Pulse or digital communications – Repeaters – Testing
Patent
1992-09-21
1995-02-14
Chin, Stephen
Pulse or digital communications
Repeaters
Testing
370 9, 332112, H03K 708
Patent
active
053902130
DESCRIPTION:
BRIEF SUMMARY
This invention relates to digital FM recording of a signal, such as the digital recording of an FM video signal
BACKGROUND
Analog generation of an FM video signal for recording presents various problems, including aging of components, tolerances in the circuitry generating the carrier and the carrier deviation, and pre-emphasis circuitry which vary significantly from the nominal. Digital FM modulating techniques have, heretofore, been cumbersome requiring the use of a substantial amount of circuitry and excessive memory storage.
A feature of the invention is a digital synthesizer technique and system which avoids the disadvantages of analog modulation, without introducing the complexity of typical digital signal processing.
SUMMARY
A digital synthesizer generates a sequence of digital output samples representing modulation of a carrier by an input signal. The synthesizer is supplied with digital input samples representing the input signal. A calculator stage, responsive to the digital input samples calculates a sequence of pulse periods defining a pulse duration modulated signal that represents the modulation of the carrier by the input signal. An output sample generating stage operates on the sequence of pulse periods for generating the sequence of digital output samples representing the carrier modulated output signal.
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1a and 1b illustrate exemplary waveforms of a modulating input signal and a corresponding modulated output signal, respectively, including representative digital samples in both signals, derived in accordance with inventive aspects;
FIG. 2 illustrates a flowchart of an inventive method used to calculate the pulse periods of the FM modulated signal illustrated in FIG. 1b;
FIG. 3 illustrates a flowchart for calculating FM output samples from the calculated values of the FM signal pulse periods; and
FIG. 4 illustrates in block diagram form an FM record system which incorporates an inventive digital FM synthesizer technique.
DESCRIPTION OF THE INVENTION
In an FM modulated system, the FM modulated output signal, FM(t), may be defined as is the deviation.
The solid line waveform of FIG. 1a illustrates the waveform of an exemplary video signal, V(t), such as a luminance signal, that is to be FM modulated onto a carrier prior to recording. The solid line waveform of FIG. 1b illustrates the resultant FM modulated signal, FM(t).
A digital synthesizing technique according to the invention may be used to generate FM(t) from V(t). The synthesis method interprets the FM signal as a pulse duration modulated signal. That is to say, the FM output signal of FIG. 1b may be considered as a pulse sequence, 1,2,3, j . . . , each pulse having a period, T(j), that is determined by the modulating video signal V(t), and by the various FM system parameters such as carrier frequency, CAR, and deviation, DEV. Additionally, the signal may be considered a bipolar one, in the sense that the pulses alternate in polarity.
To ascertain the value of each pulse period T(j) using a closed form equation is possible for only a few simple modulating signals. In general, the modulating video signal V(t) is much too complex for closed form expression.
Advantageously according to an inventive arrangement of a digital synthesizing technique, one need consider only the average frequency, Freq.sub.AV, of FM(t) over an interval .DELTA.t. Taking Equation (2) into account, the average frequency, Freq.sub.AV, may be defined as ##EQU1## i.e. V.sub.AV is the average value of the modulating video signal V(t) over the interval .DELTA.t.
Because the amplitude of the FM modulated output signal, FM(t), of FIGURE 1b is constant, only the frequency, i.e. the time T(j) between zero crossings, is the variable of importance. For this reason, the smallest time interval, .DELTA.t, to consider in evaluating Equations (3) and (4), for any given pulse j, is the corresponding pulse period T(j). In accordance with an inventive feature, the calculation of the intervals T(j) is greatly simplified by providing
REFERENCES:
patent: 4051539 (1977-09-01), Geddes et al.
patent: 4181817 (1980-01-01), Gallo
patent: 4906942 (1990-03-01), Nakai et al.
patent: 4912524 (1990-03-01), Nakamura et al.
IEEE Transactions on Consumer Electronics, Digital Signal Processing in Video Tape Recorders, Sonke Mehrgardt, vol. CE-31, No. 3, Aug. 1985, pp. 374-377.
IEEE Transactions on Consumer Electronics, Experiments on VTR Digital Signal Processing, Kaoru Kobayashi, vol. CE-32, No. 3, Aug. 1986, pp. 355-361.
Chin Stephen
Fried Harvey D.
Laks Joseph J.
Thomson Consumer Electronics Inc.
Tripoli Joseph S.
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