This question is mostly aimed at Brits who have digital TV. I’ve had it for a while now, through the aerial in my old house, and via cable in my new one. However, I can still receive the good old analogue signal, too.
The other day I was watching TV - the episode of University Challenge where the journalists beat the politicians 225-25 or something - and noticed that there were really annoying “shimmery” patterns in areas of fine detail, eg people’s hair. It didn’t look like the normal moiré pattern that you get usually - it was much more noticeable, almost like lo-res computer graphics. I’ve noticed it before on digital - both on hair and on smoky areas on music performances, etc - but not to this extent.
So I switched over to analogue, and hey presto, it disappeared. Perfect picture. So, my question is this - why are we being sold digital TV as a great technological step to the future, when the quality is demonstrably worse than a decent analogue signal?
Am I right in thinking that the shoddy quality is down to over-compression, in order to squeeze as many channels as possible into the bandwidth? Or is it inherent in digital TV technology? Or is there something wrong with my set-up and it looks fine to everyone else?
I don’t know what the problem is. But it should be metioned that there’s a difference between data compression and data reduction. And it’s usually (but not always) the latter that causes quality problems…
r-k If you go to the following discussion forum you should find plenty of people to answer any technical questions about digital television. You have to register but, once on there ,you will find it a very useful site not only for technical questions but also all aspects of digital television. :-
Every digital signal I’ve seen (not counting HDTV) in the US has been ludicrously overcompressed. I’d wager that’s what the problem on this particular UK show, too.
The answer in the case of BBC signals carried as part of the BSkyB package on Astra is that they have come to a compromise between the number of TV channels carried on each transponder (typically around 6) and the quality. The way this was determined was by reducing the quality until the number of complaints reached a certain level - they figured that it was still good enough for the 99% of people who would never notice.
As you increase the number of channels, you reduce the bandwidth available for each one, and the quality reduces.
Typical visible effects are:
diagonal boundaries of high contrast show a pixellated or staircase effect
fast moving images (such as in a football game where the ball is kicked a long distance through the air, and the shot pans rapidly across the crowd) break up, sometimes dramatically
A strobe light (this often happens on Top of the Pops) causes the picture to break up badly.
The last two are examples of a more general effect where a frame is so different from the previous frame that there is not enough bandwidth to carry the new information.
I live in the US and have noticed the same thing- in large areas of light or dark with small tone variations, I get splotches which look like low-quality JPEGs. I noticed this when I first got digital cable (I got it because the non-digital cost just as much but I couldn’t get TCM). I became convinced that I wasn’t hallucinating when I watched The Others on Showtime (all that fog gave me plenty of time to look at the effect.
Correct. Just to add some more detail: Digital TV is broadcast using a compression technique called MPEG (Moving pictures expert group). The way it works is that the first frame is sent in its entirety. After that only the difference to the previous frame is transmitted. MPEG works best for stable pictures where not much changes from one frame to the next.
Disclaimer: The above explanation of MPEG is somewhat simplified
Well this seems ridiculous. Doesn’t that defeat the purpose? Where are the benefits, and for whom?
I hear that when they decide to go completely digital at last, they’ll shut down all analogue signals completely so we’re forced to make the change if we want to watch TV - that sucks if the image quality is going to be inferior from then on!
Although there is some loss of quality from compression, it’s minor compared to analog broadcast TV. Also, the quality is exactly the same for everyone, even if they’re sitting in a basement and using a coat hanger for an antenna.
The broadcasters or cable operators can control the quality by changing the amount of compression. If people are willing to pay a few more bucks for crystal-clear channels, then companies can offer that by using more bandwidth per channel or using better compression hardware (to give more bandwidth to the parts of the picture that need it more).
One advantage of digital broadcasting is that you can get more stations on the same bandwidth. The system used on terrestrial digital in the UK means that there are either four or six stations on each channel ( or multiplex ) plus some radio and text output.
Digital is a much more “sturdy” means. I work for a cable company and, from a technician’s view, analogue sucks. With an analogue signal, almost everything interfears with your picture. Digital has two basic problems. Tiling (little boxes) or no picture. With ana, you get snow, sparkles, humbars, vertical/horizontal lines, ect… a major pain. Plus the bonus of offering more channles on digital (bandwith issues) means more mind numbing reality programs to watch. As for the big switch… The FCC has made it mandatory that after Jan 1, 2005 all broadcasts will be digital. Don’t freak out, this does not mean you will have to have a Digital t.v.
I have also noticed and been annoyed by compression artifacts. Apparently different channels broadcast at different bitrates and resolutions, so quality can vary widely among them. According to this FAQ from dvdrhelp.com:
- and noticed that there were really annoying “shimmery” patterns in areas of fine detail, eg people’s hair. It didn’t look like the normal moiré pattern that you get usually - it was much more noticeable, almost like lo-res computer graphics. I’ve noticed it before on digital - both on hair and on smoky areas on music performances, etc - but not to this extent.