The trend towards larger-screen and HD Ready TVs merely exacerbates these problems, because such sets are more revealing of picture deficiencies.
It's interesting to compare between 544 x 576i and 704 x 576i. if you have Sky, Freesat and Freeview available in your location, switch between ITV or UKTV History on satellite and digital terrestrial. You'll notice that the satellite 'simulcast' doesn't fare as well as the higher-resolution Freeview broadcast in terms of finer detail. A bitrate guide to some of the more popular satellite channels is featured right.
Compression depression
Ofcom originally specified that digital-terrestrial services must have a resolution of 704 x 576 or 720 x 576, and some of the Channel Four and Five services break this rule. But the Ofcom rules don't apply to satellite.
Downscaling the resolution prior to broadcast reduces the amount of data that has to be compressed. Like other standard-def DVB-S broadcasters, the Sky and Freesat platforms use the long-established (but inefficient by modern standards) MPEG-2 codec.
Resolution-reduction is one of the methods used to reduce bitrates, thereby allowing more channels to be accommodated onto a transponder. A second, typically used in parallel, is to increase the amount of compression that is applied. Unfortunately, you don't get something for nothing here.
Higher compression ratios mean that artifacts like macroblocking and mosquito noise (edge feathering) become more visible with movement, stark outlines and areas of higher detail. The video bitrates used by satellite channels typically lie in the 2-3Mbps range. Compare this with the 6Mbps-8Mbps of commercial DVD, which, of course, is also based on MPEG-2.
And what of the audio? The main soundtrack that accompanies most satellite channels is 192kbps MPEG-1 Layer-2, although some channels go as low as 128kbps. The BBC, Sky One and Sky's movie and sports channels can and do carry an alternative high-quality 384kbps Dolby Digital 5.1 soundtrack when appropriate.
Pushing the limits
One benefit of economising the bitrates of SD programmes is that you can record more onto a PVR. And there's a commercial imperative, too; renting transponders is an expensive business. Freesat channels and Sky will pay Astra millions of pounds per year to use its satellites, and so their potential needs to be maximised.
The upshot is that it's not uncommon to find ten or more digital TV channels on a single transponder. Compare this state of affairs with one of the Sky-receivable BBC Astra 2A transponders (10773/H), which only carries five free-to-air channels. The public-service Beeb uses Full-D1 resolution, with average video bitrates of 3.5Mbps and 256kbps audio. As a result, the channels look and sound better.
While I'm disappointed to see some reduction in the picture quality of standard-definition satellite TV, there's some light at the end of the tunnel. Modern MPEG-2 encoders have advanced considerably since the late 1990s.
E4+1's Freeview service, for example, can deliver more-than-acceptable results at 720x576i with bitrates of between 2 and 2.5Mbps. With encoders like these, improved quality could be achieved across all current standard-def satellite channels. The real key to better image quality though, remains high-definition. And it's here that satellite holds an ace card.
With only a limited number of channels sanctioned for Freeview HD by Ofcom, Freesat and more significantly Sky, look destined to claim the high-performance high-ground in broadcasting for some time to come.
First published in Home Cinema Choice, Issue 162
