Reference time detecting circuit

Details Reference time detecting circuit Reference time detecting circuit

1983-06-20

1986-10-21

Chin, Tommy P.

Facsimile and static presentation processing

Facsimile

Specific signal processing circuitry

358154, 358155, 358167, H04N 505, H04N 593

Patent

active

046188916

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to a reference time detecting circuit suitable for use with a ghost signal cancelling apparatus which cancels out a ghost in, for example, a video signal stage.


BACKGROUND ART

As, for example, shown in FIG. 1, a signal received by an antenna 1 is supplied through a tuner 2 and a video intermediate frequency amplifier 3 to a video signal detecting circuit 4 by which a video signal is detected. This video signal is supplied through a delay circuit 5 the delay time of which corresponds to the period of duration necessary for cancelling out a pre-ghost signal to a composer 6. Also, a ghost imitating signal from a transversal filter which will be described later is supplied to the composer 6 and thus the composer 6 supplies therefrom a video signal with a ghost signal eliminated to an output terminal 7.
The video signal derived from the video signal detecting circuit 4 is supplied to a delay circuit 8 which constructs the transversal filter. The delay circuit 8 is formed such that delay elements each of which has a sampling period (for example, 10 nano seconds) taken as a unit of delay are connected in plural stages (n in number) so as to achieve a delay time equal to the pre-ghost signal eliminating period and n taps are led out from the respective stages. The signals from the respective taps are supplied to weighting circuits 9.sub.1, 9.sub.2, . . . 9.sub.n, each of which is formed of a multiplier.
The signal from the last stage of the delay circuit 8 is supplied to a terminal 10f of a mode switch 10, while the output signal from the composer 6 is supplied to a terminal 10b of the switch 10. The signal from the switch 10 is supplied to a delay circuit 11. The delay circuit 11 is formed such that delay elements, each of which has a sampling period taken as a unit of delay, are connected in plural stages (m in number) so as to achieve a delay time equal to the delay-ghost eliminating period and m taps are led out from the respective stages. The signals from the respective taps are supplied to weighting circuits 12.sub.1, 12.sub.2 . . . 12.sub.m, each of which is formed of a multiplier.
The video signal from the composer 6 is supplied to a subtracting circuit 13. The video signal from the delay circuit 5 is supplied to a synchronizing separating circuit 14 and the separated vertical synchronizing signal therefrom is supplied to a standard waveform generating circuit 15 and a low-pass filter 16 wherein a standard waveform approximate to a step waveform of a rising edge VE of the vertical synchronizing signal is formed. The standard waveform is supplied to the subtracting circuit 13.
The signal from the subtracting circuit 13 is supplied to a differentiating circuit 17 which detects the ghost signal.
As a signal for detecting and measuring the ghost signal, used is such one that is contained in a standard television signal and not affected by other signal during a period of a duration as long as possible, for example, the vertical synchronizing signal. That is, as shown in FIG. 2, the portion between the rising edge VE of the vertical synchronizing signal and the period of .+-.1/2H (H represents the horizontal period) before and after it is not affected by other signal. Therefore, the afore-said standard waveform is subtracted from the signal of such period and the subtracted signal is differentiated to thereby detect a weighting factor.
If a video signal contains a ghost signal having, for example, a delay time .tau. and a phase difference .phi.(=.omega..sub.c .tau. where .omega..sub.c is a video carrier angular frequency in a high frequency stage) of 45.degree. between it and the video signal, a video signal of a waveform shown in FIG. 3A appears. On the other hand, if such signal is differentiated and inverted in polarity, a ghost detecting signal of a differentiated waveform shown in FIG. 3B is obtained. This differentiated waveform can approximately be regarded as an impulse response of the ghost signal.
The ghost detecting signal of differentiated waveform appearing

REFERENCES:
patent: 3567860 (1971-03-01), Oliver
patent: 3881055 (1975-04-01), Gerot
patent: 4357630 (1982-11-01), Odaka
patent: 4517601 (1985-05-01), Yamada

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