HD sound 101

What you need to know to understand high definition sound

We’ve already given you a brief outline of high definition sound, but to help you work out what you need into your home, we need to look at it in a little more detail.

A brief history of HD sound

In the history of technological change in the movies, far more attention has been paid the picture than the sound. Colour. Widescreen. These new presentations were considered pivotal technologies in the art form, and became established standards in filmmaking during the 1950s. Less well known were the improvements in sound.

It was in 1927 that Al Jolson uttered “You ain’t heard nothin’ yet” in The Jazz Singer, and ushered in the age of the talkies. From then until the mid-1950s, most movies used mono sound, and this was decidedly anything other than high fidelity. It even had a name – ‘Academy Sound’ – and it was marked by a pinched tonal quality, due to the lack of the upper two octaves of treble. Just watch any black-and-white 1940s or 1930s movie and you will soon hear what we mean. This was necessary because of the limitations of the optical recording track used on film: the soundtrack was photographically printed on the edge of the film and read by the projector.

But through the 1950s and 1960s, improved sound systems gradually came into use. So called ’70mm’ film had a magnetic soundtrack on its edges capable of carrying six channels of sound. It wasn’t used often, but some of the bigger musicals and huge sweeping dramas – for example, ‘The Sound of Music’ and ‘West Side Story’ and ‘The Bridge on the River Kwai’ and ‘Spartacus’ – employed it. Which is why they have been able to be re-released on DVD (and eventually, we expect, on Blu-ray) with relatively high quality surround sound.

When they were released, however, these movies were rarely heard in all their glory because the overwhelming majority of cinemas did not have the equipment needed to reproduce their multichannel soundtracks. Instead, regular 35mm distribution prints were sent out with mono Academy Sound.

Surround is born

It wasn’t until the mid-1970s that surround sound became a norm in the cinema, and that was thanks to Dolby Laboratories. Already it had developed noise reduction systems that were starting to allow optical soundtrack films to sound quite decent. But in 1975 it released ‘Dolby Stereo’, the full four channel surround system (in technical cinema-speak, ‘stereo’ means ‘surround’) which was very similar to the later home system, called Dolby Pro-Logic, that became common on VHS movie releases. This also incorporated noise reduction to allow improved sound quality.

Dolby Stereo provided sound channels for the front left, front centre, front right, and surround or rear. The front centre and surround channel were ‘matrixed’ into the front left and right channels. This is a technique that mixes two things together in a way that allows them to be extracted again, albeit imperfectly. The techniques involved reduced the bass and treble of the surround sound, limiting its effectiveness.

By far the easiest way to deliver a sense of sounds coming from all around you is to actually use multiple speakers all around you. Thus the need for actual loudspeakers, with an actual ‘channel’ of sound for them, located behind and to the sides of you. The front centre channel performs a more subtle service: it ties the dialogue – the voices of the actors – to the centre of the screen. Without the centre channel, if you were sitting over towards the left side in a cinema, or even in your lounge room, the dialogue would sound like it was coming from the left speaker and not from the screen itself.

The beauty of Dolby Stereo was that it provided real surround sound, yet it was ‘backwards compatible’ with existing cinema equipment. In older cinemas the dialogue would still be delivered, along with music and so forth. It’s just that there would be no effective surround sound.