GB2367678A

GB2367678A - Data writing and reading

Info

Publication number: GB2367678A
Application number: GB0023803A
Authority: GB
Inventors: Phil Haylor
Original assignee: SWEET JAMES ANTHONY; Trix SRT Ltd
Current assignee: SWEET JAMES ANTHONY; Trix SRT Ltd
Priority date: 2000-09-28
Filing date: 2000-09-28
Publication date: 2002-04-10
Also published as: GB0023803D0

Abstract

Method and apparatus for writing data on a magnetic writing medium and writing data to a computer bus in which the data is divided into words of a predetermined length, each bit of the data word is modulated onto a respective signal, each with a different predetermined frequency, to form a plurality of bit-modulated signals which are then mixed and recorded to the magnetic writing medium or written to the computer bus. Complementary method and apparatus is provided for reading the data from the magnetic writing medium and the computer bus.

Description

DATA WRITING AND READING The invention relates to writing and reading data, for example within a computer system. Data is required to be written and read in many sub-systems within computer systems. For example, in magnetic storage systems, data is written to a magnetic recording medium by a transducer which leaves a magnetic signature within the magnetic material of the recording medium. The data is then read from the magnetic material at a later stage by a further transducer which detects the magnetic signature within the magnetic recording medium. Components within a computer system also pass data via bus systems. Such bus systems may interconnect internal adaptor cards, memory, video and peripheral host adaptor cards for example.

Other bus systems may interconnect internal or external peripheral units such as image scanners, printers or magnetic storage devices for example. A first component writes the data to the bus in the form of a series of changes in voltage or current.

Components requiring the data read the data from the bus by detecting these changes.

In all aspects of computer technology there is a continuous requirement for more data storage, for data to be processed faster and for it to be communicated faster. In presently known magnetic storage devices, data bits are stored on the magnetic storage medium using a variety of methods. Such methods include the presence or absence of a magnetic signature on a given area of the medium, analogue frequency representation of corresponding data bits"1"and"0"or in longitudinal patterns of magnetic particles with North-South or South-North magnetic polarity.

Such an arrangement of representative data bit"1"and"0"in accordance with the longitudinal magnetic orientation. In order to increase the amount of data stored on a given area of magnetic medium, the data footprint size must be decreased, the number of recording sides must be increased and the recording tracks must be decreased in width and located in closer proximity to each other. However as the areal density increases so does the likelihood of adjacent bits degrading each other's magnetic signatures. In addition, if the bit area is to be reduced, the accuracy of the magnetic head and its positioning system must be correspondingly increased. A further problem for present magnetic storage devices is performance. Faster reading

and writing operations can increase performance. Performance increases can be achieved by increasing the speed of which the media surface (also known as the substrate) is passed under the transducer used for writing and reading the magnetic data signatures. Currently, data words are written and read sequentially. Performance increases can be achieved by increasing the rate at which the substrate is passed under the transducer used for writing and reading the magnetic data signatures.

However by writing and reading whole data words at a time would result in a significant performance increase.

In buses of the type presently known bits are transmitted by being written onto the bus at a first location by a first component in the form of the presence of an electric voltage or current and by being read at a second location by a second component which detects changes in electric voltage or the direction of the electric current. The time interval required to be left between bits is dependent on the signal degradation, which in turn is dependent on the cable length, signal shaping and electrical characteristics. As the bus length is determined by several factors, such as signal shaping and electrical characteristics the achievable bit rate on the bus is limited. Therefore in order to increase the data throughput on a bus it is necessary to increase the bus size, i. e. provide further wires or enhance signal processing, etc.

This requires more equipment which increases the cost and the complexity of the bus.

It is an aim of the present invention to alleviate some of the problems described above with conventional data reading and writing methods.

According to a first aspect of the first embodiment of the invention there is provided a method of recording data onto a magnetic recording medium, said method comprising : arranging input data into words of a predetermined length; modulating each bit of a data word onto a respective signal, each with a different predetermined frequency, to form a plurality of bit-modulated signals; mixing said bit-modulated signals to form a recording signal; and

recording said recording signal on the magnetic recording medium. According to a second aspect of the first embodiment of the invention there is provided a method of reading data from a magnetic recording medium, said method comprising : reading successive signals recorded on a magnetic recording medium; demodulating each successive read signal by a predetermined number of signals, each with a different predetermined frequency, to obtain respective demodulated bits; combining said demodulated bits obtained from a respective read signal to form a data word.

This is advantageous as it increases the bit density of the magnetic storage medium (and hence the capacity of a given magnetic storage medium) without decreasing the size of the bit storage areas, or increasing the areal density of the magnetic medium. This means that the risk of adjacent bit areas degrading each other's magnetic signatures is not compromised and the accuracy of the magnetic head positioning does not need to be increased. The invention further allows the read-write speed of the storage device to be increased without having to increase the speed at which the magnetic medium is passed under the magnetic head. Similarly, the speed of the substrate can be reduced to prevent large generations of heat or Buffer overload. The speed of the substrate may need to be reduced from current levels in order to prevent the generation of heat from affecting the integrity of the data word.

In a preferred embodiment of the invention, said mixed bit-modulated signals are modulated onto evenly spaced phase intervals of a common carrier signal.

This is advantageous it means that several data words may be recorded onto each carrier signal, further increasing the capacity of the storage device and the possible read-write speed.

According to further aspects of the first embodiment of the present invention there is also provided apparatus corresponding to the methods described above.

According to a first aspect of the second embodiment of the invention there is provided a method of writing data to a line of a computer bus, said method comprising : arranging input data into words of a predetermined length; modulating each bit of a data word onto a respective signal, each with a different predetermined frequency, to form a plurality of bit-modulated signals; mixing said bit-modulated signals to form a writing signal; and writing said writing signal on the line of the computer bus.

According to a second aspect of the second embodiment of the invention there is provided a method of reading data from a line of a computer bus, said method comprising: reading successive signals written on a line of a computer bus; demodulating each successive read signal by a predetermined number of signals, each with a different predetermined frequency, to obtain respective demodulated bits; combining said demodulated bits obtained from a respective read signal to form a data word.

This embodiment of the invention is advantageous as it increases the data rate attainable on each line of a computer bus. Therefore a computer bus of the present invention may replace a conventional computer bus using fewer lines but maintaining the same total data rate or the computer bus may be installed with various numbers of lines up to or more than the original number of lines providing a larger data rate.

This is advantageous it means that several data words may be recorded onto each carrier signal, further increasing the attainable data rate on each line of the bus.

According to further aspects of the second embodiment of the present invention there is also provided apparatus corresponding to the methods described above.

The invention will now be described by way of non-limiting examples with reference to the accompanying drawings, in which: Figure 1 shows a schematic representation of apparatus according to the first embodiment of the invention for writing data to a magnetic recording medium; Figure 2 shows a schematic representation of apparatus according to the first embodiment of the invention for reading data from a magnetic recording medium; Figure 3a shows a magnetic recording medium of a first type for use with the first embodiment of the invention; Figure 3b shows a magnetic recording medium of a second type for use with the first embodiment of the invention; Figure 4 shows a schematic representation of apparatus according to the second embodiment of the invention for writing data to a computer bus; Figure 5 shows a schematic representation of apparatus according to the second embodiment of the invention for reading data from a computer bus.

In the present invention a stream of input data is arranged into data words which may be of the same length as or a different length from the word length of the input data. Each data word is represented by groups of modulated signals at frequencies, which are unique to the data bits. A preferred method of modulating each bit-representing signal is to simply switch the signal on if the bit is a"1"and switch the signal off if the bit is a"0", or vice versa. However many other modulations can be used, such as representing one state with a higher amplitude signal and the other state with a lower amplitude signal, and may have additional advantages.

The complete data word can therefore be represented by a signal which is formed by mixing the signals representing each bit. Therefore a mixed signal representing an 8 bit data word, for example, will be comprised of a signal in which the amplitude at eight specific frequencies will be set to specific levels to represent the value of each bit. Since the signals representing each bit have different frequencies they will not interfere with one another, assuming that a sufficient bandwidth is left between them. Clearly the size of the data words that can be represented by such a signal is only limited by the available bandwidth and the resolution to which the amplitudes of different frequencies can be measured. Data word sizes are not limited by the invention but may be limited by other factors such as: the precision and quality of electronic components used to construct the apparatus; tolerance of the magnetic medium to facilitate recording of a spectrum of amplitude steps; heat factors and their effect on the integrity of the data This mixed signal representing the complete data word can then itself be modulated onto a carrier frequency, for example for recording to a magnetic recording medium. In a preferred embodiment a number of these mixed signals representing a complete data word are generated and modulated to the carrier frequency in such a manner that they are each modulated onto an equal phase portion of a common carrier signal. For example if four data words are to be recorded, they are transposed on to a carrier signal. The first mixed signal, representing the first data word, is modulated to the first 900 phase of the carrier signal, the second mixed signal, representing the second data word is modulated to the second 900 phase of the carrier signal and so on.

Additional data may be added to the mixed signal by adding signals modulated onto carrier signals with frequencies which do not interfere with the frequencies of the bit-representing signals or with the main carrier signal. This additional information may include, depending on the apparatus to which it is

applied, information such as sector number, track number, address mark or error check information. Alternatively, such information can be modulated into one phase portion of the carrier signal and the data words be modulated into the remaining phase portions.

Thus the system is able to write or read the signal several bits at a time (determined by the size of the data words) compared to conventional systems which read and write one bit at a time. This may improve, by a multiple, the rate at which data can be written and read. In the case of writing and reading data to and from a magnetic recording medium this also increases, by the same factor, the data storage capacity of the medium. Larger data words can be used to secure larger performance improvements. However, the size of data words may be limited as previously mentioned.

Figures 1 and 2 respectively show schematic representations of the apparatus for recording and reading data to and from a magnetic recording medium according to a first embodiment of the invention. In this example binary data is written to and read from a hard drive of a computer. It should be noted however that the present invention is applicable to any circumstance in which data is recorded onto a magnetic recording medium.

In the reading apparatus 1, data is inputted at the data input 10. The data may be in serial or parallel format. The data may be compressed in a data compression unit 2 which may, for example, be a"Compress, Store and Forward"type or a"Real Time and Forward"type. The data compression unit 2 is an optional feature which will provide additional apparatus performance improvements. The data from the data compression unit is passed to a data buffer 3 in which the data is divided into data words of a predetermined length. In the case that the data is in parallel format with the same number of bits as is contained in the data words then the division performed in the buffer will be implicit. In some cases it may be desirable to add predetermined bits (for example"O"s) to data words which are smaller in length than the predetermined length.

In a preferred embodiment of the invention the data words will further be divided into smaller data words (in the example shown into quarter data words) in data buffers 4a, 4b, 4c, 4d. The purpose of this further division will be explained below.

A quarter word selecting circuit 5 selects a data word from one of the data buffers 4a, 4b, 4c, 4d as required. The data word, in this example a 16 bit data word with bits dO, dl, d2,..., d 5, is then passed to a bit modulating circuit 9. A frequency generator 6 generates signals, with a different frequency fgO, fg 1, fg2,..., fg 15 for each bit of the data word, in frequency generating sections 7a, 7b, 7c,..., 7p. Each of the signals from the frequency generator 6 are modulated according to the value of the corresponding bit of the data word and the bit-modulated signals are passed to a signal mixing unit 11.

In the present example the signals are simply modulated by analogue switches 8a, 8b, 8c,..., 8p in which the signal is passed to the signal mixing unit 11 if the bit is a"1"but not if the bit is a"0" (i. e. the signal passed to the signal mixing unit 11 has zero amplitude). However there are many other ways in which the modulation may

be performed. The bit-modulated signals are then mixed in the signal mixing unit 11 which also, in this example, modulates the mixed signal onto a carrier signal. The carrier signal is generated in a carrier frequency generating section 7z of the frequency generator 6.

The modulated mixed signal is then recorded onto the magnetic recording medium 13 by a recording transducer 12. In addition to the bit-modulated signals the signal mixing unit 11 may also add additional signals to the carrier signal. These additional signals may be used to contain additional information such as sector number, track number, address mark, error check information and the like.

The recording apparatus of this example also contains a reading transducer 14 which reads a pre-recorded synchronisation signal from the magnetic recording medium. The synchronisation signal may alternatively be generated by means of a

regenerating clock. The use of a synchronisation signal provides timing of the phase portions of the modulation carrier. This synchronisation signal is used to synchronise the carrier signal generated in the signal mixing unit 11 and may also be used in the frequency generator 6 to synchronise the generated frequencies.

In a further improvement, the synchronisation signal is used to control the quarter word selecting circuit 5. The quarter word selecting circuit 5 switches between the outputs of each of the data buffers 4a, 4b, 4c, 4d to select each for a 90 phase portion of the synchronisation signal. This has the effect of successively reading each of the quarter data words and producing corresponding bit-modulated signals. By this means the signal mixing unit 11 modulates the bit-modulated signals corresponding to each of the quarter data words onto a 900 phase portion of the carrier signal and therefore the complete data word is recorded on a single, common, carrier signal.

Figure 2 shows the reading apparatus 100 which is complementary to the recording apparatus 1 described above. Complementary components are assigned similar reference numerals. A magnetic signal recorded on the magnetic recording medium 13 is read by a reading transducer 115 and passed to a frequency separator 116. This unit demodulates the read signal to remove the main carrier signal and separates the resulting mixed signal into the bit-modulated signals (each with a different frequency) representing each bit of the data word. Each of these signals is subsequently demodulated to determine the state of each bit dO, dl, d2,..., dl5. In the present example this is performed by determining whether or not a signal at the respective frequency is present or not. If the signal is present the bit is outputted as a "1"but if it is not present then the bit is outputted as a"0"or vice versa.

If, as described above, additional information (containing sector number, track number, address mark error check information or the like) has been added to the carrier signal then this will also be separated from the carrier signal in the frequency separator 116.

The decoded bits are passed to the quarter word selecting circuit 105 which transfers the bit data to the respective data buffer 104a, 104b, 104c, 104d where the bits a re-combined to form the quarter data words. The use of a synchronisation signal, again either generated from a pre-recorded synchronisation signal or from a regenerating clock, allows the quarter word selecting circuit to transfer the bit data to the correct data buffer depending on which 900 phase portion of the carrier signal the data has been decoded from. The synchronisation signal is also used in the frequency separator circuit 116.

The data buffer 103 receives the quarter data words from the data buffers 104a, 104b, 104c, 104d, and outputs the data in the required format, serial or parallel (which may be of a different size to the size of the complete data words read from the magnetic recording medium). If data compression has been applied to the information then the data is passed to a decompression unit 102. The decompression unit is a type which is complementary to the data compression unit 2 used before recording the information.

It should be noted that in an integrated reading and recording apparatus some

of the components described above in respect of both the apparatus 1 of Fig. l and the apparatus 100 of Fig. 2, such as the data buffer 3, 103, may be common to the reading circuit and the recording circuit. In the case of the buffer 3,103 it may therefore be a bi-directional buffer.

Figure 3a shows a section of a magnetic tape 20 which forms an example of a suitable medium for recording signals as described above. The tape 20 is split into discrete recording tracks 21. In a preferred embodiment the majority of the tracks may be data-containing tracks 22 and the remaining tracks or sections of tracks may be used for synchronisation, address mark or error check information tracks 23. The synchronisation tracks 23 may contain pre-recorded synchronisation signals comprised of a reoccurring signal. Figure 3b shows a corresponding disc-shaped magnetic recording medium 30 which has recording tracks 31 which may be used as data-containing tracks 32 or as synchronisation tracks 33 with a pre-recorded

synchronisation signal. In an arrangement with a plurality of disc-shaped magnetic recording mediums, a clocking signal recorded on one side of one disc may be used to synchronise all the data recording surfaces. The magnetic discs or tapes may be used in hard drives or be contained in removable cassettes for use in computers or other electronic equipment, such as"floppy disks", "Zip disks","Jaz disks", "Jazdisks"and "DAT tapes".

The improvements of this invention to devices for recording data on magnetic storage mediums may be applied to designing entirely new magnetic data storage devices. However it is also intended that existing devices can be modified to incorporate the present invention. For instance this may be possible by installing a new/replacement electronic circuit board which interfaces with the internal data bus and utilises the existing magnetic recording medium, transducers and other mechanics.

A second embodiment of the invention is used to read and write data to and from a computer bus system. The invention may be used with either internal or external computer buses. Internal computer buses are used to host Adaptor and Addon system enhancement cards which are used, for example, to provide additional memory or improved graphics processing. External, or peripheral, buses are used to connect peripheral devices such as image scanners, printers and magnetic tape backup devices. Physically the computer buses are comprised of a set of wires (lines) to which all of the components or devices on the bus are attached. Typically some of the wires are used as control lines for controlling access to the bus and the remainder are used as data lines. The buses used in the present invention may require some RF shielding in order to compensate for attenuation and signal degradation.

The data writing circuit 200 and the data reading circuit 300, shown in Figures 4 and 5 respectively, are identical to those used for writing and reading data to a magnetic storage medium described above except for the replacement of transducers 12,14, 115 and 114 by connections to the data bus. Therefore the description thereof is not repeated. Thus in the case of the data writing circuit the

signal outputted from the signal mixing unit 11, and represents the inputted data word is written onto one of the data lines 240 of the bus. The signal from a data line 340 of the bus is read into the frequency separator circuit 116. In this manner each line of the bus is capable of transmitting a complete data word rather than either having to transmit the data word one bit at a time or use a several lines to transmit the entire data word in one go. Although not shown in Figures 4 and 5, the data writing circuit 200 and the data reading circuit 300 will also have connections to the bus control lines and address lines.

The synchronisation signals for controlling the data reading and writing circuits 200,300 may either be read from a dedicated synchronisation line 241,341 of the data bus, or may be generated from clock signals conventionally used to control the timing of a data bus.

As with the embodiment for recording data onto magnetic recording mediums, it is intended that this embodiment of the invention is applicable to the upgrading of existing computer buses as well as to the design of new computer bus systems.

Claims

CLAIMS 1. A method of recording data onto a magnetic recording medium, said method comprising : arranging input data into words of a predetermined length; modulating each bit of a data word onto a respective signal, each with a different predetermined frequency, to form a plurality of bit-modulated signals; mixing said bit-modulated signals to form a recording signal; and recording said recording signal on the magnetic recording medium.
2. A method of recording data onto a magnetic recording medium according to claim 1, wherein said step of modulating each bit of said data word onto a respective signal is performed by passing the respective signals through a respective switch controlled by the corresponding bit of the data word.
3. A method of recording data onto a magnetic recording medium according to claim 1 or 2, wherein said step of mixing said bit-modulated signals to form a recording signal comprises the step of modulating said bit-modulated signals onto a recording carrier signal.
4. A method of recording data onto a magnetic recording medium according to claim 3, wherein said step of modulating said bit-modulated signals onto a recording carrier signal comprises modulating said mixed bit-modulated signals onto evenly spaced phase intervals of a common carrier signal.
5. A method of recording data onto a magnetic recording medium according to claim 4, wherein the number of said evenly spaced intervals within a period of the

carrier signal is four.
6. A method of recording data onto a magnetic recording medium according to any one of claims 3 to 5, said step of modulating said bit-modulated signals onto a

recording carrier signal comprises generating said recording carrier signal in synchronism with a synchronisation signal.
7. A method of recording data onto a magnetic recording medium according to claim 6, wherein said synchronisation signal is pre-recorded on said magnetic recording medium and said step of modulating said bit-modulated signals onto a recording carrier signal further comprises reading said synchronisation signal from the magnetic recording medium.
8. A method of recording data onto a magnetic recording medium according to any one of the preceding claims, wherein: additional signals are mixed with said bit-modulated signals at said step of mixing said bit-modulated signals to form recording signals; and said additional signals represent additional information.
9. A method of recording data onto a magnetic recording medium according to any one of the preceding claims, further comprising, before said step of arranging the data into words of a predetermined length : receiving data from a parallel input; and converting it to a serial data stream.
10. A method of recording data onto a magnetic recording medium according to any one of the preceding claims, wherein the magnetic recording medium is a magnetic tape.
11. A method of recording data onto a magnetic recording medium according to any one of claims I to 9, wherein the magnetic recording medium is a magnetic disk.
12. A method of recording data onto a magnetic recording medium according to any one of claims 1 to 9, wherein the magnetic recording medium is contained within a removable cartridge.
13. A method of reading data from a magnetic recording medium, said method comprising : reading successive signals recorded on a magnetic recording medium ; demodulating each successive read signal by a predetermined number of signals, each with a different predetermined frequency, to obtain respective demodulated bits ; combining said demodulated bits obtained from a respective read signal to form a data word.
14. A method of reading data from a magnetic recording medium according to claim 13, wherein said step of demodulating a read signal by a predetermined number of signals is preceded by the step of demodulating the read signal from a carrier signal.
15. A method of reading data from a magnetic recording medium according to claim 14, wherein said step of demodulating the read signal from a carrier signal comprises demodulating successive read signals from evenly space phase intervals of a common carrier signal.
16. A method of reading data from a magnetic recording medium according to claim 15, wherein the number of said evenly spaced intervals within a period of the carrier signal is four.
17. A method of reading data from a magnetic recording medium according to any one of claims 14 to 16 wherein said step of demodulating the read signals from a carrier signal uses a synchronisation signal, which is in synchronism with said carrier signal.
18. A method of reading data from a magnetic recording medium according to claim 17, wherein said synchronisation signal is pre-recorded on said magnetic recording medium and said step of demodulating the read signals from a carrier

signal further comprises reading said synchronisation signal from said magnetic recording medium.
19. A method of reading data from a magnetic recording medium according to any one of claims 13 to 18, wherein: in the step of demodulating successive read signals additional signals are also extracted from the read signals; and said additional signals represent additional information.
20. A method of reading data from a magnetic recording medium according to any one of claims 13 to 19, further comprising: converting said sequence of data into a parallel format; and outputting said parallel data.
21. A method of reading data from a magnetic recording medium according to any of claims 13 to 20, wherein the magnetic recording medium is a magnetic tape.
22. A method of reading data from a magnetic recording medium according to any of claims 13 to 20, wherein the magnetic recording medium is a magnetic disk.
23. A method of reading data from a magnetic recording medium according to any of claims 13 to 20, wherein the magnetic medium is contained within a removable cartridge.
24. An apparatus for recording data onto a magnetic recording medium, said apparatus comprising: means for arranging input data into words of a predetermined length; means for modulating each bit of a data word onto a respective signal, each with a different predetermined frequency, to form a plurality of bit-modulated signals; means for mixing said bit-modulated signals to form a recording signal; and means for recording said recording signal on the magnetic recording medium.
25. An apparatus according to claim 24, wherein said means for modulating each bit of a data word onto respective signals comprises a switch which is controlled by the corresponding bit of the data word and through which respective signals pass.
26. An apparatus according to claim 24 or 25, said means for mixing said bitmodulated signals to form a recording signal comprises means for modulating said bit-modulated signals onto a carrier signal.
27. An apparatus according to claim 26, wherein said means for modulating said bit-modulated signals onto a carrier signal further comprises means for modulating said mixed bit-modulated signals onto evenly spaced phase intervals of a common carrier signal.
28. An apparatus according to claim 27, wherein the number of said evenly spaced intervals within a period of the carrier signal is four.
29. An apparatus according to any one of claims 26 to 28, wherein said means for modulating said bit-modulated signals onto a recording carrier signal comprises means for generating said recording carrier signal in synchronism with a synchronisation signal.
30. An apparatus according to claim 29, wherein said synchronisation signal is pre-recorded on said magnetic recording medium and said apparatus further comprises means for reading said synchronisation signal from the magnetic recording medium.
31. An apparatus according to any one of claims 24 to 30, wherein: said means for mixing said bit-modulated signals to form a recording signal comprises means for mixing additional signals with said bit-modulated signals to form the recording signal; and said additional signals represent additional information.
32. An apparatus according to any one of claims 24 to 31, further comprising : means for receiving data from a parallel input ; and means for converting the data to a serial data stream, said means being arranged to pass the serial data to the means for arranging input data into words of a predetermined length.
33. An apparatus according to any one of claims 24 to 32, wherein the magnetic recording medium is a magnetic tape.
34. An apparatus according to any one of claims 24 to 32, wherein the magnetic recording medium is a magnetic disk.
35. An apparatus according to any one of claims 24 to 32, wherein the magnetic recording medium is contained within a removable cartridge.
36. An apparatus for reading data from a magnetic recording medium, comprising : means for reading successive signals recorded on a magnetic recording medium; means for demodulating each successive read signal by a predetermined number of signals, each with a different predetermined frequency, to obtain respective demodulated bits; and means for combining said demodulated bits obtained from a respective read signal to form a data word.
37. An apparatus according to claim 36, wherein said means for demodulating a read signal comprises: means for first demodulating the read signal from a carrier signal; and means for then demodulating the read signal by the predetermined number of signals, each with a different predetermined frequency to obtain respective demodulated bits.
38. An apparatus according to claim 37, wherein said means for demodulating the read signal from a carrier signal comprises means for demodulating successive read signals from evenly space phase intervals of a common carrier signal.
39. An apparatus according to claim 38, wherein the number of said evenly spaced intervals within a period of the carrier signal is four.
40. An apparatus according to any one of claims 37 to 39, wherein said means for first demodulating the read signal from a carrier signal uses a synchronisation signal, which is in synchronism with said carrier signal.
41. An apparatus according to claim 40, wherein said synchronisation signal is pre-recorded on said magnetic recording medium and said apparatus further comprises means for reading said synchronisation signal from said magnetic recording medium.
42. An apparatus according to any one of claims 36 to 41, wherein: said means for demodulating successive read signals further comprises means for extracting additional signals from the read signals; and said additional signals represent additional information.
43. An apparatus according to any one of claims 36 to 42, further comprising: means for converting said sequence of data into a parallel format ; and means for outputting said parallel data.
44. An apparatus according to any one of claims 36 to 43, wherein the magnetic recording medium is a magnetic tape.
45. An apparatus according to any one of claims 36 to 43, wherein the magnetic recording medium is a magnetic disk.
46. An apparatus according to any one of claims 36 to 43, wherein the magnetic medium is contained within a removable cartridge.
47. A method of writing data onto a line of a computer bus, said method comprising : arranging input data into words of a predetermined length; modulating each bit of a data word onto a respective signal, each with a different predetermined frequency, to form a plurality of bit-modulated signals; mixing said bit-modulated signals to form a writing signal; and writing said writing signal on the line of the computer bus.
48. A method of writing data onto a line of a computer bus according to claim 47, wherein said step of modulating each bit of said data word onto a respective signal is performed by passing the respective signals through a respective switch controlled by the corresponding bit of the data word.
49. A method of writing data onto a line of a computer bus according to claim 47 or 48, wherein said step of mixing said bit-modulated signals to form a writing signal comprises the step of modulating said bit-modulated signals onto a writing carrier signal.
50. A method of writing data onto a line of a computer bus according to claim 49, wherein said step of modulating said bit-modulated signals onto a writing carrier signal comprises modulating said mixed bit-modulated signals onto evenly spaced phase intervals of a common carrier signal.
51. A method of writing data onto a line of a computer bus according to claim 50, wherein the number of said evenly spaced intervals within a period of the carrier signal is four.
52. A method of writing data onto a line of a computer bus according to any one of claims 49 to 51, said step of modulating said bit-modulated signals onto a writing

carrier signal comprises generating said writing carrier signal in synchronism with a synchronisation signal.
53. A method of writing data onto a line of a computer bus according to claim 52, wherein said synchronisation signal is written on a line of the computer bus and said step of modulating said bit-modulated signals onto a writing carrier signal further comprises reading said synchronisation signal from said line of the computer bus.
54. A method of writing data onto a line of a computer bus to any one of claims 47 to 53, wherein: additional signals are mixed with said bit-modulated signals at said step of mixing said bit-modulated signals to form writing signals; and said additional signals represent additional information.
55. A method of writing data onto a line of a computer bus according to any one of claims 47 to 54, further comprising, before said step of arranging the data into words of a predetermined length: receiving data from a parallel input; and converting it to a serial data stream.
56. A method of writing data onto a line of a computer bus according to any one of claims 47 to 55, wherein the computer bus is an internal bus.
57. A method of writing data onto a line of a computer bus according to any one of claims 47 to 55, wherein the computer bus is a peripheral bus.
58. A method of reading data from a line of a computer bus, said method comprising : reading successive signals written on a line of a computer bus; demodulating each successive read signal by a predetermined number of signals, each with a different predetermined frequency, to obtain respective demodulated bits;

combining said demodulated bits obtained from a respective read signal to form a data word.
59. A method of reading data from a line of a computer bus according to claim 58, wherein said step of demodulating a read signal by a predetermined number of signals is preceded by the step of demodulating the read signal from a carrier signal.
60. A method of reading data from a line of a computer bus according to claim 59, wherein said step of demodulating the read signal from a carrier signal comprises demodulating successive read signals from evenly space phase intervals of a common carrier signal.
61. A method of reading data from a line of a computer bus according to claim 60, wherein the number of said evenly spaced intervals within a period of the carrier signal is four.
62. A method of reading data from a line of a computer bus according to any one of claims 59 to 61 wherein said step of demodulating the read signals from a carrier signal uses a synchronisation signal, which is in synchronism with said carrier signal.
63. A method of reading data from a line of a computer bus according to claim 62, wherein said synchronisation signal written on a line of said computer bus and said step of demodulating the read signals from a carrier signal further comprises reading said synchronisation signal from said line of the computer bus.
64. A method of reading data from a line of a computer bus according to any one of claims 58 to 63, wherein: in the step of demodulating successive read signals additional signals are also extracted from the read signals; and said additional signals represent additional information.
65. A method of reading data from a line of a computer bus according to any one of claims 58 to 64, further comprising : converting said sequence of data into a parallel format; and outputting said parallel data.
66. A method of reading data from a line of a computer bus according to any of claims 58 to 65, wherein the computer bus is an internal bus.
67. A method of reading data from a line of a computer bus according to any of claims 58 to 65, wherein the computer bus is a peripheral bus.
68. An apparatus for writing data onto a line of a computer bus, said apparatus comprising : means for arranging input data into words of a predetermined length; means for modulating each bit of a data word onto a respective signal, each with a different predetermined frequency, to form a plurality of bit-modulated signals; means for mixing said bit-modulated signals to form a writing signal; and means for writing said writing signal on the magnetic writing medium.
69. An apparatus according to claim 68, wherein said means for modulating each bit of a data word onto respective signals comprises a switch which is controlled by the corresponding bit of the data word and through which respective signals pass.
70. An apparatus according to claim 68 or 69, said means for mixing said bitmodulated signals to form a writing signal comprises means for modulating said bitmodulated signals onto a carrier signal.
71. An apparatus according to claim 70, wherein said means for modulating said bit-modulated signals onto a carrier signal further comprises means for modulating said mixed bit-modulated signals onto evenly spaced phase intervals of a common carrier signal.
72. An apparatus according to claim 71, wherein the number of said evenly spaced intervals within a period of the carrier signal is four.
73. An apparatus according to any one of claims 70 to 72, wherein said means for modulating said bit-modulated signals onto a writing carrier signal comprises means for generating said writing carrier signal in synchronism with a synchronisation signal.
74. An apparatus according to claim 73, wherein said synchronisation signal is written on a line of the computer bus and said apparatus further comprises means for reading said synchronisation signal from said line of the computer bus.
75. An apparatus according to any one of claims 68 to 74, wherein: said means for mixing said bit-modulated signals to form a writing signal comprises means for mixing additional signals with said bit-modulated signals to form the writing signal; and said additional signals represent additional information.
76. An apparatus according to any one of claims 68 to 75, further comprising: means for receiving data from a parallel input; and means for converting the data to a serial data stream, said means being arranged to pass the serial data to the means for arranging input data into words of a predetermined length.
77. An apparatus according to any one of claims 68 to 76, wherein said computer bus is an internal bus.
78. An apparatus according to any one of claims 68 to 76, wherein said computer bus is a peripheral bus.
79. An apparatus for reading data from a line of a computer bus, comprising: means for reading successive signals written on a line of a computer bus;

means for demodulating each successive read signal by a predetermined number of signals, each with a different predetermined frequency, to obtain respective demodulated bits; and means for combining said demodulated bits obtained from a respective read signal to form a data word.
80. An apparatus according to claim 79, wherein said means for demodulating a read signal comprises: means for first demodulating the read signal from a carrier signal; and means for then demodulating the read signal by the predetermined number of signals, each with a different predetermined frequency to obtain respective demodulated bits.
81. An apparatus according to claim 80, wherein said means for demodulating the read signal from a carrier signal comprises means for demodulating successive read signals from evenly space phase intervals of a common carrier signal.
82. An apparatus according to claim 81, wherein the number of said evenly spaced intervals within a period of the carrier signal is four.
83. An apparatus according to any one of claims 80 to 82, wherein said means for first demodulating the read signal from a carrier signal uses a synchronisation signal, which is in synchronism with said carrier signal.
84. An apparatus according to claim 83, wherein said synchronisation signal is written on a line of the computer bus and said apparatus further comprises means for reading said synchronisation signal from said line of the computer bus.
85. An apparatus according to any one of claims 79 to 84, wherein: said means for demodulating successive read signals further comprises means for extracting additional signals from the read signals; and said additional signals represent additional information.
86. An apparatus according to any one of claims 79 to 85, further comprising : means for converting said sequence of data into a parallel format ; and means for outputting said parallel data.
87. An apparatus according to any one of claims 79 to 86, wherein the computer bus is an internal bus.
88. An apparatus according to any one of claims 79 to 86, wherein the computer bus is a peripheral bus.
89. A method of recording data on a magnetic recording medium substantially as herewith described with reference to the accompanying drawings.
90. A method of reading data from a magnetic recording medium substantially as herewith described with reference to the accompanying drawings.
91. An apparatus for recording data on a magnetic recording medium substantially as herewith described with reference to the accompanying drawings.
92. An apparatus for reading data on a magnetic recording medium substantially as herewith described with reference to the accompanying drawings.
93. A method of writing data on a computer bus substantially as herewith described with reference to the accompanying drawings.
94. A method of reading data from a computer bus substantially as herewith described with reference to the accompanying drawings.
95. An apparatus for writing data on a computer bus substantially as herewith described with reference to the accompanying drawings.
96. An apparatus for reading data from a computer bus substantially as herewith described with reference to the accompanying drawings.