GB2058410A - Improvement in Tape Transport Systems for Magnetic Tape Recorders and Reproducers, Particularly of the Cassette Type - Google Patents

Abstract

A speed control system for a tape recorder comprises two reading heads TL TA spaced a preset distance apart along the tape, delay line LR for delaying the signal from the first head by an amount corresponding to the head separation and the desired tape speed, and a phase comparator CP which produces a tape drive control signal for driving the tape at the desired constant speed. <IMAGE>

Description

SPECIFICATION Improvement in Tape Transport Systems for Magnetic Tape Recorders and Reproducers, Particularly of the Cassette Type This invention relates to a recorded magnetic tape transport system particularly apt for use on cassette reproducers. Although the invention will further on be described with reference to cassette reproducers, the system is of a general application to other types of tape apparatus, for example the open reel to reel type.

It is common knowledge that in reading recorded magnetic tapes, the latter must run before the playback head at a constant speed which is generally equal to the recording speed.

The great majority of the mechanical systems so implemented makes use of a transporting spindle or capstan which rotates at constant speed and which transmits by friction its own motion directly to the tape while the reels around which it is wound are mechanically driven by transport systems fitted with friction idle wheel with or without self-contained motor, in order to guarantee the variable angular velocity necessary to ensure the take up and the tension of the magnetic tape itself.

In co-pending patent application (8025646) provision has been made to transmit the motion to the tape acting directly on the axis or on the hubs of the reels supporting the tape at variable speed such that the peripheral speed and hence the tape running speed on the playback head, is constant.

In this case, due to the high precision and speed constancy required it is necessary to slave the reel transport system angular velocity to the tape actual speed, and it is therefore necessary to set up a system fit to measure it with the required precision.

As to the patent application previously mentioned, such system again makes use of a capstan as a function however of an element transported by the tape and not transporting whose angular velocity is consequently the function of the tape itself running speed and which can thus be measured, and the measurement used for slaving the instantaneous angular velocity of the transport system by rotation of the coils.

By the same token it is conceivable to use instead of the idle capstan as a speed detector, service signals prerecorded on tape on a constant frequency service track or sub-audio superimposed on the main track.

It will further on be described a tape speed measurement system and slaving of the reel angular velocity, completely electronic, which does not make use neither of an idle capstan nor of service signals prerecorded on tape.

This invention will henceforth be described with reference to presently preferred embodiments, which are intended to be illustrative but not restrictive and on the basis of the figures shown in the attached drawings, in which: Figure 1 shows the transport system basic diagram in accordance with this invention; Figure 2 shows the basic diagram in a second embodiment of the system in accordance with this invention; With reference to Figure 1 let us consider a general system comprising a recorded magnetic tape N which, unwinding from the supply reel D, winds up on the take-up reel C transported by a motor M at a controllable speed. The controllable speed motor can be direct current motor with variable supply voltage or an alternating current motor fed by variable frequency electric current.

On such tape N is in contact, in addition to the standard playback head TL, another auxiliary head TA, similar to the preceding one and placed at a present distance d, from head TL. Both heads TA and TL read the tape N on the same track.

It is evident that the signals detected by the two heads are identical but are detected in different times. In the case shown in the figure the signal coming from head TA will be anticipated with respect to the signal coming from head TL of a time: d T=~ V in which v is the tape N speed and dthe distance between the playback heads.

Consequently, once distance D is determined, time T is an univocal function of the tape running speed v.

It will therefore be sufficient to use the measurement of# T to correct the angular velocity of motor M and hence of take up reel C in order to achieve the desired constancy of the tape N running speed v.

Again with reference to Figure 1, such purpose is achieved by applying the signal coming from head TA to the input of a delay line, LR having between input and output a constant delay time equal to Tt. The signal outgoing from the delay line LR is applied to a comparator CP to be compared with the signal coming directly from the playback head TL.

If the tape N speed is the desired one, the lead T of the signal coming from head TA is exactly compensated by the delay Tl introduced by the delay line LR (in this case will be T=Tt) and the comparator CP will give a zero output. If on the contrary the situation in which T+TI occurs, comparator CP will provide a difference signal, positive or negative, which suitably detected by detector R is applied to motor M speed control system SC. Thus each deviation of motor M speed from the speed necessary to maintain constant the tape running speed on the heads is readily corrected and nullified since a closed loop feedback system is involved.

Reference is now made to Figure 2. Even taken for granted the aforesaid description, in an exemplificative practical application, the delay line LR will be made up of a COD (Charged Coupled Device) technology analog delay line of a well known type.

This allows to transfer analog signals with the long delays necessary in applications of this type (a few tenths of a second at least). Its delay constant is in addition controllable by an external clock oscillator CK.

This arrangement is shown in Figure 2 in which it should be noted that the clock oscillator CK represented as variable constant element allows to adjust the line LR delay constant. Hence, it is evident that it is possible to control within given limits the running speed v of tape N varying the frequency of the oscillator CK and hence the delay Tt of LR.

Since the signal used by the whole system is the analog one recorded on tape (for instance musical or speech sounds) the same is of a highly variable amplitude. Consequently, between comparator CP and detector R an amplifier A is inserted whose gain is automatically controlled in a known mode so as to provide a constant amplitude signal to detector R input.

Further, in order to simplify the implementation of the comparison of the signals and the detection of the difference signal it will be necessary that these signals be band limited. Two exactly identical band-pass filters F1 and F2 will then be inserted at heads TL and TA output.

It should be noted that the pass bands of filter F1, F2 can be selected so as to fall also within the frequency area in which there exists a high energetic content of the electric noise inherent in the grain magnetic structure of the tape itself, thus solving the problems arising from the recorded signal standoffs, sub-audio or ultraacoustic frequency be recorded.

Once the band limitation is made, the detector R can be one of the several known types of peak detectors or phase discriminators implemented in a well known mode by analog or digital techniques.

Taken for granted the above, the following can be observed: a) the system described is not the only one for measuring delay T. For instance the signal instead of being band limited, could be sampled with very short pulses at a preset frequency and processed by digital techniques.

b) Provision may be made to implement a speed control acting on the clock generator CK frequency or mechanically on the distance d between the two heads. In the first case, the clock frequency shift can allow for example the fast forward function.

c) Electrical switchings evident to an expert ins the field, for instance switching of the delay line LR position from head TA to head TL or output polarity inversion of detector R allow also reverse or fast rewind reading functions having of course devised a suitable mechanical connection coupling between motor M and coil D.

This invention has been described with reference to presently preferred embodiments but, at as it will be readily understood, changes and modifications can be made in practice without departing from scope of the appended

Claims (3)

claims. Claims

1. A tape transport system for magnetic tape recorders and reproducers, particularly of the cassette type, characterized by the fact that provisions are made for motor driven devices acting directly and exclusively on the take up reel devices for sensing the running speed including a first and a second playback head of the signal recorded on tape placed at a mutually preset distance, one of the said heads being connected to a delay line having the delay constant equal to the reading delay as defined by the distance between the two heads with reference to a preset tape running speed; means to detect the shifting of the delays between the direct and the delayed signal for the speed control of the take up or rewind reel driving motor.

2. A system in accordance with Claim 1 characterized by the fact that the said delay line is constituted by an analog delay line, preferably of the COD type controlled by a clock generator.

3. Tape transport system for magnetic tape recorders or reproducers particularly of the cassette type in accordance with one or more of the above mentioned claims and substantially as illustrated and described in the figures of the attached drawings.