 On
Friday 30 November 2001 at 7:30pm, 12 members and 13 guests of
the AES Singapore Section gathered at the auditorium of A.C.E Daikin
(Singapore) Pte Ltd for a seminar on “Wireless Digital Audio Technology”
by Mr Chan Chee Oei, Director of Engineering at
FreeSystems Pte Ltd. FreeSystems develops technology that enables digital
wired and wireless transmission of digital audio. Such technologies are used
in developing chip-sets that for integration into various digital consumer
products. FreeSystems is a sustaining member of the AES.
Starting with a brief introduction
of the company, Chan said that one important mission of the company
is to develop wireless systems and solution for digital audio, taking
advantage of modern technologies to replace older technologies currently in
use such as the FM technology. He went on to state the ideal wireless
transmission attributes, which includes low idle noise, at least CD quality
audio, immunity to interference, and good mobility.
In the build up to present
FreeSystem’s technology, Chan gave a technical insight into the
history of wireless headphones. The first wireless headphones were
introduced in the market in the 1970s. These first generation models had
their dynamic range limited by the source or the transmission medium
available at that time. The then current transducers, as well as the
transmission medium also limited the bandwidth of those headphones.
These systems exhibited an SNR
in the range of only 30 – 40dB. As Compact Discs were introduced in the
1980s, the dynamic range of the source medium increased to 96dB, but that of
wireless systems were still limited by the transmission medium.
In the years 1995-1997, slight
improvements in wireless technologies increased the SNR of wireless
headphones to the 50 – 60dB range, but were still well below the 96dB
quality the source medium could deliver. Due to the lack of major
improvements in wireless technology in this field, the transmission medium
became the major bottleneck of the quality of wireless headphone systems.
Chan also briefly mentioned the existence of wireless speaker systems,
with Reccoton producing such a system using the AC power line as the
transmission medium in 1987. Chan then showed the main advantage of a
digital transmission over analog transmission. Using digital transmission
technology, the SNR of the transmission medium can be increased to 96dB,
removing the bottleneck experienced in wireless headphone systems of the
past.
In presenting FreeSystems’
technology, Chan explained the various stages involved. The signal
had to be coded, modulated before transmission, and then demodulated,
decoded, and errors corrected on reception. Using FreeSystems’ FS1901/2
Transmitter/Receiver model as an example, the isochronous data rate of the
system is 2Mbps, with a stereo 48khz/24bit signal. The modulation method
used is based on Pulse Position Modulation, known for its power efficient
coding. For error correction coding, a form of Reed-Solomon coding was
employed, using a hardwired DSP engine to perform the computationally
intensive algorithm. Soft muting, using a gentle entry and recovery in the
muting profile, was implemented to overcome unpleasant clipping sounds
whenever a non-correctable error occurs.
As for the transmission medium,
Chan talked about the implementation for wireless audio using analog and
digital IR, as well as analog and digital RF. He compared their range,
ability to work in enclosed areas, EMI issues, and error correction
capabilities. Here, it could be clearly seen that digital was superior over
analog, giving better figures for frequency response and also for total
harmonic distortion (THD+N). The digital IR system maintained the high SNR
with respect to distance, compared to its analog equivalent.
Chan listed some advantages
of the Freespan technology over an existing analog system. The noise floor
of Freespan was a hundred time (40dB) lower, frequency response 2 time
better, and it also maintains very well the THD+N with respect to
both amplitude and frequency.
After briefly mentioning his
company’s current product line, Chan talked about the current and future
development direction that FreeSystems was taking. He said that they were
studying the implementation using the RF medium for transmission, which
could overcome the line of sight requirement of an IR system. Setting his
objective to make a wireless transmission system that is as good as a wired
system, Chan touched on the challenges involved in using RF, one of which
was a higher transmission bit rate requirement.
Finally, Chan proceeded
with a demonstration of the FreeSystems’ digital IR system compared to a
commercially available analog IR system. The hissing noise was very obvious
in the latter system, compared to no audible noises in the system using
FreeSystems’ technology. With about a dozen sets of wireless headphone
receivers and one transmitter, the audience enjoyed a short clip of a recent
action movie, and experienced for themselves the audio quality of the
Freespan technology. The meeting ended after a session of question and
answers.
The AES Singapore Section thanks
Mr Chan Chee Oei for his presentation and A.C.E Daikin (Singapore)
Pte Ltd for kindly allowing the use of their premises for this AES event. |
