GB2271481A - Receiver for TV sound only which shares motorcar loudspeaker - Google Patents

Abstract

A TV sound receiving equipment which is filled to a car or motor vehicle has a window mounted flat aerial and an intermediate frequency filter and detector means which enhances signal to noise ratio. Also there is provided a means for conveniently sharing any loudspeaker filled in a car with the TV sound receiving equipment. <IMAGE>

Description

IMPROVEMENTS IN TV SOUND RECEIVING EQUIPMENT The device relates generally to a radio receiver for receiving television sound without using the vision part of the broadcast signal.

In particular, the device is specialised for use in a vehicle such as a car.

At this time, many people have become addicted to programs on television and desire not to miss the next gripping episode of a soap opera (say) while on an essential journey by car. A TV sound receiver in a car is required to be different from the conventional design for various reasons.

It would be convenient for the receiver to be suitable for fitting to a vehicle by a person with no special radio/TV skills.

Such a TV sound receiving equipment requires an aerial, a means of amplification of the radio signal and detection of the amplified signal to produce sound in a loudspeaker or headphone. The signal to noise ratio of a radio signal received at a vehicle mounted aerial varies over a very wide range. This is due to the variety of signal paths between transmitter and receiver encountered during a typical vehicle journey. Some of these paths are so long or complex that often a very low or negligible signal is detected: for example behind hills, under a bridge or in a tunnel. Thus there is a need to maximize the signal to noise ratio so that the signal can be heard for as much of the journey as possible. The aerial cannot be directional or large, conveniently.Also, it is convenient for the receiver to be able to use an existing loudspeaker already fitted in a car without interfering with an existing conventional car radio.

A suitable TV sound receiver with the claimed improvements which enable it to be used in a motor vehicle is described below with reference to the accompanying drawings in which: Figure 1 shows the aerial and stripline aerial lead and the aerial mounted in a car.

Figure 2 shows a block diagram of the receiver.

Figure 3 shows a schematic diagram of the first IF amplifier, filter and detector.

Figure 4 displays a graph of the gain of the first IF amplifier and filter against frequency.

Figure 5 shows a schematic diagram of the loudspeaker relay switches.

The aerial is made in the form of a flat folded dipole 1, as shown in figure 1 so that it can be fixed to the inside of a vehicle window. The two connection points of this are connected to a piece of transmission line in the form of a half wave stripline 2, with ground plane connected to point B, such that the impedance at the points A and B over the frequency range in use is about 50 to 75 Ohms, the same as the characteristic impedance of the cable 3, and the input of the first RF amplifier 4 at points A' B'. The amplifier 4, feeds a TV tuner 5, of conventional design which outputs an intermediate frequency which contains the vision carrier and sound signals. These are set at different sub-carrier frequencies according the standards of signal of the TV transmitter.The difference between the vision carrier and the sound sub-carrier frequencies is fixed in most of the TV transmission standards in the world. The vision signals which contain luminance and chrominance are not required by the sound receiver and would tend to interfere with the sound producing an undesirable noise when the signal strength at the aerial is low. Therefore these specific signals are removed by filter 7, and amplifier 6, which has peak gain at vision carrier and at sound carrier frequencies. The signal current output is amplified in 8, and detected by diode 9. Filter 10, is tuned to the difference frequency between sound and vision carriers. This is further amplified by second IF amplifier 11, and mixed with local oscillator 12, amplified by third IF amplifier 13, filtered by narrow band pass filter 14 and detected by limiter & FM detector 15.

The audio output signal is amplified by 16 and fed through relay switches 17 and 18 to loudspeaker 19. The relay 20, is energised when the TV sound receiver is switched on by switch 21 such that the switches 17 and 18 connect the loudspeaker 19, to the output of power amplifier 16. When the receiver is switched off, loudspeaker 19, is connected to an external sound source 23, for example, a normal car radio.

A circuit diagram of the IF amplifier 6, filter 7, IF amplifier 8 and detector & mixer 9, is shown in Fig 3. The collector of Q1 in amplifier 6 is connected to parallel tuned circuit formed by L1 and C2 which are tuned to the vision carrier intermediate frequency. The combination of C5, c4 and L2 and the base capacitance of Q2 together with the components of amplifier 6, form a circuit which resonates at the sound sub-carrier frequency with an anti resonance at the approximate mid-point of the frequency band containing the luminance and chrominance vision signals. For example, a graph of gain of amplifier 6 and filter 7 against frequency is shown in Fig 4. This shows a peak of gain 26 at the vision intermediate frequency 24.5 Mhz and another peak 27 at the sound sub-carrier frequency, 30.5 MHz.The chrominance signal would be centred on about 28.9 MHz and the luminance signal covers the range 24.5 MHz to 30 MHz in this example.

The gain of these signals is significantly less than the gain of the sound signal. Output of filter 7 is current amplified by emitter follower 8. Transistor Q2 and diode D1 are advantageously made from the same material, silicon or germanium (say) such that the DC voltage at emitter of Q2 matches the forward voltage drop of diode D1. With silicon types, the DC voltage at the emitter of Q2 is about .65 Volts for example. This matches the forward voltage dropped by diode D1.

Thus, D1 acts as an efficient detector and the AC signal at Q2 emitter is rectified and filtered by R5 and circuit formed by L3 and C6 which is tuned to the difference between vision and sound carrier frequencies which is 6 MHz in our example. This is the second intermediate frequency. This is amplified by IF amplifier 11 and following stages 12 to 16 which are of conventional design.

Advantageously, a relay circuit 20 with switches 17 and 18 can be used to connect a loudspeaker to the output of the power amplifier 16 when the TV sound receiver is switched on and then disconnect the loudspeaker when the receiver is switched off. A suitable circuit for achieving this is shown in Fig 4.

Advantageously, a correlator or similar device 24 is connected to the output of the last IF amplifier stage 13 and 14, such that the periods of successive cycles of the signal can be compared continuously and when found different in phase by more than a predetermined amount, a squelch circuit 22 can stop the audio output signal from reaching amplifier 16. Then the incidence of a very low RF signal level will not cause a loud noise.

Claims (4)

1. A TV sound receiving equipment using a flat folded dipole aerial with a half wave stripline impedance matching transmission line connected as in figure 1.

2. A TV sound receiving equipment with a filter and detector as shown in figure 3 where the vision intermediate frequency and sound sub-carrier intermediate frequencies are both enhanced and the vision and chrominance signals are attenuated.

3. A TV sound receiving equipment with an automatic means which connects a shared loudspeaking output device to the sound power output of the receiver when the receiver is switched on, after disconnecting the other sharing sound power output. On switching the receiver off, the same means disconnects the loudspeaking device from the receiver and re connects the sharing sound power output.

4. A TV sound receiving equipment substantially as described herein with reference to Figures 1 - 5 of the accompanying drawing.