Patent GB-1465873-A (GB)
Title: Multiple access message retrieval system
IPC: G09B7/04
Filed: 1975-05-05
Abstract: 1465873 Selective signalling; educational appliances EDUTRON Inc 5 May 1975 18742/75 Headings G4H and G5G [Also in Division H4]  In an information storage and retrieval system, a message of duration Nt is divided into N (a plurality) segments of duration t which are multiplexed into a message of duration t, a plurality of output and demultiplexing means each being capable of selecting any desired segment and of demultiplexing the message into the N segments. In Fig. 4, an audio message from a source 102 is time-segmented in a first multi-channel recorder 104 which by using nine disc stores, for example, for respective segments, supplies ten successive segments together to a first multiplexer 122 which heterodynes each with a respective oscillator frequency, the difference frequencies being passed by filters to an adder, so that ten segments are superimposed in time. A second multi-channel recorder 134 and multiplexer 146 repeat the procedure so that now 100 segments are superimposed. This process may be continued further. This audio message, together with a video message (from a source 153), are passed via further recorders and multiplexing as shown, to a plurality of classrooms (language labs are also mentioned). A lesson selector 160 in classroom no. 1 has a filter with selectable pass-band (or an arrangement for heterodyning the signal with a selectable frequency, followed by a fixed-pass-band filter) to select a lesson for supply to control units 162a- 162n each of which can select a desired segment for audio output. Classroom no. 2 has a single audio and corresponding video output. Classroom no. 3 has a plurality of audio and corresponding video outputs, with means whereby different lessons can be supplied to each. Classroom no. 4 differs from no. 3 in that the same (selectable) lesson is supplied to each. Fig. 7 shows a control unit for selecting a segment of the audio message. A BCD counter 260, 262 controls the frequency of a voltage-controlledoscillator 268 via a D-A converter 264, the oscillator frequency being heterodyned with the message so that the difference frequency arising from the desired segment passes through a filter 270. The counter 260, 262 can be set to the first segment by a start manual control 300 and is thereafter automatically advanced by sync signals derived from the message by demodulator 292 to select successive segments. Forward and reverse manual controls 302, 304 enable the student to step forward or back, and a pause manual control 308 is also provided. Automatic frequency control is provided via a demodulator 276 and a differential amplifier 266. The signal from which the audio segment is selected can be one of four audio subchannels (as when TV receivers are used), selected by manual controls 380a-380d (at the left) selecting, as shown, one of four crystals in an oscillator 398 providing a frequency with which the audio input is to be heterodyned to feed a fixed-passband filter 254. Control of this frequency selection can alternatively be from an extension 400 of the BCD counter 260, 262. Control pulses included in the audio message are loaded into a shift register 334 and specify whether the system is to be halted to permit the student to answer a question (bit C), if so what the answer is (bits E, F), wheterh in the event of a wrong answer the system is to proceed to a particular other segment (bit D) and which this latter segment is (bits G-P). The question was previously presented at the audio or video output or in a book. The answer, entered by actuating one of manual controls 346a-346d (lower right), is compared in gates 348a-348d with the bits E, F, "correct" and "incorrect" lamps 358, 362 being provided. If appropriate, bits G-P are loaded into the counter 260, 262, 400 to select the next segment. Fig. 6 (not shown) shows a simpler, slightly-modified embodiment.