GB2052128A

GB2052128A - Improvements in or relating to tape recording and playback systems

Info

Publication number: GB2052128A
Application number: GB7922707A
Authority: GB
Original assignee: University of Glasgow
Current assignee: University of Glasgow
Priority date: 1979-06-29
Filing date: 1979-06-29
Publication date: 1981-01-21

Abstract

A magnetic tape recording and playback system 10 is disclosed in which the message recorded on the tape is in the form of spaced signal bursts of predetermined frequency and the playback system 12 delivers the recorded message to a gate 20 the output of which is fed to an output stage 24 and the gate is controlled by a comparator 21 having one input predetermined and the other input derived from the message recorded on the tape by means of a level detector 22. The gate 20 is opened only when the detected level of the message exceeds the level of the predetermined input to the comparator 21 and this eliminates unwanted audio echo signals. The reference input to the comparator may be adjustable. In the recording circuit 11, a signal from a source 17 is periodically passed through a gate 15 to a recording stage 18, the gate 15 being controlled by an adjustable- period integrated circuit timer 16. The gates 15 and 20 may be F.E.T.'s and the stages 18 and 24 may be operational amplifiers. <IMAGE>

Description

SPECIFICATION Improvement in and relating to tape recording and playback systems This invention relates to tape recording and playback systems.

It is well known that existing tape recording and playback systems cover a range of performance qualities according to the complexity and frequency responsiveness of the various components and that cassette tape systems, which are relatively inexpensive are also relatively poor in performance quality. A principal advantage which cassette tape systems have, however, is portability as a result of which such systems are extremely popular. In one particular application of tape recording and playback systems used in psychophysiology predetermined signals are recorded on tape and played back to a subject in order to assess the response of that subject to the audio signals and it has been found convenient to utilise a portable tape system for this purpose.

Howe-#r, whether in portable or non-portable systems it has been found that where the tape containing the recorded signal is wound on a reel (such as is contained within a cassette) there is a tendency for the recorded signal to strike through from one convolution of the tape to an adjoining convolution which results in an output audio signal different from that which was intended.In I parlicular, where the recorded signal consists of a plurality of spaced bursts of constant frequency it has been found that the audio output contains echo signals both before and after the intended signals and this negates the usefulness of such taps systems for psychophysiological studies because the subject receives a warning of the oncoming audio burst and the termination of the audio burst is not abrupt It is an object of the present invention to provide an improved tape system which obviates or mitigates the above disadvantage when used in psychophysiological studies.According to the present invention there is provided a tape playback system wherein she message delivered by the tape is applied to a gate the output of which is fed to an output stage, the gate being controlled by a comparator having one input predetermined and the other input derived from said message by means of a level detector, the arrangement being such that the gate is opened only when the detected level of the message exceeds the level of the predetermined input to the comparator.

Preferably the predetermined comparator input is adjustable in level whereby the system can be set so that signals of low amplitude forming part of the message delivered by the tape can be prevented from reaching the output stage of the system by the gate.

The output stage of the system includes a loudspeaker preferably incorporated in a headset which may be worn by a subject Preferably the gate has a rapid turn-on turn-aff response and is linear in amplification. These characteristics are available in field-effect transistors (FETs).

In Phe case where it is desired that the recorded message be in the form of spaced signal bursts of predetermined frequency it is preferred that the tape recording system includes a gate having its output connected to the recording stage and its input connected to receive the signal to be recorded, the gate being controlled by a timing device, the arrangement being such that the gate is opened only for a predetermined time interval.

The time interval may be selectively adjustable and the gate may be in the form of an FET.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which Fig. 1 is a block diagram of a tape recording and playback system, Fig. 2 Illustrates a preferred arrangement of the playback system of Fig. 1, Fig. 3 illustrates a preferred arrangement of the recording system of Fig. 1; and Fig. 4 illustrates a preferred form 0A power supply for the Fig. 2 and Fig. 3 arrangements.

In Fig. 1 of the drawings the tape recorder 10 has a recording system 11 and a playback system 12. The recording system 1 1 includes a gate 15 which is controlled by a timer 16 to permit a signal from a source 1 7 to be transmitted to a recording stage 18. The playback system 12 has a gate 20 which is controlled by a comparator 21 and a level detector 22 to permit a signal on line 23 emanating from the recorder 10 to pass to an output stage 24 having a headphone amplifier 25 and a headset 26.

Fig. 2 illustrates a preferred arrangement of the system 12 from which it will be evident that the gate 20 is in the form of an FET (BF 244) to which the signal from line 23 is a.c. coupled by a 1 OHF capacitor. The output of the gate 20 is formed by the drain of tile FET which is coupled to operational amplifier (307) which forms part of the output stage 24. Control of the gate 20 is effected by an inverting drive transistor (BFY 51) which is connected to the output of the comparator 21 which is in the form of FET (BF 244) whose source is connected through a resistor chain to the negative supply voltage rail (~15 volts).This resistor chain incorporates a 1 Okohm potentiometer whereby the set level of the comparator can be adjusted. The drain of this FET is directly connected to the zero volt supply rail and the bias input is supplied through a 1 .Okohm resistor from the level detector 22 which comprises a diode (IN918) feeding a 22kohm resistor in parallel with a 2.2#F capacitor. The level detector 22 is driven by a transistor (BYF 51) having its base coupled to line 23.

Fig. 3 illustrates a preferred arrangement of the system 11 from which it will be evident that the gate 1 5 is formed by an FET having its drain (D) connected to the input of the recording stage 18 which comprises an operational amplifier (307).

The source of the FET is a.c. coupled by a 1 O,uF capacitor to terminal C to which the signal source 17 is connected and the bias for the FET is applied from the timer 16 which comprises a monostable (555) with an external duration setting 47kohms potentiometer and manually actuable start button.

The gate 15 opens with zero volt bias and closes with negative voltage (-1 5 volts).

The circuit arrangement of Fig. 3 has an amplification factor of approximately 5 and a turn off rejection ratio of at least 70 dB. There appears to be no gating transients at switch on and switch off and rise and fall times are extremely fast (in the context of audio work). The 555 timer has a duration accuracy of 1%, a temperature stability of 0.005% per centigrade degree, and a rise and fall time of 100 nanoseconds.

The circuit arrangements of Figs. 2 and 3 require the provision of + 15 volt regulated voltage supplies and this is conveniently provided by the circuit of Fig. 4 from the standard 250 50Hz a.c. mains. The components 78L15 and 79L15 are series regulators commercially available from R.S. Components.

Claims

1. A recorded tape playback system wherein the message delivered by the tape is applied to a gate the output of which is fed to an output stage, the gate being controlled by a comparator having one input predetermined and the other input derived from said message by means of a level detector, the arrangement being such that the gate is opened only when the detected level of the message exceeds the level of the predetermined input to the comparator.

2. The system of claim 1 wherein the predetermined comparator input is adjustable in level whereby the system can be set so that signals of low amplitude forming part of the message delivered by the tape can be prevented from reaching the output stage of the system by the gate.

3. The system of claim 1 wherein the gate has a rapid turn-on turn-off response and is linear in amplification.

4. A recorded tape playback system substantially as hereinbefore described with reference to the accompanying drawings.