GB2164527A - High speed cassette tape player - Google Patents

Abstract

A computer data recorder and player uses standard compact cassettes at normal density, but saves and loads the data at four times the normal speed. Both data transfer and tape speed are increased. The data is decoded by measuring the output of a monostable (38) which is triggered by the upgoing edges of the data stream. Means (22, 32) are provided to invert the signal polarity, if necessary, allowing pre-recorded cassettes of either polarity to be used. <IMAGE>

Description

SPECIFICATION High speed cassette tape player The invention relates to a high-speed cassette tape player particularlyfor use in association with a microcomputerforsaving and loading data in digital form.

Normal domestic cassette player/recorders are in widespread useforstoring data and programs produced on microcomputers, particularly those microcomputers intended for personal use in the home. The main reason for such widespread use is the low cost and availability of such recorders. The main disadvantage, however, isthe generally slow data transfer speed between the computer and the recorder, and vice versa: typical baud rates being between 250 and 2000 giving a load or save time of approximately 5 to 30 minutes for a 48K program. The data transfer speed is presently limited by two main factors. Firstly, when a conventional recorder is used, the tape travel speed is fixed at 1 7/8 inches per second (47.6mm per second). Thus, in order to increase the data transfer rate, one has to increase the density at which the data is stored on the tape.This is generally not practicable beyond a certain point, since too high a density would means that the signal could not be adequately resolved on a poor quality recorder. The data density is therefore chosen to be one which can be dealt with by almost all recorders in common use. The second limitation is due to the fact that the common method of decoding the signal on the tape relies on the use of timing loops bythe processor to distinguish between zeros and ones (to be described in more detail below).

If the data transfer rate is to be made significantly htgherthan can presently be achieved with a cassette recorder, this method can no longer be used since the processor becomes too slow to resolve the incoming data.

It is one aim ofthe presentinventionto provide a dedicated cassette player/recorder for use in association with a microcomputer which will use entirely standard, pre-existing Philips, compact cassettes while at the same time allowing significantly higher datatransfer rates (forexamplefourtimes higher) than is presently possible with such cassettes. The recorder is to be able to use any cassette tape which could be loaded intothecomputerbya standard cassette player, that is, it uses suitably formatted normal-density cassette tapes, such as the prerecorded program tapes presently on the market.A ROM integrated circuit provides the computer-with specific instructions, and this may be programmed in different ways to enable the recorderto communicate with computers having varying protocols. In each case, the device is user-transparent, and is driven by the standard recorder-control commands provided in the computer operating system.

According to one aspect ofthe present invention, a tape cassette reader (which may also be a recorder) is arranged to play a tape cassette at a speed substantial ly greaterthan the normal play speed. Thus, a standard player may be adapted to play at aobut several times the normal speed. Many conventional tape cassette player/recorders play at 4.75cm per second and in accordance with the invention the recorder may be arranged to play at perhapsfour times that speed. That could easily be achieved by replacing the drive pulley by one of larger diameter or otherwise gearing up the drive. However, it is preferable to avoid tape damage, that the fast forward and rewind speeds are not substantially increased.

According to another aspect ofthe invention a tape cassette reader which may be arranged to read faster than is usual, as described above, is also provided with means for automatically detecting the signall polarity on the tape, and righting it if it is inverted.

Tape cassettes carrying data representing computer programs may be recorded with either polarity that is to say where the data is in the form of a square wave, the first component after a synchronisation pulse may be high or low, whereas the programme foe reading the data may require to have a signal of a specific polarity. The invention enables the parityto be detected and inverted if necessary.

In particular, tape cassette reading means suitable for reading data on a tape preceded by a synchronisation pulse in the form of a high and a low (in either order) constituting a single square wave, may include means for detecting the arrival of a synchronisation pulse and polarity means responsive to detection of that arrival for determining whether the signal is high or low at a present moment in the synchronisation pulse and including an inverter arranged to invert or not invert the data signal read from the tape in dependence on the determination ofthe polarity means.

Such a cassette reader can detect incorrect polarity andcorrectforitwithoutrequiring anyadditional components.

According to a second aspect of the invention, tape cassette reading means suitable for reading data on tape inthe form of 'O's and '1's in which '0' is represented by a single square or otherwave of one length, and a '1' by a single wave of a different length includes a circuit having a preset time constant initiated by detection of a characteristic part of a read singlewavefordistinguishing between waves of one length and waves of the different length.Where the cassette reading means has been adapted to have a fasterthan normal speed, as described abovetthe time constantwill be related to the tape speed.

Conveniently the circuit is in the form of a monstable circuitarrangedto be triggered into a first state at the beginning of each single square (or other) wave and arranged to return to a second state (unless retriggered) afteratime interval greaterthan one of the said lengths, and less than the other.

There may then be means responsiveto the state of the monostable circuit at retriggering or at the The drawing(s) originally filed was (were) informal and the print here reproduced is taken from a later filed formal copy.

The claims were filed later than the filing date within the period prescribed by Rule 25(1) of the Patents Rules 1982.

beginning of-each single wave for determining whether the previous single wave represented a '0' or a '1'.

The invention includes a tape cassette player arranged to drive the tape ata speed greaterthan the normal speed (orsubstantiallygreaterthan 47.6mm per second) and incorporating inversion detecting and correcting meansand/orsignal discriminating means incorporating a circuit with a time constant related to the tape speed, wherebythe reader can be used directly with a computerfor loading a program at substantially greaterthan the normal rate of loading forthatcomputer.

In orderto assist in understanding the background to the invention,theformat of a typical recording on a standard cassette tape will now be described with reference to the accompanying figures in which: FIGURE 1 showsthetypical representation of binary one and binary zero on the tape; FiGURE 2 shows the leader and synchronisation pulsewhich precedethedataonthetape; and FIGURE 3 illustrates the start of a typical file on the tape.

Dataisstoredonthetapein binaryfrequencymodulated form. As shown in Figure 1, both '0' and '1' are-represented by a single square wave thatof'0' being of length 2T2 and that of '1' being twice as long.

Each data file is preceded by a waveform of the type shown in Figure2.This consists of leader pulses 10 and a synchronisation pulse 12. The purposeofthe leader isto indicate to the control system that a data filefollows, and it comprises a square wave (of which only two cycles are shown) of period length T2. T2will typically be longerthanT1.The leader isfollowed by a single cycle of a square wave of shorter period 2T#, known as the synchronisation pulse. The data follows immediatelyafterthis. Figure 3 illustratesthe start of a typical data file on tape beginning 001.

Although agiven cassette recorderwill always record data in the same sense, different recorders may record with opposite polarity, and any device which is designed to use existing tape cassettes must be able to read them whatevertheir polarity.

As will be explained below, the polarity may only become a problem when the data transfer rate is high enough to preventthe system of the prior art being used to decodethe tape. Different computers will also work with data at different densities on the tape, althoughfora given computerthe data density will be constant.

When decoding a sequence of 'O's and '1's on a standard cassettetape played at normal speed, the computercan distinguish between the two simply by timing them (the loading programmegenerallycontains a timing loop). This method, which depends only upon thetime between changes of state, is not influenced bythe polarity ofthe signal on thetape.At high speeds, however, the changes of state follow one anothertoo rapidly in relation to the programme instruction times,forthem to be timed by this method.

The present invention, therefore, is not simply concerned with providing a highertape speed. It is also concerned with a different decoding method. This bringsthe associated problem that such~ a method is sensitive to the signal polarity and the invention is further concerned with a method ofautomatically detecting the polarity and inverting the signal as necessary.

Otherfast methods of recording data to tape have been developed, but these rely upon altered data formats. Data can for example be transferred a; high speed if it is stored on one track of a tape along with a clock pulse stored on a second track. In this case the data is not in frequency modulated form but simply in binary form with high = 1, low = 0. The data is read into the computer along with the clock signal, the latter indicating to the processorthetimesatwhich thesignal isto besampled. Other methods have included the use of a closed tape-loop. All these methods, however, sufferfrom the disadvantage that theyarenotableto use standard formatcassettes.

This is a serious problem since proprietory programs available on cassette are frequently protected against copying and may not be available otherthan in this standard format. Further since each of the devices of the priorartcan only be usedwithtapestheyhave written themselves (or which are provided in the relevantformat)they do not have to deal with-the problemoftwo possible polarities which occurs when using standard cassette tapes.

The invention maybe carried into practice in various ways, but one specific embodimentwill now be described by way of example, with reference to the further accompanying drawings in which:~ FIGURE 4 is a schematic block diagram of the electronics within the recorder/player used when loading from a tape; and FIGURE 5 is a sketch illustrating the method of decoding the recorded signal.

The microcomputerwill generally contain a cassette interfacethrough which data is passed when saving or loading a program. The interface,when saving, converts the data to be stored into a suitable format and sends itto the recorderatthe correctspeed.The process is reversed when loading. Naturally the interface provided cannot be used when data is to be transmitted ata higher speed, and the recorderofthe present invention contains its own interface: the interface provided is bypassed. The action occurring when a "Save" command is issued are asfollows; a control circuit within the recorder is alerted to this fact bythe relevantaddress being place on the address bus (which it is continuously monitoring).Control is then diverted from the computer operating system to thatwithinthe ROM associatedwith the recorder which then issues the relevant commandsforthe data transfer to proceed via the replacement interface, at fou rtimes the normal speed. Since the cassette also runs atfourtimes the normal speed, a standardformat, standard-density cassette is the result. When the "Save" has been completed, control is passed back to the computer operating system.

The construction and operation ofthat part of the recorder concerned with loading will now be described with reference to Figure 4. As with the "Save" command, the operation of the device during loading is under control of its ROM 16. The recorded signal is readfromthetapebythetape head 18 from which it passes to an amplifier and signal processor20which ensuresthatthe signal to be operated upon is a good square wave. It then passes to one of the inputs of an exclusive or (XOR) gate 22.

The signal passes via a gate 28 and is monitored by the computer following the new loading system having instructions in the ROM 1 6.The signal will initially be characteristic of the leader pulses, a square wave of period 2T2. This is recognised by timing using the monostable circuitorthe more conventional timing loop system.

When the time period suddenly changes to the much shortervalue of T3 the synchronisation pulses have been reached.

The program within the ROM then determines the signal state during the second half-cycleofthis pulse (i.e. at a time 3T3/2 from its commencement- point 30 in FIGURE 2. If this is low, the signal on the tape is clearly in the desired form of FIGURE 2. If it is high, the signal is inverted. According to what is found,the computer issues a command to a lock control 32.

During the second half of the synchronisation pulse a signal is sentto the recorderto trigger a flip-flop which then locks a signal at the second input to the XOR gate such that if tape signal was originally not inverted a low level is introduced and maintained so thatthe output retains the same polarity as the input.

Likewise if the signal was inverted a high level is introduced and maintained such thatthe output is inverted with regard to the input, and thus changed to being of the correct polarity.

The signal in line 26 is therefore the correct way up regardless of the way it appears on the tape. Once the correct level has been determined from the synch ronisation pulse at described, it remains at that level at least until the loading is complete.

The decoding of the output signal in the line 26 is performed with the aid of a monostable circuit 38. This is arranged so that once triggered its output on a line 36 remains low for a fixed period T4 (about-3T1) after which it switches to high. In use, the monostable is triggered along a line 40 by and upgoing part ofthe output signal. An example is shown in FIGURE 5 where the upper trace represents part ofthe data section of the output signal in the line 40. The signal is coded for 001010as can be seen by reference to FIGURE 1. Below thetrace its effect on the monostable 38 is indicate by arrows. These are the points at which the signal is upgoing, and the monostableis retriggered at each of these points. The lower trace shows the monostable output, along the line 36.This remains lowfor a time T4 after being triggered when, unless it has been reset in the interim, it goes high. The combination ofthe upper trace (on the line 34) and the lower (along the line 36) enables the computerto decode thetrace. It is programmed to look atthe monostable stabe and determine its value just prior to the rising edge ofthe data line. If it is low, the previous wave was an 'O', if high then a '1'. This is indicated in the lower line of FIGURE 5. Having decoded the signal the computerthen stores it as required.

When loading is complete, control is handed back to the main computer operating system.

The player/recorder is simple and cheap to construct, most parts being those used in standard cassette recorders. The only electronic additions are those shown in FIGURE 4 along with interfacing to be used for control. The mechanical components are entirely standard, but are geared in such a way as to cause the tape to run atfourtimesthe normal speed.

The most convenient way is by using a tape drive pulley which has a circumference four times that of the normal pulley. The fast forward and fast rewind must, however be regeared; if these were to cause the tape also to run atfourtimes the normal speed it would be likely to break.

Claims (10)

1. Atape cassette readerarrangedto play a standard compact cassette at a speed substantially greaterthanthe norml play speed, having means for producing an output signal representative of data recorded on the tape, and means for selectively inverting the output signal in dependence upon the polarityofthe recorded data.

2. Atape cassette reader as claimed in Claim 1 in which the means for inverting the output signal comprises an XOR gate having inputs representative ofthe recorded data, and of the polarityofthe recorded data, and an output representative of the output signal.

3. AtapecassettereaderasclaimedinClaim 1 or Claim 2 including means for detecting a synchronisation pulse in the recorded data, means for determining, at a preset moment in the synchronisation pulse, whetherthe signal is high or low, and in which the means for selectively inverting the output signal is arranged to act in dependence upon the said determination.

4. Atape cassette reader arranged to play a standard compact cassette at a speed substantially greaterthan the normal play speed, having means for reading data recorded on the tape inthe form of "0" 's and "1" 's in which a "0" is represented by a single square wave of a first length and a "1 " by a single square wave of a second length, having a circuit with a presettime constant initiated by detection of a characteristic part of a read single wave for distinguishing between waves ofthefirst length and of the second length.

5. Atape cassette reader as claimed in Claim 4 in which the circuit is a monostable circuit arranged to be triggered into a first state by the characteristic part of the read wave, and to return to a second state (unless retriggered) afterthe preset time constant,the time constant being shorter than one ofthe said lengths but longerthan the other.

6. Atape cassette reader as claimed in Claim Sin which means are provided for detecting the state of the monostable at retriggering for determining whether the previous wave represented a "0" or"1".

7. Atape cassette reader as claimed in any one of Claims 1 to3 and as claimed in anyone of Claims4to 6.

8. Atape cassette reader substantially asspecifically described herein, with reference to the accompanying diagrams.

9. A method of decoding binary data in which a "0" is represented by a single square wave of a first iength and a "1 " bya single square wave of a second length, comprising detecting a characteristic part of a wave,triggering a circuit of presettime constant in responsethereto, and distinguishing between a "0" and a "1" by detecting the state ofthe circuitwhen itis next retriggered, or at the arrival ofthe characteristic partofthe nextwave.

10. A method as claimed in Claim 9 in which the circuit is arranged to be triggered into a first state by the characteristic part of the wave and to return to a second state (unless retriggered) after the preset time constant, the time constant being shorterthan one of the said lengths but longerthan the other.

I 1. A method of decoding binary data substantially as specifically described herein, with reference to the accompanying diagrams.