Surround sound explained

One of the things that goes with high definition TV, as far as many people are concerned, is surround sound. If you’re watching a film or a great drama in high def, then adding 5.1 sound makes the experience even better.

But if you don’t shop wisely, you may find that you don’t get the experience you expect. I’m going to explain why, to help you make sure you make the right choice.

First, what is surround sound anyway? What’s all this 5.1 stuff? Well, normal standard definition TV in the UK is just stereo, with a left and a right speaker. Surround sound is described as 5.1 because there are five main channels, plus one extra. That’s left and right at the front, left and right at the rear, a centre speaker (traditionally under the TV, for the voices), and the extra channel is the ‘sub woofer,’ which is for the deep chest rumbling bass noises, like explosions, passing trucks and so on.

You don’t have to have all those speakers – I have a great pair of left and right front speakers for example, and they produce all the bass I need, so I don’t have a subwoofer, nor do I have a centre speaker; my surround decoder is set to split the centre channel between those two instead.

And there I’ve just introduced another element – the ‘surround decoder.’ This is a bit of kit that takes the digital information from a broadcast (or a DVD, or BluRay) and decodes it, sending the right information to each speaker. You can buy an ‘AV Amplifier’ which includes a surround decoder, or you can buy a separate decoder to add to an existing hifi system (though there aren’t many available these days, and they tend to be expensive). Other alternatives are AV Receivers, which are like AV Amplifiers, but with a radio built in, and all-in-one gadgets from some companies like Panasonic and Yamaha.

These tend to take the form of a ‘sound bar’ which is a long box to sit under the TV, with the electronics and centre speaker, and then satellite speakers to go around the room.

Some sound bars, and some TVs too, include ‘pseudo’ surround modes, which try to create the same effect, but without extra speakers and wires, by bouncing sound off walls, or using fancy digital effects to trick your ears. But, in my view, they’re never going to really match up to having all those speakers positioned around your living room.

OK. Now, we’ve got a surround decoder, we’ve got speakers, and we’ve got something that produces surround sound – a BluRay player, or a high definition receiver. All set? Not quite.

Surround formats

Now, we get into the tricky world of formats. When it comes to surround sound, the name most people associate with it for home systems is Dolby Digital, which is also known as AC3, and sometimes when people say ‘5.1 surround sound’ they’re assuming Dolby Digital. Turns out that’s not always a good assumption.

Some DVDs use a system called DTS instead, and there are newer versions of Dolby and DTS used on BluRay, which can have seven main channels; but you don’t need to worry about that for high def broadcasts.

What you do need, however, is to make sure that your surround decoder understands the information from the HD source you’re watching. So, if you have a decoder that only works with Dolby Digital then you have to make sure that that’s what you feed it. It won’t make sense of anything else; that’s why some BluRay players have ‘downmix’ or ‘downconvert’ options, allowing them to convert, for instance, the 7.1 surround sound on the latest discs to 5.1 Dolby Digital, so even if you have older kit, you can still get the effects.

There’s one other thing to note – most surround decoders will also understand a format called ‘PCM’, which is just a digital version of stereo.

Making the connection

Where are we now? Take an HD source, like BluRay or an HD receiver, connect it to a surround decoder, and then you get great effects through all the speakers. So, how do you connect it?

There are a couple of ways. Most people will be connecting their BD player, Freeview HD box or Freesat box to a TV via HDMI. And then what? If you want surround sound, you’ve got to get that out of the box too, unless your TV happens to have a surround or pseudo-surround mode, and you’re happy with that.

If not, the way lots of people do it is via a digital audio output on the box – most of them have at least one; it’s either an optical connection (you can usually tell from the glow coming from the socket), or an electrical one (often referred to as ‘co-axial digital’). Some have one or the other, some units have both, and the generic term is “S/PDIF” after Sony and Philips who came up with it.

Just about every surround decoder has several S/PDIF inputs, so you connect the HD box or player to the decoder this way, hook up the speakers, and you’re done – as long as the box is sending a signal your decoder understands. Many decoders will have connections for the video too, acting essentially as a switchbox.

Some more modern surround decoders (and especially recent AV Amplifiers and Receivers) also have HDMI sockets. The idea is that you connect all your HD bits of kit to the AV amp via HDMI, and then a cable goes from the amp to the TV. So you get a single cable for each device, sound and vision are both switched by the AV amp, and you use its remote control to select what you want to watch, and how loud it is – all very neat and straightforward.

So, now you know the basics of what surround sound is, and how to hook it up. In the next two parts, I’ll look at exactly what you need to do to get surround sound from your Freesat or Freeview HD box.

Next: Freesat and surround sound

Part 3: Freeview HD and surround sound

4 thoughts on “Surround sound explained

  1. Thanks for this clear and concise article. I always found surround sound formats confusing but this explains it very well.

  2. “most surround decoders will also understand a format called ‘PCM’, which is just a digital version of stereo”

    I thought PCM could be ‘multi-channel’ i.e it could transport the decoded 5.1 audio and wasn’t mixed down, so PCM isn’t just stereo. Is that not the case?

  3. PCM can be multi-channel, but when we’re talking about the optical or co-axial connections to home surround systems, they don’t have the capacity for multi-channel.

    So, if you’re connecting via either of those (‘S/PDIF’) then it’s either a supported surround format or PCM stereo.

    Sorry – a bit of an oversimplification in the original.

  4. I’ve just found your site, which is clear and instructive on various topics. I was very interested to read about your use of existing speakers. I’m new to home cinema and am planning to get an AV receiver along the lines of the Sony STR-DH810. I have a bundle of decent speakers in the house and wonder if it’s possible – or sensible – to try to use them instead of buying a speaker package. Have you written in detail about this by any chance? My other AV equipment is a Panasonic TX-L37G15B TV, a Humax Foxsat-HDR and a Toshiba DVR19DTKB DVD recorder/VCR.

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