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Tape Rap | |
Article from Phaze 1, June 1989 |
the subtle differences explained
Recording tape... We all use it, and as it unravels on the floor, we all curse it. But if you want to know how it works, sit back and unwind with PHAZE 1...
WHEN YOU SPEND weeks modifying your instruments and searching for the room with perfect acoustics, you'd be forgiven for not wanting to worry about recording tape. I mean, you've got enough problems trying to stick your effects setup together with gaffer tape! Everybody takes recording tape for granted but it's actually a high precision engineering product and should be treated with respect.
As you probably know, recording tape works on the principle of "magnetism". The surface of any recording tape is composed of countless crystals of magnetic oxide, which is mixed with a flexible binder and fixed to a plastic backing tape. As the tape passes over the recording head in the tape machine, a magnetic field is produced which permanently magnetises the surface of the tape. The end result is a magnetic pattern along the length of the tape, representing the original sound. When the tape is replayed, it passes a playback head and the magnetic information is once more converted back into electrical energy — from which point it can be amplified and eventually fed to a loudspeaker.
Simple? Maybe. The truth is that a great deal of research has gone into finding the right "formula" for recording tape — the best materials, the best particle size and shape — to give us the standard of audio fidelity we've come to expect.
Before going any further, there is one popular misconception that must be cleared up. There is no such thing as an eight-track tape, or 16-track tape, or whatever — the surface of any tape is the same all over, and is not divided up in any way. What creates the individual "tracks" is the way in which the tape machine's head is constructed; this is what dictates how many tracks of information can be laid side by side on the tape's surface. A quarter-inch (wide) reel of tape, for example, could be used to record in mono, stereo, four-track or even eight-track formats. It all depends on the recorder. A professional stereo machine has just two tracks: one for the left-hand signal, one for the right. Most semi-pro or domestic stereo machines, on the other hand, tend to record four tracks and this includes our friend the audio cassette.
The reason this is done is so that the tape can be turned over and played the other way to double the playing time. This is often referred to as playing the 'other side' but in reality, we're playing the same side but different tracks. Imagine the tape's surface as being a bit like a dual carriageway with two lanes being reserved for traffic going one way and two lanes for traffic going the other. Figure 1 shows how the tracks are actually recorded on a compact cassette from which you can see chat the dual carriageway analogy isn't quite accurate — but it puts the idea over.
The great advantage of compact cassettes is that they are compact (!), relatively durable and far more convenient than open reel tapes. But this convenience brings with it a cost.
Cassettes run at a slow speed compared to open reel machines 1⅞ ips as opposed to 7½ or 15 ips (inches per second), and their track width is narrower which means fewer crystals of oxide pass the tape heads each second. As you might imagine, the more individual crystals are used to represent a given piece of sound, the more accurate the result will be. Think of it like a mosaic picture, the smaller the individual pieces, the more 'in focus' the end result. Cassette tape is also fiddly to handle which makes it nearly impossible to splice. Not really a problem for home use but in the studio, tape is being spliced all the time. Because of this limitation in both sound quality and flexibility, cassette is strictly a domestic and home studio format.
So we've discovered two very important facts — the faster the tape speed, the higher the quality and the wider the tape tracks, the higher the quality. But what do we mean by "quality" - what does an "out of focus" sound actually sound like?
In general, low tape speeds mean increased hiss and very low tape speeds mean a poor high frequency performance which results in a dull sound. You can hear the hiss on a brand new tape — you don't have to record anything — so how do we keep it to an acceptable minimum?
The trick is to make sure that the music you record onto tape is much louder than the hiss, but there's also a limit to how much magnetism you can put onto a tape before it can't handle any more. This is called tape saturation and it's effect on sound quality shows up as distortion on the loud sections. Fortunately, tape manufacturers have improved the performance of their tape coating significantly over the past few years. There are the so-called 'chrome bias" tapes which use a special formulation that gives a better high frequency response (brighter tone) and the more recent metal cassettes which can accept even greater levels without distorting. However, these must be used in machines designed to take advantage of them or you won't reap the benefits. These improved formulations are used almost exclusively in cassettes and go some way to compensate for the slow tape speed and narrow track width.
Another weapon in the war against hiss is noise reduction, Dolby being the most popular example. Audible hiss contains a significant proportion of high frequencies so what the Dolby system actually does is to boost the most vulnerable low level, high frequency audio signals during the recording process so that they are recorded onto tape well above the background noise level. When the tape is being played back, exactly the opposite process occurs and another circuit puts the levels back the way they were originally, the outcome being that the natural sound is restored and the hiss reduced in the process. This only occurs when the music is quiet, by the way, because when it is loud, it masks the hiss anyway.
Because Dolby works both on record and playback, it is known as an encode/decode system and to get a faithful sound, you must either record and play with the Dolby on, or record and play with it off. If you record with Dolby but play back without it, the result will be unnaturally bright whereas if you play back a non-Dolby'd tape with the Dolby switched on, the sound will be dull. Bet you knew that anyway!
The most popular Dolby system in commercial use is Dolby B, but over the past few years, the more effective Dolby C has started to appear on many machines. This works on a similar principle but is less likely to cause dulling of the sound, is less susceptible to overload distortion and reduces the hiss level further than Dolby B does. If you're wondering what happened to Dolby A, that is a strictly professional system used in large studios which actually divides the audio spectrum into four bands and then processes each separately. This does produces fewer side effects but for a higher cost — the inevitable trade-off.
Dolby's most recent addition is Dolby SR or Spectral noise reduction. This is unbelievably complicated, and at around £1000 per track to fit, you can imagine that only very serious pro studios can afford to use it. Even so, it does claim to give analogue tape machines the ability to achieve or even exceed the sound quality available from digital machines — so it may be commercially viable to keep your old 24-track and add Dolby SR rather than buy a complete digital system.
There is another noise reduction system around (well, there would be, wouldn't there!) called dbx but this is seen mainly on Portastudio type recorders or professional open-reel multitrack machines. It never really caught on as a domestic format. This system progressively turns up the level of all quiet sounds during recording regardless of whether thay are bass sounds or bright sounds and like Dolby, it performs the exact opposite function during replay. This is said to be a "compander" type noise reduction because it compresses the signal during recording and expands it during playback.
The dbx system gives us a terrific degree of noise reduction during quiet sections of music but it can cause side effects such as the dulling of percussive sounds and noise pumping. This latter effect really shows up worst when the music consists of only bass sounds — as the music gets louder, you can hear the hiss come up in level too because there is little in the way of high frequencies to hide it. On good tape machines, the side effects are quite acceptable, but if dbx has been used to try to hide the defects of a budget tape system, then the side effects are likely to be quite noticeable.
The Compact Cassette was originally designed purely for stereo use but the introduction of the Portastudio found us recording all four tracks in one direction rather than two in one direction and two in the other. Doubling the tape speed to 3¾ips improved the sound quality and dbx noise reduction kept the noise down to a reasonable level. This simple idea was the basis of the home studio. Last year, two machines appeared on the market that managed to cram eight tracks onto the humble cassette and, because of the improvements in tape formulations and the advances made in electronics over the past few years, the result was surprisingly good.
For the open-reel user, quarter inch tape can now handle stereo, four-track or eight-track recording to a pretty high standard while half inch tape is used for professional stereo mastering, eight-track, and 16-track semi-pro recorders.
Fully professional studio multitracks still tend to use two inch tape for both 16 and 24-track work and it isn't uncommon for these machines to run at tape speeds of 30 ips to get the best audio quality. However, a reel of two inch tape costs around £100 and at that tape speed a reel would only last around 12 minutes!
There are one or two other oddball formats around, the most famous of which is Akai's half inch tape cassette used in their 12-track recorders. These offer the best of both worlds in that they give a better sound quality than compact cassettes yet retain the convenience. Their disadvantages are their limited playing time and relatively high initial cost.
We're now in the era of digital recording and if we exclude the fabulously expensive open-reel studio digital multitracks for the moment, we are left with DAT or Digital Audio Tape. This records the music as a sequence of numbers in much the same way as the tape machines used to store computer programs — but much faster. To achieve this speed, the current DAT machines use a rotating head rather like that in a video recorder which scans diagonal stripes across the tape's surface. This has the same effect as a tape speed of around ten feet per second, even though the tape itself moves very slowly. The outcome is a tiny cassette, even smaller than a conventional Compact Cassette that can hold up to two hours or more of CD quality sound.
To make good recordings, a tape machine must be kept clean, be it analogue or digital, open-reel or cassette. DAT machines can be cleaned by using a special head cleaning cassette but for analogue machines, you can't beat a bottle of head cleaning alcohol (isopropyl alcohol) and a few cotton buds. You simply use an alcohol soaked swab to thoroughly wipe the heads and all other metal parts in the tape path and then use the dry end to remove any excess. Leave the machine to dry out for a minute or two and than play your tape.
Cleaning should be carried out before each important recording because tiny particles from the surface of the tape stick to the head and prevent the tape from running over it smoothly. If left unattended, you'll soon find that your tapes sound dull and may even suffer fron dropout — a random fading of the sound. An added incentive to clean your heads is that it extends the head life considerably and on a multitrack machine, the heads may account for up to half the total cost!
Tape machines should be demagnetised every month or so to prevent the build up of a permanent magnetic field which can even damage existing recordings as well as degrading the quality of new ones. For cassette machines, the electronic demagnetisers built into cassette shells works very well and are easy to use. For open reel, a hand held demagnetiser is essential and you should read the instructions very carefully before using it.
How you store your tape will greatly affect its life. About the worst thing you can do is leave cassettes on the parcel shelf of your car on a sunny day as the heat will eventually spoil the recordings and may even warp the cassette shell itself. Tapes prefer to be stored at a fairly constant room temperature without ever getting very hot or very cold. A cupboard in your bedroom is ideal.
Tapes also hate dust because it gets between the tape and the head causing loss of tone, dropout and accelerated head wear. Cassettes should be put back in their storage cases while open-reel tapes should be stored in plastic bags before being replaced in their boxes and stored stood on end. Professional tape stores are temperature and humidity controlled but for the home user, a little common sense goes a long way.
It's also a good idea to wind open-reel tapes right through at play speed before storage to ensure that the tape is neatly wound. This prevents the edges sticking out where they can easily be damaged — particularly serious if you use a narrow format multitrack machine, as you could damage the outside tracks to the extent where serious dropouts occur.
Tapes are a magnetic recording medium so another way to guarantee problems is to store them close to magnets. Always keep your tapes at least a couple of feet away from speakers and try to anticipate less obvious sources of high intensity magnetism such as guitar pickups, electrical equipment with transformers such as power amps and effects and, of course, TV sets.
Simple eh? Absorb all of this and you'll get the best out of whatever tape you choose to use. All you've got to do then is record some great music. Well, it couldn't all be this easy could it?
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