Pulse or digital communications – Repeaters – Testing
Patent
1989-04-21
1991-05-28
Olms, Douglas W.
Pulse or digital communications
Repeaters
Testing
455 73, H04B 138
Patent
active
050200771
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The present invention relates to a modem and data communication system, particularly envisaged for use in (HF) transmission media although it is also very well suited to operate in higher frequency media.
FIELD OF THE INVENTION
In general, data communication in non directive medium such as air is conducted over relatively short distances and at relatively high speed and relatively high frequency. The form of data communication channels employed in air tend to limit the distances that can be transversed, usually to line of sight, as in microwave links and the like. Larger communication distances have been attained with lower frequency communication channels, such as, for example, very high frequency (VHF).
Typically, prior art modems have been designed to operate at VHF or ultra high frequency (UHF) or microwave frequencies or over guided media such as telephone and coaxial cables. Such prior art modems were designed primarily to attain good transmission speed. The electrical conditions and characteristics of the electromagnetic media used by the prior art modems are substantially constant.
In contrast communication over HF media is complicated by constantly varying and unreliable electrical conditions and characteristics. These complications include burst noise, flutter fading, frequency drift, delay distortion or multi pathing and a generally greater level of noise than other more conventional media. Such noise includes uncorrelated random noise originating predominantly from stratospheric disturbances of the propagation wave, receiver input thermal processes, and intermediate frequency thermal processes. This noise is similar to band limited white noise. Noise also includes correlated random noise originating from atmospheric disturbances, adjacent channel cross talk, ignition noise and the like. This noise is characterized by strong burst wise occurrences and manifests itself in the form of short and strong periods of noise superimposed on the signal.
Conventional modem designs are generally incapable of compensating for the complexities of HF media and consequently attempts to modify conventional modems to HF use have generally been unsuccessful.
The attraction of modem design to HF media is that data may be transmitted between base and remote or mobile terminals over long distances (in excess of 10,000 kms) and at a considerable cost advantage to other media.
Another attraction of HF media is less costly installations and less crowded communication band.
However, the problem to date has been recovery of transmitted data and privacy of such data.
SUMMARY OF THE INVENTION
The present invention provides a modem and data communication system employable on high frequency channels to communicate data over relatively long distances. In accordance with one aspect of the present invention there is provided a modem for a data communication system, A modem for a data communication apparatus, said modem being arranged for connection between a source of band limited audio logic such as a radio transceiver and a host computer, the modem characterized in that it comprises:
(A) a data receiver having:
(a) a band pass filter having an input arranged to be connected to the source of band limited audio logic;
(b) a squaring circuit connected to an output of the band pass filter to produce a substantially square wave signal comprising a plurality of cycles with rapid zero crossings, said cycles forming logic bits of data with changes of state between a logical high and a logical low value and said logic bits of data forming logic bytes of data;
(c) an audio period timer connected to an output of the squaring circuit and configured to develop a count indicative of the time delay between consecutive zero crossing in the same direction;
(d) a bandwidth window configured to receive the count from the audio period timer, and to filter the counts;
(e) a bit rate timer configured to develop a further count and to issue an interrupt signal to predict when the bits of data have one of the changes of state from
REFERENCES:
patent: 4425665 (1984-01-01), Stauffer
patent: 4475216 (1984-10-01), Noguchi
patent: 4551846 (1985-11-01), Takeda et al.
patent: 4596022 (1986-06-01), Stoner
patent: 4627078 (1986-12-01), Stoner
patent: 4785255 (1988-11-01), Lucak et al.
Chin Stephen
Olms Douglas W.
Transcom Australia Limited
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