Pulse or digital communications – Pulse code modulation
Patent
1997-06-30
2000-05-16
Chin, Stephen
Pulse or digital communications
Pulse code modulation
375246, H04B 1404
Patent
active
060646977
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method of and equipment for modulating an input signal composed of data bits into a pulse signal, and pulse demodulating equipment.
DESCRIPTION OF THE RELATED ART
There have heretofore been known, as pulse modulating methods of this kind, pulse amplitude modulation (PAM), pulse width modulation (PWM) and pulse position modulation (PPM) methods which change the pulse amplitude, pulse width and pulse position of the pulse wave used as a carrier, respectively, in accordance with the data bits.
In optical communication systems which use infrared rays and similar rays of light as carriers, the pulse amplitude modulation (PAM) which represents data bits by light intensity, in particular, has a problem with its reliability; therefore, the pulse width modulation (PWM) or pulse position modulation (PPM) has been used in combination with other modulation methods.
FIGS. 13(a) and (b) show signal waveforms by an ASK (Amplitude-Shift-Keying) system which is an example of such a combined use of different modulation methods. Digital data composed of a plurality of data bits is converted into a pulse signal (a) by an NRZ (Non-Return-to-Zero) data modulation scheme which makes the digital data "L" (low) or "H" (high), depending on whether the data bits is "0" or "1" and the pulse signal (a) is used as a modulating wave for ASK modulation of a burst signal into a modulated wave (b).
In the case of demodulating the modulated wave (b), the envelope waveform of the burst signal is extracted from the input signal and is waveform-shaped into the pulse signal (a), from which the plurality of data bits is demodulated by marking and spacing portions of the phase signal a "1" and a "0", respectively.
The ASK system represents 1-bit data by a pulse signal of a waveform of the same duration as that of the data, and hence it permits transmission of data in large quantities and is suitable for high-speed data transmission.
However, since the pulse width of the pulse signal (a) modulated by the NRZ data modulation varies under the influence of frequency characteristics of the light emitting element, the light receiving element and the modem circuit and the transmission line used, the pulse width of the demodulated pulse signal does not always become the same as the pulse width of the modulated signal.
For example, the received waveform at the demodulating side may sometimes expand its pulse width, because of a delay in the fall of the pulse waveform as indicated by a received waveform 1 in FIG. 14, or narrow its pulse width, because of a delay in either of the rise and fall of the pulse waveform as indicated by a received waveform 2 in FIG. 14. That is, the pulse width C.sub.0 of the transmitted waveform does not necessarily become the same as the pulse width C.sub.1 or C.sub.2 of the received waveforms 1 or 2.
In the demodulation of data bits from the pulse signal of such a varied pulse width, a shift in its pulse position causes a demodulation error.
On the other hand, there has also been known a PPM system which changes the time interval A.sub.0 between leading edges of the modulated pulse signal according to the data bits, through utilization of the fact that the time width A.sub.0 between leading edges of the pulse signal is equal to the time intervals A.sub.1 and A.sub.2 between the leading edges of the demodulated signals. As shown in FIG. 13(c), according to the PPM system, each data bit of the digital data is preceded by a pulse of a width T and the subsequent pulse quiescent period is represented by a time interval T or 3T, depending on whether the data bit is "0" or "1" and the thus converted pulse signal (c) is used as a modulating wave for the ASK modulation of the burst signal.
With the PPM system, demodulation errors decrease, but since each 1-bit data is represented by the pulse quiescent period, the frame transmission period for sending a plurality of data bits in its entirety is long, giving rise to the problem of low transmission rate.
As a solution to this
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patent: 4472686 (1984-09-01), Nishimura et al.
patent: 5140256 (1992-08-01), Hara
patent: 5185766 (1993-02-01), Cho
patent: 5315299 (1994-05-01), Matsumoto
patent: 5640160 (1997-06-01), Miwa
Chin Stephen
Ghayour Mohammad
SMK Corporation
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