Studio Sound Techniques (Part 5)
Nearly all studios use analogue tape machines of one sort or another. The fact that many of them are more than ten years old and still in working order shows that with proper maintenance they are basically reliable.
While digital recorders have many advantages over their analogue counterparts, cost is not one of them. In fact, the price of these machines with editing facilities puts them well out of reach of all but a handful of top studios, let alone home studios. Still, it probably won't be that long before they become commonplace. Until then we will have to continue trying to get the best out of our analogue machines.
Because the magnetic information on the tape is directly related to the instantaneous level and frequency of the input signal, any deviation from the correct path of the tape past the heads will result in poor reproduction. It follows then that maintenance falls into two main areas — mechanical and electrical. These are often closely connected insofar as the mechanical shortcomings may only manifest themselves on an electrical meter or as an audible disturbance on the output. Conversely, an electrical fault may manifest itself in poor tape handling.
One of the first points to note about magnetic information on tape is that the record head impresses the majority of the signal on the full depth of oxide (magnetic material); whereas the playback process is largely a surface phenomena, particularly at high frequencies. This is why hybrid tapes can use chromium dioxide for the high frequencies and ferric oxide for low frequencies. In practice this means that the reproduce (playback) side of things will generally be more sensitive than the record side. This is good news from the viewpoint that prompt action on hearing degraded playback performance can save fruitless recording attempts.
It is often tricky to ascertain whether loss of signal is due to a failure in the record or playback circuits. This leads us to a syndrome known as: What's on the tape? Putting faults to one side for a moment, a good machine will have many preset adjustments accessible from the front so that the frequency response of the machine can be optimised for a particular tape. Make no mistake that different reels of the same make and type of tape will possibly require different settings of bias and high frequency equalisation which are closely related. Some types of tape are more stable in this respect that others; Ampex 406 and 454 are, from experience, very good. So if your machine has inaccessible bias or high frequency record equalisation controls you would be better off paying more for a higher quality tape. On the other hand if you are running your studio commercially, you will be required to handle tapes recorded elsewhere, in which case convenient adjustments to equalisation and bias are mandatory. There are several equalisation standards for recorders; notably NAB and AES in the States for 15 ips and 30 ips respectively, plus IEC and CCIR in Europe, both for 7½ ips and 15 ips. It is important to know which type your machine is and obtain a calibrated alignment tape. Some have switchable equalisation which is very useful.
Alignment tapes or any master tapes are best stored having been played through so that you would normally place them on the take up reel and rewind them. This is common practice because the tape is protected from several potential sources of trouble when stored 'tail out'.
If the tape has been fast wound onto a reel, you will notice that some tape edges appear to be 'leafy' or sticking up. In the normal course of handling, these edges can get bent over which will result in poor tape to head contact when that portion of the tape passes the heads. These type of drop outs can be eliminated by playing the tape through at normal speed and obtaining a flat wind.
People often ask why print through should be better by leaving the tape tail out rather than head out. After all, at first sight there appears little difference. The reason is coupled to two facts. Firstly, as mentioned before, the magnetic information is recorded into the full depth of oxide, whereas playback information is mainly picked up from the surface. Secondly, pre-echo is more upsetting psycho-acoustically because dramatic entrances are given away losing the effect altogether. Remember that a pause in music, where the full dynamic is perceived, is often crucial to evoke the right feeling. On the other hand we are well used to hearing post echo as provided by any normal environment or enclosed space.
Thus, when tapes are stored tail out, the magnetic corruption to create pre echo has to take place through the thickness of polymer backing and oxide (which tends to form a magnetic barrier) to get to the surface of the previous turn. When stored head out the magnetic corruption has only to pass through the polymer backing to reach the playing surface. See Figure 9.
Although this analysis is only looking at pre-echo print through, by reversing the direction of the arrows in Figure 9 a similar argument can be constructed to show that post-echo print through will be worse when storing tapes tail out. Of course in practice both take place simultaneously and they are both reduced by using noise reduction systems which we will come to later.
Mechanical: The main points of interest on the mechanical side involve how the machine handles the tape in the various modes of operation. There should be no sudden snatching of the tape when changing modes. The specific procedure for adjusting the tensions on the take up and supply reels in the different modes on your machine, should be in the manual supplied with it. If you don't have a manual and you intend keeping the machine it is worth trying to get one, otherwise here are a few checks you can carry out.
With equal amounts of tape on each reel, slip a loop of string over the pinch roller stem (Fig. 10). Press the 'play' button and pull on the string so that the pinch roller does not meet the tape. If the tensions are equal the tape will remain stationary which is best, or creep very slowly one way or the other. If it moves moderately in either direction the chances are that the tape speed will vary evenly over the length of a reel. This means that songs recorded at the beginning of a reel will run at a different speed if spliced to the end of a reel.
Another way of checking this is to record a tone at the beginning of a reel, splice it to the end and compare it with the original frequency of the oscillator. If you find a difference but no way of adjusting the 'play' tensions, make a mental note to try to keep recorded material in the same relative position (i.e. beginning, middle or end) on the reel. That way at least you will get back from the machine songs at the same tempo and tuning. Though they may well sound quite different on another machine!
A crude but useful way of checking how much back tension there is in a given mode involves displacing the tape between the reel and tension arm or guide using your index finger. See Figure 10. Although you cannot make a measurement like this at least it gives you a feel of how much tension there is. After some experience of using this method you can make a fair assessment of normal tensions. Take care in the fast winding modes not to upset the tape path too much or get your fingers caught on the reels! Table 1 lists some common faults and their possible causes.
Electrical: The following description is a generalised one so various points may not apply in your specific case. However, you will need:
1. Cotton buds and head cleaner.
3. Assorted screwdrivers and spanners.
4. Audio oscillator.
5. Alignment tape.
6. Some way of metering the input and output.
Firstly, clean all the heads and guides so that they are bright and shiny. Check the heads for the first signs of uneven wear (see Figure 11). Figure 11a shows a normal wear pattern i.e. equal amounts of flat either side of the gap down the centre of the head.
Under most circumstances this should be less than half of one centimetre altogether. Any more than this may result in poor tape to head contact at the gap which is vital for correct operation. Any dust or oxide stuck in the gap will impair the frequency response. Oxide is a particular nuisance since it acts as an alternative route for the magnetic information, preventing the magnetic flux appearing across the gap. If the heads are badly worn Figure 11b and c there are several companies who will relapp them, giving the machine a new lease of life. This is especially useful on old multitrack machines where the cost of relapping is marginal compared to replacing heads.
Next demagnetise all the parts in the tape path (heads, guides and rollers). Make sure to switch the demagnetiser on at least three feet from the machine and any master tapes. Failure to observe this precaution can result in transient magnetic fields adversely affecting these objects. Asa general rule it is worth trying to take the demagnetiser through the machine following the tape path; from the supply reel, past the heads, to the take up reel, taking care not to stop in one spot or go back over any section. This is done with the power to the machine switched off.
Feature by P.A. Becque