Digital quality


On a normal television, you will generally experience the same level of quality as you would get from a budget DVD... Not as good as the good DVDs, but far better than an off-air signal. Marginally better than analogue satellite, and certainly no sparklies.


Now the problem comes if you have a bigger TV, or your work with computers makes you 'sensitive' to the compression artefacts. Once you know what they look like, you'll see them all over the place - most noticably on the FTA music channels and (sadly) The Horror Channel, as is shown in the following picture:

Visible JPEG artefacts

You see, digital television is supposed to make things a lot better. For us, the viewers. At least, that is the theory!

In practice, on the Sky platform, it seems that channels with clout have good quality pictures. You don't often see artefacts on BBC. I would be very surprised if you had any at all on Sky's movie channels. Meanwhile the freebie offerings (Reality TV, The Vault, Classic FM TV, THC) have artefacts. It isn't usually visible until the picture starts moving quickly - if you're watching a NASCAR wreck on Reality TV, you can't help but notice the blocking. MPEG works using a mosaic of blocks, and in the fast movement, you can see these blocks.

Why does this happen? I am not aware of a similar thing on the 19.2°E transmissions. On an earlier version of this document I discussed the patterning inherent in using a mathematical computation over an analogue signal, and to this end I will show the original two images:

Analogue example.   Digital example.
On the left is the analogue picture, Mario Bartiromo co-hosting CNBC's "Closing Bell" programme.
On the right is the digital picture, Estibalitz Ruiz de Azua on Euskadi Irratai TeleBiste.

Now it is easy to spot the difference between the analogue and the digital. The analogue is soft and sharp contrasty divisions blur slightly. Estibalitz's yellow pullover, against the dark background, is a harsh contrast and the digital reception reflects this. Additionally the 'noise' in the analogue picture is random. In the digital picture, it forms subtle patterns. As I type this, This Week is on BBC 1 and I can see the same patterning there (but you'll have to look damned hard!).



So what is going on? One of the main answers is to look at what is going on behind the scenes:

The bandwidth has to be shared between all of the channels on a particular transponder. The MPEG information is sent as 'packets' and the digital receiver gets all of the packets. It must sort out which packets go to which channel, so it can dispose of the many unwanted channels and keep the good stuff.
Now if you have seven channels, as in the second list, each can have nearly 4 Mbaud.
But if you have ten channels, as in the first list, this drops to 2.7 Mbaud.
An average DVD streams data at 4Mbaud to 7Mbaud (some less, some more).
This is a very simplistic example (DVDs do not need the enhanced error correction that satellite TV requires), but it goes to show that only a few more channels gives you quite a reduction in bandwidth. Lower bandwidth, lower quality.

Ever used a streaming webcam (or WebRadio) on ADSL?
Tried it on ISDN?
Tried it on a dialup, anything over 28k8?
It is the same basic idea. To get the same thing down less bandwidth, you have to compromise and accept a drop in quality.


This becomes an issue on the SkyDigital platform due to the vast tonnes and tonnes of channels, most of which the average viewer won't see. Got money to blow? There are so many direct-sell channels that I'm not going to list them.
Need God? Many channels will coax happiness back into your spirit, provided you are willing to part with some cash.
Are you of 'Indian' origin? As you can see above there are a number of channels for you too. I think Urdu and Tamil and Punjab and Bangla are languages. Don't quote me on that though...



It is a horrible double-edged sword. You see, the easy solution is to ditch the unnecessary channels to free up space for the desired channels - though I notice a number of transponders are active but not carrying anything the Digibox can 'see'.
The hard part of the easy solution is in defining what is a ditchable channel. I would nominate all of the 'freaky Americanised religion' channels, and randomly half of the direct sales channels, and anything that broadcasts 'BabeCast'. But, then, somebody might actually get fulfilment out of watching 'Nation217' and decide that a channel showing nothing but a bunch of horror movies sounds dumb. Obviously I'd disagree.

So on the one hand, unless you are willing to pay the premium, you will often come away with a worse picture than you'd have seen on analogue.
On the other hand, you can take your pick from more channels than you could possible remember, something totally impossible on analogue (even Sky analogue!).


I was going to say that I think the freebie channels get a bit of a raw deal, but Sky even treats some of its minority channels badly. Just look at 11817 V 27500 2/3!!!

In general, I think digital methods are a good move forward. If we compare DVDs against videos... well, there isn't much of a comparison there, really!


And finally, a little bit of fun.

This is Leslie Laroche. She is presenting the results of the NASDAQ on CNBC Europe... not as good as they'd all like to see, but much better than the day previous where pretty much everybody suffered "P/E compression".
Thanks to digital technology, and a less-than-perfect signal, we can watch in mock horror as Leslie mutates into...

Animated GIF, about 380K!
...a Buffy vampire? That thing from Aliens? Ruby Wax?
Oh my God! It's Sharon Osborne!!!

And that, obviously, with plenty of apologies to Leslie, the poor person who happened to be talking as I flipped to CNBC Europe!
I feel kinda guilty for doing this to her (turning anybody into Sharon Osbourne is a cruel and unusual thing to do!), so here is a picture of her as she really is...

Leslie, normally; JPEG 12K


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Copyright © 2005 Richard Murray