GB2225501A - Television apparatus - Google Patents

Description

2225501 K1 400 1 TELEVISION APPARATUS This invention relates to television

apparatus, and in particular it relates to a television receiving set capable of receiving television signals both in the PAL and SECAM systems.

Different systems of television broadcasting are used in different countries. For instance, the SECAM system is used in France, while the PAL system is adopted in the Federal Republic of Germany.

Figure 3 of the accompanying drawings is a block diagram of a previously proposed television receiving set capable of receiving television signals both in the B,G-PAL and L-SECAM systems. In Figure 3, reference numeral 1 denotes a tuning circuit ts for resistivity designed to satisfy the requirement against input terminal interference in the Federal Republic of Germany. In this set, the television signal, which comes by way of an antenna la, is - in the detected in the heterodyne mode and sent out form of an intermediate frequency signal. Television signals in different modes received from different stations are sent out as intermediate frequency signals from the tuning circuit 1, as indicated in Table 1 below in terms of picture intermediate frequency (PIP) and sound intermediate frequency (SIP).

Table 1

PIPs and SIPs of Signals in different Modes mode B,G-PA1 L-SECIAM PIP 38. 0.M.H.z 38.9MHZ SIP 33.4MHz 32.4MHz Reference numeral 2 denotes a preamplifier for amplifying intermediate frequency signals.

K1 400 Numeral 3 denotes a band-pass filter that utilizes surface elastic waves (SAW) and is designed to pass a _L band having a middle frequency Of 38.gMHz. Since the band-pass filter 3 has a large attenuation rate, it can deteriorate the image signal to noise ratio (SIN) as well as the sensitivity of the TV-set when the intermediate frequency signal sent out from the tuning circuit 1 is directly introduced into this filter. The preamplifier 2 is located in front of the band-pass filter to eliminate deterioration of the characteristics of the TV-set. Numeral 4 denotes a video-detection circuit which detects intermediate frequency signals whose bandwidth has been narrowed by the band-pass filter 3 and the detected videosignals are sent to the output terminal 5 regardless of whether the received television signals are in B,G-PAL mode or in L-SECAM mode.

Reference numeral 6 denotes a band-pass filter designed to pass intermediate frequency bands L having a centre frequency of 5.5MHz. Numeral 7 denotes an FM detection circuit. The output from the video-detection circuit 4 carries PAL audiointerClarrier signals having a frequency of f=5.5MHz, which are selected by the band-pass filter 6 and detected for FM signals by the FM detection circuit 7. The output or the audio-signals from the detection circuit is then brought to the output terminal 8. The audio-signals in PAL mode are demodulated by the use of intercarrier detection techniques.

On the other hand, television signals in SECAM mode, unlike signals in PAL mode or NTSC mode, carry amplitude modulated audio-signals. Therefore, when a television signal in L-SECAM mode is received, the tuning circuit 1 sends out a corresponding 1 1 K1 400 - 3 intermediate frequency signal carrying an audio AM signal wave having a frequency of f=32.4MHz. The audio-AM wave passes through a preamplifier 9 and a band-pass filter 10 with a centre frequency of 32.4MHz and is detected by an AM wave detection circuit 11. Then the audio signal in L- SECAM mode is forwarded to the output terminal 12.

With such an arrangement as described above, a received television signal can be distorted by the signals found in the intermediate frequency amplifier (or the preamplifier 2 in Figure 3) of the adjacent channel which can produce interference and beat effects. Therefore it is desirable to prevent interference by the adjacent channel. In the Pederal Republic of Germany, rigorous requirements for resisitivity against input terminal interference are stipulated in the PTZ standards. In a television receiving set as illustrated in Figure 3, while the detection circuit 1 meets the requirements for resistivity against input terminal interference, a trap 13 is provided in front of the preamplifier 2 to eliminate any possible interference due to the the signals in the adjacent channel, to sal. requirements for resistivity against input terminal interference. The trap 13 typically comprises a capacitor C and a coil L and is designed to have a trap frequency which is determined by (the PIF of the adjacent channel) - 0.5MHz = 32.4MHz.

In Prance the audio-intermediate frequency or SIP for the L-SECAM system coincides with the trap frequency off f =32. 4MHz as shown in Table 1. Therefore, if the television set is used in Prance, it is necessary to keeD the trau 13 inactivated. A switch means comprising a diode D, a resistor R, a switch SW and a power source E is provided for such K1 400 4 circumstances.

More specifically, if the television set is used in the Pederal Republic of Germany (where PTZ is applicable), the user turns on the switch SW to activate the diode D and hence the trap 13. This causes the 32.4MHz component of the intermediate frequency signal to be attenuated, which in turn enhances the resistivity against input terminal interference. On the other hand, in Prance (where the FTZ is not applicable), the user turns off the switch SW to inactivate the trap 13. This enables demodulation of audio-signals for SECAM mode.

A television receiving set of the abovedescribed type requires a switch means for activating/inactivating the trap 13 depending on the geographical area where it is used, entailing a cumbersome switching operation to be carried out by the user of the television set.

Theredfore, it is an object of the invention 'AM two-way television receiving to provide a PAL/SEr'. set that does not require switching operations for activating or inactivating an attenuating trap.

According to the present invention, there is provided a PAL/SECAM two-way television receiving set comprising; a video-detection circuit for receiving intermediate frequency signals and sending out video-detection signals and audio-intercarrier signals in PAL mode, an audiodetection circuit for receiving said intermediate frequency signals and sending out audio-signals in SECAM mode, and a bridge T-type trap for 32. 4MHz provided at the input terminal o-f said video-detection circuit.

In use, even iff the signal frequency is 32.4MHz, the impedence of the bridge T-type trap of a television set according to the invention will not be E 1 1 K1 400 - OA as seen from its input side. Therefore, when television signals in SECAM mode are received, the corresponding intermediate frequency audio signals will be introduced into the audio detection circuit with a sufficient amplitude. Besides, signals with the frequency of 32. 4MHz will be sufficiently attenuated by the bridge T-type trap and therefore will not be introduced into the video detection circuit. Consequently, the resistivity against input terminal interference will be sigificantly improved for receiving television signals in PAL mode.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a block diagram of a - according PAL/SECAM two-way television receiving set to the invention; Figure 2 shows an equivalent circuit diagram of a bridge T-'Uype trap used for the embodiment of Figure 1; and Figure 3 shows a block diagram of a AM two-way television receiving conventional PALISEC set.

Ret"erring to Figure 1 which illustrates a block diagram of an embodiment of the B,G-PAL/L-SECAM two-way television receiving set according to the invention, a bridge T-type trap 14 is located in front of a preamplifier 2 in place of a trap 13 as shown in Figure 3. The bridge T-type trap 14 typically comprises a resistor R1, capacitors C, and C2, and a coil L1, of which the capacitances of capacitors C, and C2 are equal. It will be apparent that the rest of the composition of the - of a television set is identical with that conventional set as shown in Figare 3 and therefore 9 K1 400 6 - corresponding components are indicated by identical reference numerals and will not be further described.

Figure 2 is an equivalent circuit diagram of the bridge T-type trap 14, where a 4 -Y conversion is carried out. As shown in the diagram, the bridge T-type trap of Figure 1 can be replaced by an equivalent Y-type circuit having impedances Z1, Z2 and Z3 Reference signs Ra and Ca respectively denote the resistance and the ^ the resistor Rl capacitance of the equivalents oL and the capacitors Cl and C2, while reference sign r denotes the resistance of the coil L,.

When the frequency of the incoming signal is 32.4MHz, the bridge T-type trap 14 resonates with the signal and hence resistance Ra will become negative, its absolute value being determined so that it is equal to the resistance r of the coil L,. Ehus, the 32.4MHz component of the intermediate frequency signal will be sufficiently attenuated before it enters the preamplifier 2. kConsequently, the resistivity of the set against input terminal interference for television signals in B,G-PAL mode will be significantly improved.

On the other hand, the impedance Z, of the bridge T-type trap 14 will be effective for a signal with the frequency of 32.4MHz. Therefore, an intermediate frequency audio signall (32.4MHz) will be satisfactorily demodulated when the set receives a television signal in L-SECAM mode provided the signal is introduced into the AM detection circuit 11 with a sufficiently high level, although it may undergo a slight attenuation of the level.

- from the above description, As is apparent

1 1 K1 400 7 a television set according to the invention has the advantages of having no switch means for the trap, thus simplifying the overall circuit and of having operational simplicity for receiving signals in PAL or SECAM mode without requiring external switching on/off operations for the trap.

K1400

Claims (2)

1. A PAI/SECAM two-way television receiving set comprising a video-detection circuit for receiving intermediate frequency signals and sending out videodetection signals and audio-intercarrier signals in PAL mode, an audio detection circuit for receiving said intermediate frequency signals and sending out audio-signals in SECAM mode and a bridge T-type trap for 32.4MHz provided at, the input tion circuit.

-ect terminal of said video det

2. A television receiving set as claimed in Claim 1 wherein the bridge 17-type trap comprises a resistor, two equal capacitances and an inductive Coil.

Television apparatus substantially as hereinbefore described with reference to, and as illustrated by, Figu-re 1 or Figure 2 of the acClompantying drawings.

Published 1990 a, The Patent Office. State House. 66 71 High Holbcrn. LondcnWClR4TF_ Further copies naybeobtained from The Patent Office Sales Branch. St MaT:v Crky. Orpington. Kent BRS 3RD Printed by Multiplex techniq,,ies ltd. St Marj Cray. Kent. Con 1'87 1