GB2105556A - A signal transmission system - Google Patents

Abstract

In a stereo radio transmitter, should one of the stereo input channels fail, e.g. by being short- circuited, it may be desired to isolate that input from the transmitter and to feed a mono-signal through the other input. In order to do this some form of switch must be provided to isolate the faulty channel from the transmitter and to feed the mono-signal through both channels. The switches S1 to S6 would fulfil this function, but mechanical switches are considered unacceptable and so semiconductor switches need to be used. In accordance with the invention switching devices are inserted in the unbalanced lines downstream of the balanced-to-unbalanced transformers 1A, 1A'. This has the advantage of reducing the number of switches and also minimising the effects of distortion because the switching impedances of the switches are less than the input impedances of the following high-impedance amplifiers 1B and 1B'. This contrast with the much lower input impedances of the transformers 1A, 1A'. The switching devices 1E to 1 H are controlled by signals from a control circuit 6 which can be set to either one of three possible states for normal stereo broadcasting, for conditions where the lower channel is faulty and a mono-signal is being fed through the upper channel and where the upper channel is faulty and a mono-signal is being fed through the lower channel. <IMAGE>

Description

SPECIFICATION A signal transmission system This invention relates to a signal transmission system and particularly, though not exclusively, to an F.M. radio transmitter.

A conventional transmitter will now be described with reference to the accompanying drawing, but ignoring parts 5, 1 E, 1F, 1 G, 1 H and 6 which will be referred to later. Each of two stereo channels are, as in normal in radio transmission studios, presented on a pair of balanced lines. The input signals, which are typically audio signals from microphones, are first fed to an attenuator 1 which adjusts their amplitudes to values which the rest of the transmitter is designed to handle. The attenuator 1 comprises two balanced-to-unbalanced transformers 1 A and 1 A'. Each of these have a secondary winding, one end of which is earthed.

The outputs of the transformers 1A and 1A' are thus in unbalanced form, this being required in order to simplify the circuitry of the transmitter.

Each of the signals is passed to an amplifier 1 B or 1 B' which has a high input resistance, e.g. of the order of 1 OB ohms. The output of each amplifier is applied to a variable, resistive, attenuator 1C or 1 C' terminated by a resistor 1 D or 1 D'. The output of each variable attenuator is applied to a stereo encoder 2 which serves to multiplex the two unbalanced signals onto a single channel.

The multiplexed signals are used in a modulator 3 to frequency modulate an R.F. signal which is amplified by an amplifier 4 and then transmitted via a suitable antenna (not shown), The present invention arose when considering the need for a system such as that which has been described to be modified to transmit a mono broadcast without loss of deviation in the modulator in the event that one of the two stereo inputs should fail. One proposal which was made for meeting this requirement is shown in broken lines by the block 5 on the drawing. This block 5 contains two mechanical switches S1 and S2 which can be closed so as to join the two channels together.If the faulty input channel is the lower of the two channels shown dn the drawing the mono-signal to be transmitted will be fed through the upper channel, through switches S3 and S4, whilst the switches S5 and S6 are opened in order to isolate the faulty input channel from the attenuator 1. Conversely, if the fault is in the upper of the two channels, the monosignal will be applied to the lower channel through switches S5 and S6, whilst the upper channel will be isolated by opening switches S3 and S4. The switches Sv1 to S6 were originally proposed to be mechani.cal switches, but these suffer from problems of reliability in that, owing to oxidation of the contacts, their impedance can increase over a period of time.Solid state switches were also considered unsatisfactory because their impedance, perhaps as much as 1 50 ohms, is comparable to the input impedance, perhaps 600 ohms of the balanced-to-unbalanced transformers 1A, 1A' and therefore results in undesirable losses.

Furthermore, because the solid state switches are non-linear, i.e. the impedance varies according to the input signal, unacceptable distortion arises.

Yet another problem of employing solid state switches would be that under faulty conditions when a mono signal is applied to one channel only, this signal has to pass through two sets of switches in order to reach one of the transformers but only through one set of switches to reach the other transformer. This produces an imbalance due to the impedances of the switches.

This invention provides a signal transmission system comprising a first and a second balancedto-unbalanced transformer, each having a relatively low input impedance, for receiving respective first and second signals from first and second balanced pairs of lines and applying the signals to respective first and second unbalanced lines; and further circuitry having first and second inputs of relatively high impedances for receiving the respective first and second signals from the unbalanced lines and multiplexing them onto one channel; characterised by switching means for connecting the unbalanced lines together.

By employing switching means in the aforementioned manner its input impedance and variations thereof are insignificant or at least small compared with the input impedance of the - said "further circuitry". They therefore cause little or no undesirable effects.

The aforementioned switching means is preferably semiconductor switching means because of its reliability in operation over a period of time.

When the aforementioned switching means is closed, its impedance of, for example, 1 50 ohms is smaller or insignificant compared with the greater input impedances of the further circuitry as exemplified by the amplifiers 1 B, 1 B' shown in the drawings. Any nonlinear behaviour of the solid state switching means therefore has very little effect and losses are minimised.

One way in which the invention may be performed will now be described with reference to the accompanying drawing and in particular to the component 6 and respective first, second, third and fourth switching devices 1 E, 1 F, 1 G and 1H.

The two unbalanced lines are joined by the first semi-conductor switching device 1 E and the fourth semiconductor switching device 1 H.

Second and third semiconductor switching devices 1 F and 1 G are provided to isolate the respective amplifiers 1 B, 1 B' from their associated transformers. The switches 1 E, 1 F, 1G and 1 H are controlled by switching signals supplied by a control circuit 6 which has a threeposition manually operable switch 6A.

For normal operation of the transmitter the switch 6A is in a position which ensures that the control circuit 6 supplies control signals to the semiconductor switches such that the first and fourth switches 1 E and 1 H are open and the switches 1 G and 1 F are closed. The transmitter thus operates in a conventional manner except for the addition of the small impedance of 1 G and 1 F.

This is, however, negligible compared with the input impedances of amplifiers 18 1 B' and therefore has no or little effect.

If the lower of the two input channels (this will be referred to as the first channel) should fail, e.g.

by becoming short-circuited, a mono-signal is applied to the upper of the two input channels (this will be referred to as the second channel) and the switch 6A is moved to a second position such that the control circuit 6 produces control signals which serves to close switches 1 E and 1 G and to open switches 1 F and 1 H. The monosignal is thus passed through switches 1 G and 1 E to both of the amplifiers 18 and 1 B'. Again, because of the high input impedances of these amplifiers the impedance of switches 1 E and 1 G is of little significance. In this condition the switch 1 F serves to isolate the amplifiers 1 B and 1 B' from the faulty channel.

if the upper of the two input channels should fail the mono-signal is applied instead to the lower of the two channels and the switch 6A is moved to a third position whereby the control circuit 6 produces control signals serving to open switches 1 E and 1 G and to close switches 1 F and 1 H. The mono-signal is thus passed through switches 1 F and 1 H to the amplifiers 1B and 1 B'.

It will be appreciated that the illustrated embodiment of the invention requires only four switches 1 E, 1 F, 1 G and 1 H in contrast to the six switches required in the arrangement shown at 5 on the drawing. If semiconductor switches of the type which allow current flow in both directions are used then the switches 1 E and 1 H can be replaced by a single switch connected between the input to the amplifiers 1 B and 1 B'. In such an arrangement only three switches are required.

However the illustrated arrangement (where the switch 1 E is connected directly, i.e. not through switch 1 G, to the amplifier 1 B'; and where the switch 1 H is connected directly to the amplifier 1 B) is preferred since it means that, under faulty conditions, the mono signal is applied to each of the amplifiers 1 B, 1 B' through only one switch.

The imbalance referred to previously is thus avoided.

Whilst the invention arose in the design of a radio transmitter it will be appreciated that it could be applied to other systems where two channels of information are multiplexed onto a single channel.

Claims (7)

Claims

1. A signal trayismission system comprising a first and a second balanced-to-unbalanced transformer, each having a relatively low input impedance, for receiving respective first and second signals from first and second balanced pairs of lines and applying the signals to respective first and second unbalanced lines; and further circuitry having first and second inputs of relatively high impedances for receiving the respective first and second signals from the unbalanced lines and muitiplexing them onto one channel; characterised by switching means for connecting the unbalanced lines together.

2. A signal transmission system according to claim 1 in which the switching means includes a semiconductor switch.

3. A signal transmission system according to claim 1 or 2 comprising a second switching means for isolating the first unbalanced line from the said first input of the further circuity.

4. A signal transmission system according to claim 3 comprising third switching means for isolating the second unbalanced line from the said second input of the third circuitry.

5. A signal transmission system according to claim 4, wherein the first switching means is connected between an upstream end of the third switching means and a downstream end of the second switching means.

6. A signal transmission system according to claim 5 and including a fourth switching means connected between an upstream end of the second switching means and a down stream end of the third switching means.

7. A radio transmitter comprising a signal transmission system substantially as described with reference to the accompanying drawing and substantially as illustrated therein.