How It Works - Tape Machines (Part 1)
You can always get more out of your equipment if you know more about it. David Mellor begins a new series that explores the innermost workings of various studio devices. This month: the Tape Recorder.
Do you know what goes on inside your tape recorder? Do you know how to make almost any recorder run backwards? Magnetic maestro David Mellor spills the beans.
One day someone will write the definitive history of the myths and legends of the tape recorder. There are certainly enough home-truths and half-truths around to keep its author well supplied. My aim in this article is to gather together a few simple facts about tape recorders - some you may know already, some you may not; but put together they will provide a clear insight into the workings of the machine that is central to the livelihood of everyone involved in sound recording.
The purpose of the tape recorder is to translate variations in an electrical signal in a piece of wire into variations in a magnetic signal in a length of tape - simple as that. To do this the recorder needs certain basic components: a recording head to convert electricity into a varying magnetic field; a playback head to convert the magnetic field back into electricity; and a tape transport mechanism for moving the tape past the heads at a constant, even speed. There are, of course, numerous ancillary bits and bobs needed to help the basic components do their work, but the fundamentals are that simple.
Figure 1 shows the process represented in graph form which should be self-explanatory. Figure 2 shows a block diagram of the tape recorder's electronics. Let's see what happens to the incoming signal.
The first block the hapless signal meets is the input amplifier which adjusts the level to suit the particular machine's internal operating requirements. Next along the line is the record head driving amplifier which mixes the signal with a high frequency 'bias' signal before sending it to the record head.
The bias is simply a tone of around 100kHz which helps the wanted signal to get onto the tape accurately. Think of it as the back-hander you give to the club secretary so that he will book your band!
Any bias signal that accidentally finds its way back into the replay head, either from the tape or through leakage, is simply filtered away. The oscillator which creates the bias tone also serves the erase head which disperses any existing magnetism, leaving the tape clean for a new recording.
The playback head works in the reverse manner by turning the magnetic signals on tape into a weak electrical signal which is then amplified sufficiently to be useful to the outside world. There are tape recorders which combine the record and replay functions into one head. This is not an easy process to do well and it is only recently that the manufacturers have been able to make heads that can perform each function as good as separate heads.
You will probably notice that Figure 3 looks a bit like a tape recorder. Good! I like to know I'm dealing with intelligent people. It doesn't look like all tape recorders, but it strongly resembles most.
In a professional tape recorder (other than Nagra and Stellavox machines which are a special case), there will be three motors. One driving the capstan and one for each of the two spools. These motors can be very powerful, up to half a horsepower or more, so the unwary tape-op would be well advised to keep his or her fingers away from the spools when the engineer is using the remote control. (Oops, there goes another one!) The spool motors will have both electronic and mechanical brakes to keep matters in hand at all times.
The way the motors function is as follows: the prime mover is the capstan motor, which drives the tape past the heads at a constant speed, with the pinch wheel holding the tape against the capstan. The (right) take-up spool motor will supply sufficient tension to wind up the tape as it emerges without having any tendency to pull the tape through. The (left) supply spool motor is energised in the reverse direction which has the effect of holding the tape firmly against the heads. This back-tension has to be correct, and in advanced tape recorders with motion-sensing facilities, the machine can tell by the speed of rotation of the supply spool how much tape it is carrying and therefore how much tension to apply in order to compensate for the weight of tape. The two tension arms adjust for any short-term variations which might otherwise cause tape snatching.
Older recorders and most domestic models do not use the back-tension technique but instead have a pressure pad to hold the tape to the heads. If you currently have a machine like this, my advice would be either to sell it quick or put it in the loft until it becomes an antique! The pressure pad is a cheap and cheerful device which, as I shall explain later, causes certain problems.
A better alternative to back-tension is the 'closed loop' principle where there is an additional capstan and pinch wheel situated before the heads. The diameter of this secondary capstan is slightly smaller than the main capstan and thus generates the necessary tension to control the tape.
Tape recorders, as you know, come in a variety of formats, but one feature they do have in common is that their standard playback speed or speeds will always be multiples or sub-multiples of 15 inches per second (ips), and that the tape width will always be a multiple of a quarter-inch, eg. half-inch, two inches etc. Here are some of the formats you are likely to encounter:
As it says, this is a single channel format which spreads the audio over the full width of the tape, less a small 'guard band' at either edge. You will rarely see one of these machines in a music studio, but they are more common in the film world.
Yes, they are different and this point is confusing to many people. Good old historical reasons are at the root of the discrepancy and I am afraid problems can and do arise.
The original idea behind two-track recording was that you could either have stereo or two separate mono recordings on your tape. Of course, if you have two mono recordings, one of the Bolton Philharmonic and the other of Bert Rumpus in his garden, you don't want one leaking into the other, so an unrecorded 'guard band' of 1.85mm is left between the two tracks for safety.
What a waste of tape! Leaving such a wide area unrecorded meant that noise performance was degraded (more about noise later) and so some bright spark in Germany (schpark?) decided that stereo recordings would be better done by leaving a 0.75mm guard band instead, a little bit of crosstalk between the two tracks being unimportant in this case. As it is not possible to have two mono recordings on one tape, a full-track erase head is provided.
Problem number one. If you record a tape on a stereo machine and then decide to re-use it on a two-track recorder, the guard band on the two-track machine's erase head will leave part of the old recording unerased. It will be OK played on the two-track, but take it back to your stereo machine and you will hear snatches of the old stereo recording coming through.
Problem number two. If you have recorded a live session using two machines, one stereo and one two-track, and you wish to edit between the tapes then because the track width changes abruptly at the edit point you will get a jump in level when playing on the stereo machine.
My advice, ideally, would be to use a machine with a stereo record head (then there will never be a problem with a tape you send out to someone) and a two-track playback head (then there will never be a problem with an incoming tape). Incidentally, you can tell the difference between the two head types just by looking. Take a peek at Figure 4.
Just to complicate things further, there has been a recent return to the old two-track format in order to accommodate a timecode synchronisation track that is recorded down the centre of the tape. Not to be confused with the old American three-track format. Who's confused? I'm not confused!
KEEP WELL OFF!! This format is a disaster area. If you ever see one of these machines I would advise you to dial 999 and ask for the quarter-track tape recorder disposal squad! The quarter-track format is a bit like two-track except that when you come to the end of the reel of tape you can turn it over and use it the other way just like a cassette. I shall leave you to imagine the horrendous problems that this might cause. Be warned!
Back to saner territory, though there are still a variety of formats here which fall into three groups - professional, semi-professional and obsolete.
4-track on half-inch tape
8-track on one inch tape
24-track on two inch tape
4-track on quarter-inch tape
8-track on quarter-inch tape
8-track on half-inch tape
16-track on half-inch tape
16-track on one inch tape
4-track on one inch tape
16-track on two inch tape
I think it is fair to describe the 16-track on two inch tape format as obsolete, although it has been popular until recently. It is possible to play old 4-track one inch tapes on modern 8-track machines by selecting alternate tracks for replay (eg. 2,4,6,8). I am told you can play 16-track tapes on 24-track machines in a similar way but I have never seen it done.
What all these multitrack machines do for you is allow you to play back tracks already recorded and to record new material synchronised with the old, keeping each track physically separate on the tape. Where would we be without them?
What good is a tape recorder without the tape? You might not know it but there is quite a variety available. Some people have got the distinct impression that there is only one sort of tape these days - Ampex 456 - but there is more to life than that.
Tape comes in a range of thicknesses, which simply means that the thinner the tape, the more you get on the reel. The normal tape in daily professional use is termed 'standard play'. Portable recorders, and other machines such as the Fostex A8, which only take small size spools may use 'long play' tape which lasts one and a half times as long for a given spool size.
There are trade-offs, of course - the recording quality is not quite as good and the tape is not as robust. 'Double play' tape is half the thickness of 'standard play', and is very tricky to edit. 'Triple play' is also available but not advisable as it can stretch easily.
Tape also comes in different coatings, the normal variety not being given any special name, the more expensive type being labelled 'high output'. The first professional high output tape was Ampex 456. It has since been joined by Scotch 226, Agfa 469, and most recently by BASF's Studio Master 911. The idea is that you can put more level on the tape, improve the dynamic range, and so get a better performance as regards noise and distortion. The problem is that this only applies at low and mid frequencies, so if you take advantage of a high output tape's good points, you may have slight difficulties in other areas. The moral is to shop around and find what is best for you.
The back of the tape also deserves attention. Nearly all tape now has a matt finished back coating. The idea of this is to reduce static and also to help air escape from between the layers of tape as it is fast wound onto the spool. Not all back coatings are equally efficient and you will find that some tapes wind more evenly than others. Obviously, if a tape is unevenly wound it stands a greater risk of edge damage.
Where would we be without a few problems to keep us interested? Although tape recorders are an awful lot better than they were, they are not perfect and one or two small gremlins can still leap out and surprise you on occasion. Let's look at the potential problem areas.
Because the size of the magnetic particles on tape is not infinitely small, there is bound to be a certain granularity in the recorded sound which manifests itself as noise. A good tape recorder can achieve a noise level of around 1/2000th of that of a loud signal. This is expressed as a signal-to-noise ratio of 66 decibels (usually written as: S/N = 66dB).The larger the number, the better.
Most modern tape recorders have a signal-to-noise ratio approaching the theoretical maximum, but one point to bear in mind is that the wider the tape, the lower the noise level. A 16-track recording on half-inch tape will have a signal-to-noise ratio only half as good as 16-track on two inch tape. Yer dunna get owt fer nowt, as we say in Woking. Noise reduction systems such as dbx, Dolby A or C can be, and frequently are, used by tape recorder manufacturers to improve noise considerably (how else did you think Fostex managed to cram 16 tracks onto half-inch tape?), but that's another story...
What comes off is what went on, plus a bit of distortion. Unfortunately, as far as distortion goes, the tape recorder is the weakest link in the chain of studio equipment and it will add as much as 3% of dirt to the nice clean signal you have recorded. (Some people prefer to call it 'a nice warm sound'.) The ear is more accommodating than some hi-fi buffs would have us believe, and such a level of distortion is actually quite tolerable on an original recording. The more times the recording is copied, however, the more the distortion and noise builds up. Once again, most modern tape recorders approach the theoretical limit in this area.
From the deepest thud of the bass drum to the tinniest tinkle of the triangle, we want all frequencies to come back from the tape at the right level. This is usually the case, give or take, although if the recorder is not maintained regularly and lined-up correctly, performance in this area will quickly deteriorate. [Refer to the 'Tape Line-up' feature in our September and October 86 issues for guidance.]
Optimum tape speed for good frequency response is 15 inches per second. Any slower and the high frequencies become more difficult to record. Any faster and bass frequencies tend to get a bit uneven.
Wow and Flutter
This is what Duran Duran fans get when they see their favourites perform! It also refers to speed variations of the tape. I don't need to explain why this is undesirable. One point that does need to be mentioned however is that, particularly in older designs, the tape speed can change slightly from one end of the tape to the other. Not so much that you would notice it when playing all the way through mind you, but if you try to edit two bits together from opposite ends of the reel, you're in big trouble. Best to check on this if you are buying an antique.
Not a topic that is on everyone's lips but important nevertheless. Modulation noise, or mod noise for short, is a 'shushing' sound that comes and goes with the recorded signal. It is at its worst in cassette decks and any recorder with a pressure pad. When there is no signal present there is no mod noise, but it mushes up anything you try to put onto tape.
If you look at a quality tape recorder such as a Studer or Nagra, you will notice that there are rollers in the tape path that do not appear to do anything. Their secret function in life is to damp out any vibrations in the tape.
If you have the opportunity to try out the following little experiment, you will be able to hear modulation noise quite clearly, and realise what it can do to your recordings.
Record a tone of around 10kHz at a healthy level and listen to the playback while you are recording. Put your fingers on the roller nearest the heads and stop it turning. Hear what I mean? That nasty hissing sound in the background is modulation noise.
If you do not have these damper rollers on your recorder then you will have to live with the problem, although employing Dolby noise reduction (types A, B or C) will help; dbx will not.
This brings up the real old chestnut of whether you should store your tapes 'head out' or 'tail out'. The answer is 'tail out' and this is why:
Adjacent layers of tape on a reel will tend to transfer magnetism from one layer to another causing an effect known as print-through. Here you get sounds where you don't expect them and probably don't want them. (I have yet to hear of an artistic use for print-through.)
Take a look at Figure 5. It shows three layers of tape on a spool. On the centre layer are recorded a series of numbers which will print through onto the adjacent layers. The numbers will print onto the bottom of the magnetic coating on layer A and onto the top of the coating on layer C.
Obviously, it is worse for the print to be on the top of the coating as this will be nearest the playback head when the tape is replayed and will produce the loudest false signal. Print-through will happen like this whichever way the tape is wound. The difference is that if the tape is wound 'head out', the print will be worse on the layer which comes before the wanted sound, and if it is wound 'tail out' the print will come after. If it comes after, then it gets mixed up with any natural echo or reverb and generally passes unnoticed. No more arguments now, OK?
So that's it, you now know as much about tape recorders as I do - well, there's a bit more to it than that but I have explained the basic principles and problems.
But what about making a tape recorder run backwards? OK, if your machine has three motors and no pressure pad, then what you do is to wind the tape round the wrong side of the capstan, then in reverse S-shape around the pinch roller, and on to the take-up spool. Hey presto! It sorts out its own tensions and runs backwards! I shall leave you to work out why you can't record this way - and no, I won't pay for a new motor if you burn yours out! That's the price of creativity. The technique seems to work fine with my Revox, so give it a try.
Feature by David Mellor
Previous article in this issue:
Next article in this issue: