RF modulator requiring no test pattern generator

Abstract: An RF modulator according to this invention is con structed so as to allow channel setting without a test pattern generator. Specifically, the RF comprises an A/D converter (25) for digitizing the TV channel setting data, an oscillator control circuit (24) for controlling both the audio carrier oscillation and the RF carrier oscillation on the basis of the channel setting data digitized by the A/D converter (25), a signal adder (22) for combining the video signal (S1) amplitude-modulated at the frequency of the RF carrier oscillation with the audio signal (S2) frequency/amplitude-modulated at the frequency of the audio carrier oscillation, channel picking-up means (17), for picking up a portion near the center of each channel in the channel setting data, and a clipper circuit (15) for cutting of the video signal (S1) at both ends of each channel setting range that have not been picked up by the channel picking-up means.

Description: This invention relates to an RF modulator, and more particularly to an RF modulator built in the VCR (video cassette recorder) which performs TV channel setting by the frequency synthesizer technique. In addition to its main signal processing system, the VCR (video cassette recorder) generally incorporates an RF modulator that RF (radio frequency)-converts the signal from the signal processing system to produce a signal suitable for TV (television). In replaying videotape, the video signal is amplitude-modulated by the amplitude modulator at the frequency generated at the RF carrier oscillator. The audio signal is frequency-modulated by the frequency modulator at the frequency generated at the audio carrier oscillator. The frequency-modulated audio signal is further converted, by the frequency converter, into the frequency generated at the RF carrier oscillator. The amplitude-modulated video signal and the once-frequency-converted audio signal are added to form an RF signal output, which is in turn transmitted to the TV system. Channel setting for a playback of videotape is made by changing the oscillating frequency of the RF carrier oscillator at its variable capacitor. The purpose of channel setting is to put video and audio signals in a vacant TV channel. Each country requires its unique channel setting in the UHF band. To deal with this situation, the variable capacitor is used to cover a wide channel setting range. For the channel setting, the user operates the channel setting mode switch to activate the test pattern generator, whose output signal is connected to the video signal system, with the result that the generatorcreated test pattern is converted into an RF signal that is supplied to the TV system. On the other hand, the TV has received a vacant channel previously. The variable capacitor is adjusted so that the test pattern appears on the vacant channel. After the channel setting is complete, the test pattern generator is disconnected to switch to the normal mode. In an RF generator with such an arrangement, the variable capacitor is used to change the oscillating frequency of the RF carrier oscillator for TV channel setting. This raises a problem: use of the variable capacitor results in the susceptibility to temperature drift and change with passage of time in terms of mechanical accuracy, making it impossible to provide stable reception irrespectively of the proper channel setting. Recent VCRs produce signals usable in place of the test pattern for channel setting, such as a signal that makes the entire screen blue or what is called a blue back. This makes it unnecessary to operate the test pattern generator to supply the test pattern. US-A-4,500,920 shows an RF modulator with a carrier generator circuit for both audio and video, and means for combining modulated video and audio signals. However there is no disclosure of an A/D converter, or a channel picking-up means or signal cut-off means. Accordingly, the object of the present invention is to provide an RF modulator without a test pattern generator which enables stable TV reception by channel setting. The present invention provides an RF modulator as set out in claim 1. With this configuration, the channel picking-up means picks up a portion near the center of each channel in the A/D converter-digitized channel setting data, while the clipper circuit substantially cuts off the video signal at a suitable clipping level at both ends of each channel setting range that have not been picked up. This makes the channel setting immune to temperature drift and change with passage of time in terms of mechanical accuracy. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing the composition of an embodiment according to the present invention; Fig. 2A is a characteristic diagram of the channel setting data with respect to the A/D converter input voltage received by the clipper control circuit of Fig. 1; Fig. 2B shows a signal waveform of the control signal with respect to the A/D converter input voltage received by the clipper control circuit of Fig. 1; Figs. 3A and 3B show waveforms each used to explain the operation of the clipper control circuit of Fig. 1; Fig. 4 is a block diagram illustrating the arrangement of a second embodiment in accordance with the present invention; and Fig. 5 is a block diagram for an application according to the embodiment of Fig. 1. Referring to the accompanying drawings, the present invention will be explained using embodiments. Fig. 1 is a block diagram showing the structure of an RF modulator according to a first embodiment of the present invention. In this invention, a varactor diode-based voltage-controlled oscillator is used in place of a variable capacitor difficult to use for stable TV reception. The oscillator is connected into the PLL (phase-locked loop) to compose a frequency synthesizer-type RF modulator that digitally controls the control voltage of the varactor diode. In replaying videotape, the video signal S1 supplied to terminal 11 is amplitude-modulated, by a modulator unit 12, at the frequency generated by an RF carrier oscillator 13. The modulator unit 12 is composed of a clamper circuit 14, a clipper circuit 15, and an amplitude modulator 16. The clipper circuit 15 is controlled by a clipper control circuit 17. The audio signal S2 supplied to terminal 18 is frequency-modulated, by a frequency modulator 19, at the frequency produced at the audio carrier oscillator 20. This frequency-modulated audio signal S2 is then converted into an RF frequency by frequency converter 21 which also receives the frequency generated by the RF carrier oscillator 13. The amplitude-modulated video signal S1 and the once-frequency-modulated frequency-converted audio signal S2 are added together at an adder 22 and the result appears as the RF signal S out at terminal 23. Both the RF carrier oscillator 13 and the audio carrier oscillator 20 are controlled by an oscillator control circuit 24. The analog input signal from a variable resistor VR serving as an external adjusting element is converted into a digital signal at an A/D converter 25. This digital signal is supplied to both the oscillator control circuit 24 and clipper control circuit 17. The oscillator control circuit 24 determines the frequency dividing ratio and the channel setting data for changing its output frequencies on the basis of the digital signal from the A/D converter 25. The RF carrier oscillator 13 compares its own frequency with that of the oscillator control circuit 24 at a tuning varactor diode 26, which is comprised in a PLL. In this way, the oscillating frequency for each of the audio carrier oscillator 20 and RF carrier oscillator 13 is set digitally. In the clipper control circuit 17 receiving the digital signal from the A/D converter 25, previously prepared channel data is compared with A/D-converted channel setting data to pick up the center of the voltage range to which each channel is set. The action for channel setting will be explained hereinafter. As shown in Fig. 2A, the A/D converter 25 supplies individual channel setting data, in the form of the input voltage of a specific value from the external adjusting element VR and with a specific range of values, to the clipper control circuit 17. Then, the clipper control circuit 17 outputs a control signal that functions in such a way that the channel setting may not be made at both ends of each channel voltage range set at the A/D converter, these ends adjoining the preceding and succeeding ranges. That is, as shown in Fig. 2B, with the clipper control circuit 17, when the setting voltage is present near the center of the channel setting range, the signal V H is supplied, but otherwise, the signal V L is supplied. Receiving the signals V H and V L from the clipper control circuit 17, the clipper circuit 15 changes the clipping level as shown in Figs. 3A and 3B. For the input of signal V H , the circuit 15 performs the normal white clipping of video signal S1 to prevent overmodulation, whereas for the input of signal V L , it substantially cuts off the video signal S1 with the clipping level at a low value. In such an operation, only the video signal near the center of the channel setting voltage range is output. Adjusting the variable resistor VR so that the video signal thus obtained may appear on the screen causes the voltage to be set near the center of each channel. The way for the user to set the output frequency for each channel using the above-mentioned arrangement will be explained hereinafter. It is assumed that the VCR signal system supplies a video signal distinguishable from other stations's broadcasts, such as a blue back. First, the TV reception channel is set for a vacant channel not occupied by a TV station. The channel setting mode switch 27 (shown in Fig. 1) of the RF modulator is then turned on. After this, the variable resistor VR is adjusted so that the video signal from the VCR signal system such as a blue back may appear on the screen. The RF modulator's channel setting switch 27 is then turned off. With this configuration, even if there are fluctuations in the supply voltage, a temperature drift, and the change with passage of time, the adjusting voltage is prevented from straying from near the center of the channel setting voltage range. This allows the construction of an frequency synthesizer-type RF modulator that needs no test pattern generator and provides stable TV reception. Fig. 4 is a block diagram showing the structure of an RF modulator according to a second embodiment of the present invention. The arrangement of Fig. 4 is the same as that of Fig. 1 except that a switch 28 is connected between the clamper circuit 14 and the clipper circuit 15 and a switch control circuit 29 is provided in place of the clipper control circuit 17 to control the switch 28. As with the control signal of Fig. 2B, the switch control circuit 29 produces the signal V H when the setting voltage is near the center of the channel setting range, but otherwise it generates the signal V L . The channel setting range for each channel is supplied in the form of the input voltage from the A/D converter 25. The switch control circuit 29 turns the switch 28 on during the existence of signal V H , causing the clipper circuit 15 to carry out the normal white clipping of video signal S1 to prevent overmodulation. During the period of signal V L , it turns the switch 28 off to cut off the video signal S1. Although both embodiments described above are designed so that the audio signal S2 may be frequency-modulated by the frequency modulator 19, a frequency/amplitude modulator 30 capable of both frequency and amplitude modulations may be used instead of the frequency modulator 19. In this connection, Fig. 5 shows an application of the frequency/amplitude modulator to the Fig. 1's arrangement. The same holds true for the Fig. 4's arrangement. Accordingly, in the VCR using a signal usable as the test pattern signal, such as a blue back, use of the arrangements according to the above-mentioned embodiments makes it possible to provide an RF modulator that functions as a frequency synthesizer without a test pattern generator and assures stable TV reception in channel setting. Reference signs in the claims are intended for better understanding and shall not limit their scope.

Claims: An RF modulator that performs TV channel setting, comprising: an A/D converter (25) for digitizing a channel setting signal, which can take a range of values for each channel; an audio carrier oscillator and an RF carrier oscillator; an oscillator control circuit (24) for controlling both the audio carrier oscillation and the RF carrier oscillation on the basis of the channel setting signal digitized by said A/D converter (25); signal synthesising means (21, 22) for combining the RF carrier oscillation amplitude-modulated by a video signal (S1) with said audio carrier oscillation, frequency/amplitude-modulated by an audio signal (S2); channel picking-up means (17, 29) for determining when the digitized channel setting signal is in the range portion at or near the center of the range of channel setting signal values for each channel; and signal cut-off means (15, 28) for cutting off said video signal (S1) when said channel picking-up means determines that the digitized channel setting signal is outside said range portion for any channel. An RF modulator according to claim 1, characterized in that said channel picking-up means (17, 29) compares the channel setting signal from said A/D converter (25) with the previously prepared channel signal to produce a signal (VH,VL) representing whether said channel setting signal is in or outside said range portion, for any channel. An RF modulator according to claim 1, characterized in that said signal out-off means includes a clipper circuit (15) for cutting off said video signal (S1) by clipping said video signal (S1) at a suitable clipping level. An RF modulator according to claim 1, characterized in that said signal cut-off means cutting off said video signal (S1) includes a switch circuit (28) for cutting off the transmitting path of said video signal (S1). An RF modulator according to claim 3, characterized in that said channel picking-up means comprises a clipper control circuit (17) that controls the clipping level of said clipper circuit. An RF modulator according to claim 4, characterized in that said channel picking-up means comprises a switch control circuit (29) for controlling the switching of said switch circuit (28).

Inventors: MORI HIROYOSHI, SHIGETAKA SAWA, MORI, HIROYOSHI, SHIGETAKA, SAWA, MORI, HIROYOSHI, C/O INTELLECTUAL PROPERTY DIV., SHIGETAKA, SAWA, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA AVE KK, TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA, TOSHIBA AVE CO., LTD
Application year: 1991
Main classification: H04N5
IPC codes: H04N5