Monochannel Audio Teaching Device

Abstract

A monochannel audio teaching device comprises an audio sender which multiplexes several audio signals into a single composite signal for transmission over single channel audio circuits for recording on the single channel magnetic tape, and an audio receiver which recovers the various audio signals from the single composite signal received. The sender provides inputs for two or more sources of audio information, related to, for example, multiple choice questions. Two frequency-multiplexed channels are provided and into each of these channels one or more audio signals can be fed on a time-multiplexed basis. Control signals precede each unit of information being time multiplexed. The control signals and Frequency/Time-multiplexed audio signals are combined to form a single composite signal or recording for subsequent use at the receiver. At the audio receiver the signals are demultiplexed and their control signals cooperate with student operated switches to select which unit of information is to be heard.

Description

This invention pertains to studio training devices and, more particularly, to such devices wherein multiple units of information are transmitted over a single channel.

There are presently available audio training devices in the form of multitrack tape players which teach a student by means of multiple choice questions. A first track carries the multiple choice question while parallel tracks carry statements associated with the possible choices to tell the student whether he made the correct choice and, if not, why.

While such presently available devices perform the required teaching function they require expensive multihead tape recorders and players. Most inexpensive tape players are single track devices. Since many tape players are required in any broad based training program it is desirable to use single track tape players. Furthermore, for many systems only a single channel is available.

It is, accordingly, a general object of the invention to provide an audio teaching device wherein several related units of information are carried on only one channel.

Briefly, the invention contemplates an audio teaching apparatus which comprises a source of at least three audio signals, each representing a different but related unit of teaching information. Associated with each of the audio signals is a unique control signal. Signal multiplexing means frequency multiplex a second and a third of the audio signals to provide frequency multiplexed signals. Means serially transmit, on a single channel, the first of the audio signals and an associated control signal followed by the frequency multiplexed signals and a control signal associated with the second and third audio signals.

There is further contemplated an audio receiver which receives the signals on the single channel and demultiplexes them back to first, second and third audio signals which are selected for listening by subject-operable switches cooperating with the control signals.

Other objects, the features and advantages of the invention will be apparent from the following detailed description when read with the accompanying drawing which shows apparatus for practicing the invention.

In the drawing:

FIG. 1 shows a block diagram of an audio sender in accordance with the invention;

FIG. 2 shows a block diagram of an audio receiver in accordance with the invention;

FIG. 3 shows a schematic of a control switch utilized in a control panel of the audio sender of FIG. 1;

FIG. 4 shows a block diagram of the switching circuits of the audio sender of FIG. 1;

FIG. 5 is a block diagram of an audio multiplexer in the audio sender;

FIG. 6 is a block diagram of a tone generator in the audio sender;

FIG. 7 are waveforms indicating signals at particular points in the tone generator of FIG. 6;

FIG. 8 is a block diagram of an audio demultiplexer in the audio receiver of FIG. 2;

FIG. 9 is a block diagram of a tone receiver in the audio receiver; and

FIG. 10 is a block diagram of the selection circuits in the audio receiver.

The disclosed system shows an audio sender AT which receives audio signals representing multiple choice questions and answers and records these signals on the single channel of a magnetic tape of a cassette, for example. By way of example, a question posing four choices is presented and is followed by statements concerning the correctness of each of the four possible choices. After a complete tape is prepared the cassette, or preferably a cassette which is a copy of the master can be inserted in an audio receiver AR which can be a modified cassette tape recorder. A student then runs the audio receiver and in response to each question selects one of the possible choices by depressing an appropriate switch. The switch causes the audio reproduction of the statement associated with the choice.

More particularly, the format employed is to generate and record a first tone control signal which is unique to questions and general comments and then record the question or general comments, then to generate and record a second tone control signal unique to first and second choices and start recording simultaneously the statements for the first and second choices. At the end of the first and second statements, the routine is repeated for the third and fourth choices. Now, the statements for the first and second choices are frequency multiplexed into one channel and are followed by the third and fourth choices which are frequency multiplexed into the same channel. It should be noted that the channel now carries in three time multiplex slots: first, the question preceded by its tone control signal; second, the frequency multiplexed statements associated with first and second choices preceded by their tone control signal; and third, the frequency multiplexed statements associated with the third and fourth choices preceded by their tone control signal.

By way of example and not limitation, the following parameters will be used assuming a channel passband of 9 kHz. The first and third statements will be time shared in a band from zero up to 3 kHz., the second and fourth statements time shared in a band from 3 to 6 kHz. and the control tones as coded bursts of 9 kHz. carrier. However, it should be realized that other frequencies could be used.

Now, refer to FIG. 1 which shows five audio sources AS1 to AS5 which can be instructors with microphones under the control of a director or can be prerecorded tape units or the like which transmits audio signals, via lines FAS1 to FAS5, to switching circuits SKS. Circuits SKS switch the signals from sources AS1, AS3 and AS5, onto line SLO under the control of signals from control panel CP; and switch the signals from sources AS2 and AS4 onto line SHI by control signals on the lines from the control panel CP.

The signals on lines FCPl-2, FCP3-4 and FCP5 are generated by selectively depressing the switches CP1, CP2 and CP3, respectively, on control panel CP. Whenever any one of these three switches is depressed a pulse signal is also transmitted via a related line T1-2, T3-4 and T5 to tone generator TG.

Tone generator TG in response to these pulse signals transmits coded combinations of bursts of 9 kHz. signal via line FTG and audio multiplexer AM to tape recorder TR. Audio multiplexer AM receives the time-multiplexed signals on lines SLO and SHI and frequency multiplexes these signals. These frequency multiplexed signals are transmitted via line FAM and amplifier A1 to the tape recorder TR for recording.

The recording of a typical question will now be described assuming the tape recorder TR is operating. A director depresses switch CP3 of control panel CP. In response thereto, a pulse signal is fed via line T5 to tone generator TG which in turn generates a 100 millisecond burst followed by three spaced 10 millisecond bursts of 9 kHz. carrier which pass through audiomultiplexer AM to tape recorder TR. Hereinafter, the 100 millisecond burst will be called a "dash" burst and the 10 millisecond burst a "dot" burst. The depression of switch CP3 also generates signals on line FCP5 which opens paths in the switching circuits SKS from line FAS5 to line SLO. The director then directs the instructor at audio source AS5 to state the multiple choice question which is transduced to an audio signal in the base band (0 to 3 kHz.). The audio signal passes through switching circuits SKS and multiplexer AM and is recorded in the base band range by the tape recorder TR.

At the end of the question, the director releases switch CP5 and depresses switch CP1 which causes the generation of a pulse signal on line T1-2 and a signal on line FCP1-2. The pulse signal on line T1-2 causes tone generator TG to generate a dash burst followed by one dot burst of 9 kHz. carrier which passes through multiplexer AM into tape recorder TR. At the same time, the signal on line FCP1-2 causes switching circuits SKS to close a path from line FAS1 to line SLO and a path from line FAS2 to line SHI. When the director depressed switch CP1 he ordered the instructors at sources AS1 and AS2 to make simultaneously the statements concerning the first and second possible choices, respectively, the audio signals of the statement from source AS1 pass via circuits SKS and line SLO into multiplexer AM in the base band range. The audio signals from source AS2 (in the 0 to 3 kHz. base band) pass via circuits SKS and line SHI into multiplexer AM. It should be noted that now the audio signals from sources AS1 and AS2 are simultaneously entering the multiplexer AM, via lines SLO and SHI, respectively. Audio multiplexer AM frequency multiplexes these signals. In particular, all audio signals of line SLO remain in the base band. However, the signals on line SHI are frequency converted up to the band of 3 to 6 kHz. The up-converted signals from line SHI and the base band signals from line SLO are amplitude added by the multiplexer to provide a frequency multiplexed signal having a band of 0 to 6 kHz. which is fed to tape recorder TR.

When both instructors have finished their statements the director releases switches CP1 and depresses switch CP2 causing the generation of a pulse signal on line T3-4 and a signal on line FCP3-4. The pulse signal on line T3-4 causes tone generator TG to emit a dash burst followed by two dot bursts of the carrier signal which are recorded in the usual manner. The FCP3-4 signal causes circuits SKS to close a path from line FAS3 to line SLO and a path from line FAS4 to line SHI. The director orders the instructors at sources AS3 and AS4 to speak simultaneously the statement associated with the third and fourth choices, respectively. The associated audio signals from source AS3, at base band, enter multiplexer AM via line LSO. The associated audio signals from source AS4, at base band, enter the multiplexer AM via line SHI. It should be noted that they are in parallel with the signals from source AS3 on line SLO. The signals on line SLO pass through the multiplexer at base band while the signals on lines SHI are up converted to the 3 to 6 kHz. band. The base band signals from line SLO and the up converted signals from line SHI are amplitude added and recorded on the tape recorder TR. At the end of the statements by sources AS3 and AS4 the director releases switch CP3 and the recording cycle for one question is completed.

Before describing the playback of the recorded information the various elements of the audio sender AS will be described.

The control panel CP comprises three control switches CP1 to CP3. Since switches CP1 to CP3 are alike only control switch CP1 is shown in FIG. 3. Switch CP1 comprises a pair of ganged single-pole single-throw switches SW1 and SW2 whose movable contacts are connected to a voltage source +v. The fixed contact of switch SW1 is connected, via a pulldown resistor R1, to a voltage source -v, and to line FCP1-2, when switch SW1 is open, line FCP1-2 is at a negative voltage (assumed to be the absence of signal), but when switch SW1 is closed, line FCP1-2 carries a positive signal. The fixed contact of switch SW2 is connected, via differentiating capacitor C1, to line T1-2. The junction of capacitor C1 and line T1-2 is connected via resistor R2 to voltage source -v. Whenever switch SW2 is open, line T1-2 is at a negative level. However, when switch SW2 is closed, a pulse is transmitted onto line T1-2.

The switching circuits SKS, shown in FIG. 4, comprises five similar gates G1 to G5 which can be conventional analog gates, each having a signal input, a signal output and a control input. Signals can only pass from the signal input to the signal output when a signal is present at the control input. The signal inputs of the gates G1 to G5 are connected to the lines FAS1 to FAS5, respectively. The signal outputs of the gates G1, G3 and G5 are connected, via mixer M1, to the line SLO, while the signal outputs of the gates G2 and G4 are connected, via mixer M2, to line SHI. The control inputs of gates G1 and G2 are connected to line FCP1-2. The control inputs of gates G3 and G4 are connected to line FCP3-4 and the control input of gate G5 is connected to line FCP5.

The audio multiplexer AM, shown in FIG. 5, centers around adding operational amplifier AOA having four inputs. The amplifier can be a conventional amplifier analog adder which adds the signals fed to its inputs to transmit the amplitude sum of these signals onto line FAM.

The first input receives the base band audio signals from line SLO via amplifier A2, low-pass filter LP1 and amplifier A3. Filter LP1 insures that only signals below 3 kHz. enter the first input. The base band signals of line SHI pass, via amplifier A4 and low-pass filter LP2, to the modulating signal input of conventional lattice modulator LM. Filter LP2 insures that only signals below 3 kHz. enter the modulating input. The carrier input receives a 3 kHz. signal from carrier oscillator CO. Modulator LM up converts the signals on line SHI to signals in the band of 3 to 6 kHz. which are fed, via amplifier A5, to the second input of amplifier AOA. The third and fourth inputs to amplifier AOA are connected to oscillator CO and line FTG, respectively.

Tone generator TG comprises three set-reset flip-flops F1 to F3 whose set inputs S are connected to lines T1-2, T3-4 and T5, respectively, and whose reset inputs R are connected to sequential outputs 1 to 3 of step counter SSK. The "1" output of each of the flip-flops F1 to F3 is connected to an input of OR-circuit OR1. The output of OR-circuit OR1 is connected to the input of one-shop OS1 and to the reset input of set-reset flip-flop F6. Thus, whenever one of the flip-flops F1 to F3 is set the signal "1," line FL jumps from a low voltage to a high voltage. The signal will remain at the high voltage until the flip-flop that was set is reset.

One-shop OS1 can be a one-shot multivibrator that is triggered on when the voltage at its input switches from a low voltage to a high voltage. When triggered on, it transmits onto line FOS a positive pulse having a duration of 100 milliseconds. See FIG. 7. The signal on line FOS passes through one input of OR-circuit OR2 to keyed oscillator KO. Keyed oscillator KO is a sinusoidal oscillator which emits a 9 kHz. carrier as long as a signal is present at its input. The carrier is transmitted via amplifier A6 to line FTG. At the same time the signal of line FOS is fed to the reset input of step counter SSK. Step counter SSK can be a four state step counter which is locked in a zero state as long as a signal is received at its reset input R and is stepped state-by-state for each pulse received at its count input C. Thus, in response to the triggering of one-shot OS1 a dash burst of 9 kHz. carrier is transmitted to line FTG and step counter SSK is locked in the zero state with no signals from any one of its state outputs 1 to 3.

The FOS line is also connected to the set input of flip-flop F6. Flip-flop F6 is of the type which triggers on negative going signals. Thus, at the end of the 100 millisecond pulse on line FOS, flip-flop F6 is set and starts transmitting a positive signal on line FO1 to the input of keyed free-running multivibrator KM. See FIG. 7. Free-running multivibrator KM can be a symmetrical astable multivibrator which oscillates only when a positive signal is present at its input and then transmits 10 millisecond pulses onto line FKM.

Since line FKM is coupled, via a second input of OR-circuit OR2, to the input of keyed oscillator KO a dot burst of carrier is transmitted onto line FTG for each 10 millisecond pulse on line FKM. Furthermore, since line FKM is coupled to the count input of counter SSK, the counter steps one position for each pulse on line FKM. The first of such pulses steps the counter off the zero state into the 1 state and the "1" output transmits a pulse to the reset terminal of flip-flop F1 to reset the flip-flop if it had been set by a signal on line T1-2. However, it will be assumed that a signal on line T5 had set flip-flop F3. Therefore, at the third such pulse, the "3" output of counter SSK will reset flip-flop F3 and the signal on line FL will switch back from a high voltage to a low voltage. This voltage level switch is received by the reset input R of flip-flop F6 which triggers to the reset state terminating the signal on line FO1 and shutting off the keyed free-running multivibrator KM, terminating the tone control signal cycle. Thus, whenever, one of the T1-2, T3-4 and T5 lines carries a pulse signal to the tone generator TG, it generates a dash burst of carrier followed by from one to three dot bursts dependent upon which of the flip-flops F1 to F3 had been set by the pulse signal.

The playback or receive mode will now be described with respect to FIG. 2. In a normal teaching program, a multiple choice question is presented which lists four possible choices. Thereafter, statements concerning each choice are given. Therefore, the first information transmitted from tape player TP via line FT is the tone control signal for a question. This tone control signal, as well as every other tone control signal, is fed from tape player TP, via line FT, audio demultiplexer AD and line TTR, to tone receiver TNR. Tone receiver TNR decodes the signal as a signal associated with questions and transmits a TR5 signal to selector circuits SG which connect a path between lines DLO and TAO. The audio signals related to the question then follow from tape player TP and pass via audio demultiplexer AD, line DLO, selector circuits SG and line TAO, to audio output AO which can be an audio amplifier and speaker.

The next information is the tone control signal for the statements associated with the first and second choices. This signal is fed from tape recorder TP, via line FT, demultiplexer AD and line TTR to tone receiver TNR where it is decoded to generate a signal on line TR1-2. If the student had made the first or second choice by pushing switch SG1 or switch SG2 in selector circuits SG a path would be closed from line DLO to line TAO or from line DHI to line TAO, respectively. If he depressed either of the switches SG3 and SG4 no path would be open at this time. In any event, tape player TP then transmits the audio signals associated with the first and second choices via demultiplexer AD and via lines DLO and DHI, respectively, to selector circuits SG. Whether either of these signals pass to the audio output depends on whether the student is depressing switch SG1 or SG2.

There then follows the tone control signal and the audio signals representing the statements for the third and fourth possible choices and these are handled in a manner similar to those of the first and second possible choices, respectively. The tone control signal for the third and fourth choices result in a signal on line TR3-4 while the audio signals representing the statements of the third and fourth choices appear on lines DLO and DHI, respectively. The depression of switch SG3 closes a path between lines DLO and TAO, while the depression of switch SG4 closed a path between lines DHI and TAO.

Thus, it is seen that the question is unconditionally fed to the audio output AO, but the choice statements depend on which switch the student depressed in selector circuits SG.

The various elements of the audio receiver AR will now be described.

In FIG. 8, the audio demultiplexer AD receives all the signals from tape player TP, via line FT which is connected to amplifier A16 which feeds band-pass filter BP, high-pass filter HP and low-pass filter LP3. High-pass filter HP passes signals at 9 kHz. and above. Thus, only the tone control signals (bursts of 9 kHz. carrier) can pass via filter HP and amplifier A7 to line TTR. Low-pass filter LP3 is designed to pass signals only in the 0 to 3 kHz. range. Thus, the audio signals for statements associated with the first and third possible choices pass through low pass filter LP3 and amplifier A8 to line DLO.

Band-pass filter BP passes signals in the 3 to 6 kHz. range. Thus, the audio signals associated with the statements of the second and fourth possible choices can pass via the filter BP and amplifier A9 to full-wave detector FWD and low-pass filter LP4. Low-pass filter LP4 passes signals in the 0 to 3 kHz. band. Hence, the combination of detector FWD and filter LP4 demodulate or down convert the audio signals associated with the second and fourth possible choices to base band. These signals are fed, via amplifier A10, to line DHI.

The tone receiver TNR is shown in FIG. 9. The tone bursts are received from line TTR by tuned amplifier TNA, of conventional design, and fed to detector DT, of conventional design, to recapture the envelope of the carrier bursts. The pulse shaper PS, which can be an overdriven amplifier with clipping, shapes the envelope into sharp leading and trailing edge pulses similar to those shown on lines FOS and FKM of FIG. 7. (It should be noted that the envelope of the tone control signal comprises a 100 millisecond or "dash" pulse followed by one to three 10 millisecond or "dot" pulses.) The output of pulse shaper PS feeds in parallel to one-shot OS2 (a monostable multivibrator which emits a 50 millisecond pulse when triggered) and inverter IV (simply an inverting amplifier) whose outputs feed NOR-circuit NOR. The combination of these three elements comprise a pulse-width detector which emits a "clear" pulse onto line L1 only when the dash pulse is received. This clear pulse is fed to the reset input R of step counter RSK (similar to previously described step counter SSK of FIG. 6) to clear and hold the counter in the zero state until the pulse terminates. It will be assumed that three dot pulses follow. These are fed to the count input C of counter RSK which steps to the third state and transmits a signal on line TR5. This signal remains until the counter is cleared at the next tone control signal. A lesser number of dot pulses would have stepped the counter to a lower state. For example, two dot pulses would step the counter to state 2 and a signal would be transmitted on line TR3-4.

The selection circuits SG, shown in FIG. 10, center around two-input adding operational amplifier AOA1 (similar to amplifier AOA of FIG. 5) whose output is connected to line TAO. The first input is connected to line DLO via gate G5A and the second input is connected to line DHI via gate G6A. Gates G5A and G6A, which are similar to gates G1 to G5 of FIG. 4, have their control inputs connected to the outputs of OR-circuits OR3 and OR4, respectively.

OR-circuit OR3 has three inputs connected to line TR5, to line TR1-2, via single-pole single-throw switch SG1, and to line TR3-4, via single-pole single-throw switch SG3, respectively. OR-circuit OR4 has two inputs connected to line TR1-2, via single-pole single-throw switch SG2 and to line TR3-4, via single-pole single-throw switch SG4, respectively. Thus, gate G5A is open and the signal on line DLO can pass to amplifier AOA1 only at three possible times: (1 ) when a signal is present on line TR5 (when a question is being presented); (2 ) when a signal is present on line TR1-2 associated with statements of the first and second possible choices and switch SG1 is closed indicating the selection of the first possible choice; and (3 ) when a signal is present on line TR3-4 associated with the third and fourth possible choices and switch SG3 is closed indicating the selection of the third possible choice. Similarly, gate G6A is open when a signal is present on line TR1-2 and switch SG2 is closed or when a signal is present on line TR3-4 and switch SG4 is closed.

There has thus been shown improved apparatus for compressing question and answer information on a single channel by a combination of time and frequency multiplexing.

Although the specific embodiment contemplates single channel magnetic tape devices, the invention could be used over a single channel telephone line or a radio channel. In addition, it could be incorporated in the audio portion of an educational television system, such as shown in U.S. Pat. Nos. 3,180,931 and 3,256,386.

Furthermore, while switches SG1 to SG4 are shown as freely operable by the student, they could be expanded to rows of four switches each, where the operation of the rows of the switches is under the control of further control signals as shown in U.S. Pat. No. 3,345,758.

Finally, while only single channel operation has been shown it should be realized that the inventive idea can be used for a plurality of such single channels, each of which is time and frequency multiplexed in different combinations to increase the versatility of the teaching device.

While only one embodiment of the invention has been shown and described in detail, there will now be obvious to those skilled in the art, many modifications and variations satisfying many or all of the objects of the invention, but which come within the spirit of the invention as defined by the appended claims.

Claims

What is claimed is:

1. Audio teaching apparatus comprising a source of at least three audio signals, each of said audio signals representing different but related units of teaching information, means for generating at least two different control signals, one of said control signals being associated with a first of said audio signals, the other of said control signals being associated with the second and third of said audio signals, signal multiplexing means for frequency multiplexing said second and third of the audio signals to form a frequency multiplexed signal, and means for serially transmitting on a single channel said first of the audio signals and the control signals associated therewith and followed by frequency multiplexed signal and the control signal associated with said second and third audio signals.

2. The teaching apparatus of claim 1 wherein all of said audio signals are in a first given frequency range and said control signals have a given frequency, and wherein said signal multiplexing means comprises a first input means for receiving said first and second audio signals, a second input means for receiving said third audio signal, means for frequency converting the audio signals received at said second input means to signals in a second and different frequency range, means for connecting said frequency converting means to said second input means, and a signal combining means having at least two inputs and an output for transmitting a signal which is a combination of the signals received at said inputs, one of said inputs being connected to said first input means and the other of said inputs being connected to said frequency converter means.

3. The audio teaching apparatus of claim 2 wherein said signal combining means is an analog adder means having at least three inputs, a first of said inputs being connected to said first input means, and wherein said frequency converting means comprises a modulator having a modulating signal input connected to said second input means, a modulated signal output connected to a second input of said analog adder means, and a carrier signal input, and a source of carrier signals connected to said carrier signal input and to the third input of said analog adder means.

4. The audio teaching apparatus of claim 3 wherein said analog adder means has a fourth input connected to said means for generating the control signals.

5. The audio teaching apparatus of claim 4 wherein said control signal generating means includes means for generating coded bursts of signals having a given fixed frequency which is distinguishable from the signals in said first and second frequency ranges.

6. The audio teaching apparatus of claim 1 wherein said means for serially transmitting comprises a magnetic tape recorder.

7. The audio teaching apparatus of claim 2 further comprising means for receiving the signals transmitted on the single channel, said receiving means comprising demultiplexer means having an input for serially receiving said first audio signal and the associated control signal and said frequency multiplexed signal and the control signal associated therewith, and three outputs, said demultiplexer means further including first means for selecting out said control signals for transfer to said first output and second means for selecting out said first audio signal and said second audio signal from said multiplexed signal for transfer to said second output, and said third audio signal from said multiplexed signal for transfer to said third output, an audio output means for converting audio signals to audible information, a controlled switching means comprising at least two subject operable switches, each associated with a different one of said second and third audio signals, and a signal controlled switching means for selectively connecting said second and third outputs of said demultiplexer means to said audio output means, said signal controlled switching means being controlled by said control signals so that said second or third audio signal is switched to said audio output means only when the switch associated therewith is operated at the time that the control signal associated therewith is received.

8. The audio teaching apparatus of claim 7 wherein the first selecting out means of said demultiplexer comprises a first signal filter connected between the input and said first output for only passing signals having said given frequency, and said second selecting out means comprises a second signal filter connected between the input and said second output for only passing signals in said first given frequency range and the serial combination of a third signal filter, a full-wave detector and a fourth signal filter connected between the input and said third output for only converting signals in said second given frequency range to signals in said first frequency range.

9. The audio teaching apparatus of claim 7 wherein said controlled switching means comprises an analog adder means having first and second inputs and an output connected to said audio output means, first and second gates, each of said gates having a signal input, a signal output and a control signal input, the signal input of each of said gates being connected to one of said second and third outputs of said demultiplexer means, each of the signal outputs of said gates being connected to one of the inputs, respectively, of said analog adder means, means for seperating the control signals associated with each of said audio signals, means for transmitting the control signal associated with said first audio signal directly to the control signal input of one of said gates, means for transmitting the control signal associated with said second and third audio signals via the subject operable switch associated with said second audio signal to the control signal input of said one gate, and means for transmitting the control signal associated with said second and third audio signals via the subject operable switch associated with said third audio signal to the control signal input of the other of said gates.

10. The audio teaching apparatus of claim 7 wherein said means for serially transmitting comprises a magnetic tape recorder.