each station (rather than transmitted) - then the BBC these days usually carry the News Channel on their network feed for the longer opts on the feed send to each regional centre - which they opt away from. This is known as 'Network sustaining' and is designed to be passed on to the transmitters should a region fail to opt-out.
ITV don't run an opt-out system - so they don't have to send ITV to each local studio, instead the local studio output is fed to ITV Playout North and South, and they cut them into the correct transmitter feed at the correct time. As coding and mux is now centralised for transmitters this makes sense.
Is it just color bars or in the case of BBC One the tune to SD Barker? In the US during commercial breaks some networks may transmit nothing but I think NBC and ABC Show an animated logo slide. During local time it’s usualky just black with some sort of channel identification or color bars with the number to NOC.
US stations run with a central network feed sent to affiliates for local modification. UK commercial broadcasters no longer really do that as they are all played out centrally (though STV in Scotland hang off an ITV feed I guess)
Finally, when people discuss a TV show do they say I saw it on BBC One or just BBC?
Either 'I saw it on BBC One' or 'I saw it on THE BBC'
Unlike US three-letter broadcasters (ABC, NBC, CBS, PBS), the BBC takes a definite article, but ITV and Channel Four don't...
"I saw it on THE BBC" "I saw it on ITV"...
(Yep - doesn't make sense...)