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Wireless audio system


Doremi Labs, part of Dolby, provides playout equipment for digital cinema. To meet accessibility requirements in many regions for the hard of hearing, it needed a way to provide audio description and hearing assistance tracks through earphones to any customers who require them. Retro-fitting cinemas with this equipment can be expensive, so the solution had to be ‘plug in and go’.


We deployed our Salix™ reference design, which is based on very low-cost digital cordless telephone chips. These use the worldwide popular DECT system, and can take advantage of the billion-plus chips sold and the radio spectrum available in over 100 countries.

Salix uses the on-chip signal processing functions to implement a modern high-quality codec algorithm, which delivers good-quality stereo audio, whilst using only modest radio spectrum. The DECT system provides fully-automatic radio management: at switch-on the transmitter unit seeks the quietest part of the allocated spectrum, and receivers seek the pre-allocated transmitter identity. If interference is encountered, the receivers report this fact back to the transmitter, which will move to another channel without interruption in the service.

Implementing the codec on to a small low-cost phone chip required considerable DSP skills, to optimise both code size and processor cycles.


Doremi has a transmitter unit which can be deployed anywhere in the auditorium, as it does not require a line-of-sight to the receivers. The indoor range of the system is more than 50 metres, and the system allows broadcast of two stereo channels simultaneously.

When a receiver is requested by a cinemagoer, the attendant docks it into a programming unit, and chooses the auditorium and programme. The user then only has to be concerned with on-off and volume controls.

Wireless audio system for hearing assist

Alternative technologies include infra-red transmission, which requires line-of-sight. In a cinema with its non-reflective walls, this means that four or more emitter devices must be installed together with their wiring, and much of this work must be done at height with consequent cost and disruption. The Salix radio technology offers all the advantages of infra-red from a small low-cost transmitter and antenna which could be installed in minutes in the projection room or on the back wall.