One of the assertions often found around the internet is that FreeviewHD must be compromised, because there’s not much bandwidth available. It’s true that there’s less capacity than there is on satellite (though for other reasons, there aren’t as many free HD channels there as people might like). But in watching Freeview HD, I have been pleasantly surprised with the picture quality, both from the tuners in TV sets and from stand-alone set top boxes connected via HDMI.
The question that lots of people have been asking is “What’s the bitrate of the channels.” The easiest way to answer that question would be with a PC tuner card, which would let you see the instantaneous rates; unfortunately, there aren’t any available at the moment, which rules that out.
So, I’ve done the next best thing, which is to look at a selection of recordings from the BBC and ITV that I made on the Digital Stream Freeview HD PVR that I’ve just been testing for Register Hardware. I’m afraid that, since nothing actually caught my eye on Channel 4, there aren’t any recordings from that in the list below.
The Freeview HD multiplex is broadcast using what’s called a variable bit rate, or VBR. The total bit rate available for all the HD channels on Freeview is around 40Mbits/second – which is better than was expected when the DVB-T2 system was being designed. The mux is set up so that each channel has a variable amount of capacity, of between 3 and 17Mbits/second. The VBR and statistical multiplexing works is by analysing the video stream, and a channel that needs more capacity, like a fast moving picture of a football match will get more of it than one that, for example, is showing a lovely picture of a painting in a gallery, which isn’t changing at all.
So, how does it work out in practise? The headline figures for HD, based on looking at the duration and disk space indicated on the Digital Stream PVR, are that the BBC are using around 3.2GB per hour, while ITV is coming in at around 2.75GB per hour. I won’t directly convert those to the bit rate because it’s not quite so simple – not all the data in the file is video, for instance. Some is audio, and there may or may not be audio description too. And, of course, that gives you an average figure.
It’s also important to note that, since in my view ITV makes very few HD programmes worth watching, let alone recording, all the ITV recordings I made were at the same time, each week, and so all will have been affected to the same degree by what was on the other channels, assuming no major scheduling upsets. On the other hand, the BBC HD recordings were from a variety of times, though there are repeated recordings in there and, on the whole the bit rate doesn’t really change much.
For comparison, I also have a couple of SD recordings from the BBC, which are included in the list below.
Updated: 9th August, additional data added for ITV and Channel 4, plus some more BBC programmes.
These, then, are the BBC recordings, with running time and disk space used.
- That Mitchell and Webb Glimpse, 850MB, 16’28”
- The Thick of It, 1.6GB, 30’3”
- Doctor Who, 2.7GB, 55’14”
- Mongrels, 1.6GB, 29’36”
- Mongrels, 1.6GB, 30’10”
- Mongrels, 1.6GB, 28’17”
- Great British Railway Journeys, 1.7GB, 29’24”
- Top Gear, 3.2GB, 1h 3’24”
- Sherlock, 3.6GB, 1h 29’46’
- FAQ about Time Travel, 3.3GB, 1h 20’17”
- Jonathan Creek, 4.6GB, 1h 35’13”
- Sherlock, 3.5GB, 1h 30’1”
- Doctors (SD, BBC1) 818MB, 30’9”
- BBC 1 (SD), 1.1GB, 41’19”
ITV HD recordings
- Identity, 2.8GB, 1h 0’4”
- Identity, 2.9GB, 1h 2’33”
- Identity, 2.6GB, 1h 0’31”
- Identity, 2.7GB, 1h 0’13”
- Three Kings, 8.6GB, 2h 10’37”
- Cry Wolf, 4.4GB, 1h 38’58”
Channel 4 HD recordings
- Misfits, 2.9GB, 1h 4’54”
- The Core, 7.2GB, 2h 34’53”
- Smallville, 2.4GB, 54’54”
- Hollyoaks, 1.5GB, 29’56”
- The Big Bang Theory, 1.2GB, 24’57”
From this, it looks like ITV are habitually using a slightly lower bit rate than the BBC, but that will of course depend on what’s scheduled against Identity, and the demands of those programmes on the bit rate. So, I’ll find some more HD material from ITV, and update this post with more figures, together with some from Channel 4, to give a more comprehensive overall picture.
Meanwhile, the other useful thing that you can learn from this is that, roughly, you’ll be able to record 100 hours of HD programming on a FreeviewHD PVR with a 320GB hard drive.
One Reply to “Bitrates on FreeviewHD”
Certainly, since digital switchover in the Mendip transmitter coverage area, the picture quality and overall signal strength has dramatically improved.