Since my original post about surround sound on Freeview HD and the publication of my roundup of set top boxes on Register Hardware, quite a few people have asked me more about this issue, and I’ve also had some feedback from Freeview. So, now it’s time to update and explain a little more. There’s a mix of good and bad news, really.
First, though, let’s get some jargon out of the way, because there’s a bit too much of it, and some of the terms sound very similar, even though they refer to different things. Here, in ComputerActive style, is a quick guide to some of the important abbreviations and terms.
- 5.1: Shorthand for surround sound with five channels (Left, Right, rear L, rear R, centre, and the .1 is the subwoofer)
- AAC: A type of audio encoding, which can be understood by the HE-AAC decoders in Freeview HD boxes
- AC-3: Another name for the Dolby Digital audio encoding understood by most home cinema systems
- AD: Audio Description. An additional soundtrack with information to help the partially sighted.
- ARC: Audio Return Channel. Part of the latest version of HDMI, an option that allows a TV to feed its own audio signal (eg from Freeview HD) back to the AV amplifier.
- AV: Audio Visual. Often shorthand for surround sound kit, in the home audio context, eg an ‘AV amplifier’
- Co-axial: A type of connector used for digital audio, with a ‘photo’ type plug on each end
- Dolby Digital: The most common format understood by home AV gear
- Dolby Digital Plus: An updated version of Dolby Digital, which can support audio description, and may be used on Freeview HD
- Dolby Pulse: A Dolby version of the HE-AAC format
- DTS: Another fairly common surround sound format, understood by most home AV gear
- E-AC3: Another name for Dolby Digital plus
- HDMI: The connector used for high definition video. It can also carry digital audio signals too
- HE-AAC: High Efficiency AAC. One of two types of audio encoding that can be used on Freeview HD
- MPEG: A type of video and audio encoding; on standard definition channels, MPEG audio is used
- Multi-channel: Another way of describing surround sound – audio that has more than just left and right channels
- Optical: A type of connector used for digital audio, made of fibre-optic material
- PCM (aka L-PCM): Yet another type of audio encoding, which most AV gear understands. Can be used for stereo, or multi-channel sound
- S/PDIF: A standard for digital audio connections using Optical or Co-axial connections
- Transcoding: Converting audio from one format to another, for example from Dolby Digital plus to Dolby Digital, or from HE-AAC to Dolby Digital
Ok, so where are we?
Jargon out of the way, what’s the current situation? Well, if you want to buy a Freeview HD box and plug it into your existing AV setup, then the way I think that most people are likely to connect up the sound is via S/PDIF. For instance, you might connect the box to your TV via HDMI, and then use the S/PDIF to link the audio output to your surround sound system.
Unfortunately, if you do that, then you probably won’t get surround sound from the Freeview HD programmes that include it (and not all do; a good source of information about what’s in surround is DigiGuide, incidentally).
That’s because of two things. Firstly, all the channels broadcasting on Freeview HD at the moment are using AAC audio, even for surround sound programmes, instead of Dolby Digital plus. (In earlier pieces I’ve referred to this as HE-AAC; Freeview has pointed out that it’s actually AAC, and will be HE-AAC in future, but frankly that technical point makes no difference to the overall outcome).
The second factor is that the majority of boxes available at the moment do not transcode from AAC/HE-AAC to Dolby Digital. And without that, you’re not going to get surround sound via the S/PDIF link. All you’ll get is stereo PCM.
There are some rays of light, however. Firstly, I’ve put together a list of equipment that can convert the signal to Dolby Digital. Secondly, at least some kit can be updated to do the transcoding, so even if you’ve bought a Freeview HD box that doesn’t, like the Humax, it may do in future. As I hear about planned updates, I’ll be adding those to my list too.
There’s a second bit of fairly good news, in this response from Freeview to my questions:
“It is mandatory now to either transcode to AC-3, DTS or Linear PCM with the same number of channels as the broadcast.”
What this means, effectively, is that if a box isn’t outputting Dolby Digital (AC3) or DTS – and most of them aren’t – it should at least be creating an L-PCM signal, with the same number of channels. So a 5.1 AAC or HE-AAC broadcast will give you 5.1 channels of PCM audio.
Unfortunately, the S/PDIF connection isn’t designed to support anything more than stereo PCM; it doesn’t have the capacity for surround sound in PCM format. So you’re out of luck if you want to connect via co-axial or optical links. But if you have an AV amplifier or receiver with HDMI inputs, you should be able to connect your Freeview HD box to that, and receive multi-channel audio.
I say should for two reasons. Firstly, I don’t have AV kit with HDMI inputs to verify this, and secondly, not all kit with HDMI inputs will support multi-channel audio via HDMI, so you need to check the specs before you buy anything.
The normal way of connecting up like this will be to plug the Freeview HD box into your AV system, and then the output from that into your TV. However, it’s also possible, with some TVs, to have the TV pass the audio out through the connection back to the AV gear. Typically, you’d use that option if you have a TV with Freeview HD built in and want surround sound from it, but you’ll need to make sure both TV and AV kit can work together like that.
So, in summary, still not great news for people who want to connect via an optical or co-axial S/PDIF link – you need to choose your kit carefully. But if you do have equipment that supports HDMI multi-channel audio, then you should get surround from Freeview HD. Please do let me know how you get on.