[AndyFurniss]

I don't know what I am talking about really :-)

Not sure I do either :-)

Possibly working with decoded raw video is just easier from a generic design POV.

Yes - definitely - and works with any codec.

AIUI mpeg don't specify anywhere what an encoder should do (apart from producing a compliant bitstream). Which means variety between encoders then extra variety between mpeg2/h264/VC1/whatever. Perhaps for specialised silicone it's just far more robust to work with the decoded output.

Yep - and I think deinterlacing is not a high priority for many designers who are more interested in progressive content (interlacing is slowly disappearing - but it will still be a while before it has gone completely)

For TVs they are going to be doing far more than de-interlacing anyway. I recall reading of phase plane correlation as being one way motion interpolaters worked, but don't know anything about Alchemist or Quasar.

Phase Correlation is a motion detection algorithm. Unlike pure block-matching it can also cope with scaling and rotation - not just linear translation. It is often used with block matching though ISTR. https://en.wikipedia.org/wiki/Phase_correlation

It's extensively used in high-end broadcast frame rate converters - such as the Snell (now SAM) Alchemist range of standards converters. For near-transparent conversion between 50 and 60Hz TV systems you need very high quality motion detection to allow frames to be interpolated with minimal artefacts. Alchemist is still the gold standard for this. Quasar is the equivalent up converter which takes SD interlaced video and up converts it to HD. Deinterlacing to maximise the vertical resolution is a key part of this, and AIUI phase correlation is used to ensure the highest quality interpolation of the missing fields in an interlaced signal.

Alchemist : https://s-a-m.com/products/alchemist-ph.c-hd/c-24/p-209
Quasar : https://youtu.be/oltaH2Ez82g?t=120
Quasar : https://www.youtube.com/watch?v=7kZGZeCOsL4