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october 2009
The channel crunch Want to see more?
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The channel crunch The picture quality on mainstream free-to-air satellite and Freeview can vary considerably as our investigation shows
© SES-Astra
In most respects, and for most viewers, the switch from analogue to digital TV has brought marked improvements. Not only is it more efficient – several digital TV services can be crammed into the same spectrum occupied by a single analogue one, thereby increasing our potential choice of viewing – but there’s also freedom from ‘ghosting’, availability of multiple soundtracks (usually for audio-description purposes), radio stations, advanced ‘digital teletext’ and PVRs with playback that has the AV quality of the original broadcasts. Then there’s the promise of HDTV; there’s already a good range of hi-def pay-TV services on satellite, courtesy of Sky, plus a few free-to-air ones. HDTV via digital terrestrial should be with us in time for the 2012 Olympics. But even as we approach the switchover deadline not everyone can get terrestrial digital reliably – especially if their aerial is in poor condition or in any other way unsuitable. To such unfortunates the ‘noise’ and other unwanted effects of poor reception is preferable to annoying dropouts. DTT transmitter powers are still low relative to 2 What Satellite & Digital TV October 2009
Toolkit Want to investigate digital TV quality for yourself? You’ll need a PC equipped with a digital tuner card, plus some of the following: n TSReader: www. tsreader.com n Bitrate Viewer: http://www. winhoros.de/docs/ bitrate-viewer/ download n DVBViewer (can be configured to record singlechannel transport streams): www. dvbviewer.com n ProjectX: project-x. sourceforge.net n Java (needed to run ProjectX choose the version that’s appropriate for your platform) www.java.com n Winamp: www. winamp.com
the analogue ones; this situation should change for the better as the analogue transmitters are closed down. Reception should become more reliable as a result, although a decent aerial installation is still recommended. There will probably be reception ‘black spots’ – these have been with us since analogue broadcasting ruled the roost – but those living in such areas can, as an alternative, receive TV via satellite. The main UK channels are now FTA and can thus be received using Sky, Freesat or a FTA receiver, thanks to the extensive footprints of the Astra 2x and Eurobird satellites. In densely urban areas digital cable represents another choice. The future may be HD but, as we discovered, broadcasters aren’t getting standard definition entirely right. There’s inconsistency between the digital terrestrial and digital satellite platforms – the upshot of which is that it may be better to watch a certain channel on Freeview rather than satellite (or, occasionally, vice versa!). Precious spectrum is being wasted through unnecessary duplication (odd, given that the desire to give us more channels has made spectrum more valuable) and use is being made of compromise technical
standards that detract from the viewing experience. Larger screens – especially HD ones – are able to resolve the deficiencies.
Fragmentation Broadcasting is a lot more fragmented than it was 20 years ago. Successive governments obsessed with market reforms have, for example, taken transmission responsibilities away from the BBC and transferred them to multiple private contractors. In contrast, the BBC originally kept everything ‘in house’ and thus had complete control over all aspects of its operations – including technical standards. Channel Four and the ITV companies – of which there used to be plenty – used to rely on the high standards of the Independent Broadcasting Authority, until it was disbanded, courtesy of the disastrous 1990 Broadcasting Act. The recession may have affected the broadcasting sector, but even the digital terrestrial platform presents us with dozens of radio and TV channels. The number has grown constantly since digital terrestrial was launched in 1998. As DAB listeners without cloth ears know, if more services are to be crammed
TV bandwidth How the tests were done We set up a number of PCs with Hauppauge digital terrestrial (DVB-T) and satellite (DVB-S) tuner cards, and used them to record entire multiplexes (‘muxes’). A mux, carried via a DVB-T channel or DVB-S satellite transponder, contains several channels. For example, the Astra 2 10.773/H transponder carries several BBC services and Five to Freesat/FTA/Sky satellite viewers, while the 481MHz Crystal Palace ‘Mux 2’ carries ITV1/2/3/4, Channel 4, More4, E4 and More4+1 to Freeview viewers. It’s thus far more efficient than analogue broadcasting, with which only one channel could be carried in the same spectrum.
TS Reader
To record these muxes TSReader software was used. We recorded the DVB-S and local (in our case, Crystal Palace) DVB-T muxes simultaneously – thus the need for multiple PCs – for eventual comparison purposes. The file sizes were enormous because they contained multiple channels and to ensure meaningful results we recorded for at least an hour. It’s just as well that large hard discs are so cheap nowadays! The next step is to isolate the individual channels from the muxes. Again, this is possible with TSReader – from each mux, you end up with several transport streams – each of which contains a single channel. Using ProjectX, we then demultiplexed these transport streams to yield MPEG-2 video (.m2v), MPEG 1, Layer 2 audio (. mp2), along with any other data (e.g., teletext or DVB subtitles). Usually there are multiple soundtracks, one of which is provided for audio description
when available. We then used a bitrate analyser to determine the peak and average MPEG-2 bitrates, plus information like resolution. The versatile Winamp program will play the .mp2 files – sampling frequency (48kHz for UK digital TV) and bitrate are both displayed. We’ve concentrated on the BBC, ITV and Channel 4 services, all of which carried on a free-to-air basis to UK viewers. The only Five service currently available in this way is Five itself. Our results are shown in the tables on page 5.
Bitrate Viewer In spite of Freeview’s bandwidth limitations, ITV1 is broadcast at a higher resolution than on satellite
into the same bandwidth then something must give. That ‘something’ is channel bitrate. Since digital terrestrial began (as OnDigital) bitrates have been shaved as more channels have joined the platform. Sky’s option of booking more transponders isn’t an option for digital terrestrial (note, out of interest, that the number of services carried per transponder has also increased since digital satellite broadcasting began). One could assume that reducing the bitrate would result in increased ‘artifacting’, including obvious blocking and edge ‘fuzz’. Thanks to improvements in broadcast MPEG-2 encoding technology during the intervening period, though, such deterioration has been less noticeable than expected. Indeed, the terrestrial transmissions of E4+1 manage perfectly watchable pictures with video bitrates as low as 2.5Mb/s. In contrast commercial DVD, which employs the same MPEG-2 codec, tends to range from 5Mb/s to 7Mb/s. Commercial PAL DVDs employ a resolution of 720 x 576i, which is known professionally as ‘full D-1’. Some UK digital broadcasters also use this resolution for at least a portion of their digital TV channels. However, other services use a lower resolution to initially reduce the data that has to be MPEG-encoded. This ‘compromise’ resolution of 544 x 576i (known professionally as ‘sub-sampled D-1’) may suffice for 4:3 material, but some broadcasters are using it for widescreen material as well. Because there’s less horizontal resolution, detail is markedly lower – especially on modern large-screen TVs. Which leads us neatly to the aforementioned lack of consistency. ITV1’s satellite service is 544 x 576i, but on Freeview it’s 704 x 576i (this is known as ‘cropped D-1’, which in subjective terms is barely distinguishable from full D-1). Given the greater bandwidth potential of satellite, one would have expected the converse to be true. Some have noticed that ITV is visibly ‘softer’ on satellite than it is on Freeview – this lower horizontal is the most likely reason.
Timeshifted services
Fuzzy logic: ITV1’s satellite picture comes off a little worse, yet there’s enough bandwidth for improvement
But what of ITV’s other satellite channels (such as ITV2, 3, and 4 and CITV)? ITV2 terrestrial also uses 544 x 576i, but, bizarrely, the ‘time-shifted’ ITV2+1 version is 704 x 576i. And so if ITV2’s televisual output is your bag, our recommendation is to wait an hour and watch ITV2+1 instead if quality takes precedence over timeliness! We asked ITV for an explanation, but it was unable to meet our deadline. Only the BBC boasts full D-1 resolution for all of its services (apart from BBC Parliament), whether satellite or Freeview. The BBC’s October 2009 What Satellite & Digital TV 3
Static images The screengrabs below are of a Staples advert broadcast at around 6pm on August 26, 2009, by Channel 4. These were resized to 1024 x 576 with Photoshop to ensure correct picture aspect ratio. We deliberately chose source images that were ‘static’ to avoid the need for deinterlacing. These images were captured using a PC equipped with satellite and DTT cards, DVBViewer (for recording the transport stream) and ProjectX (for demultiplexing and frame grabbing).
We favoured analysing static images (like this ad on Channel 4 grabbed above on Freeview and below on satellite) to avoid the need for de-interlacing
The resolution of some Channel 4 channels actually improved on satellite while we were writing this feature. Coincidence?
minority interest coverage of Parliament is the exception, being broadcast at 544 x 576i terrestrially and 720 x 576i on satellite. Sensibly, Five uses 720 x 576i for both Freeview and satellite. But its other Freeview services, Fiver and Five US, are carried at 544 x 576i. The latter channels are not yet available on FTA satellite. Channel 4 is broadcast with a resolution of 704 x 576i on satellite and Freeview. E4 is broadcast in 544 x 576i but, in an echo of the ITV situation, its E4+1 service is 720 x 576i. Both are 704 x 576i on satellite. Film4 is 544 x 576i on satellite – disappointing, given the potential picture quality of recent and digitally remastered movies – but is fortunately 720 x 576i on Freeview (as an aside, I should give Sky subscribers the bad news – its standarddefinition movie channels are also 544 x 576i). I asked Channel 4 to explain itself, but all I got was the following statement. ‘Your observations as to the resolution of our services on digital satellite and DTT are correct, and you are also correct that it is a matter for Channel 4 to determine these values to make best use of our transmission resources’. But if the BBC can use full D-1 throughout, why can’t the rest? Red button interactive services take up additional channels that do nothing of any use when such content isn’t actually being broadcast – a case in point being the BBC’s interactive streams on 12441/V (satellite). Odd duplications abound on satellite, certainly as far as Channel 4 (and Channel 4 +1) is concerned. Apparently the only differences are regional ad-break variations. Then there are the audio channels. The DVB (digital video broadcast) standard, which underpins satellite, cable and terrestrial, makes provision for multiple soundtracks. Foremost is the main (default) soundtrack that’s always present,
usually at 192kb/s – although the BBC specifies 256kb/s for everything except News 24 (192kb/s) and Parliament (128kb/s). Bear this in mind if you’re listening to live TV/radio simulcasts – Proms concerts, for example – because these bitrates exceed those of all current digital radio broadcasts.
Compromised picture The additional (‘alternative’) DVB soundtracks could be 5.1 Dolby Digital (as used by some of Sky’s movie channels), different languages (Euronews is a case in point) or – as in the case of Freeview and the majority of ‘The Big Five’ – ‘audio descriptions’ for those with poor eyesight. Sadly, many of the channels offering these second soundtracks via satellite merely ‘duplicate’ the main audio on 256kb/s or 128kb/s streams if such narrations aren’t available – which is most of the time (the BBC and Channel 4 are aiming to audio describe 10 per cent or so of their output). Others, noticeably DTT broadcasts, simply give you 64kb/s of mono silence if audio description isn’t available. On those occasions when audio descriptions are present, the 64kb/s soundtrack is ‘mixed’ with the main audio by compatible receiving equipment. This is wasteful. If audio description isn’t available why isn’t the soundtrack ‘switched off’ and the capacity redeployed for higher-bitrate video? An alternative would be to use it for 5.1 Dolby Digital soundtracks, as offered by many films and TV shows (including British coproductions). Whether this happens remains to be seen. Although many digital receivers are compatible, broadcast infrastructure would need modification. Our conclusions? Channels spend a fortune on high-quality programming and so why do they compromise picture quality? There may be a ‘push’ towards high definition but shouldn’t broadcasters make the most of standard-def first? n Martin Pipe
Definitions Resolution
Expressed here as the number of horizontal pixels (e.g. 720), followed by the number of vertical pixels (e.g. 576) and the refresh type (‘i’ for interlaced – used for all current UK standard-def broadcasting or ‘p’ for progressively-scanned).
Full D-1
A standard-definition resolution of 720 x 576i (PAL/50Hz) or 720 x 480i (NTSC/60Hz). It was named after the first system to use it widely – Sony’s D-1 broadcast digital videocassette format
Artifacting
Objectionable effects caused by lossy compression technologies like MPEG-2. Such effects include blockiness or break-up with fast movement, fuzziness around outlines and blurring. Trying to compress video at too low a data rate usually causes noticeable artifacting.
Transport stream
This bitstream contains the multiplexed (‘muxed’) audio, video and supplementary data of one or more channels, plus timing and
4 What Satellite & Digital TV October 2009
other information. The transport stream modulates a radio-frequency carrier (typically a cable/DTT channel or satellite transponder).
DVB
Digital Video Broadcasting – a set of internationally agreed open standards for digital TV maintained by the DVB Project, which has nearly 300 members. There are several of these – they include DVB-S/S2 (satellite), DVB-T/T2 (terrestrial), DVB-C (cable) and DVB-H (handheld devices). DVB-S2 and the recentlyratified DVB-T2 standard cover more
bandwidth-efficient services, including HDTV (with H.264 compression)
Kb/s and Mb/s
Used to express data transfer rates – respectively, kilobits per second and megabits per second. A BBC One satellite broadcast might have a video bitrate of 3.5Mb/s and an audio bitrate of 256kb/s. All other things being equal, channels with higher bitrates suffer from less artifacting – in other words, the higher the bitrate, the better!
TV bandwidth BBC
+ Shared capacity with CBBC * Shared capacity with CBeebies
Channels
Video
Bitrate peak
Bitrate average
Audio
BBC One London (satellite)
720 x 576i
7.1Mb/s
3.3Mb/s
256kb/s stereo (main) 256kb/s stereo
BBC One London (terrestrial, Crystal Palace)
720 x 576i
6.7Mb/s
3.6Mb/s
256kb/s stereo (main), 64kb/s mono (audio description)
BBC Two England (satellite)
720 x 576i
7.1Mb/s
3.6Mb/s
256kb/s stereo (main), 256kb/s stereo
BBC Two England (terrestrial, Crystal Palace)
720 x 576i
5.1Mb/s
2.8Mb/s
256kb/s stereo (main), 64kb/s mono (audio description)
BBC Three (satellite)
720 x 576i
7.9Mb/s
3.7Mb/s
256kb/s stereo (main), 256kb/s stereo+
BBC Three (terrestrial, Crystal Palace)
720 x 576i
5.5Mb/s
3.0Mb/s
256kb/s stereo (main), 64kb/s mono (audio description)+
BBC Four (satellite)
720 x 576i
8.4Mb/s
2.9Mb/s
256kb/s stereo (main), 256kb/s stereo*
BBC Four (terrestrial, Crystal Palace)
720 x 576i
5.4Mb/s
3.3Mb/s
256kb/s stereo (main), 64kb/s mono (audio description)*
BBC News 24 (satellite)
720 x 576i
6.7Mb/s
3.0Mb/s
192kb/s stereo (main), no 2nd audio
BBC News 24 (terrestrial, Crystal Palace)
720 x 576i
5.0Mb/s
3.6Mb/s
128kb/s stereo (main), no 2nd audio
BBC Parliament (satellite)
720 x 576i
3.8Mb/s
2.4Mb/s
128kb/s stereo (main), no 2nd audio
BBC Parliament (terrestrial, Crystal Palace)
544 x 576i
4.0Mb/s
1.9Mb/s
128kb/s stereo (main), no 2nd audio
Channels
Video
Bitrate peak
Bitrate average
Audio
ITV1 London (satellite)
544 x 576i
7.0Mb/s
3.2Mb/s
192kb/s stereo (main), 128kb/s stereo
ITV1 London (terrestrial, Crystal Palace)
704 x 576i
5.7Mb/s,
2.9Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
ITV2 (satellite)
544 x 576i
7.1Mb/s
3.4Mb/s
192kb/s stereo (main), 128kb/s stereo
ITV2 (terrestrial, Crystal Palace)
704 x 576i
4.8Mb/s,
2.3Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
ITV2+1 (satellite)
544 x 576i
4.5Mb/s
2.1Mb/s
192kb/s stereo (main), 128kb/s stereo
ITV2+1 (terrestrial, Crystal Palace)
544 x 576i
4.5Mb/s
2.2Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
ITV3 (satellite)
544 x 576i
3.7Mb/s
3.3Mb/s
192kb/s stereo (main), 128kb/s stereo
ITV3 (terrestrial, Crystal Palace)
704 x 576i
5.8Mb/s
2.3Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
ITV3+1 (satellite)
544 x 576i
3.8Mb/s
3.3Mb/s
192kb/s stereo (main), 128kb/s stereo
ITV4 (satellite)
544 x 576i
7.0Mb/s
3.1Mb/s
192kb/s stereo (main), 128kb/s stereo
ITV4 (terrestrial, Crystal Palace)
704 x 576i
4.8Mb/s
1.8Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
ITV4+1 (satellite)
544 x 576i
6.7Mb/s
3.3Mb/s
192kb/s stereo (main), 128kb/s stereo
CITV (satellite)
544 x 576i
7.3Mb/s
3.0Mb/s
192kb/s stereo (main), 128kb/s stereo
CITV (terrestrial, Crystal Palace)
544 x 576i
5.3Mb/s
2.0Mb/s
128kb/s stereo (main), 64kb/s mono (audio description)
Channels
Video
Bitrate peak
Bitrate average
Audio
Channel 4 (satellite)
704 x 576i
5.3Mb/s
2.9Mb/s
192kb/s stereo (main), 128kb/s stereo
Channel 4 (terrestrial, Crystal Palace)
704 x 576i
5.3Mb/s
3.8Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
Channel 4+1 (satellite)
704 x 576i
9.1Mb/s
2.9Mb/s
192kb/s stereo (main), 128kb/s stereo
Channel 4+1 (terrestrial, Crystal Palace)
544 x 576i
4.1Mb/s
1.9Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
E4 (satellite)
704 x 576i
6.3Mb/s
2.6Mb/s
192kb/s stereo (main), 128kb/s stereo
E4 (terrestrial, Crystal Palace)
544x576i
2.5Mb/s
1.6Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
E4+1 (satellite)
704 x 576i
6.3Mb/s
3.1Mb/s
192kb/s stereo (main), 128kb/s stereo
E4+1 (terrestrial, Crystal Palace)
720x576i
4.3Mb/s
2.1Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
More4 (satellite)
704 x 576i
6.5Mb/s
3.7Mb/s
192kb/s stereo (main), 128kb/s stereo
More4 (terrestrial, Crystal Palace)
544 x 576i
3.8Mb/s
2.0Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
More4+1 (satellite)
704 x 576i
6.9Mb/s
3.6Mb/s
192kb/s stereo (main), 128kb/s stereo
FilmFour (satellite)
544 x 576i
6.7Mb/s
3.3Mb/s
192kb/s stereo (main), 128kb/s stereo
FilmFour (terrestrial, Crystal Palace)
720 x 576i
5.4Mb/s
1.9Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
FilmFour+1 (satellite)
544 x 576i
5.4Mb/s
2.4Mb/s
192kb/s stereo (main), 128kb/s stereo
Channels
Video
Bitrate peak
Bitrate average
Audio
Five (satellite)
720 x 576i
8.1Mb/s
3.4Mb/s
256kb/s stereo (main), 256kb/s stereo
Five (terrestrial, Crystal Palace)
720 x 576i
7.0Mb/s
3.1Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
Fiver (terrestrial, Crystal Palace)
544 x 576i
4.9Mb/s
1.8Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
Five US (terrestrial, Crystal Palace)
544 x 576
5.1Mb/s
1.9Mb/s
192kb/s stereo (main), 64kb/s mono (audio description)
ITV Channels
Channel Four
Five Channels
October 2009  What Satellite & Digital TV  5