I definitely know Fox US fires trigger pings to affiliates for things such as local watermarking.
Yes - Fox is the odd-one-out in the US markets as it provides affiliates with a pre-encoded 'network' feed rather than a high bitrate, high quality mezzanine 'contribution quality' feed. The Fox stations all have 'splicers' installed.
These splicers take the pre-encoded MPEG2 network feed, and do a very clever partial decode to insert a local bug, by decoding just the macroblocks in the stream that need to be decoded and re-encoded to do so. The rest of the picture just passes straight through and is not decoded and recoded (avoiding further compression artefacts) This also allows Fox to tightly handle 5.1 Dolby audio on their network shows I believe - which some other networks historically struggled with (or more accurately their local stations did)
The Fox splicer also has an MPEG2 encoder for local material played out from the station (commercials, local news etc.) which is synchronised to the splicer's incoming network feed, allowing clean junctions to and from network. This gives Fox tighter control, mandates a uniform bug insertion system that can be remotely triggered etc.
This system was introduced when Fox switched from running a 480i 16:9 SD network feed - which was encoded at the local stations in 480p - and introduced 720p HD. (Fox - unlike ABC, NBC and CBS didn't initially run HD on digital OTA. Their 480p was 'good enough' for them)
This was the situation a few years ago - I think Fox may have since upgraded their splicer tech further.
ABC, CBS and NBC work differently and distribute a higher quality network feed BUT this is then permanently decoded to baseband video and then re-encoded after playout master control in each station. This requires more equipement and personnel than Fox stations I believe.
Bingo, I've had it explained to me before and it's gone in one ear and out the other, however your explanation is roughly what I've heard previously. It's seriously impressive tech.
What I always wonder is how local stations seamlessly merge their local news 'previews' into a national network ECP
, along the lines of the preview BBC One viewers are treated to at 5:56pm before the Six. I know local NBC affiliates have been doing that since the late 90s, and it always looks so slick.
You can see the Fox Splicer equipment details at www.hdrollout.com
NBC distributes their signal as a 50 Mbps MPEG-2 signal. Per the public affiliation agreements on the FCC website they expect it to average around 12-15Mbps out of 19.39Mbps ATSC signal. With better encoders I think they expect a 10 Mbps average - allowing them carry another 720p channel and maybe an SD channel.
All the networks have equipment installed allowing them to switch the correct channel change to automatically for special programs or sports or the local station can handle it and triggered commercials. Part of the hardware that's used for logo insertion, severe weather tickers, the tickers during the morning shows is manufactured by EVERTZ and for NBC it's called NameDropper HD and CBS is LIDIA (not sure about abc). The functions of this hardware is triggered by the network. The tickers itself can be edited in ENPS
or a simple text file where every new line is interpreted as a new headline with a divisor such as •. Some stations like Hearst have several Viz Engines at their use and they use it for snipes during programs, a much better and visual emergency alert system (better than just a line on screen), and closing. Additionally my NBC I think has coded it to have the station logo on screen in the corner where other NBC stations logos appear and disappear.
The previews you are alluding to are likely a type of soft opt.
NBC doesn't do the ECP ends where they can superimpose video above the credits but some ABC stations do. The stations have the exact timings and the dimensions of video feed so they just shrink it to fit.
NBC for example ends their primetime exactly at 10:59:30 and cuts to black and affiliates can't start early given their way of playing out the end of show credits overtop a full screen promo.
I don't watch ABC often but I think they do the same but they may have a coverable promo for video above the credits like a ECP.
For CBS I don't think would be possible as the ECP credits show a preview in a U type shape with static pillar bars on the side, and in them 4:3 safe zone there's a bottom graphic running the credits and above it in a shrunken 16:9 preview feed appears. Additionally CBS goes right up to the top of the hour with another ad break.
Hope that makes some sense.