Sky’s HD platform hampered by STBs1 June 2006
BSkyB’s new HD service got off to a quiet start last week, because set top boxes weren’t delivered to it on schedule: it is estimated that more than a third of the 40,000 subscribers reported to have pre-ordered will have their installation delayed, writes David Fox.
However, initial Sky HD reviews declare that the viewing experience will be well worth the wait. This will be encouraging news for Chris Johns, BSkyB’s chief engineer, who spoke at the recent HD Masters conference in London of the need to deliver the best possible pictures to justify Sky HD’s position as a "premium platform".
It launched with nine channels, including Sky One (with programmes like 24), Sky Sports HD, Artsworld, Discovery, National Geographic, two movie channels, and Box Office, which offers HD movies at the same time as they come out on DVD. It is also carrying the BBC HD trial service. Sky has chosen to use progressive for movies and interlaced for almost everything else, but he said of 720p and 1080i: "There really is very little difference between the two of them." On a 1366×768 display (most LCD panels), he believes that 720p looks slightly better, whereas 1080i, naturally, looks better on a 1080 display. However, overall, "I feel that 1080 shades it," he said. There are bandwidth advantages for 720p, but these are marginal. "You only really see those errors [compression artefacts in 1080i] when it is highly compressed."
If it encoded HD in MPEG-2, it would need 12 to 18 Mbps per channel. With MPEG-4 (H264), it expected that it could encode each channel in 8-12Mbps. However, it is currently more like 12-18Mbps, and Johns believes that encoder manufacturers are over promising the bandwidth benefits of H264 at the moment, although there is scope for greater efficiency in future.
Because there is sufficient bandwidth on satellite, it can cope with the less efficient compression of interlaced to deliver 1080 services, but Sky is looking at doing more with 720p in future to deliver more services in the same bandwidth.
The set top box can cope with either 1080i or 720p, can do up and down conversion, and contains a hard disk recorder.
Sky is acquiring on HDCAM. "You’ve got to make sure that the quality is high at the point of acquisition, to ensure it’s high at the destination." If material is shot on a more highly compressed format, such as HDV, transferred to HDCAM, then Avid during post, and then encoded for transmission, the effects of concatenation will be visible. "You are going to get degradation all the way down the chain. You have to start with the highest quality," he said.
For mastering, Sky uses HDCAM SR, to limit any compression effects, as he maintains that HDCAM starts to degenerate after about five or six generations. SR also allows it to maximise the number of audio channels, especially for sport.
It has upgraded a sports studio for HD and SD production in 1080 or 720, using Sony 1500 cameras, which are 1080p 50 capable for possible future use.
For outside broadcasts, Sky has some 11 HD production trucks, with more on the way, with about 250 HD cameras. Out of about 1,000 OBs it expects to cover this year, more than 400 will be in HD. A key problem for most broadcasters speaking at the HD Masters conference was audio synchronisation. To counter this Johns has instigated a zero tolerance policy. They have gone through the entire production chain to ensure that lipsync is optimal throughout, so that there is a zero difference on transmission. It is not good enough for it to be on spec, he said, because errors build up to become noticeable during transmission.