Mpeg - Killing Quality.

Not sure if this is the case - certainly I think some cable service just re-broadcast the DTT feeds at MPEG2 level. I think this was one of the causes of a cable problem when the BBC changed their statmuxing system, as it meant some services went at too high a data rate for the cable system to cope with.

Well if you want bad MPEG compression you should have seen Eurosport back in the days of ITV Digital. They overloaded their multiplexes did they not.

Yep - ITV Digital tried to squeeze too many channels into each 24Mbs 64QAM mux - meaning even with Statmuxing the quality was very low. The BBC/Crown Castle moved to 16QAM - which only provide 18Mbs data capacity - but also radically reduced the number of channels carried in each Mux. I think the Beeb limit to 4 full screen channels in Mux 1, with 4 and a bit in Mux B.

ISTR that ITV Digital were squeezing 7 or 8 into each Mux?

That's about right - they really were very unstable and blocky in the slightest of bad weather. There are the tales of people opening fridges doors and the signal going aswell.

What does ISTR stand for again?

I seem to remember...

That was not directly to do with the number of channels being compressed into a mux. It was more to do with the lower power, and less robust 64QAM system used.

ITV/C4 and SDN/Five still use 64QAM - but in common with all DTT broadcasters they have been allowed to increase their transmitter power in many areas. This has meant that more people get a better signal, so receivers are less susceptible to impulse noise (like poorly suppressed electrical equipment - fridge lights, thermostats, pressure washers etc.)

Yes, when the effects of analogue interference are removed, digital looks better - until you look closely and see that the effects of overcompression are much worse.

I remember when we had Sky Digital installed, I initially thought the pictures looked stunning, no more sparklies or the poor colour saturation introduced by VideoCrypt decoding. Then when I actually sat down and watched something properly for the first time - en episode of Cheers (a set which heavily uses wood panelling really doesn't look too great on digital TV) and then I could see it all, blocky images, slight background movement being displayed as a static image jumping about 2 points, it was all there. By the end of it I wondered quite how this could be considered progress when a few analogue sparklies and (very slight and very occasional) ghosting on terrestrial channels are imo far less annoying.

As was said, there's nothing particularly bad about MPEG2 (DVD's really do look brilliant), it's just that it can't be compressed as much as broadcasters want and still deliver decent quality. MPEG4 can compress a full length film at a resolution of 512x288 with full frame rate into just over 700MB which (at it's native resolution) looks better than the average digital TV channel. MPEG2 could not hope to achieve that - it can either deliver high quality or high compression, not both. When MPEG4 can do both, changing to it (or a successor format) seriously needs to be looked into.

Incidentally, is there a reason why there isn't an MPEG3? Was it a new format which didn't get off the ground, or did they jump straight to 4 to avoid confusion with the MP3 audio format?


You should have seen Granada Plus. A digital channel playing material originated from noisy old PAL video really needs a decent amount of bandwidth, and it didn't get it on On Digital. Particularly in the final days when they had squeezed 6 or 7 channels onto 1 multiplex, there were times when it really did look like a Real Video clip played full screen.


It can also stand for 'I seem to recall'

Yep - though your MPEG4 example is cheating slightly - as 512x288 source images are likely to be progressive (or single field sourced from 576 line video?) rather than interlaced and thus a little easier to process. I suspect that this also means they are running at half the field rate (25fps rather than 50fps) ? This isn't an issue with 25fps (24fps shot) sourced film material - but it is for broadcast video which has 50fps temporal motion. Much MPEG4 material still seems to struggle slightly with interlace - but I think this is not the algorithm - more the users fault.

Of course for a given data rate a lower resolution picture will always look better anyway... 512x288 is less than half the resolution of a 720x576 or 540x576 digital TV channel.

DVDs also look better than broadcasts using similar MPEG2 technology (as do many MPEG4 sources) for a number of reasons. They are often processed in a number of passes, to optimise the encoding, something not possible with live broadcasts. DVDs are also increasingly being sourced from high resolution film scans, or HDTV transfers, rather than 720x576 telecines. MPEG2 achieves significantly better results if a higher source resolution is used as the motion tracking improves, as there is more picture detail to correlate with, and any noise is often above the aliasing frequency of the output stream

MPEG3 did exist as a working party - it started at roughly the same time as the MPEG2 group I believe - and was aimed at HDTV compression. Both groups were building on the original MPEG - renamed MPEG1 - spec. It became clear that the requirements for high quality SDTV and HDTV compression were not that dissimilar - so the two groups rolled their standards into MPEG2, and MPEG3 didn't really happen.

This is why ATSC in the US, and the DVB-HD systems are based on MPEG2 at higher resolutions.

(Roll on Euro1080 - currently testing 25Mbs 1080/50i transmissions on Astra 1 at 19.2. When I get my dish pointed the right way I should be able to receive their FTA loop on my PC!!! I believe that one of the two services is likely to remain FTA - and they are also experimenting with HDTV MPEG4 using Windows Media 9? )

I had begun to accept the compression artefacts on DTV, and had resigned myself to 'creeping flesh' syndrome on actors' faces and shiny or checked suit jacket material turning into something that looks like it's covered in animated fluff.

Then, I watched analogue TV for the first time in months at my Grandma's house the other day, and it was such a breath of fresh air to see sweeping pans, shiny curtains, detailed objects and other such things updating fluidly and seamlessly on the screen.

Apart from the obvious downside of colour subcarrier, analogue PAL is soooo much better for a clean, fluid picture. Of course, it hinges on decent reception, but then, even ghosting (which my own analogue reception suffers from) causes less headache and perceptive 'forgiveness' because it's applied uniformly to the entire frame, rather than at 'pinch points' randomly distributed around the frame, which inevitably occur around the exact bits of the image (i.e. the moving bits and detailed bits) which your brain automatically notices more than the background.

I think that you aren't fairly comparing like with like. If you allow the same amount of RF bandwith for a DVB-T signal carrying MPEG2 as required for a single analogue PAL signal then you would have 18-24Mbs available to encode a single SDTV signal. This would allow pretty nearly lossless transmission (50Mbs I-frame only MPEG2 is pretty much agreed to perform as well as DigitalBetacam - which most shows are now edited to, and if you use B- and P- frames as well you can improve compression) The Euro HD channel is using 25Mbs to carry HDTV pretty well.

I agree that MPEG2 as used on DTT is overcompressed - but that is not a fault of the system, more a fault of the compression rates used to broadcast extra channels.

So maybe it's not a fair technical comparison but he is comparing like-for-like given that they are both forms of domestic TV transmissions. If Joe Public goes out and buys a massive 69" Plamsa telly only to discover that when he gets it home the allegedly soon-to-be-turned-off analogue picture looks better than digital then he's not really going to care that if they used the full bandwitdh it would look good.

There's a lot of talk about this HDTV satellite, but what channels are actually going to be on it?!

Exaaaaaaactly !