GB2123630A

GB2123630A - Signal modulating circuitry

Info

Publication number: GB2123630A
Application number: GB08320705A
Authority: GB
Inventors: Frank Woodcock
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-09-21
Filing date: 1983-08-01
Publication date: 1984-02-01
Also published as: GB8320705D0

Abstract

A signal modulating circuit, especially for incorporation in home computers of the kind provided with an internal speaker for providing an audio output, serves to transform the computer-generated audio signals into a form suitable for onward transmission to a TV receiver via a video modulator incorporated in the computer. The modulatable oscillator comprises parallel varicap diodes D1, D2 and a transistor oscillator having a tuned circuit L1, C4, C5 tuned to the intercarrier sound frequency. The audio input is also directly injected over a rectifier diode D3, to improve gain. <IMAGE>

Description

SPECIFICATION Signal modulating circuitry This invention relates to signal modulating circuitry especially for use in conjunction with microprocessor-based signal sources such as home computers and video game units employing television receivers and video displays.

Various devices have been proposed with the object of providing home computers at the cheap end of the market, e.g. SinclairZX-81 and ZX Spectrum, with an audio facility so that, for example, keyboard operation is accompanied by an audible output. On the whole, such devices have tended to be bulky because they employ a built-in speaker and amplifier or alternatively they have required the provision of a separate amplifier and speaker.

The Applicant has previously marketed a device which avoided these drawbacks by making use of the sound channel of the TV receiver used as the video display for the home computer. This device incorporated signal modulating circuitry for producing a suitable carrier signal for input to the TV receiver, the carrier being modulated with an audio frequency signal derived from the home computer.

Although this latter device is considerably more compact than those devices employing internal speakers, it has been found to be undesirably large for incorporation within the compact housings of home computers such as the ZX-Spectrum. Also, the audio output from the TV receiver is subject to a discernible hiss which is undesirable.

The object of the present invention is to provide improved signal modulating circuitry.

According to the present invention there is provided signal modulating circuitry comprising an oscillator arranged to generate a high frequency carrier signal suitable for input to the sound or video demodulator of a TV receiver, the osci!lator tuning circuit including at least one variable capacitance diode to which the modulating signal is to be applied via input means of the circuitry whereby the output of the oscillator comprises said carrier signal modulated in accordance with the fluctuations in the voltage level of the applied modulating signal.

If desired, in addition to applying the modulating signal to the diode or diodes, the input means may be arranged to couple the modulating signal directly into the tuning circuit via a rectifier with resulting increase in signal gain.

The tuning circuit conveniently includes a variable capacitor for manual adjustment whereby the carrier signal can be tuned to the desired frequency. The inductive component of the tuning circuit may be a fixed value inductor.

By employing variable capacitance diodes for effecting modulation of the carrier signal, it has been found that the capacitive elements of the tuning circuit may have relatively low values with the consequence that the circuitry can be kept very compact, to the extent that when the circuitry provided by the present invention is used as an audio modulator, may be easily accommodated within the limited confines of the ZX-Spectrum housing.

Examples of the invention are illustrated in the accompanying circuit diagrams to which reference is now made.

Referring firstly to Figure 1, the transistor Q2 in conjunction with circuit elements C2, C3, C4, C5, L1, D1 and D2 forms an oscillator whose output is coupled via resistor R3 to an amplifier stage including transistor 01. The circuit components forming the tuning circuit of the oscillator stage are selected so that the carrier frequency produced by the oscillator is suitable for input to the sound channel of a TV receiver via the output terminal "OUT" of the circuitry.

Typically for use in the UK, the oscillator frequency will be set to 6 MHz (this being the inter-carrier sound frequency used for TV transmission in the UK) by appropriate adjustment of trimmer capacitor C5.

The carrier signal is modulated by the application of an audio frequency signal to the "AUDIO INPUT" of the circuit. This signal may be derived from a computer such as the ZX Spectrum and will typically be in the form of a pulse train whose mark-space ratio may be varied to represent different frequencies. For example, the keyboard of the computer may be used in the manner of an electronic organ keyboard, the computer being appropriately programmed so that it produces a different mark-space ratio for each of a series of keys of the keyboard.

The pulse trains so produced are applied via coupling capacitor C6 to one or more varicap diodes D1, D2 and also to the junction between capacitors Cl and C2 via diode D3. The effective voltage level of the applied signal will depend upon the mark-space ratio and consequently the capacitances of diodes D1 and D2 will vary according to the mark-space ratio of the signal. In this manner, ths oscillator frequency is modulated according to the input signal derived from the computer. The direct injection of the input signal a diode D3 has been found to afford improved gain. However, such direct signal injection, while preferred, can be omitted if desired.

Figure 2 illustrates a simplified form of the circuit of Figure 1 which, inter alia, omits the amplifier stage and direct injection of the audio signal. In this embodiment, it has also been found possible to omit the supply rail and thereby reduce the number of connections to external circuitry. Thus, in the embodiment of Figure 1, four external connections are needed, namely connection of the high and low voltage rails to a voltage source and earth, connection of the circuit input to for example a terminal of the loudspeaker of the computer and connection of the circuit output to the video modulator.

In Figure 2, the collector of the transistor Q2 is connected directly to the output of the circuit and the circuit is driven by power drawn via the circuit input and output connections with the loudspeaker and video modulator. In other words, the circuit is powered by the voltage source or sources associated with the video modulator and loudspeaker and does not require a separate connection to a voltage source. In practice, the power consumption of the circuit of Figure 2 may be very low.

The elimination of a voltage rail for the circuit along with the elimination of the amplifier stage and direct signal injection allows considerable economies to be made in manufacture of the circuit.

From the foregoing, it will be seen that the audio modulator described above enables a sound output to be derived from a home computer (or other signal source such as a video games unit or tape recorder) by translating the computer audio frequency electrical signal into a modulated high frequency carrier signal compatible with the sound channel of a TV receiver. The output of the audio modulator may be connected to the video input of a UHF video modulator forming part of the computer so that a combined audio/video output is fed to the TV receiver from the computer (or other source) to provide both sound and vision.

As described above, the circuitry of the invention is employed as an audio modulator.

However, it is envisaged that it may function as a video modulator by effecting appropriate tuning of its tuned circuit. For instance, a combined audio/video modulator may be produced by using two circuits each substantially as shown in Figure 1 for example, the two circuits being respectively tuned to the appropriate sound and video carrier frequencies. In such an embodiment, the circuits will be operated in tandem with the output of one connected to the input to the other. Thus, for instance, the first circuit may constitute the audio modulator to which the audio signal is applied and the second circuit may constitute the video signal to which the video signal and the output of the audio modulator are applied.

Claims

1. Signal modulating circuitry comprising an oscillator arranged to generate a high frequency carrier signal suitable for input to the video or sound demodulator of a TV receiver, the oscillator tuning circuit including at least one variable capacitance diode to which the modulating signal is to be applied via input means of the circuitry whereby the output of the oscillator comprises said carrier signal modulated in accordance with the fluctuations in the voltage level of the applied modulating signal.

2. Circuitry as claimed in Claim 1 in which in addition to applying the modulating signal to the diode or diodes, the input means is arranged to couple the modulating signal directly into the tuning circuit via a rectifier.

3. Circuitry as claimed in Claim 1 or 2 in which the inductive impedence of the tuning circuit is constituted solely by a fixed value inductor.

4. Circuitry as claimed in Claim 1, 2 or 3 in which an input terminal of the circuitry is connected to the tuning circuit via a pair of variable capacitance diodes.

5. In combination: a computer incorporating a video modulator for providing a video signal to a television receiver or monitor and means for generating audio and video signals of which the latter are directly coupled to the video modulator; and signal modulating circuitry as claimed in any one of Claims 1~4 for coupling said audio signals to the video input of the video modulator whereby a combined video and audio signal can be supplied to the receiver or monitor.

6. The combination claimed in Claim 5 wherein the computer incorporates an audio output device such as a loudspeaker to which said audio signal generating means is coupled and in which the input of said circuitry is connected so as to tap the audio signals supplied to the output device.

7. The combination as claimed in Claim 4 or 5 in which said circuitry is so arranged that it derives at least part of its power requirement from a voltage source which drives said video modulator, such power being drawn by said circuitry through the agency of the connection between its output terminal and the video input of the video modulator.

8. The combination claimed in Claim 5 in which said circuitry is so arranged that it derives its total power requirement from a voltage source or sources serving to drive said video modulator and said audio output device, such power being drawn by said circuitry through the agency of the connection between its input terminal and said output device and the connection between its output terminal and the input of the video modulator.

9. The combination claimed in Claim 5 in which the video modulator also comprises signal modulating circuitry as claimed in Claim 1 but tuned to a higher carrier frequency than the audio modulator.

10. Signal modulating circuitry substantially as hereinbefore described with reference to, and as shown in, Figure 1 or Figure 2 of the accompanying drawings.