GB2024576A - Apparatus for displaying waveforms on a television receiver - Google Patents

Abstract

A vertical waveform is generated on a television receiver in response to, for example, an audio signal by pulses from a monostable 17 which is triggered by line sync pulses from a generator 10 and which resets when a capacitor 20 has charged sufficiently through a transistor 22. The monostable 17 resets to a variable datum level which is determined by a catching diode 23 in accordance with the modulating signal applied to a terminal 28. The modulating signal thereby shifts the location along the line scan of the point of reset. The pulses can be used to modulate luminance and/or colour on a TV receiver. A second monostable 15 generates short pulses for drawing a band on the screen. Two circuits can be used with simple logic to form three coloured patterns on the screen. <IMAGE>

Description

SPECIFICATION Apparatus for displaying waveforms on a television receiver The present invention relates to apparatus for displaying waveforms on a television receiver. The apparatus may be sound-on-vision apparatus, that is to say apparatus for providing changing visual patterns in response to an audio signal. The apparatus according to the invention is an attachment for a television receiver on which the visual display appears, and the invention is defined in claim 1.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit and block diagram of an embodiment of the invention, Figure 2 shows explanatory waveforms, Figure 3 shows the type of display provided by the circuit of Figure 1, Figure 4 shows an alternative type of display in colour, and Figure 5 shows a logic circuit used in providing the display of Figure 4.

In Figure 1 a sync generator circuit 10 utilizing, for example, commercially available integrated circuits such as type LM555 generates line and field synchronizing pulses of standard format.

These pulses are combined at a video output terminal 11 with video pulses generated in a manner to be described and which cause segments of each line scan to be displayed at positions which vary from line to line, so that sinuous vertical bands appear on the screen of a television receiver to which the combined signal is applied. These bands are effectively oscilloscope type traces of an audio signal but with the time axis vertical.

The video pulses can be utilized as simple luminance pulses but can be arranged by conventional techniques to introduce colour information to a colour receiver, so that the bands are displayed as a selected colour on a background of a different contour.

The video pulses have a fixed duration of say 5 us established by a monostable circuit 1 5 formed conventionally by two cascaded NOR gates 12 and 13 and a feedback capacitor 14. The monostable circuit is triggered at a variable time during each line scan by a trigger pulse on one input 1 6 to the gate 12. Blanking pulses from the sync generator 10 are applied to the NOR gate 13 to prevent any video pulses being generated when they could interfere with the sync pulses at terminal 11.

The trigger pulse is provided by a monostable circuit 1 7 formed conventionally by two cascaded NOR gates 1 8 and 1 9 and a feedback capacitor 20. The monostable circuit 1 7 is triggered by sync pulses from the generator 10 applied to the gate 18. The generator applies a blanking signal to the gate 1 9 to prevent the circuit 1 7 being triggered during field flyback, so that the video pulses are then not developed.

The instant at which the monostable circuit 17 resets and provides a trigger pulse is determined by two factors. One is the datum voltage to which the capacitor 20 discharges when the circuit 1 7 is triggered. The other is the rate at which the capacitor recharges. The capacitor recharges at a fixed rate from a constant current source formed by a suitably biased transistor 22.

This transistor may be replaced -by a resistor without departing substantially from a linear charging rate if operation is restricted to the approximately linear region of the exponential charging curve.

The datum voltage is not constant but is determined by the potential on the anode of a catching diode 23 at the instant of triggering the circuit 1 7. A mean value for this potential is established by a resistor 24 and potentiometer 25 and this determines the mean position of the aforementioned vertical band. A varying voltage is superimposed on the mean potential via a capacitor 26 from a transistor amplifier stage 27 to which the audio signal is applied from an imput terminal 28. The amplitude of the audio signal at the instant at which the circuit 17 is triggered thus determines how long elapses before the circuit resets.

Referring to Figure 2, waveform (a) shows the line sync pulses which trigger the circuit 1 7.

Waveform (b) is the blanking pulses applied to the gate 1 3. Waveform (c) represents the voltage at point (c) in Figure 1 on the capacitor 20. The first illustrated sync pulse sets (c) to the datum voltage which has an illustrated mean value of 2v. (c) then rises substantially linearly as the capacitor charges as illustrated at 30. When the threshold of the gate 1 8 is reached, assumed to be 3v, the circuit 1 7 resets. The resulting output waveform from the circuit 1 7 is illustrated at (d) and triggers the circuit 1 5 which provides output pulses (e).

Since the datum level in (c) is modulated by the audio signal, as indicated by arrow 31, and since the ramp segment 30 of (c) is substantially linear, the instant at which the circuit 17 resets varies as is indicated by arrow 32. This varies the timing of the rising edge af (d) as indicated by arrow 33 and the position of the pulses (e) varies, arrows 34.

The position of the vertical band on the television receiver is thus modulated from line scan to line scan by the audio signal.

The circuit as described will produce one band.

Additional bands may be produced by duplicating the circuitry with the exception of the sync generator.

The circuit described is preferred because simple CMOS logic can be used for the gates 12, 13, 18, 1 9. To this end the datum level is varied by way of the catching diode. In an alternative circuit the datum level would stay as fixed by the potentiometer 25 and the threshold level would be varied by using the amplified audio signal as that level. This would require the use of a differential comparator responsive to the threshold level and voltage at (c) to provide a signal to the gate 1 8 for resetting the circuit 1 7.

Figure 3 shows the sort of display which results using the circuit of Figure 1. The pulses 34 generate a band 35 of contrasting colour or luminance. Figure 4 shows another possibility: two channels produce demarcation lines 36 and 37 which form boundaries between red, blue and green areas (R, B, G). Each channel can comprise the circuit of Figure 1 up to the output of the monostable 17. The monostable 15 is not required as each line 36, 37 corresponds to the rising edge of waveform (d), Figure 2.

Let the outputs of gates 18 and 19 be A and A respectively for one of the two circuits and B and B respectively for the other circuit. Different audio or other modulating signals have of course to be applied to the input terminals 28 of the two circuits. The logic circuit of Figure 5 combines A, A, B andg to provide R, G and B signals which will create the coloured areas shown in Figure 4.

Claims (4)

1. Apparatus for displaying waveforms on a television receiver, comprising a line and field synchronising pulse generator, a video pulse generator arranged to generate a video pulse each time it is triggered, and a trigger circuit for the video pulse generator comprising a monostable circuit which includes an DC time constant circuit and which provides the trigger pulse when the voltage on a capacitor of the time constant circuit reaches a threshold level, means responsive to each synchronising pulse to reset the said voltage to a datum level, and a modulating circuit responsive to a modulating signal to vary the threshold level and the datum level relative to each other in dependence upon the amplitude of the modulating signal.

2. Apparatus according to claim 1, wherein the datum level is determined by a catching diode in accordance with a voltage which is modulated by the modulating signal.

3. Apparatus according to claim 1 or 2, wherein there are two of the video pulse generators for two modulating signals, and a logic circuit responsive to pulses from the two generators to provide first and second further pulses in the presence of pulses exclusively from one circuit and the other circuit and to provide third further pulses in the presence of pulses from neither or both of the two circuits.

4. Apparatus according to claim 1 and substantially as hereinbefore described with reference to and as illustrated in Figure 1 of the accompanying drawings.