GB2162009A - A tuner unit - Google Patents

Abstract

A tuner for a video deck or a television set for detecting and outputting video signals and audio signals from signals broadcast by different broadcast systems includes a wide-band tuner 1 which can cover frequencies received from both the PAL system and the NTSC system and outputs an intermediate frequency set to be the lowest frequency of the two television systems. Video IF detectors each of which is composed of a video detection circuit 4, 9 and an audio detection circuit 6, 11 are provided separately for the two television systems. By changing-over the mode with a switch 12, a single tuner can deliver audio/video signals from both the PAL and the NTSC systems. <IMAGE>

Description

SPECIFICATION Tuner unit This invention relates ta a tuner unit for a video deck or television set for detecting and outputting video signals and audio signals from signals transmitted by different television broadcast systems.

Ordinary television sets and video decks have been developed in accordance with the broadcasting frequencies of the countries in which they are to be used and it has been only possible to arrange for reception of broadcasts of a single television broadcast system. However, it is to be expected that a single television or a single video deck for special use in, for example, diplomatic or military service needs to be able to receive the broadcasts of a plurality of television broadcast systems irrespective of the country in which it is used. As such televisions and video decks have been required in the market, the development of tuner units which can meet the requirements of different broadcast systems, such as NTSC and PAL, has been rendered necessary.

The structure of the conventional tuner unit is illustrated in the block diagram shown in Fig. 2. A signal is received and a channel is selected by a narrow band tuner 13, and the signal of the selected channel is converted to a predetermined intermediate frequency signal peculiar to the country of use, such as for example, 45.75 MHz in the USA or 58.75 in Japan. The converted intermediate frequency signal consists of a picture intermediate frequency signal P and a sound intermediate frequency signal S and is amplified in an intermediate frequency amplification circuit 14 and passed through a SAW filter 1 5 such that it is allowed to possess a desired frequency selectivity characteristic.The picture intermediate frequency signal P and the sound intermediate frequency signal S are detected in a video detection section 16, and the detected video signal is amplified in a video amplification circuit 1 7 having a high pass characteristic for intensifying the high frequency range and is output. The frequency modulated sound signal which is obtained by detection in the video detection section 1 6 is demodulated in an audio detection section 1 8 and is output as an audio signal. In the prior art, the tuner unit is operated only in a geographically predetermined intermediate frequency range and it provides a detector for only that one television broadcast system.

Thus, only broadcasts from a single television broadcast system can be received. In order, therefore, to cope with the needs of the market in the prior art, different tuner units which were developed according to the particular local television broadcast systems had to be combined, which brought about problems in terms of increased costs, complications in manufacture and so forth.

Accordingly, it is an object of the invention to provide a tuner unit which can receive television broadcasts from television systems of any country.

It is another object of the invention, rather than simply providing a tuner unit in which conventional tuner units have been combined, to provide a tuner unit which eliminates the deleterious effects of intermediate frequency interference with low frequency channels received in the NTSC system.

It is a further object of the invention to provide a tuner unit which decreases costs and simplifies the manufacturing process.

To this end, in a tuner unit according to the invention there is used a wide-band tuner which can cover frequencies received both from the PAL and the NTSC television broadcast systems. Video IF detectors, each of which consists of a video detection circuit and an audio detection circuit, are provided separately for the two television systems, whereby the tuner can receive broadcasts of either the PAL television broadcast system or the NTSC television broadcast system by changing-over the mode with a switch.

A tuner unit embodying the invention will now be described by way of example with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a block diagram showing an embodiment of a tuner unit according to the invention; Figure 2 is a block diagram showing the conventional tuner unit; Figure 3 is a graph showing an attenuation characteristic in relation to the frequency of a SAW filter used in a video IF detection circuit for the PAL system; and Figure 4 is a graph showing an attenuation characteristic in relation to the frequency of a SAW filter used in a video IF detection circuit for the NTSC system.

Referring now to Fig. 1, there is shown a block diagram of the embodiment. The signal from an antenna is received and a channel is selected by a wide-band tuner 1 and the signal of the selected channel is converted to an intermediate frequency signal by the tuner 1. The tuner 1 uses a wide-band tuner accom modating Europe such as to cover all the broadcasting frequencies of the PAL and the NTSC systems. Since it is a tuner which is used in Europe, the picture intermediate frequency is 38.9 MHz. The intermediatefre- quency signal which has been converted by the tuner 1 is output into both a PAL video IF detector for receiving PAL video/sound sig nals and a NTSC video IF detector for receiv ing NTSC video/sound signals. Reception for the PAL system will first be explained.The intermediate frequency signal which has been converted to an intermediate frequency by the tuner 1 is amplified in an intermediate frequency amplification circuit 2 and is passed through a SAW filter 3 to be allowed to possess a desired frequency selectivity characteristic.

The characteristic of the SAW filter 3 is shown in Fig. 3. In the Figure, the referential symbol P denotes a picture intermediate frequency, S a sound intermediate frequency and C a frequency of chrominance subcarrier.

As is obvious from the Figure, the SAW filter 3 is set such that P and C attenuate from the peak by about 6 dB and S attenuates by about 20 dB. After passing through the SAW filter 3, the picture intermediate frequency signal P and the sound intermediate frequency signal S are detected in a video detection section 4, and the detected video signal is output after the high frequency range of the frequency characteristic is amplified in a video amplification circuit 5 having a high pass characteristic. The frequency modulated sound signal which is obtained by detection in the video detection section 4 is demodulated in an audio detection section 6 and the video signal from the video amplification circuit 5 and the audio signal from the audio detection section 6 are input into a PAL/NTSC changeover switch circuit 1 2.

Reception for the NTSC system will next be explained. The intermediate frequency signal which has been converted by the tuner 1 is amplified in an intermediate frequency amplification circuit 7 and is passed through SAW filter 8 such that it is allowed to possess a desired frequency selectivity characteristic for separation of the P and S signals.

The characteristic of the SAW filter 8 on the NTSC side is shown in Fig. 4. In the Figure, P denotes a picture intermediate frequency, S a sound intermediate frequency and C a frequency of the chrominance subcarrier. As is obvious from the Figure, the SAW filter 3 is set such that P and C attenuate from the peak by about 6 dB and S attenuates by about 20 dB. In the NTSC system, the picture intermediate frequency of a conventional IF detector is 58.75 MHz in the case of Japan, and 45.75 MHz in the case of America, but since this embodiment uses a wide-band tuner for Europe, the intermediate frequency of this embodiment is 38.9 MHz, which is the lowest of the standard intermediate frequencies of both PAL and NTSC systems.After passing through the SAW filter 8, the picture intermediate frequency signal and the sound intermediate frequency signal are detected in a video detection section 9, and the detected video signal is output after the high frequency range of the frequency characteristic is amplified in a video amplification circuit 10 having a high pass characteristic. The frequency modulated sound signal which is obtained by detection in the video detection section 9 is detected in an audio detection section 11 and the video signal from the video amplification circuit 10 and the audio signal from the audio detection section 11 are input into a PAL/NTSC change-over switch circuit 1 2. By the PAL/NTSC change-over switch circuit 1 2 the output signals of PAL and NTSC are switched over and either of the signals is selected to be output in the next step.In this way broadcasts of both the PAL and NTSC systems are processed by a single tuner unit.

In the tuner unit according to the invention a wide-band tuner which can cover frequencies received from both the PAL and the NTSC television systems is used, and a plurality of video IF detectors, each of which consists of a video detection circuit and an audio detection circuit provided separately for processing audio/video signals of the different television broadcast systems, permits broadcasts of either the PAL television system or the NTSC television system to be receivable by changing-over the mode with -a switch.

Accordingly, in spite of the differences in the intermediate frequency, the frequency separation difference between the received frequencies P and S, (6.0 MHz in the case of PAL; 4.5 MHz in the case of NTSC) and the different received frequency ranges, this tuner can receive all television broadcasts of both the PAL and the NTSC systems with the need for parts minimized. Furthermore, by setting the intermediate frequency to be 38.9 MHz, which is the standard intermediate frequency in Europe, the wide-band tuners and SAW filters which are now in use for a tuner directed to Europe can continue to be used as they are. In addition, this invention is free from the following drawback in the prior art: that is, when broadcast of the NTSC system is received in North America, the separation in frequency between the standard intermediate frequency 45.75 MHz and the received frequency of the Channel 2 (55.25 MHz), is only 9.5 MHz whereby, if the characteristics of the tuner are not accurate, the intermediate frequency signal immediately interferes with the input signal when receiving Channel 2. In this invention, since the intermediate frequency is set to be 38.9 MHz, which is the standard intermediate frequency in Europe, and thus is lower than the standard intermediate frequency 45.75 MHz of North America, any interference by the intermediate frequency signal can be removed very practically and effectively.

Claims (4)

1. A tuner unit comprising: a wide-band tuner which can cover frequencies received from both the PAL broadcast system and the NTSC broadcast system and which outputs an intermediate frequency which is set to be the lowest frequency of said television broadcast systems; first and second video IF detectors coupled to the wide-band tuner output, each of which is composed of a video detection circuit and an audio detection circuit for extracting video and audio signals broadcast by said television broadcast systems; and a switch for changing-over the mode such as to allow either the PAL system or the NTSC system signals to be obtained.

2. A tuner unit according to Claim 1 wherein said first video IF detector includes a SAW filter having a frequency selectivity characteristic substantially as shown in Fig. 3.

3. A tuner unit according to Claim 1 wherein said second video IF detector includes a SAW filter having a frequency selectivity characteristic substantially as shown in Fig. 4.

4. A tuner unit substantially as hereinbefore described with reference to Figs. 1, 3 and 4 of the accompanying diagrammatic drawings.