ITV Digital and News Corp's NDS

ON/ITVDigital and the rest of the UK DVB-T system launched with 2k 64QAM modulation. When Freeview launched they switched the BBC and Freeview muxes from this to 2k 16QAM modulation - which was a lot more robust (though delivered less data in the same bandwith) - so this improved coverage areas and allowed less-good aerials to receive the picture. They also reduced the compression a bit, so the bitrate for each channel improved (and encoders had improved as well by then). ITV/C4 and Five/SDN kept with the old transmission scheme - and so for a long time those 2 muxer were trickier to receive in some cases than the 4 BBC and Crown Castle Freeview muxes.

When the digital switchover kicked in we all switched to 8k 64QAM - which delivered similar bitrates to 2k 64QAM but is more robust and at the same time transmission power was increased (as there were no analogue services to interfere with)

I'd say that since the earrly 00s SD picture quality has dropped though. Off-air recordings I have of BBC Three from 2003 look a lot less artefacty than the channel does today for instance - but then we now have HD services, and I guess they want HD to look better than SD

My experience of IPTV in the UK is that it is pretty basic in picture quality terms. Sky/Freesat and Freeview are similar for SD services - though ITV, C4 and Five were able to broadcast at lower resolution on satellite than on Freeview for a long time (they are legislated to be full res on Freeview but that doesn't cover satellite) - but they've since upped their satellite resolution ISTR. Sky dropped the resolution on some of their SD services (from 720/704 to 544 x 576) when they launched HD I believe - again to make the difference between SD and HD more marked.

As for your spoiler - not sure what you mean. It's a standard term in UK English.

I agree, I actually think the best picture quality on DTT was C5 in 99/early 00s, it took a huge
dive when SDN changed their mux/coding arrangements in 2002ish ?

Ah yes - when they switched to the dreaded very Long GOP encoding - and wound the bitrate down hugely. Went from looking great to dreadful.

My country's terrestrial TV always broadcasts in HD. (Terrestrial multi channels like Freeview in UK is yet to start, though. )

In the late-90s/early-00s when most countries started introducing digital terrestrial TV - some introduced HD (US, Australia, Korea etc.) whilst others stayed SD (though many went 16:9 SD). This was all based around MPEG2 compression (now very out-dated) and first generation modulation (ATSC 8VSB and DVB-T COFDM mainly)

This was because HD production in the late-90s was still incredibly expensive - and many countries had already begun to upgrade to SD digital component production (a significant improvement over analogue composite) and most countries couldn't afford to switch to HD that early. (Australia was a notable exception - but native HD production wasn't widespread once their transmissions launched - so most HD content was US imports...) Japan launched their ISDB-T (using HD MPEG2 for broadcast TV) in the early 00s. Because MPEG2 and ATSC/DVB-T/ISDB-T were relatively inefficient it was really only feasible to carry one decent quality 1080i service in a single mux, possibly with an additional couple of 480i/576i SD services. You could just about do 2 x 720p services in a single mux if you were willing to accept sub-par, but probably still watchable pictures.

Some European countries switched later (notably France, Ireland and Norway), as did NZ, so they were able to adopt the newer H264 video compression system (which is a lot more efficient) for HD broadcasts, or for SD and/or pay-SD. (Some countries kept MPEG2 SD for free-to-air broadcasts, but used H264 for encrypted pay-TV stuff) However these countries retained the first generation DVB-T modulation system. H264 compression, and 720p video, allowed 2 or 3 HD services to be potentially carried in a single mux.

Sweden, the UK and a few more countries waited for DVB-T2 (a more efficient modulation scheme) to be developed (effectively it was pushed heavily by the UK) before introducing H264 and HD broadcasts, allowing us to use spectrum to carry more HD content. This has allowed 4 or 5 services in reasonable 1080i quality to be carried in a single mux.

Yes - I remember there was a time (shortly after launch on Freeview IIRC) that E4+1 looked _A LOT_ better than E4 on Freeview.