FreeviewHD and surround sound

If you’re not familiar with the basics of surround sound, I recommend you go back to the first part of this feature, before reading on about how it works with Freeview HD.

If you have read that already, then welcome to part three. It’s time to take a look at what’s happening with surround sound on Freeview’s new HD service, and why it’s not quite as straightforward as it is with Freesat.

Fun with formats

For technical reasons, fitting HD onto Freeview is a bit of a squeeze; there’s just not enough space (or ‘bandwidth’) to play with as there is on satellite. And so, one of the decisions taken by broadcasters has been to use a system called ‘HE-AAC’ to broadcast the sound.

One reason for doing that is to save space – HE-AAC needs less bandwidth to produce the same quality as Dolby Digital – but there’s another, too. That’s the need to provide audio description, which is the live description like “The Doctor hits a dalek with a spanner” intended for people with sight problems, and required by broadcasting rules on at least 10% of programmes.

You can’t do audio description with Dolby Digital, though it should be possible with the brand new Dolby Digital plus system, which can also be transmitted on Freeview HD. That’s why you’ll see the Dolby logo, or mention of Dolby Digital plus on many Freeview HD receivers.

However, connect a Freeview HD box to your surround decoder at the moment, and there’s actually a very good chance that you won’t be watching Dr Who in anything other than stereo. Choose the wrong options setting up your Freeview HD system, and you might just be treated to the silence instead.

What’s going on here, then?

Well, it’s that HE-AAC format. It may be efficient, but it’s also relatively new, and a lot of kit doesn’t understand it. So, plug a Freeview HD box into your surround decoder, and if it doesn’t know what to do with HE-AAC, you’ll have to dive into the menus and make sure that the box is sending PCM audio instead, giving you just stereo sound.

If you’re connecting via HDMI, then – as long as your AV amp or receiver understands multi-channel audio via HDMI, and the Freeview HD box outputs it – you will get surround sound. But older kit might not support HDMI multi-channel audio either, and again simply give you stereo. When I wrote this, there were still AV receivers on sale at Richer Sounds that fell into this category, for instance. And as far as I can see, there’s no home AV kit currently available that directly supports the HE-AAC format; it’s all Dolby, DTS and the various variations of that, or PCM for stereo, though I’d love someone to prove me wrong on that.

Even programmes like Doctor Who that, on Freesat, are broadcast in Dolby Digital 5.1 surround don’t use Dolby on Freeview HD. They’re still in 5.1 surround, but using HE-AAC, because of the need to provide an audio description as well. (And, presumably, it’s much simpler for the broadcasters to use one format all the time, rather than switching between HE-AAC and Dolby, which may cause glitches on some equipment.)

Solving the problem

All is not entirely lost, however. While the ‘Dolby Digital plus’ logo on the front of most Freeview HD receivers generally means that if that format were to be transmitted , it would be passed on to your decoder, some boxes can do more. Specifically, they can ‘transcode’ or convert surround sound from HE-AAC to Dolby Digital, on the fly. That means you can feed it into your existing surround sound system, and wallow in all the glorious audio effects. They’ll also convert Dolby Digital plus to ordinary Dolby Digital, meaning that too will work with older equipment.

So if, like me, you want to get surround sound from Freeview HD through an existing surround system, you really do need to make sure that the box you buy is one of those that can create the Dolby Digital signal for you – and it’s not always easy to find out which those are. Even in briefings with some of the manufacturers, I’ve met with blank faces when I ask this question.

I’m testing a selection of boxes at the moment for RegHardware, and I’ll compile a list of which ones do transcode HE-AAC to Dolby Digital; some may apparently just need a software tweak to enable transcoding, depending on the chips they use – I’m trying to find out more about that.

It’s a shame that transcoding HE-AAC is not a mandatory part of the Freeview HD specification – as it is in other European countries – because without it, it seems to me that the vast majority of the surround sound systems already in UK homes simply won’t be able to work with Freeview HD in anything other than stereo. For a lot of people, that’s going to take away a big part of the pleasure of watching high definition television.

Buying advice

If you have a surround sound setup that you want to use with Freeview HD, you really need to do your homework first, to make sure the box you buy will work with the kit you already have. That means either a box that can transcode to Dolby Digital, or a surround decoder (AV amp, receiver and so on) that accepts multi-channel audio via HDMI. Personally, I’d go for the former, mostly because I can’t afford to replace my amplifier.

Meanwhile, if you’re buying new kit, at least you don’t have to worry about backwards compatibility. My advice is to buy an AV amplifier or receiver that understands multi-channel audio via HDMI and, if possible, that supports HE-AAC as well, and also Dolby Digital plus, which may be used in future.

If any readers have home AV kit that does understand HE-AAC audio, please let me know the make and model number. If you’re buying Freeview HD gear, you should probably check my list of kit that transcodes HE-AAC to Dolby Digital. And don’t despair – it’s possible that some boxes can have transcoding added via an update.