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Simple Computer Voice (1976)

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Generates those "Dalek"-type voice effects


ORIGINALLY intended to generate a computer sounding voice for an amateur dramatic society, this device can be used to make "Dalek" type voices, and as such can provide hours of entertainment for children.

The circuit is simple, easy to set up, uses little current, can be battery operated, and is suitable for fitting into a child's space suit or Dalek outfit, as well as its original application.

The output level is 500mV, hence it is compatible with both the AUX input on most amplifiers (for stage ruse) and the many available i.e. power amplifiers for battery operation in a child's toy.

CIRCUIT DESCRIPTION

The usual way of producing a mechanical voice is by synthesiser techniques such as ring modulation. An oscillator giving a sine wave output is used to amplitude modulate the audio signal.

A circuit similar to this was tried, but whilst it worked and gave good results, it was somewhat tricky to set up, and there were doubts about its long term stability.

The final circuit behaves in a similar way to a conventional amplitude modulator, but the modulating waveform is a square wave (Fig. 1).

The circuit diagram of the final circuit is shown in Fig. 2. ICI is a 741 operational amplifier arranged as an inverting amplifier. The circuit is designed for a single power supply, hence R3, R4 provide a mid rail voltage at their junction. The gain of the amplifier is determined by the ratio R5/R2. The microphone used has an output of 15mV so the gain is set to 30 to give the required 500mV output.

When relay RLA I contact closes, the gain is reduced and is given by the ratio (VR1 + R6 in parallel with R5)/R2.

RVI therefore controls the "depth" of the modulation and thus the amount of distortion.

MICROPHONE

The moving coil 'microphone used was somewhat ibassy, so capacitor Cl 'was included to give a certain amount of bass cut. As the gain of the amplifier is determined by negative feedback, it is very easy to add shaping should such features as bandpass filtering be required.

If a ceramic or other high output microphone is used, resistors RI and R2 should be increased in value to reduce the gain of the amplifier.

CONSTRUCTIONAL DETAILS

The circuit is constructed on 0.1in Veroboard, and layout and track cutting diagrams are given in Figs. 3 and 4. These are straightforward and should present no problems.

Trim pots are used on the circuit although there is no reason why the pots should not be mounted remotely.

To sum up: VR1 controls the depth of 'the distortion, VR2 the volume, VR3 the rate of distortion. In theory VR1 should affect the volume, but in practice at the levels of distortion necessary, the effect is not unduly noticeable.

MODULATION OSCILLATOR

The relay is "buzzed" by an oscillator (shown in Fig. 3) constructed from the ubiquitous 555 timer. The frequency of oscillation is controlled by R7, R8, C4 and VR3.

VR3 controls the "rate" of the modulation. The relay can be driven up to 250Hz but it was found that the best results were given in the range 20-60Hz.

Diode DI clips any inductive spikes generated as the relay coil de-energises. This is included in the reed relay used in the prototype.

It might 'be thought that the life of the relay would be very short 'being maltreated in this manner, but as the relay used (and most reeds) have a mechanical life in excess of 10 million operations the author did not feel this posed any problem.

Contact life is not so easy to assess as it is determined by two conflicting factors. A large switched current causes contact 'burn, conversely a small arc helps to clean oxidation off the contacts. The current being switched in this circuit is infinitesimal, so the contact life will be determined by the dirt on the. contacts. It is impossible to say when failure will occur, 'but the prototype has been working for several months without showing any signs of iminent death.

The supply can be anywhere in the range 6 to 15 volts (with suitable choice Of reed relay). The prototype was built for 9 volts operation.

With a 9 volt supply, the current consumption is about 12mA.

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  • APA 6th ed.: Parr, E. A. (May 1976). Simple Computer Voice. Practical Electronics p. 406.
  • MLA 7th ed.: Parr, E. A.. "Simple Computer Voice." Practical Electronics [add city] May 1976, 406. Print.
  • Chicago 15th ed.: Parr, E. A.. "Simple Computer Voice." Practical Electronics, edition, sec., May 1976
  • Turabian: Parr, E. A.. "Simple Computer Voice." Practical Electronics, May 1976, section, 406 edition.
  • Wikipedia (this article): <ref>{{cite news| title=Simple Computer Voice | url=http://cuttingsarchive.org/index.php/Simple_Computer_Voice | work=Practical Electronics | pages=406 | date=May 1976 | via=Doctor Who Cuttings Archive | accessdate=18 November 2017 }}</ref>
  • Wikipedia (this page): <ref>{{cite web | title=Simple Computer Voice | url=http://cuttingsarchive.org/index.php/Simple_Computer_Voice | work=Doctor Who Cuttings Archive | accessdate=18 November 2017}}</ref>