GB2175436A - Fast edit point and cue location - Google Patents

Abstract

To facilitate fast location of edit-points in a disc-based audio recording system, one or more editing tracks ("spooling" and/of "aux.data") are recorded in conjunction with the digital audio signal ("audio"). The spooling track is a reduced sample rate, reduced bandwidth version of the audio signal. This can be replayed at increased speed to simulate "spooling" on an analogue tape recorder, without exceeding the maximum data reading rate of the disc. The auxiliary data file ("aux data") includes edit markers indicating points in the audio signal. These points are selected by an operator during recording as potential edit points or cue locations. The edit markers can be displayed when the recording is replayed. Preferably a directory of edit markers is also generated, so that a particular edit point can be located quickly. <IMAGE>

Description

SPECIFICATION Fast edit point and cue location The present invention relates to a method of digitally storing an audio signal on magnetic disc, for subsequent fast location of edit points and/or cue points. Sound editing and cueing performed by most known methods can only be satisfactorily monitored by listening. An edit point is conventionally found by first 'playing' through the material at up to twenty times normal speed (spooling). Accurate location then proceeds by the 'rock-and-roll' method, i.e. manually moving the tape back and forth across the reply head. The same technique is used for cueing, though automatic devices using time-codes may give some assistance.

In a digital audio tape recorder (DATR) the data tracks cannot be satisfactorily replayed over such a wide range of tape speeds. Edit point location and cueing may be provided by the use of an auxiliary analogue audio track.

In known recorders with analogue tracks, (including DATRs) the replayed signal can be monitored at up to twenty times normal speed by adjusting the tape guide so that the tape passes close to the replay head. This requires a degree of manual dexterity, and gives an unpredictable quality, but the technique is nevertheless of great value in, for example, finding a pause during an interview. Some recorders have built-in time-code location equipment, whereby a time-code sequence recorded throughout the length of the tape can be examined momentarily so that an intelligent control system can wind the tape to the desired time code location. Clearly this requires a knowledge of timings, and a data entry system for the operator. It has the disadvantage of requiring the operator to estimate a time code value with no indication of success or failure in finding the required location.

In a disc based audio recording and editing system, such as one based on Winchester disc technology, the data reading speed at the heads is constant, and speed variation can be achieved by 'picking' data as required with appropriate signal processing to restore the sampling rate to a constant value. U.K. patent application no. 8421378 entitled 'Variable speed digital audio replay' describes a mechanism to do this.

To provide high speed location and cueing in disc-based systems, two additional methods have been reported. In the first, the high data reading speed of magnetic disc systems is exploited to give a four times replay speed (eight times if only one channel is replayed). See "Editing digital audio" by McNally G.W. and Gaskell P.S. (lCASSP-84, San Diego, March 1984.

Conference Proceedings pp 12B4. 1-12B4.4). In the second, short excerpts are replayed at normal speed while 'skipping' through the material. In the first method, a fixed limit is reached at the maximum data transfer rate from disc, while in the second the subjective effect of listening to the interrupted material is rather disturbing, and of course the desired material may be missed.

The present invention provides a method of recording an analogue audio signal, in which the analogue signal is sampled at a first rate to generate a digital audio signal which is recorded in association with at least one other signal derived simultaneously in response to the analogue audio signal and used during playback for locating potential edit points and/or cue locations in the audio signal.

In one preferred method, the said other signal is a digital editing signal derived by sampling the the analogue audio signal at a second, slower rate, the editing signal being recorded for replay at increased speed to simulate spooling through the digital audio signals.

In another preferred method, the said other signal comprises indicating data indicating points in the digital audio signal selected during recording as potential edit points or cue locations.

The invention further provides apparatus for recording an analogue audio signal, comprising signal proceesing means for sampling the analogue signal at a first rate to generate a digital audio signal, and for simultaneously deriving at least one other digital signal in response to the analogue audio signal for use during playback for locating potential edit points and/or cue locations in the audio signal.

In a preferred embodiment, the other signal is a digital editing signal and the processing means comprises a sampling means operable to sample the analogue audio signal at a second, slower rate to derive the digital editing signal for replay at increased speed to simulate spooling through the digital audio signal.

In another preferred embodiment, the apparatus further comprises indicating means for incorporating indicating data in the said other signal, the indicating data indicating points in the digital audio signal selected during recording as potential edit points or cue locations.

Other preferred features of the invention are defined in the subsidiary claims set out below.

In this application, two methods of fast location are described. In the first, the editing signal is a specially processed "spooling" file generated at the time of the original recording. This permits digital replay at up to thirty-two times normal speed (in the arrangement described) with fully variable speed control.

The 'spooling file' is a reduced bandwidth, reduced sampling rate version of the original audio signal. This low data rate signal can then be replayed at a much higher speed before the disc transfer rate becomes a limitation. Since the file is an audio file it can be replayed through a varispeed device such as that described in the U.K. patent application referred to above, to give a very large range of speed control. It may also be used to generate a waveform display which is useful in some applications.

In the second method, the editing signal comprises 'edit markers' which are generated by an operator during recording or reply for subsequent cueing. The edit markers are recorded on the disc in an auxiliary data file with the audio signal, and are also recorded as a simple list (or directory) for fast searches. The resulting digital signal then contains a record of its edit history.

The second method involves the participation of a sound editor or assistant during recording to generate the 'edit markers' at relevant times. 'Edit markers' can also be added later. These are logged by a disc-based system in an 'auxiliary data file' which serves several purposes discussed later. The information can be organised in such a way that fast retrieval is possible-avoiding the need to search through the audio recording, which may be two hours long.

The auxiliary data file may contain formatted users data-('Labels') to further assist in this process, such as the labels described in "Labels and their formatting for digital transmission and recording" by Lagadec R. and McNally G.W. (74th AES Convention, New York, October 1983).

Both methods involve the creation and maintenance of additional data files over and above the original audio data. To achieve this, four separate issues must be addressed:~ 1. The disc format and file organisation. By "disc format" is meant the pattern of sectors created by a formatting program before the disc is used. Each sector consists of header information and null data written at desired physical positions on the disc.

2. Fife formats for audio, spooling and auxiliary data.

3. Signal processing for generating and replaying spooling files.

4. Data processing for fast retrieval of cueing information.

The proposal described below simultaneously implements both methods of fast location discussed above. Naturally, there may be circumstances in which it is only required to implement one or other of the methods.

One embodiment of apparatus for implementing the method of the invention will now be described by way of example, with reference to the accompanying drawings, in which:~ Figure la to 1d show schematically the operation of the apparatus for performing various functions of recording and playback; Figure 2 shows schematically the format of a sector of the auxiliary data file; Figure 3 shows schematically the data buffers in the RIO unit shown in Figs. la to id; and Figure 4 shows the characteristic of a decimating filter used during recording of the spooling file.

It is convenient to adhere to conventional disc-formatting into sectors, tracks and cylinders so that commer cially available disc controllers can be used. Sectors are blocks of data, conventionally 512 or 1024 bytes with identifying header information and error correction data. Tracks are sequences of sectors making up one complete revolution of the disc on a single surface.

Cylinders are combinations of tracks, on different surfaces of the disc but at the same radial distance from the disc axis. For most efficient use of the disc, a 'scatter storage' system would be used so that as files are deleted and created the disc does not become chequer-boarded.

However, there are also powerful reasons why audio files should be contiguous in the present situation. Principally, this permits high speed replay to be guaranteed and it gives a direct relationship between time-code and disc address.

In this proposal, three contiguous files are built during recording and are known as audio, spooling and auxiliary data files. Special signal processing and data formatting hardware is needed for this task and is described later. Since the data rates can be specified and fixed, areas of the disc can be allocated to audio files, spooling files and auxiliary data files. For a given time-code, the addresses of the corresponding audio, spooling and auxiliary data will then be in strict proportion, greatly simplifying file management and data processing. For example, a storage allocation program will simultaneously manage all three files, and for a given time-code, addresses in the three files may be calculated without rounding error.

An improvement in performance can be achieved by offsetting sector 0 on different cylinders-a process known as skew sectoring which is done when the disc is originally formatted.

The time taken to electronically switch between heads in a multi-head drive is usually sufficiently short that there is no data interruption, and data on several tracks can be read continuously as if it were a spiral track. However, when a seek (head movement) to an adjacent cylinder is initiated, the access time caused by head movement (typically 2-10 ms) will incur a latency. By latency is meant a delay in data read out, caused by the time taken for the required data on the new track to reach the head. In the worst case, the latency time is the time taken for one complete revolution of the disc. By using contiguous files, the access time can be anticipated by skewing the disc formatting so that a sector 0 is ready to be ready after a seek. The advantage is greatest in disc systems with many tracks of low capacity.Skew sectoring can also be used in scatter storage systems, but the guaranteed performance is difficult to analyse, and the disc scheduling can become over-complex. See "Efficient editing of digital sound on disc" by C.

Abbott (Journal AES, Vol. 32, No. 6, June 1984, pp 394-402). Disc controllers offering socalled zero latency are becoming available and are a preferred solution, with the advantage that data transfer times are reduced for all head movements.

There are many advantages to be gained by configuring the audio, spooling and auxiliary data files so that they have a defined relationship with the timing of an original digital signal such as one arriving via an AES/EBU interface. Particularly relevant are time-code, and the block structure defined by the BSYNC preamble. The AES/EBU interface and BSYNC preamble are described in the ANSI Standard document ANSI S.40-1985, "Serial Transmission Format for linearly represented digital audio data".

Attainment of the desirable properties given by choosing structures for the three files which have a simple relationship, can be combined with the need for a known timing by defining recording units'. A recording unit is the minimum interval of time in which an integer number of BSYNCs, audio sectors, spooling sectors and auxiliary data sectors occur on the disc.

A suitable value for the length of a recording unit is 640 ms for a 48kHz rate. This gives a storage breakdown as follows: The 'recording unit' is defined as 640 ms. (160 BSYNCs).

This contains: Audio - 240 sectors of 512 bytes Spooling - 15 sectors Auxiliary data - 32 sectors The spooling and auxiliary data can be written to disc in multiples of 15 sectors and 32 sectors respectively. A convenient buffer size is then 24064 bytes.

Note that for other sampling rates (such as 44.1kHz) the relationship remains the same. The scheme is easily adapted to other commonly found sector sizes.

The formats of the various files will now be described, beginning with a description of the audio file.

In a stereo editor, a sample by sample multiplex of the two audio signals has the advantage that a single disc access before and after an edit produces all the necessary data. However, special purpose hardware is needed to create this multiplex and the data is inefficiently transferred if, for example, only a single channel is to be processed, or a delay is needed to one channel relative to the other.

Alternative techniques, such as the use of 'recording units' in which blocks of data of each audio channel are collected and written to the disc permit efficient single or multi-channel working. However, larger buffers are needed to smooth out the interruptions to data flow caused by additional seeks on the disc, particularly at the edits.

As stereo editing constitutes the major application for the equipment described here, a sample multiplex format is preferred, although the hardware described here can work with data in blocks if extra storage is provided.

The spooling files should have exactly the same format as the audio files so that they can be replayed and processed in the same data formatting and signal processing hardware. The hardware consists of a Real-time Input Output unit (RIO) and a Variable Speed Processor (VSP) (Fig. 1). An addressing ratio of 16:1 between the audio and spooling files gives a capability of 1 .5kHz bandwidth for two channels of the spooling file. The spooling file may be processed at up to 32 times the normal speed of the audio file, by using the variable speed apparatus described in the British patent application referred to above.

The purpose of the auxiliary data file is to increase the transparency of the editor to the AES/EBU interface and provide additional data relating to editing. Thus, storage must be allocated for: 1. User data-all 96kbit/s must be stored intact.

2. Status-a subset of 96kbit/s would be acceptable because the data is highly redundant.

3. Validity-again a subset of the 96kbit/s would be acceptable because the information is sparse and can be efficiently coded.

4. Ranging-a 32kbit/s rate for range codes would provide a means for modifying the effective dynamic range of the recorded audio.

5. Edit markers further low capacity channel for identifying to the editor operational reference points such as previous edits or cueing points.

Constraints on the auxiliary data file format include the need for a simple address ratio to the audio data and to BSYNC as described earlier, and the need for easy separation of the contributing data fields for machine processing. For example, additional features of an audio editor will include: 1. Editing of user data for use in CD sub-code applications such as karoake and graphics.

2. Manipulation of recorded status codes and timecodes.

3. Fast location of edit markers.

An 'addressing ratio' of 15:2 has been chosen to balance these requirements and an example of the data format is given in Fig. 2. Each sector on the disc which makes up an auxiliary data file has an identical format.

There is no advantage to be gained in separating the auxiliary data file into, say, five separate files for users' data, status, etc., since for editing work, the main activity is to process edit markers and users' data, which are closely related.

As an example of ratio addressing, assume that editing is to a resolution of 4 ms. (This figure would be 4.36ms in systems using a 44.1kHz sampling ratee). This means that status blocks etc. are never sub-divided. If editing is to a greater resolution, then the transfer of auxiliary data must be suspended until the start of the next valid block. This can be readily calculated from the fixed relationship between sector numbers and BSYNCS discussed above.

Consider an edit 12ms after start of file. 1ms of audio=48 stereo samples=96 words. Then to compute the location of the associated data within a disc sector:~ 1) Audio word address is 1152 1152 2) Spooling word address is =22 16

The ratio addressing technique always gives integer r#esults without rounding.

Edit markers for the auxiliary data filed will be defined here as an identifiable block of data which is notionally attached to desired points in an audio recording for subsequent location of that point. To be of practical use, edit markers must be found rapidly and the form they take can greatly influence the efficiency of a search. It can readily be arranged that the host computer logs the edit markers, initiated by a sound editor from a control panel, and stores them in a file after completion of the recording. Additionally, they are multiplexed with the recorded data during recording. Fig. la shows an incoming audio signal 10 from the AES/EBU interface being converted to audio, spooling and auxiliary data files. Edit markers generated manually at a control panel 12 are supplied to the RIO unit 14 in real time and also to a host computer 16.

Thus, a file of 'edit marker pointers' is generated (by the host computer) equivalent to a set of timecodes but specified by an address on the disc. These can be scanned very rapidly top find a particular point in the audio. The writing of markers in trhe auxiliary data file means that edit markers can be displayed during replay of the material without reference to another file. This is separate from the audio directory entry associated with a new recording.

Now an example of its use. A recording or transfer to disc is to be made and the editor enters an identifying name at the keyboard, e.g. '2nd movement, take 4'. This is logged by the host computer and also written into the auxiliary data file when the recording or transfer is started. During the recording, if the editor notes a performer error he immediately presses a 'cue' button. The host adds the event to the log and an edit marker is written into the auxiliary data file. Clearly, there is no time to annotate the event other than to assign a cue number generated by the system. The provision of a small number of separate cue buttons might assist here by allowing some differentiation between cues made for different reasons. At the end of the recording, the audio, spooling and auxiliary data files on disc are complete and the host automatically transfers the contents of the log to the edit marker pointer file. (EMP file).

To return to the point at which the performer error occurred, the editor makes a request using the 'cue' button followed by the number (in this case, number one). The EMP file is searched and the address on the disc is passed to the software routines which play the disc. In the menu system of the editor (described in the McNally and Gaskell reference referred to above), the corresponding timecode is automatically loaded into the display with an indication that 'cue' mode has been selected. The material both before and after the cue point can then be selected in the normal way.

Note that edit markers may be relevant to equipment not forming part of the editing system and may be communicated to other equipment over the AES/EBU interface. The edit markers in the auxiliary data file can be considered a scratchpad to be used while editing, but when the edited material is replayed over an AES/EBU interface, the final edit markers would be conveyed in the status channel. When recording, edit markers may need to be generated automaticaly if, for example, there are timecode discontinuities in the signal received on the AES/EBU interface such as would occur on a digital tape providing the signal had already been edited by cutting.

The methods of recording and replaying data from disc are summarized in Fig. 1. Data formatting is performed in the RIO unit which assembles data in a form suitable for fast contiguous transfers to or from disc. The unit creates builds and maintains up to three files on the disc while maintaining the throughput determined by the AES/EBU interface.

The RIO unit contains a large area of memory (512kbytes) and a high speed microcontroller operating under a program downloaded from the host system (see McNally and Gaskell). This buffers the data from the AES/EBUY interface (and processed data from the VSP unit) and orders it in three separate contiguous memory areas. Each of these areas may then be individually transferred to disc using well known direct memory access (DMA) techniques, which provide high speed, with minimal computational overheads for the host computer.

A possible alternative technique uses conventional system memory and a dual DMA channel, i.e. one channel between an input/output unit without memory and the system memory: and a second channel between system memory and the disc. However, in the present application, it is more convenient to incorporate the memory into the RIO unit and define the separate areas for audio, spooling and auxiliary data in local software.

The buffer must be designed to prevent read/write conflicts, or data late errors caused by the buffer emptying or filling. It is convenient, but not essential, to use a double buffering scheme (Fig. 3) so that throughput to the disc is not impaired and the system bus is used efficiently. It is cost effective to design the RIO unit to perform this operation.

A suitable architecture for RIO has been previously described by McNally and Gaskell with four separate data areas or 'blocks'. For the purposes of the present invention, two of these blocks may be allocated for the buffering of auxiliary data and spooling, and the other two for audio data. in the example of a recording unit given above, this results in DMA transfers of 7680bytes spooling data, 16384bytes of auxiliary data and 65,536bytes of audio. These are conveniently handled by two buffers of 24064bytes and two buffers of 65536bytes. The experimental RIO unit of McNally and Gaskell was designed with four buffers of 65536bytes and is therefore quite suitable for this task. The detailed partitioning of the buffers (Fig. 3) is carried out by software.

Figs. 1(c) and 1(d) show two replay techniques which incorporate signal processing of the audio. For variable speed replay of the audio file (Fig. 1(c)), the sampling rate of the replayed signal must be restored to 48kHz before audition; for instance by the process detailed in U.K.

patent application no. 84 21378.

The spooling file is a reduced bandwidth, reduced sampling rate version of the audio file. If the sampling rate is reduced by a factor of 16 to 3kHz, the audio bandwidth must be reduced before recording to less than 1 .5kHz to prevent aliassing. This can be achieved by a filter (a decimating filter) with the characteristic shown in Fig. 4. Such a filter can readily be implemented by hardware based on the TMS320 signal processing chip, and described in the U.K. patent application just referred to.

During replay of the spooling file (see Fig. 1(d)), the two channels are separately processed.

The file format is identical to that of the audio file.

The use of auxiliary data and/or spooling files provide a new method by which a predetermined or unknown point in audio material can be rapidly located. The increase in data storage required is acceptable (about 20%) and the methods make efficient use of data formatting and signal processing hardware which is already used for other reasons in disc-based editors.

Claims (42)

1. A method of recording an analogue audio signal, in which the analogue signal is sampled at a first rate to generate a digital audio signal which is recorded in association with at least one other signal derived simultaneously in response to the analogue audio signal and used during playback for locating potential edit points and/or cue locations in the audio signal.

2. A method according to claim 1, wherein the said other signal is a digital editing signal derived by sampling the the analogue audio signal at a second, slower rate, the editing signal being recorded for replay at increased speed to simulate spooling through the digital audio signals.

3. A method according to claim 2, in which the digital signals are recorded in respective regions of a common store, the regions being so selected that the storage addresses of editing signal samples and audio signal samples relating to the same point of the analogue audio signal are related in a pre-determined manner.

4. A method according to claim 3, wherein each address is ascribed a numerical value, and the values of the addresses of corresponding samples are in a predetermined ratio.

5. A method according to claim 4, wherein the predetermined ratio is the ratio of the first and second sampling rates.

6. A method according to any of claims 2 to 5, wherein the digital editing signal represents a reduced bandwidth version of the analogue signal represented by the digital audio signal.

7. A method according to any of claims 2 to 6, wherein the first and second sampling rates are in the ratio 16:1.

8. A method of reading a digital editing signal recorded in accordance with any of claims 2 to 7, and in which the editing signal which is read is processed to provide a signal having the said first rate of sampling and representing the analogue audio signal at the said increased speed.

9. A method according to claim 8, in which the increased speed is manually selectable.

10. A method according to claim 1, wherein the said other signal comprises indicating data indicating points in the digital audio signal selected during recording as potential edit points or cue locations.

11. A method according to claim 10, wherein indicating data is positioned in the other signal to indicate by its position the position of the corresponding selected point in the digital audio signal.

12. A method according to claim 10 or 11, wherein indicating data is generated in response to an operator command to indicate the point in the audio signal contemporaneous with the command.

13. A method according to any of claims 10 to 12, wherein indicating data is generated automatically in response to abnormal events in the audio signal, to indicate the location of the abnormalities.

14. A method according to any of claims 10 to 13, in which a directory of data is also generated and stored, the directory recording the locations of the points in the audio signal which are indicated by indicating data.

15. A method according to any of claims 10 to 14, wherein indicating data is selected from a set of distinguishable data, whereby the significance of the points indicated may be classified.

16. A method according to any of claims 10 to 15, wherein the indicating data is interspersed in the other signal among reference data relating to the audio signal and for use during editing of the audio signal.

17. A method according to claim 16, wherein the digital audio signal and the other signal are arranged to comply with a data transmission standard, whereby the other signal is transmissable with the audio signal.

18. A method of replaying a signal recorded in accordance with any of claims 10 to 17 for listening purposes, in which the other signal is read with the digital audio signal and an indicator is activated in response to the indicating data to alert the user to each indicated point in the audio signal.

19. A method according to any preceding claim, wherein the digital audio signal and the other signal are recorded on tracks of a magnetic disc, and wherein blocks of samples of each digital signal are recorded in series on the tracks, whereby the points in time represented by samples of a block and the respective angular positions of those samples on the disc are linearly related.

20. A method according to claim 19, wherein the disc has a plurality of tracks and the first sample recorded on each fresh track is angularly displaced with respect to the first sample recorded on the track filled immediately previously.

21. A method of locating edit points and/or cue locations substantially as described above, with reference to the accompanying drawings.

22. Apparatus for recording an analogue audio signal, comprising signal proceesing means for sampling the analogue signal at a first rate to generate a digital audio signal, and for simultaneously deriving at least one other digital signal in response to the analogue audio signal for use during playback for locating potential edit points and/or cue locations in the audio signal.

23. Apparatus according to claim 22, wherein the other signal is a digital editing signal and the processing means comprises sampling means operable to sample the analogue audio signal at a second, slower rate to derive the digital editing signal for replay at increased speed to simulate spooling through the digital audio signal.

24. Apparatus according to claim 23, comprising writing means for recording the digital signals in respective regions of a common store, the writing means being operative so to select the regions that the storage addresses of editing signal samples and audio signal samples relating to the same point of the analogue audio signal are related in a pre-determined manner.

25. Apparatus according to claim 24, wherein each address is ascribed a numerical value and the values of the addresses of corresponding samples are in a predetermined ratio.

26. Apparatus according to claim 25, wherein the predetermined ratio is the ratio of the first and second sampling rates.

27. Apparatus according to any of claims 23 to 26, wherein the signal processing means comprises a decimating filter for reducing the bandwidth of the signal represented by the digital editing signal.

28. Apparatus according to any of claims 23 to 27, wherein the first and second sampling rates are in the ratio 16:1.

29. Apparatus according to any of claims 21 to 28, further comprising means for reading a recorded digital editing signal at the said increased speed, and means for processing the editing signal which has been read, to provide a signal having the said first rate of sampling and representing the analogue signal at the said increased speed.

30. Apparatus according to claim 29, further comprising means for selecting the increased speed.

31. Apparatus according to claim 22, further comprising indicating means for incorporating indicating data in the said other signal, the indicating data indicating points in the digital audio signal selected during recording as potential edit points or cue locations.

32. Apparatus according to claim 31, wherein the indicating means positions data in the other signal at positions corresponding to the positions of the selected points in the digital audio signal.

33. Apparatus according to claim 31 or 32 wherein the indicating means comprises a control member operable to cause indicating data to be incorporated, indicating the point in the audio signal contemporaneous with the operation of the control member.

34. Apparatus according to any of claims 31 to 33, wherein the indicating means comprises means for generating indicating data automatically in response to abnormal events in the audio signal, to indicate the location of the abnormalities.

35. Apparatus according to any of claims 31 to 34, in which the indicating means comprises means for generating and recording a directory of data recording the locations of the points in the audio signal which are indicated by indicating data.

36. Apparatus according to any of claims 31 to 35, wherein indicating data is selectable from a set of distinguishable data, whereby the significance of the points indicated may be classified.

37. Apparatus according to any of claims 31 to 36, wherein the indicating data is interspersed in the other signal among reference data relating to the audio signal and for use during editing of the audio signal.

38. Apparatus according to claim 37, wherein the digital audio signal and the other signal, are generated to comply with a data transmission standard, whereby the other signal is transmissible with the audio signal.

39. Apparatus according to any of claims 31 to 38, further comprising means for replaying recorded digital audio signals and an indicator responsive to the other signal corresponding to an audio signal being replayed to alert the user to each point in the audio signal indicated by the other signal.

40. Apparatus according to any of claims 22 to 29, further comprising disc storage means for recording the digital audio signals and the other signal or signals on tracks of a magnetic disc, and operative to record samples of each signal in series and in blocks on the tracks, whereby the points in time represented by samples of a block and the respective angular positions of those samples on the disc are linearly related.

41. Apparatus according to claim 40, wherein the disc storage means is operable to record the signals on multiple track magnetic discs, and to record the first sample on each new track at a location which is angularly displaced with respect to the first sample recorded on the track filled immediately previously.

42. Apparatus substantially as described above with reference to the accompanying drawings.