Barry Fox

When we look at a film or TV screen, we immediately notice if the sound and picture are not perfectly synchronised.

Just one frame of lip-sync slippage, an error of 40 milliseconds, offends the ear-eye-brain mechanism.

The first cinema sound systems used a gear train to mechanically link the projector to a disc player. Because projectionists sometimes played the wrong discs and reels, the system was replaced with an optical soundtrack down the side of the film.

The cinema went back to sound-on-disc with DTS (Digital Theatre System), which uses a time code to lock a CD-Rom or hard disk containing the digital soundtrack to the film.

Every time a digital sound or visual signal is processed it gets delayed. Compression, special effects and editing all lose a few milliseconds.

Decompression and error correction lose more. Each step change is not in itself noticeable, but add them all together and the delays become apparent.

Delays at the transmission end can be corrected by the broadcaster. But in the home, plasma and LCD panels and projectors add extra delay because they convert interlaced TV signals for progressive scan display.

More delay is added if the pictures are then enhanced or upscaled to add more lines and pixels, by storing several pictures for long enough to analyse them and guess what might be missing.

The sound needs less processing, so words end up ahead of lip movement. This never happens in nature, where sound is behind vision.

When the delay hits around 30 milliseconds, we start to notice it. Modern TV delays can be 80 milliseconds or more.

German company Micronas sells a $5 chiset with onboard memory that delays audio by 40 or 80 milliseconds. The chip is inside a TV, and factory-adjusted to compensate for the delay introduced by onboard picture processing.

But when a panel or projector is working with a separate surround-sound processor and amplifier, all are adding different delays that may cancel each other out.

Last year Motorola launched a new digital wireless audio system that uses the latest version of Bluetooth. Bluetooth Class 1 extends the range from 10m to 100m and A2DP (Advanced Audio Distribution Profile) adds stereo.

Motorola’s ‘Home Entertainment Solution’ is a digital wireless streamer (DC800) and wireless stereo headphones (HT8200) costing only around £100.

‘We never did a formal press release or launch’ said Motorola, ‘but they have recently gone into stores in the UK’.

I borrowed a system and for a while was chuffed. Here was a way to listen to TV and home-cinema sound without disturbing others, using an interference-free digital wireless link.

I quickly got a dose of reality. The Bluetooth circuitry has to do so much processing that the sound heard through the headphones lags horribly behind the pictures on screen.

I asked Motorola whether this is fixable, but was ignored. This may be why the launch was low key; Moto’s engineers know they have a problem and no solution.

The same problem is troubling professional sound engineers. TV studios and stage shows use analogue FM radio microphones.

They would go digital to avoid interference, but processing delays, or ‘latency’, make it impossible, especially if the sound is being picked up by a combination of wired, analogue FM and digital radio microphones.

When direct and delayed sounds are mixed, the ear identifies a 10-millisecond difference. Just five or six milliseconds is enough to cause ‘comb filter’ effects, similar to those used to create pseudo-stereo sound from a mono signal.

Beyer sells a secure digital wireless system for conferences. It uses 128bit encryption on a 2.4GHz frequency, and the Bank of England and Ministry of Defence consider it safe against eavesdropping.

But processing adds a 15-millisecond latency. Beyer’s next chips will reduce this to eight milliseconds, and the target is two. This needs very high-speed processing, but research is under way.

Usually the broadcast industry feeds off consumer developments – think Betacam, DAT, Minidisc and laptop editing – but this time it’s the other way round.

The industry’s work on latency may help the consumer audio-visual world get what it needs: an affordable box to delay sound or pictures by an infinitely adjustable amount.

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