An acronym for "synchronized compressor and expander"
Radio signals in the High Frequency (HF) band propagate over long distances by reflecting from the ionosphere which is dynamic in nature. This causes signal fading and, coupled with the existence of strong interference in the HF portion of the spectrum, often makes the received voice difficult to understand and uncomfortable to listen to.
In the mid 1960s, the British Post Office developed the Lincompex system to overcome the aforementioned problems. Although Lincompex has proven to be successful in international radio telephone circuits, HF users with limited financial means have not been able to take advantage of it because of its high cost and because it required the use of radio with and end-to-end frequency offset of less than +/- 2Hz.
Syncompex was designed specifically with small users in mind. The objectives were:
- To develop a small and inexpensive equivalent to Lincompex (Link Compressor and Expander), using digital techniques.
- To improve the intelligibility of voice communications over HF radio.
Digital techniques were used throughout to take advantage of the potential cost saving offered by advances at the time in microprocessors/LSI technology. Syncompex is designed to operate with an end-to-end frequency offset up to +/- 20 Hz. The RF spectrum required is the same for a normal single sideband transmission. This allows Syncompex to be used with standard radios and also to be retrofitted to many older radios already in service. When radio noise is encountered, the high-level syllables of the voice can be understood, but the low-level syllables are masked, thereby degrading the intelligibility of transmitted voice. The Syncompex system circumvents the above-mentioned difficulty by increasing the amplitude of the low-level syllables, so that the dynamic range of the transmitter wave form is reduced, allowing all syllables to be transmitted at or near full power. In this way, the low-level syllables can be heard as well as the high-level, thus improving the voice quality at the expense of increased average power.
In 1975, Mr. L. Hatton, Director of the Communications Systems Directorate, assigned S.M. Chow the task of designing a small and inexpensive equivalent to Lincompex, using digital techniques. The result was the Syncompex design which, among other things, digitized the information carried on the control channel and did not require highly stable frequency sources. It could be added on to existing radio sets.
Discussions were held with industry to discuss building and testing the design. This led to a proposal in August 1975 by the Canadian Electric Company (CGE) to produce three breadboard models. A contract was awarded in January 1976 and completed by December. CGE, having no marketing mechanism for the device, decided not to proceed beyond the research and development stage. A request for proposals resulted in a six month contract with Canadian Marconi to test and evaluate the device over a radio network. This contract was completed in August 1977. The test indicated that further design work on the control channel would be required to make it more robust when selective fading was encountered. A new contract was let to Marconi to redesign the channel to overcome the observed deficiency. This included computer simulation of the coding schemes and some on-the-air tests of the digital portions. This contract ran from January to September of 1978.
Although the design and tests were successful, Marconi withdrew from further negotiations and dispersed their design team in January 1979. Further in-house work carried out at CRC in the Fall of 1978, resulted in a design model which was later transferred to Miller Communications Limited, under a contract to produce test models of the Syncompex design. These models were fully tested over actual HF links. The Department of National Defence contributed funds for this last contract and participated in tests, with the intention of possibly using these devices in the Canadian Forces.
The results of the tests may be summarized as follows. A dramatic improvement in intelligibility and corresponding reduction of factors contributing to operator fatigue was observed in the trials. The improvement was largely attributed to the silencing of noise by Syncompex during speech pauses. Thus, the listener probably felt more confident that he had not missed a low level syllable which could change the meaning of the sentence. Also, There was great improvement for circuits that have a marginal signal-to-noise ratio. When the signal is strong, a slight impairment can be heard due to the loss of spectrum pre-empted for the control channel. Finally, some tests were made with a Syncompex model having only a single control channel. The performance of this system was judged to be unsatisfactory because of its inability to withstand selective fading due to multipath. When fading conditions were encountered, the single control channel was disabled intermittently causing sudden changes in the received audio level. Test subjects registered strong disapproval of this type of distortion because it sounded "unnatural." In some cases, listening discomfort during selective fading was so acute that subjects reported that they preferred to sacrifice intelligibility. Since multipath conditions are frequently encountered, in-band frequency diversity to counteract selective fading was considered a necessary part of future designs. A later model of Syncompex included a dual control. The trials gave sufficient evidence that Syncompex could offer worthwhile improvement in performance to HF-Single Sideband (SSB) radio telephone systems.
Bhaneja, B., Lyrette, J., Davies, T.W. and Dohoo R.M. "Technology Transfer by Department of Communications: A Study of Eight Innovations." MOSST Background Paper. Ottawa; Supply and Services, 1980.
Chow, S.M. and McLarnon, B.D. "Syncompex: A Voice Processing System for Low Cost HF Radio Telephony." CRC Report No. 1316. Ottawa; Department of Communications, August 1980.
Page created on August 13, 1997 by Cynthia Boyko
Last updated on February 6, 2001 by Stu McCormick
Copyright © Friends of CRC, 1997.