» posted on Thursday, August 12th, 2010 at 11:37 by Nigel
FreeviewHD bit-rates and file sizes revisited
I first wrote about bit rates and file sizes for FreeviewHD a couple of weeks ago; at that stage, I’d mostly recorded from BBC HD. The original post has now been updated with some details of additional recordings made from ITV 1 HD and Channel 4 HD, to give a more detailed comparison. I’ll be adding some more data to that later.
This post is largely about the main figures you can take away from these sample recordings, and what that means in terms of how large a disk drive there should be in your Freeview HD recorder.
Across 32 separate recordings, the average file size comes in at 2.96GB per hour, or near enough 3GB. If you record a mix of programmes from HD channels, that’s a reasonable figure to work with, so a recorder with a 320GB hard drive will give you about 100 hours, 500GB will give you around 160 hours, and a 1TB drive will give you about 330 hours. That, of course, is assuming that you only ever record things in HD. SD programmes seldom come to more than 2GB per hour, and more often will be around 1.5 on channels like BBC1, dropping to less than 1GB on minor channels.
All that said, it’s worth breaking down these figures a little more. Many people will want to know, for example, if some channels use a higher rate than others, as hinted at in my first post (with a pretty clear caveat, too).
So, let’s look at the figures by channel: with 13 recordings, I have the largest bit of data from BBC HD, and the average is 2.97GB/hour. Channel 4 HD, with 9 recordings, averages 2.80GB/hour, while ITV – and this will surprise a few people – has the highest figure, at 3.10GB/hour.
That’s not the whole story, of course. Rates vary, because of the statistical multiplexing that’s used, where more is allocated to the pictures that need extra data.
There’s quite a variation, too. On the BBC it’s between 2.33 and 3.52, on C4HD from 2.62 to 3.00 and on ITV1HD from 2.60 to 3.94. Those lowest figures on the BBC, incidentally, were for Sherlock (not, as some might have expected, The Deep, which managed 2.46 while not actually looking much like HD at all).
What’s more interesting is to note those high figures from ITV 1 HD. To an extent, they do skew the figures overall and relate to three of the nine test recordings. Two of them were films (Three Kings, 3.94GB/hour and Total Recall, 3.82GB/hour) while one was the international football highlights at 3.68GB/hour. Strip those out, and the ‘normal’ programming averages 2.70GB/hour.
This is interesting not because it makes ITV look rubbish, but because I think it does clearly show how well the statistical multiplexing does work – when the material demands it, with action-packed films, or football, Freeview HD does manage to find the bitrate, and certainly the picture quality on Total Recall looked very good to me.
It also suggests that, rather than that figure of around 3GB per hour, if you’re intending to record mostly films or sports on a FreeviewHD recorder, you would be better off assuming an average requirement of 4GB per hour, giving 80 hours on a 320GB unit, 125 on a 500GB, and 250 hours on a 1TB recorder.
Comparing Freeview and Freesat
I’m due to have the new Humax Freeview HD recorder delivered tomorrow; in the interests of research, I’ll set up identical recordings for both Freesat and Freeview HD over the next week or so, to provide a direct comparison between the two, which will I hope interest those who are convinced that the BBC HD channel’s bitrate on Freesat has been deliberately fixed to make it no better than Freeview.
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