Is satellite TV picture quality up to scratch?

26th Oct 2008 | 10:00

Is satellite TV picture quality up to scratch?

Satellite delivery not always a guarantee of best picture quality

Satellite broadcasting has revolutionised UK television.

By providing an ever-growing choice of channels capable of fulfilling practically every whim, the technology has set a standard that rival broadcast platforms have struggled to compete against. But as we reveal here, satellite delivery is not always a guarantee of best-in-class picture quality.

Historically, satellite, and Sky in particular has often been at the forefront of key AV developments in the UK. By opting for old-tech PAL over then rival MAC, Sky got its service up and running with inexpensive receiving systems. Yet it was also the first broadcaster to champion HDTV.

Sky's 20-year reign as the nation's first-and-foremost satellite TV provider has seldom been anything other than groundbreaking from a technical standpoint: it pioneered interactive services like home-shopping and made Pay-Per-View a real alternative to movie rentals, as well as providing expanded coverage of major sporting events.

But for home cinema addicts, the real value of Sky Digital was in its widescreen potential and improved AV quality. The horrendous 'mush' and soft pictures of the original analogue service's VideoCrypt scrambling system, so noticeable on the bigger screens that were becoming popular with AV enthusiasts, were replaced by sparklie-free clarity – especially if you were using RGB Scart – while the digital service's sound quality stood head and shoulders above the old companded FM system.

But, ten years on, just what is the state of satellite TV in the UK? Specifically, how does the picture quality available from Sky and its newer rival Freesat compare to terrestrial digital TV service Freeview?

Like Freeview, Freesat is a no-subscription alternative to Sky, albeit one which offers a mix of hi-def and standarddef channels. It takes advantage of the same transmissions as Sky, allowing viewers to interchange Sky and Freesat equipment using the same dish.

So, broadcasters now have two high-profile platforms to court viewers, and both service providers offer a high-definition channel selection. But has the drive to expand viewing choice compromised the quality of standard-definition channels?

Even on satellite, spectrum is a finite resource and so something has to give if more services are to be accommodated. By shaving down bitrates, each of the satellite transponders can accommodate more channels – including the multiplicity of home-shopping channels, dating/chat/astrology services and those '+1' time-delays.

On the other hand, can improvements in encoding technology help maintain satellite's image quality?

Channel discrepancies

In the beginning, popular Sky services like Sky One specified a resolution known in broadcast circles as 'cropped-D1' (704 x 576i), which is close to DVD's full-D1 res of 720 x 576i.

Clarity was generally excellent. But a couple of years ago I noticed that frequently-repeated and thus familiar Sky One programmes like The Simpsons and Al Murray-vehicle Time Gentlemen Please seemed to have taken on a 'softer' appearance when viewed on my largescreen TV. It transpired that this channel had moved to sub-sampled D1 (544 x 576i); and the lower horizontal resolution was having an inevitable impact on visible detail.

Also using this resolution are other popular channels, including Paramount and UKTV Gold. But some are even worse. If you enjoy the documentaries that form the mainstay of Discovery (sharks and Nazis, generally), you may be surprised to learn that the channel is broadcast with a resolution of just 528 x 576i. I'd argue that such resolutions are barely acceptable for 4:3 material, but they're often used for anamorphically-squeezed widescreen fare – including the movies that subscribers often pay a premium for.

What's more, some satellite broadcasters using the Sky and Freesat platforms have been slow to adopt widescreen playout hardware, and insist on broadcasting selected 16:9 programmes in 4:3 'letterbox' format, as opposed to carrying them in anamorphic widescreen. Virgin famously did this with The Sarah Connor Chronicles, while Sci-Fi made a 4:3 hash of Season One of Heroes.

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 inHome Cinema Choice, Issue 162

Now read Freesat vs Sky: how to make the right HD choice

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