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As evidence of how far in the other direction Sky's HD film broadcasts can go, probably the most depressing example is James Cameron's Titanic. Every time I've checked out this film on Sky's HD channels, it has invariably looked so soft it's hardly any sharper than standard definition. What's worse, any underwater shots are practically ruined by excessive amounts of MPEG encoding noise, taking the form of either large picture blocks, or smearing. This is symptomatic of a film that's either not being encoded into HD with a high enough bit-rate, or of a film that's not being broadcast with a high enough bit-rate.
Therein lies the difficulty of knowing for sure where the cause of Sky HD's inconsistency lies. For without a Blu-ray version of Titanic to compare with, we just can't tell for absolute certain if the fault with the Titanic broadcasts lies with Sky not giving it enough bandwidth, or Fox for doing a shoddy HD master that Sky's had to send out. Though the fact that a few of the other films I've viewed for this test have looked a bit soft too suggests that Sky's variable bit-rate system probably isn't completely blameless.
While I might not have been able to do a straight Blu-ray/Sky HD head-to-head of Titanic, there are other films on Sky right now that do have Blu-ray versions available for comparison. Including Forgetting Sarah Marshall.
One thing that needs to be pointed out before going any further with a comparison is that the Blu-ray version of the film is presented in 1080p/24, while the Sky broadcast is 1080i. And actually, this fact is probably responsible for the biggest advantage the Blu-ray has over the Sky broadcast, for sharp edges on the broadcast, especially around facial contours, look noticeably more jagged than they do on the Blu-ray. You'll only see this in any obvious sort of way if you watch on a really big screen, or else sit very close to the picture, but on smaller screens it still results in a slightly fuzzy look to edges at times.
The other key picture advantage the Blu-ray of Forgetting Sarah Marshall has over the Sky broadcast is that it suffers marginally less with MPEG decoding artefacts. Which is to say that there are practically none on the Blu-ray, but a very small amount on the Sky broadcast.
For instance, near the start of the film, where Peter gets a call from Sarah saying she's going to be back earlier than expected, the lamp in the background and the skin on Peter's forehead both look marginally more 'alive' with noise on the broadcast than they do on the Blu-ray. To be clear on this, though, the difference in noise levels is less pronounced than that noted with Xbox 360 downloads during Blu-ray comparisons. At a push I'd also say that the Sky broadcast looks fractionally softer than the Blu-ray, though this really is a marginal thing.