by Ole Melvold [Senior Engineer, Tandberg, Norway]
The seemingly simple process of transporting tape across the electronic magnetic heads in a tape recorder, so that signals can be recorded on and played back from the tape, relies upon a complex and often critical relationship between the electrical portion of the recorder and its mechanical parts.
The mechanical portion of this process is often complicated and is usually the source of the majority of malfunctions and failures in any recorder.
The problem faced by the designer is, then, to simplify this mechanical operation and, if possible, to replace as many purely mechanical functions by electrical ones to obtain greater reliability. The use of purely electronic controls will not only yield greater reliability but, of course, will provide faster, more convenient operation including all the tape handling processes, start, stop, wind, etc. Electronic, "finger tip" activated controls can, therefore, be used to go directly from any wind mode directly to a play mode without manually engaging the conventional "stop" function and with no additional stress imposed upon the tape, and with greatly reduced acoustical and electrical noise. The tape recorder lends itself to electronic pulse control which, in turn, facilitates new applications besides the preservation of audio signals. Recorded signals can be used to control other electronic units and processes; remote control becomes available.
Electrical units can be used to replace many mechanical parts and eliminate purely manual operation of pinch roller, brakes, etc. Clutches can be replaced by motors whose speed in winding and braking can silently and easily be electronically controlled.
Solenoids can be used for operating the pinch roller, brakes, etc.
All this assumes, however, some reliable form of control. The application of logic circuits in integrated form for control purposes can produce many advantages. The so-called High-level, specially integrated logic circuits lend themselves particularly well to tape recorder control, and this circuitry will be largely immune to possible electrical noise within or from a source close to the recorder.
Control signals will originate essentially from the operating push buttons.
By carefully analyzing the desired functions, it will be possible through couplings algebra to formulate these to a mathematical formula that can be realized by electronic circuits.
For newcomers to this technique, some explanation may be needed. A logic circuit is an electrical model of a logic function which tells us that certain conditions must be present for a change of condition (or for no change). One symbol for a logic circuit controlled by DC voltage levels can be seen in Figure 1.
With high levels on both inputs A and B, output T will be high level, expressed AB = T. Conversely, when either A or B is low, T will be low, expressed ZB = T.
Additionally, AB = T or "AB = T. The symbol means in couplings algebra (Boolean Algebra) AND. The symbol + means OR. A may be called negation of A. The connective AND indicates that T is high if, and only if, both A and B are high (conjunction). The connective OR symbolizes that T is high if, and only if, either A is high or B is high or both are high (disjunction). In electronic logic circuitry terminology, circuits as above may be described as AND-gates and OR-gates.
As one in logic mathematics refers to a two-basic statement, true or not true, in electronics we refer to two levels, high or low (1 or 0). The conversion from one level to another is symbolized by a ring (NOT), as seen in Figure 2.
The practical logic circuit in Figure 2 is named a NAND-gate (NOT-AND gate). When A and B represent high levels, T will represent a low level, AB = T, T = AB. To improve the understanding as to how a NAND-gate functions, we can look at Figure 3, where such a gate is visualized with discrete components.
The circuit is designed so that the transistor will be in saturation when neither A nor B are connected, which means that the level T is low. If the base current disappears, caused by connecting A or B to ground, the level T will become high. The circuit works then as an OR-gate with a converted output. One single NAND-gate cannot maintain any given information (when the finger in Figure 3 is removed, the high level will disappear). In order to obtain memory we have to use two circuits, as shown in Figure 4. A low level on the set-input will make T remain high. The circuit resets with a low level on reset-input. By adding a RC network, the reset can be automatic, as seen in Figure 5.
Fig. 1-Symbol for logic circuit controlled by DC voltage levels
Fig. 2-Symbol for NOT-AND (NAND) gate
Fig. 3-Circuit using NAND gate
Fig. 4-Memory circuit
Fig. 5-Memory circuit with automatic re-set.
A Description of the Logic Circuit System in the New Tandberg Series 9000X
In the design of Tandberg Series 9000X, efforts were made to take maximum practical advantage of this sophisticated type of control, through the use of integrated logic circuitry.
As in any tape deck, the rotation and radius of the capstan shaft will determine the tape speed. The tape is transported by the pinch roller (engaged by the pinch roller solenoid) moving toward the capstan. See Figure 6. Forward winding is caused by a strong torque on the right side motor (counter-clockwise), and a low torque for braking on the left side motor (clockwise). Rewinding will be established conversely.
Fig. 6--9000X layout
In establishing the mathematical criteria for these functions, we will designate:
Tape movement to right as R
Tape movement to left as R
Winding speed as H
Play speed as H
The following conjunctions can then be formed:
Winding speed to right as HR
Winding speed to left as HR
Stop as HR
Play speed to right as HR
Two bistable circuits must accommodate four function criteria. The first circuit shall represent the speed H and H. the second circuit shall represent the tape movement. The diagram relating directly to the operating buttons on the recorder can be seen in Figure 7.
Fig. 7-Control circuit
Fig. 8--Complete arrangement showing all functions
This simplified control is, of course, not sufficient in a practical functional circuit design. Additional circuitry to operate the solenoid will be necessary, besides a memory to distinguish between playback and record. Further, there has to be a trigger network included to allow for braking time after completing winding before a play condition is set.
In addition to the mechanical braking, there was a requirement for electrical braking. Additionally, electronic end-stop pulses and stop-signal pulses, when turning on and off the AC switch (power-up reset-circuits), were also found necessary to eliminate the possibility of tape spillage. To prevent accidental erasing, the recording function was also given certain conditions that had to be met before becoming established.
As mentioned earlier, these functions can be formulated through couplings algebra. With a realization of this, one arrived at a solution as shown in Figure 8. This diagram fulfills the foregoing requirements.
Circuit 11 in this diagram is basically the two bistable circuits providing the information that shall control the four tape conditions.
Circuits 12 and 13 are monostable and necessary for the control of, respectively, the brake and pinch roller solenoids.
Circuit 8 is bistable and has to distinguish between recording and playback modes.
The triggering Circuits 4, 6 and 7 control the braking phase when the tape mode changes from a wind position to a play position.
Circuits 9 and 10 shall make triggering of Circuit 13 feasible by Circuit 12.
Circuit 14 shall eliminate this possibility when stop condition is set.
Circuits 2 and 3 prevent the tape deck from being set in record mode prior to the presence of the stop condition.
Circuit 1 shall be a Schmitt-trigger that gives end-stop pulse.
Circuit 5 is a "power-up" reset circuit.
Circuits 15 and 17 transfer the information to the winding motors of their respective relays.
Circuits 16 and 18 give electrical braking during the braking phase.
Conclusion
The use of logical analysis to determine a logically correct engineering solution for a consumer tape deck opens many possibilities. Besides excellent electronic and mechanical specifications, safe, fast and easy operation are just some of the advantages.
This type of logic control system can be applied toward timed and sequential automatic operations, remote controls, etc., even in environments with extensive electrical noise.
(Audio magazine, Apr. 1973)
Also see:
Tandberg open-reel tape decks (ad, Apr. 1973)
The Changing Face Of Open-Reel: market and product trends (Apr. 1973)
Tandberg Model TD 20A Open-Reel Tape Deck (Mar. 1979)
Cassette Deck Transport Problems (Sept. 1974)
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