Automatic music search system, popularly called AMSS*, is nowadays incorporated in many commercially available cassette recorders. With AMSS one can skip the present music or go to the starting point of the presently played music by pressing ‘Cue (FF)’ or ‘Rew’ keys. This reduces repeated cueing and rewing of tape in order to reach the desired point of the tape.
AMSS does not require elaborate circuitry, but the mechanical modifications necessary in the cassette deck mechanism alienate an average electronics enthusiast. In this article, it will be shown that the mechanical modifications are not as complex as imagined and that the AMSS can be implemented in almost all tape decks commercially available today.
To include the AMSS feature in your cassette deck, however, the following requirements are to be met:
1. Your deck should have Cue and Rew facilities, which are seen in almost 60 per cent of deck mechanisms. If you are buying a new recorder, ensure that its deck mechanism has these facilities.
2. The deck should be soft touch control type. This is not absolutely important, but the pressure, or rather the force required to press a key to function during Cue or Rew should not be a ‘hammer touch’ one.
3. Your deck should be able to incorporate the additional components of this project, such as relay etc, without disturbing the normal operation of the deck.
The idea of AMSS is straightforward. Holding the FF (or Cue) key or Rew key while still playing the cassette and releasing the key only when a gap in music is noticed in the tape. That’s all, and nothing more.
The block diagram is shown in Fig. 1. The left and right channel signals are mixed to form a single signal in the mixer block. This signal is amplified by a voltage amplifier—the saturating amplifier. The amplifier’s output is limited to a constant value by a limiter block and fed to a schmitt trigger cum time delay comparator, and finally given to an electromagnet through a buffer.
Fig. 2. shows the complete circuit diagram. The inputs are derived from the outputs of preamplifiers of the existing cassette recorder circuit and mixed
1 through R1 and R2 (47k) resistors. The mixed signal is attenuated by preset VR1 to an optimum level. This attenuation is necessary because the saturating amplifier should be over-driven but not excessively.
The saturating amplifier is nothing but a voltage amplifier wired around IC TBA810 in a conventional fashion, but with a higher load resistor. The high value of load resistor makes the power amplifier to closely approach the characteristics of a voltage amplifier. Any other voltage amplifier design can be substituted, but the prototype used the 810 circuit.
The voltage output across load resistor R6 is nearly equal to supply voltage whenever the tape is Cued or Rcwcd. Ill is AC voltage is rectified by diode D2, the current through which is limited by resistor R7 and filtered by CIO.
Capacitor CIO forms a memory -like element because it stores charge as long as signal is present. When there is a gap in the music, the voltage across the load drops suddenly, but the voltage across CIO drops slowly as it has only one discharge path, i.e. through high value of R9.
The combination of R9and CIO detides the lime width of gaps in music which should not be altered. These values have been selected after trying the values in almost hundreds of music albums. The gap works for all types of music.
1C 555 is wired to work as com para-, lor as well as schmitt trigger. Its comparator voltage is kept very low by RIO, a 680-ohm resistor, and the input is simultaneously given to pins 6 and 2 for comparing. As soon as CIO discharges sufficiently, the output pin (pin 3) of IC2 goes high driving off the ‘complementary pair’ formed by transistors Tl and T2. This removes power from the relay (which in this circuit is used asrr an electromagnet) and the electromagnet (relay) releases the FPor Rew key to continue to play from that point of the tape.
At pin no. 2 of IC2 there is a red LED. (It should be only a red LED. because it fund ions as a 1.65-volt regulator in this circuit). This fixing of the max. voltage across CIO ensures correct operation of the circuit, independent of the recording level of the tape.
A word about the relay. The relay is 6V, 100 ohms type critically. But the supply voltage is between 13V and 15 V. This is done purposefully. When Tl saturates, it almost sources 150 m A through relay RL1 and makes it a very powerful magnet—powerful enough to hold the FFor Rew keys. This may heat up the relay and increase residual magnetism. but both the side effects can be ignored absolutely.
Assembly of t he electronic circuitry being simple, there is hardly anything to be said about it. However, construction of the mechanical portion of the project is a bit time consuming and you may have to have patience while dealing with the fittings.
In the circuit diagram RL1 and RI.2 are shown as a single inductor for simplicity. But it is a parallel combination of 6V, 100-ohm relays—one for bolding the FF key and the other for Rew key.
Fig. 3 shows how to use relays as electromagnets. Remove the cover and terminal bases of the relays and fix them to a 4mm or so aluminium sheet as shown. Now fix the aluminium plate to the deck mechanism or close to it firmly. The restoring springs of the relays should be removed and the fulcrums should be glued with Fevicol for support. Even with the glue dried, the relay arms should move freely, to and fro. Now link the FF key to the arm of one relay and Rew key to the arm of the other with dial cords used in radios in such a way that, whenever the keys are pressed, the corresponding arm moves a bit closer to the electromagnet. See that the link cord is untwisted and tight.
When FF key is pressed during ‘play’, the arm of the relay moves forward towards the electromagnet. Meanwhile, the signal is amplified, processed and fed to the electromagnets. As now the arm is much closer to the electromagnet, it is drawn powerfully towards it and held as long as the electromagnet gets supply. The electromagnet holds the key even if you release your finger. It releases the key only when t he circuit senses a gap in the music When one electromagnet is operative, it does not cause the pulling of the other key, because the aim of the other key is far beyond the range of its corresponding electromagnet.
If your deck has a leaf switch which operates during FF and Rew only, then it may be used in series with the supply rail to avoid unnecessary current drain during normal operation.
For constructing the Music Search System (EFY, Jun.*92) I took output of record level indicator amplifier (built around TBA810 in my tape recorder) to give input to IC2. But whenever there is a beat in the music the relay starts chattering.
The author has not mentioned anything about push-to-on switch Si at pin 2 of IC2!
Besides, I found the relay coils heat up with the supply mentioned by the author.
T.SURENDRA BITS, Pilani
The author. Mr T.S.Shankar, replies: Input to the circuit should be given only from the output of a preamplifier and not from a power amplifier. Otherwise, the relay chattering can’t be avoided.
Switch Si is meant to cancel out the search function while it is operative.
As we are supplying 12V to a 6V relay coil, the coil may get slightly warm. Since it dissipates more than 1.4 watts, it can never get overheated. Mr Surendra’s relay coil must be 4.5V or 3V rated.