Researchers at The University of Hong Kong (HKU) have proved that LEDs (light emitting diodes) can be used to transmit audio signals. A small-scale prototype appliance built to illustrate capabilities of the technology to industry and entrepreneurs has an audio transmission range of about two metres.
But, says co-investigator Dr Alfred Wong, a range of 30 metres can be achieved with more powerful LEDs. Possible applications of the technology are varied.
It could be used, for example, to transmit traffic reports to motorists when they stop at traffic lights. Grocery stores could use the technology to broadcast many individual promotions at different counters for, say, rice, meat or washing powder. The usual method is to broadcast across the entire store.

Dr Wong tunes in to music from the LED-based information broadcasting system.

In a similar application, museums could use the technology to broadcast descriptive information at individual exhibits, and in any language ordered by the user. Instead of Juke Box music in, for example, a restaurant, diners could order their own music from a channel selector at their table. Advantages of the technology over wired sound systems include:
The ability to embed information with lighting which can help save on infrastructure costs.
As many as 20 channels can be delivered at the same time, the exact number depending on the degree of sound compression used.
Because the audio transmission depends on light, the technology does not require a broadcasting licence. In the HKU system prototype, input analogue sound from sources such as radio, audio cassette, compact disc, or live sound (see illustration) is converted in an encoder to a digital signal.
In being digitised, the audio signal is converted from 44.1 kilosamples per second (kbps) to about 700 kbps. As the data flow rate is high, it is compressed using the MP3 format to 128 kbps; reducing the space it occupies enables the streams to be transmitted more efficiently.
Once compressed, the streams are merged by a multiplexer. The multiplexed signals, still in electrical form, excite LEDs at the rate of 1 million pulses a second. The pulses are so rapid that human eyes cannot notice the flicker. The LED pulses are picked up by a light-sensitive receiver at the broadcasting end of the system.
The signal is separated into streams via a demultiplexer and then decoded. Operators can select from a number of channels and the original input sound is recreated. The two-year project was completed late last year with a prototype being displayed at the Innovation and Technology Exposition in Hong Kong in November.
Said Dr Wong: “Usually LEDs are associated with light and display systems as in computer gadgets. As far we can tell from available research literature, this is the first time LEDs have been used to transfer sound.”
He added: “The extent of uses for this system is extremely wide, particularly in the home where, for example, it could be used in anything from home theatres to controlling air conditioning.”

Principal Investigator
Professor E S Yang: esyang@eee.hku.hk
Co-Investigator
Dr Alfred Wong: awong@eee.hku.hk