ဤစာမျက်နှာကိုယခုလက်ရှိအင်္ဂလိပ်ဘာသာဖြင့်မရနိုင်ပါ.
Introduction
Cassettes are used within the GRN family for a number of applications. They are almost always the medium by which our recordings are distributed, in which case they have usually been copied on a high speed copier (duplicator). Therefore, the master from which these final copies are made (the Copy Master) is usually a cassette. Although the practice is to be avoided if at all possible, cassettes are also sometimes used for making Original Recordings, and/or Program Masters.
Conventional reel-to-reel and cassette recorders use analog recording. A characteristic of analog recording is that the recorded signal is degraded at each recording or copying stage. High quality professional reel-to-reel machines, such as Nagras and large studio recorders, cause significantly less degradation than domestic cassette machines. High quality professional cassette recorders give much better performance than domestic machines, but still fall short of the performance of high quality reel-to-reel machines. Cut editing is only feasible with single track reel-to-reel machines.
Use of noise reduction circuits, such as Dolby, improves the performance of cassette recorders, when it is certain that the cassette will always be played back on a machine having matching characteristics. If this is not so, then the noise reduction circuits will worsen the degradation. For this reason, noise reduction circuits must never be used when making Copy Masters. Dolby 'B' should be used if possible in the case of Original Recordings or Program Masters, provided the machines on which these recordings will need to be played back also have Dolby 'B' facilities.
The degradations of each stage are cumulative, so to achieve acceptable quality in the final product it is necessary to start with the best quality. Therefore it is desirable that the first links in the chain, ie., Original Recordings and Program Masters, should be made on high quality professional reel-to-reel machines or DAT recorders. For the same reason Copy Masters should be made on high quality professional cassette recorders. They should be recorded at normal speed using high quality cassettes. Original Recordings should only be made on cassettes in unusual circumstances where it is impossible to use Professional reel-to-reel or DAT recorders.
Types of Degradation caused by Analog Recording
Hiss
We are all familiar with the hiss which can be heard to a greater or lesser degree behind cassette recordings. Technically, this is called 'noise', and the extent to which it is present is called 'Signal to Noise Ratio'. It is a different problem to those background noises such as traffic, people coughing, animal sounds etc. which are a characteristic of the recording location. It cannot be avoided, only minimised by using good quality equipment and by keeping recording levels as high as possible consistent with other restraints.
Other 'Electrical' Noises
As well as the hiss described above, recordings may be marred by hum, crackles or buzzes introduced into the signal as the result of electrical interference. These are usually the result of either the unsatisfactory connection of leads or of faults in the electronics. Examples of unsatisfactory connections of leads include long runs of unbalanced cable, earth loops, and faulty earth connections.
Tape Saturation
When a recorder is being operated within the appropriate range of signal levels, the recording is 'linear'. This means that changes in the level of the magnetic field recorded on the tape are proportional to changes in the level of the sounds being recorded - a sound twice as loud will produce twice the level on tape. Within the linear range, when played back, the tape will produce the same change in the output sound level as occurred in the original sound. However, there is a limit beyond which the tape is no longer linear - the change in the strength of the magnetic field is less than the change in the sound being recorded. This 'compresses' the sound - on playback the peaks of sound are less than they should be.
The situation is complicated because the effect varies with frequency - the higher frequencies saturate at a lower level than the middle and low frequencies. Also, the amount of bias affects the level at which saturation occurs, and so a correct bias setting is important.
A fundamental problem of analog recording is to keep the average record level as high as possible to minimise the amount of background hiss whilst keeping the peak record level low enough to avoid serious tape saturation and distortion. Obtaining a satisfactory record level is one of the most important aspects of a field recordist's art. It is equally important in subsequent re-recording processes, including high speed copying.
Distortion
Strictly speaking, any degradation of the original sound is a 'distortion', but the term is usually applied to those changes which occur because the recording system is non-linear. This distortion has a number of effects, which are mainly audible because of the production of new components which were not in the original signal. Some of these are 'Harmonics', that is, they are harmonically related to sounds in the original signal, and these may not be obvious unless there is a very large amount of distortion. Others are the 'Intermodulation Products', which generally are more objectionable. A wide range of non-linear characteristics are possible, some of which produce more objectionable 'distortion' than others. The degree to which the 'distortion' produced by a given amount of a particular non-linearity is objectionable also depends on the type of signal being recorded.
As explained above, tape saturation is a form of non-linearity, and so produces 'distortion' as discussed here. For moderate amounts of tape saturation, the harmonic and intermodulation products are not very noticeable because it is one of the less severe forms of distortion, but they do tend to 'muddy' the signal, particularly if the signal is re-recorded a number of times. The distortion produced by overloading the electronics of the recorder or associated equipment is likely to sound much worse.
Tape saturation is affected by the level of bias being used, and if the bias is too low there will be increased distortion. Sometimes this is due to a machine being incorrectly adjusted, or having a fault. Another common cause is using a tape which is formulated for a high bias (eg., CrO2) on a machine which has provision only for normal (Fe) tape. There have also been cases where even though the recorder was equipped with a sensor to detect the presence of a high bias tape, it was ineffective because CrO2 tape had been loaded in a normal cassette by mistake. For these reasons it is recommended that only normal (Fe) tapes be used. It is also necessary that recorders have the bias adjusted correctly.
Frequency Response
A wide and constant frequency response is necessary for a recording to sound natural. If the low frequencies are not present, or are recorded at a lower level than the rest of the recording, we will sense that the sound is lacking in bass. Similarly, if the very high frequencies are affected we will consider it as dull or 'lacking sparkle'. If the higher part of the middle frequencies are reduced, we will perceive it as lacking in treble. We can also hear when the lower middle frequencies are missing - we might describe the sound as 'hollow'. A low level of the 'mid-middle' frequencies will be perceived as 'lack of presence'. If one part of the frequency response is at an excessive level compared with the rest, we find that objectionable also - excessive low frequencies sound 'boomy', excessive high frequencies 'shrill', etc.
The frequency response of a recording can be affected by a number of factors including: inferior tape, dirty heads, magnetised heads, magnetised tape guides or other components of the tape path, incorrect head azimuth, worn heads, incorrect biasing, tape saturation, incorrect playback equalisation, use of Dolby on record and not on playback or vice versa, and faulty electronics. As well as these 'technical' problems, poor frequency response can result from poor recording technique on the part of the original field recordist.
Wow and Flutter
Wow and Flutter are caused by problems in the mechanical components of tape transports. Anything which causes a variation in the speed at which the tape moves past the heads during either record or playback will change the frequencies (Pitch) of the signal being recorded. If the change occurs at a slow rate, it is described as 'Wow', and if at a faster rate as 'Flutter', the terms being descriptive of how the result sounds. Wow and flutter are most easily perceived on a continuous test tone, or on a long sustained musical note. Some instruments, eg., the piano, produce sounds which show up the effect more than others. Where the sound is complex, such as that from an orchestra, extra sound components are produced similar to intermodulation products, and the effect may sound similar to the 'distortion' described above.
Print-through
The adjacent layers of a tape are wound in close proximity to each other and over a period of time there is a tendency for the signal on one layer to 'copy' at a low level onto the next. Because, compared with the wanted signal, it is at a low level and slightly shifted in time, it is often perceived as an echo (although it might be a 'pre-echo'). Some types of tape are more prone to this effect than others, and it is also affected by the conditions under which the tape is stored. The effect is most noticeable when one layer has a section of high level signal and this is wound next to a layer with a section of low level signal.
Crosstalk
Sometimes an unwanted signal is played back as well as the wanted signal. One of the more common situations is where a small amount of the program from the second side of the cassette is heard as well as the wanted side. This may result from a misalignment of the tape guides or head on either the recording machine or the machine used for playback, so that the active part of the playback head overlaps the gap between the recorded tracks and plays back part of the second track as well as the wanted track. In this case the unwanted track will be unintelligible because it will be played backwards. Crosstalk between the left and right channels of a stereo recording can also occur, but is usually not noticeable because the unwanted track is in the 'right' direction, and usually the two signals are very similar. In the case of a true stereo program, crosstalk between right and left channels will reduce the stereo effect.
High speed copiers usually copy both sides simultaneously, and it is possible for the signal in one channel to be present in the other channel, because of a fault in the electronics or because of magnetic coupling between the parts of the multi-track recording head.
A similar effect can occur where a recording is made over a previously recorded tape, and part of the original recording remains to be played back along with the new recording. This can occur if the erase function on the recorder is inadequate, or if there is a tape/head alignment problem. It should be noted that not all high speed copiers provide an erase function. Hence, it is highly desirable that cassettes be bulk erased before they are reused.
It should be noted that a tape may play back satisfactorily when checked by the original recordist or the operator of a high speed copier, and yet produce complaints of crosstalk when played back by someone else. This would occur where the head/tape alignment of the play back machine used by the person complaining is different to that of the original recorder or copier, and to the machine on which the tape was checked. Hence it is important that GRN machines are correctly adjusted to industry standards.
Construction of Cassettes
The cassette may, at first sight, appear to be a simple device, but a lot of work has gone into its design, and unless it is well made to very close tolerances, it will not perform satisfactorily. It consists of the following components:
Shell
The plastic case which makes up the shell of the cassette should be a precisely moulded unit. The halves should fit together smoothly and there should be absolutely no warp in the sides.
Some shells are held together with screws, and this type of construction has the advantage that the case can be disassembled to recover the tape in the event of damage. Also, because welded shells can be made more cheaply, a screwed shell is almost always the mark of a superior cassette.
In many makes of cassette the two halves of the shell are held together by a weld. When properly done this can result in a good, rigid shell. However, it is not possible to get to the tape without destroying the shell and this can be a disadvantage. Cheap cassettes are invariably of welded construction. Therefore if cassettes with welded shells are to be used, care must be taken to ensure that they are of good quality.
Slip Sheets
These are the two sheets between the tape and the shell and their function is to guide the edges of the tape pack as it turns within the cassette and to reduce friction. They are usually made of laminated plastic treated with some type of low friction material such as teflon.
Tape Head Pad
Also often called 'felt pad'. This pad is spring loaded and its function is to ensure that the tape makes close contact with the head. It only requires a very small separation to occur between the tape and the face of the head for the recording or replay to be unsatisfactory.
Shield
In the area of the cassette where the head enters there is a small piece of metal located behind the pad. It is a magnetic shield and when the cassette is in use provides shielding around the front of the head. This is important to guard against interference and hum which could otherwise be picked up by the head.
Hubs
These play a big part in ensuring that the tape runs smoothly and is not damaged during operation. They need to rotate smoothly, and to have a satisfactory means of attaching the end of the tape so that it is securely anchored against the force which occurs when the end of the tape is reached. This force is particularly high at the end of fast forward or rewind, or at the end of copying in a high speed copier. The anchor tends to cause a ridge in the first few layers of tape causing damage to the magnetic coating. In order that this does not result in fades or dropouts at the beginning and end of the tape, the anchor should be arranged in such a way that it causes the least physical disturbance to the layers of tape.
Pins and Rollers
These components are used to guide the tape around the sharp turns in the cassette. Some manufacturers use nylon rollers running on stainless steel pins, and provided these are well made, this is probably the best system. If plastic axles are used, they can lead to the axle and roller welding themselves together. In operation poorly made rollers cause the transport of the tape to be wobbly and jerky. Excessive friction may lead to the tape becoming packed too tightly, leading to wow and flutter, or to the tape jamming. If the tape is not transported smoothly and evenly the edges may become damaged.
Recommended Cassette Types
Copy Masters should only be made on approved types of cassettes using Normal (Type 1) tape. At present these are: TDK 'AR', Maxell 'UDI', Maxell 'UDXL', BASF Ferro Maxima 1 (provided the cassette shells are screwed). If none of these are available locally, arrangements should be made to have one of them imported. These tapes should be either new or clean bulk erased prior to being used. If in unusual circumstances it becomes necessary to make Original Recordings or Program Masters on cassettes, these same types should be used.
Cassettes for distribution should be made using good quality cassettes designed for duplicating on high speed copiers. High speed copiers usually operate at 8 to 16 times normal cassette speed and low grade cassettes which may operate satisfactorily at normal speeds may not withstand the stresses of these higher tape speeds.
It should be kept in mind that the copy tape has to be able to survive at least one run through a high speed copier, but a Copy Master tape has to be good enough to survive many runs through the copier and still produce good quality sound.
The following are some examples of problems experienced with inferior cassettes:
- Plastic axles may cause the guide rollers and axles to weld themselves together.
- 'Out of round' guide rollers may cause tape mistracking resulting in dropouts and generally poor recording.
- If the leader tape is not adequately secured, the tape may separate from the hub when the end of the tape is reached.
- If the splicing tape is not adequately secured, the tape may separate from the leader.
- If the tape is of poor quality it will produce poor recordings. It may shed some of the magnetic coating, which at the least will require frequent cleaning of the copier and which may prevent correct head-to-tape contact during the copying of the tape. If the tape is unduly abrasive it will cause excessive wear of the copier heads.
Before large numbers of cassettes are purchased it is recommended that a few be obtained for examination, testing and assessment of the recording quality. If necessary, the tapes may be sent to the Technical Coordinator or Nominee for assessment.
Assessing the Technical Quality of Recorded Cassettes
When checking cassettes it is important that an assessment be made of the technical quality as well as of the content of the recording. In this context, 'technical quality' should be taken to mean those factors which relate to the performance of the equipment, or to the effects of incorrect operation of the equipment. When a cassette is being checked it is important to listen to it on two distinct levels of monitoring equipment:-
- High quality reproducing equipment and also;
- A typical user machine.
The use of high quality equipment is not because we aim to produce 'Hi-Fi' recordings which will impress enthusiasts, it is to ensure that any faults will be heard. It is important to listen at a high level in an area where the ambient background noise is low, because the evidence of defects is often at a low level.
It is also important to listen on a machine of the type with which the cassette is going to be used, at the level likely to be used in practice, to make sure that it will be satisfactory for field use. This might be a Messenger or a cheap commercial player. Although the lower performance of these machines may mask some deficiencies in the recording, other deficiencies can become more obvious, even to the point of reducing the intelligibility of the message.
- Examine the Structure/Manufacture of the Cassette (Tape and Shell)
- Has the cassette been kept in its case and well stored? Or is it dusty and dirty?
- Has the cassette shell started to bend in the heat?
- Is the felt pad still in position?
- Does the tape run smoothly or is it noisy on fast forward and rewind? Does it stick at all ?
- Is there any sign of excess magnetic coating coming off the tape onto the head when used?
- Can you still read the label?
- Examine the Technical Quality of the Cassette
- Listen to the cassette on good quality monitoring equipment - cassette player or recorder, amplifier and speakers.
- Also listen to it under the conditions in which it will be used in the field.
- What is the level measured on playback? (Preferably with a peak hold meter.)
- Does the Copy Master produce a good copy? (Check a copy as well as the Copy Master)
- Does the sound seem distorted in any way?
- Can you hear hiss, hum, print through, or other interference which is objectionable?
There may be dropouts or other temporary losses of the signal caused by damage to the tape at the beginning of the tape, or wrinkles sometimes develop at the edge of the tape which interfere with the sound production.
Recommended Action
Because some of our Copy Masters have been stored for many years it is important to be able to determine which ones need replacing. Ideally every Copy Master we produce should be re-evaluated ten years after the date of production. By then, if distribution has been effective, the immediate need for which the program was prepared should have been met. Future requests for copies of the recording may only be spasmodic and it may well be that it would be better to redo the recording as the technical quality becomes noticeably poorer over a period of ten years.
If you find a Copy Master is technically poor, do not use it for making any more copies. If there is a firm requirement for more copies, you can order a replacement Copy Master from wherever the Program Master is held. (See GRID)
Unless absolutely essential, sub-standard copies should not be distributed as it only brings our work into disrepute.
Storage of Cassettes
All magnetic recordings deteriorate with the passage of time. The conditions necessary for maximum cassette life are the same as for reel-to-reel tapes, and are as follows:
The magnetic recording on the tape can be erased by a strong magnetic field, so this must be avoided. It also suffers a slow degradation in the presence of weaker fields, and these are likely to increase print-through. Therefore tapes must not be stored near transformers, strong magnets including large loudspeakers, electrical wiring carrying heavy currents, electric welding equipment, etc. Bulk erasers should only be used well away from stored recorded tapes.
Tape is also affected by the climate in which it is stored, both with regard to the recorded magnetism and the chemical stability of the tape. The ideal storage conditions are a temperature of less than 18°C at a relative humidity of less than 30%. This is difficult to achieve. A temperature of 20°C at a relative humidity of less than 40% should give good tape life. Where a controlled environment can not be provided, it may help if the tape can be stored in sealed metal boxes with silica gel. An indicator silica gel should be used so that its moisture content can be monitored, and it should be dried and returned to the box as required. To be effective a comparatively large amount of silica gel would be required.