JPH0548985A - Receiving circuit for television signal - Google Patents

Abstract

PURPOSE:To easily receive a second television signal S2 without using a dedicated receiver. CONSTITUTION:A local oscillator 35 capable of oscillating a local oscillation frequency in such range as can receive a first television signal S1 present in a first frequency band, is provided. When the second television signal S2 having a frequency higher than that of the first television signal S1 as a video carrier is received, a tuner 32 is changed over to a lower side heterodine system and the frequency is converted. At the same time when the frequency conversion system is changed, the polarity of a AFT control operation is reversed in a polarity reversing circuit 38. Thus, since the conversion into the same intermediate frequency signal as in the case where the first television signal 1 is received is avialable even when the local oscillator 35 capable of oscillating only the local oscillation frequency in such range as can receive the first television signal S1 is used, the dedicated receiver is not required.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television signal receiving circuit suitable for application to a video wireless transmission / reception system or the like.

[0002]

Video wireless transceiver systems are known. This video wireless transmission / reception system is an indoor transmission / reception system which is prevalent in the United States and the like. For example, a video signal (for convenience, referred to as a second television signal S2) captured by the video camera 12 as shown in FIG. When viewed on the television receiver 26, since the cable connecting the video camera 12 and the television receiver 26 is generally short, 1 to 2 m, the video camera 12 must be operated close to the television receiver 26. ..

In order to solve such inconvenience, a video wireless transmission / reception system 10 has been developed, and the video camera 12 is operated even from a place away from the television receiver 26 to a certain degree. Is displayed on the television receiver 26.

The outline thereof is as shown in FIG. 6. For example, the second television signal S2 output from the video camera 12 is transmitted by using the video wireless transmitter 14. 16 is a transmitting antenna (rod antenna or the like). In the United States, the current frequency band of TV channels (8
Since the frequency band higher than 3 channels is addressed as a civic band, the transmission frequency is video wireless frequency (902-928MH) in this civic band.
z).

A receiving antenna (rod antenna or the like) 18 is attached to the receiver 20 installed indoors, and the second television signal S2 received by the receiving antenna 18 is transmitted to the receiver 20 through the TV channel. After being frequency-converted into a television signal of an empty channel (for example, 3 channels or 4 channels of VHF band), it is supplied to an antenna input terminal (not shown) of the television receiver 26 via the input switching means 22. It

The input switching means 22 includes an outdoor antenna 2
A television signal for broadcasting (referred to as a first television signal S1) received at 4 or the like is supplied, and one of them is selected manually or automatically. This input switching means 22 is either the receiver 20 or the television receiver 26.
It is provided inside. In addition to the video camera 12, the input source for the transmitter 14 is a stationary type VCR (VC).
R), cable TV, etc., and there is no particular restriction on the input source.

[0007]

By the way, the receivable range of the receiving tuner built in the television receiver capable of receiving the first television signal S1 is up to 83 channels as described above. On the other hand, the transmission frequency (video carrier) for video wireless approved as the video wireless transmitter 14 is in a higher frequency band (902 to 928 MHz).

Therefore, when the variable range of the local oscillation frequency of the tuner is up to the range in which the first television signal S1 can be received (as is the case with many television receivers), the tuner is used as it is. Therefore, the second television signal S2 cannot be received. Therefore, the receiver 20 having the frequency conversion function as shown in FIG. 6 is used as an adapter.

Even if such a receiver 20 is not used, the second
It is preferable that the television signal S2 can be received.

Therefore, the present invention solves the above-mentioned conventional problems, and provides a television signal receiving apparatus capable of receiving the second television signal S2 without using a dedicated receiver. It is a proposal.

[0011]

In order to solve the above problems, the present invention oscillates only a local oscillation frequency in a range capable of receiving a first television signal existing in a first frequency band. When a second television signal having a local oscillator capable of receiving a second television signal whose frequency is higher than that of the first television signal is used as a video carrier, when the second television signal is received, By switching the tuner to the lower heterodyne system and performing frequency conversion, it is converted into the same video intermediate frequency signal as when the first television signal is received, and the polarity of the AFT control operation is inverted. It is characterized by that.

[0012]

In FIG. 1, the local oscillator 35 provided in the tuner 32 is capable of oscillating only a local oscillation frequency in a range capable of receiving the first television signal S1 existing in the first frequency band. used. The first television signal S1 is mainly a signal from a broadcasting station.

With respect to this first television signal S1,
The second television signal S2 having a frequency higher than this as a video carrier can be received. The second television signal S2 may be, for example, a video wireless signal.

When the second television signal S2 is received, the tuner 32 is switched to the lower heterodyne system to perform frequency conversion, so that the first television signal S1 is converted into the same video intermediate frequency signal as when it was received. To be done. At the same time, the polarity of the AFT control operation is reversed. This is the difference between the upper heterodyne frequency conversion method and the lower heterodyne frequency conversion method.
This is because the control operation of the FT is reversed.

As described above, the frequency conversion is performed even when the local oscillator 35 that can oscillate only the local oscillation frequency in the range that can receive the first television signal S1 existing in the first frequency band is used. If the system is changed to the lower heterodyne system, the second television signal S2
Can be received in the same manner as the first television signal S1.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a case where an example of a television signal receiving circuit according to the present invention is applied to a television receiver used in the above-mentioned video wireless transmission / reception system will be described in detail with reference to the drawings.

A video wireless transmission / reception system to which the present invention can be applied is constructed as shown in FIG. 3, which is almost the same as the conventional example shown in FIG. The present invention is different from the conventional example in that the second television signal S2, which is a video wireless signal, can be received without using the receiver 20. Instead, the television receiver 26 is provided with a television signal receiving circuit 30 for implementing the function of the present invention.

It is a well known fact that the upper heterodyne system is adopted as the frequency conversion system in the tuner when receiving a normal television signal. Therefore, as shown in FIG. 4, the video intermediate frequency fIF (45, 75 MHz) is applied to the video carrier fp of the television signal.
A frequency fOSC that is as high as that of the local oscillation frequency is selected as a local oscillation frequency, whereby the first television signal S1 is frequency-converted into a video intermediate frequency signal.

FS represents a voice carrier. fP 'and fS' indicate a video carrier and an audio carrier in the IF band.

However, the receivable range of the tuner is up to 83 channels (885.25 MHz) as described above, and the transmission frequency (video carrier) for video wireless is used.
Is higher frequency range (902-928MHz)
Therefore, even if frequency conversion is performed by the upper heterodyne system, the frequency range from 902.0 + 45.75 = 947.75 MHz to 928 + 45.75 = 973.75 MHz must be covered with the second television signal S2. I won't. This is outside the capacity limits of the tuner, as mentioned above.

Therefore, when receiving the second television signal S2, the frequency conversion system is changed from the upper heterodyne system to the lower heterodyne system. Then, as shown in FIG. 5, the frequency fOSC lower than the video carrier fp of the television signal by the video intermediate frequency fIF (45.75 MHz) is selected as the local oscillation frequency. The frequency of S1 is converted into a video intermediate frequency signal. fS1 and fS2 indicate double sideband voice carriers.

As a concrete example of numerical values, a local oscillation frequency from 902.0−45.75 = 856.25 MHz to 928.0−45.75 = 882.25 MHz is obtained, which allows the existing tuner to be used as it is. Even if used, the second television signal S2
Can be converted into a predetermined video intermediate frequency signal.

In FIG. 5, fP ', fS1', fS
2'denotes a video carrier and an audio carrier in the IF band. The voice carrier fS2 'is later filtered.

FIG. 1 shows an example of a receiving circuit 30 which realizes such a frequency conversion operation.

The television signal received by the antenna is guided to the tuner 32. The tuner 32 includes a high frequency amplifier 33, a mixer 34, and a local oscillator 35, as is well known.

In this example, the local oscillator 35 is composed of a PLL, and as described above, the local oscillation frequency for each channel is selected so as to be the upper heterodyne system when receiving the first television signal S1. The local oscillation frequency for the channel is selected so that the lower heterodyne system is used when receiving the television signal S2.

The video intermediate frequency signal output from the tuner 32 is output via the video intermediate frequency amplifier 36. The video intermediate frequency signal extracted from between the stages of the video intermediate frequency amplifier 36 is supplied to the AFT circuit 37 to form an AFT signal (automatic frequency adjustment signal) for fine adjustment of the video intermediate frequency signal. It is supplied to the tuning microcomputer 39 via the.

The AFT input by the tuning microcomputer 39
A tuning signal (a tuning voltage, a frequency division ratio for a frequency divider, etc.) is generated for the local oscillator 35 based on the signal, and a control signal for the polarity control circuit 38 is generated. The frequency conversion method (upper heterodyne method or lower heterodyne method) can be easily changed by selecting the division ratio.

The polarity control circuit 38 is provided for the following reason. That is, when the tuning signal is generated from the AFT signal when the first television signal S1 is received, the following operation is performed. According to the upper heterodyne method, for example, when the local oscillation frequency shifts to the lower side, the video carrier (frequency) of the intermediate frequency amplifier 36
Shifts to the lower side of the normal value. Such a signal is AF
When input to the T circuit 37, the AFT signal (voltage) becomes higher than the reference voltage, so that a tuning signal for correcting the local oscillation frequency to the higher side is generated.

However, since the frequency conversion method of the second television signal S2 is changed to the lower heterodyne method, the AFT control operation also needs to be reversed from the above. Therefore, when receiving the second television signal S2, the polarity of the AFT signal is inverted based on the control signal from the tuning microcomputer 39.

Of course, the S-shaped detection characteristic of the AFT circuit 37 may be inverted, and the same processing may be performed, and such processing may be performed by the tuning microcomputer 39 by software.

The tuning microcomputer 39 has a built-in control program for performing the processing shown in FIG.
That is, FIG. 2 shows an example of processing up to generation of the control signal, and the channel selection mode is the second television signal S.
When the channel selection reception mode of 2, that is, the video wireless band is set (steps 41 and 42), polarity inversion processing of the AFT signal is performed (step 43).

Although the present invention is applied to the television receiver for the video wireless transmission / reception system in the above description, this is only an example.

[0034]

As described above, in the television signal receiving apparatus according to the present invention, the second television signal whose frequency carrier is higher than that of the first television signal is received. Sometimes, the tuner is switched to the lower heterodyne system to perform frequency conversion so that the first television signal is converted into the same video intermediate frequency signal as when it was received.

According to this, even if the local oscillator capable of oscillating only the local oscillation frequency within the range capable of receiving the first television signal existing in the first frequency band is used, the second television signal is generated. Can be received. Further, according to this configuration, it is not necessary to use a dedicated receiver for receiving the second television signal as in the related art, which is economical.

Therefore, the present invention is extremely suitable when applied to the above-mentioned video wireless transmission / reception system and the like.

[Brief description of drawings]

FIG. 1 is a system diagram showing an example of a television signal receiving circuit according to the present invention.

FIG. 2 is a flowchart showing an example of a channel selection processing flow when receiving a second television signal.

FIG. 3 is a system diagram of a main part showing an example of a video wireless transmission / reception system.

FIG. 4 is a diagram of a video carrier and a local oscillation frequency showing an example of frequency conversion by the upper heterodyne method.

FIG. 5 is a diagram of a video carrier and a local oscillation frequency showing an example of frequency conversion by a lower heterodyne system.

FIG. 6 is a system diagram of a conventional video wireless transmission / reception system.

[Explanation of symbols]

 10 Transmitting / Receiving System 12 Video Camera 14 Transmitter 26 Television Receiver 30 Receiver Circuit 32 Tuner 34 Mixer 35 Local Oscillator 36 Video Intermediate Frequency Amplifier 37 AFT Circuit 38 Polarity Control Circuit 39 Channel Selection Microcomputer

Claims (1)

[Claims] 1. A local oscillator capable of oscillating only a local oscillation frequency within a range capable of receiving a first television signal existing in a first frequency band, and for the first television signal, When receiving a second television signal having a frequency higher than this as a video carrier, when the second television signal is received, the tuner is switched to the lower heterodyne system to perform frequency conversion. 1. A television signal receiving circuit, characterized in that the television signal of No. 1 is converted into the same video intermediate frequency signal as that received, and the polarity of the AFT control operation is inverted.