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How It Works: The Cassette (Part 9)

Do you live in peace with your cassette recorder? Or is your life full of mangled tape misery and longings for lost high frequencies? Confirmed reel-to-reelie David Mellor explains the problems that bedevil the cassette format, and offers some advice to make the medium work for you rather than against you.


Cassettes are as much a fact of life as the birds and bees. And just as bees sting and birds do whatever they feel necessary to make life annoying for us humans, cassettes have their little aggravations. To be fair, you can get an extremely good sound from a cassette, but why can't you get it all the time? And why does a cassette that sounds great on your hi-fi system sound like rubbish on someone else's? There are answers to these questions, but first of all, a little background...

The cassette, or Compact Cassette as we should call it, has been around since the early Sixties. Philips invented it and, since they thought they could make a fat profit on sales of cassette tape, they gave away the format specification to any audio manufacturer who wanted to build a cassette deck. The rest is history.

In those far-off days, the hi-fi industry was almost non-existent, compared to the proliferation of the 1980s. Maybe your grandparents will let you look at their old Radiogram that they keep in the attic now. A nice piece of polished wood veneer, but not much in the way of sound quality. Faced with this as competition, all Philips wanted to make was a cheap and cheerful tape medium which could record speech and music to a low-to-moderate standard. When it first came out, that was all the cassette format could do. How manufacturers ever got the idea that they could turn it into a high quality sound reproduction system I'll never know, but somehow they have come pretty close.

SQUEEZE



Figure 1.

Let's look in more detail at the cassette itself. The tape is one eighth of an inch wide and (usually) four channels are crammed on to it. Tape speed is (usually) 1⅞ inches per second. Figure 1 shows the track arrangement. As you can see, the left and right channels of the stereo are placed next to each other. This makes the cassette automatically mono-compatible, as a single channel tape head will simply read both tracks at once.

Figure 2.

The plastic cassette housing (Figure 2) contains rollers which guide the tape at the correct angle past the tape heads. There are various cutouts to allow entry for the erase head, the record/playback head, and the capstan and pinch roller. To keep the tape pressed against the record/playback head is a sprung pressure pad. Behind the pressure pad is often a metal magnetic shield, which is designed to prevent stray hum fields getting into the sensitive playback head.

From the point of view of convenience, the cassette is a winner all round. But for performance, how does it compare with reel-to-reel? Let's start by considering the amount of tape used. Quarter-inch reel-to-reel tape running at 15 inches per second (ips) consumes a good deal of tape acreage. A quick calculation shows that, excluding guard bands, stereo reel-to-reel gets through 3.73 square inches of tape every second. A 40-minute album would therefore need nearly seven square yards of the stuff. That's 5.8 square metres to our continental cousins. Compare this with the cassette, which economises tape consumption down to 0.18 square metres per album. A 32-fold difference! Of course, there is a price differential too: I pay around £12.50 for a 30-minute reel of quarter-inch tape and consider it good value.

Considering how much tape is used, compared to professional formats, it's amazing that you can get any music at all on to cassette - let alone a Wagner opera with a chorus and orchestra of around 150 musicians. Of course, when the format first came out, it couldn't be done - not sensibly. But when the hi-fi manufacturers got their teeth into the problem, things started to happen.

The first major improvement to cassette sound was the Dolby B noise reduction system. One of the effects of the narrow track width on cassette tape was that random variations caused by the coarseness of the magnetic particles could clearly be heard as hiss. The Dolby B system - an adaptation of the professional Dolby A system - cut hiss by around 10 decibels. Enough to make an appreciable difference. The next improvement was the development of chrome tape.

Reel-to-reel tape is made using particles of iron oxide - rust to you and me. Not just any old sort of rust, but rust nonetheless. Makes me wonder why I buy tape when I could just take scrapings from my car and coat my own! Iron (or 'Ferric') oxide is a perfectly good magnetic material, but it's not the only one. Chromium dioxide ('chrome' for short) is better because it can accept more magnetism, and comes in smaller particles. Put simply, you can get more music onto the same area of tape.

The first chrome cassette tapes caused something of a furore in the hi-fi world. You needed to have a special cassette deck to gain any benefit from them, and there was a rumour afoot that they would wear out your tape heads more quickly. Well, it's true about the special cassette deck - and to this day we still have the 'Chrome/Ferric' switch - but in my experience it takes ages to wear down a cassette head because the tape moves so slowly. When the head is worn out, the rest of the machine probably is too. Real chrome tapes are made by only a few companies, BASF being the most prominent. The Japanese, not caring to take out a licence from the American DuPont company for chrome tape, invented 'pseudochrome' - a ferric tape with a lashing of cobalt - which is just as good.

Later on came 'metal particle' tapes. These are like conventional rust - sorry, I mean ferric - tapes, but made before the iron has turned into oxide. These metal tapes never really caught on, because although they were better performers at high levels and at high frequencies, they were also noisier than chrome (or 'pseudochrome') tapes and the trade-off wasn't all that advantageous.

On a more serious note one might ask, if chrome and metal tape is so much better than normal ferric tape, why don't recording studios use it on their reel-to-reel machines? The answer is that the benefits are mainly felt at low tape speeds, so at 15ips it wouldn't make much difference. There is also the nagging doubt that these tapes will not fare as well in storage as the conventional sort.

ALIGNMENT



The intention has been, so far, to show that it has been a tough job to get cassettes to sound anything like decent. It's a wonder that they are as good as they are, and it should not be surprising if they fall down a little every now and then.

The root of most of the cassette's problems is in the matter of alignment. Any owner of a professional tape recorder will know that the machine has to be aligned to the brand of tape he uses - either by himself, his maintenance person, or the dealer he bought it from. If it isn't specially aligned for the make and type of tape, it will not give of its best.

In the early days of hi-fi cassette decks, cassette tapes came in a variety of types. Generally each manufacturer made a good one, a bad one and an indifferent one. Has anything changed? Well, yes it has, but the point is that there have always been 20 or 30 different formulations of tape on the market - and each one needs its own optimum machine alignment, usually performed on internal 'tweakers'.

If your machine was aligned by the manufacturer for type XYZ tape, then when a different tape manufacturer brings out type PQR, even if it is a better tape in every way, it will sound worse on your machine - until you re-align it. Don't forget that when the manufacturer of type XYZ changes the formulation slightly (and he probably won't bother to tell you) you will also have to re-align your machine.

The situation sounds like a real mess. It's not as bad as it used to be, because most tape manufacturers now start on the basis of 'what alignment settings does the average machine have?', and design the tape from there. There will always be inconsistencies however and the alignment problem will not go away. 'But what effect does poor alignment have?', I hear you cry. Distortion, loss of high frequencies, excessive tape noise... Need I say more?

There is another matter of alignment which doesn't depend on tape type. This is the one that really screws up the chances of your demo cassette sounding any good in the record company's A&R office. The magic word is azimuth.

In a nutshell, the cassette tape travels in one direction, and the record/playback head has to be set at 90 degrees to this. That means 90 degrees plus or minus nothing, or virtually nothing. If this is not the case, you will be putting recordings on the tape 'at a slant'. Because the cassette deck usually has a combined record and playback head, on your own machine you will always record and play back at the same slant and so it will make no difference. On someone else's machine, however...

Figure 3. Frequency response of cassette or reel-to-reel tape recorder with incorrect azimuth setting.

The effect of off-azimuth is not only lost high frequencies. Take a look at Figure 3 and imagine what a frequency response like that could do to your music! It's a pity, but there is no real answer to this one. If manufacturers go for tighter tolerances, it pushes the price up. If you want to do your own alignment, then you need an oscilloscope and a special alignment cassette. The BASF calibration cassette costs around £100, and cheaper ones are useless. Even if you align your machine to perfection, it isn't necessarily going to make that much difference to the way your tape sounds on someone else's deck. And by the way, did I forget to mention that the accuracy of the azimuth setting also depends on the quality of the cassette shell? It's true, I'm afraid.

To be absolutely fair, if you buy a cassette deck from a quality manufacturer, use decent tape, look after the machine and don't knock it around, then you will probably get acceptable results. The unfortunate thing is that the cassette is the accepted medium for demo tape exchange, and you are relying on the person who receives your cassette to have high standards too.

DOLBY MYTHS



'I always play it with the Dolby switched out because it sounds better.' How often have you heard someone say that? Sometimes it does sound better, but we are touching on a philosophical point, on which I must expand before I go further.

The deep and meaningful question is, do you want a tape (or cassette) recorder to make a recording sound 'better' than the original, or do you want it to sound as close to the original as possible? Opinions on what 'better' means vary so much that the only acceptable way to go is for the latter proposition - that a recorder should copy its input as closely as possible. Having arrived at that conclusion, I can go on to say that unless there is some problem, the Dolby B system will always make an improvement. Exceptions to this rule will follow shortly.

So, starting from the premise that if a cassette recorder is competent in every other department, the Dolby system will make recordings sound better, what perhaps might make things sound worse?

One of the reasons for aligning cassette recorders is to adjust for tape sensitivity. Some tapes will give a higher output than others across the entire frequency range. Without noise reduction, this doesn't cause too much of a problem because you can simply turn the volume up or down to compensate. However, the Dolby B noise reduction system is level dependent, it must receive the same level on playback as it did on record. A level change of as little as 2dB can make a difference. If levels are not set right, then you might end up with an excess of high frequency or a deficiency, both of which are very noticeable. There could also be a 'noise pumping' effect. Once again, even if your cassette deck is set up correctly, it doesn't mean that everyone else's is.

From this, I hope it's clear that if your deck is properly aligned for the brand of tape you use, then Dolby B (and Dolby C, of course) should make a definite improvement to sound quality. Some people, however, may say that even when it is working perfectly, they can hear deficiencies in the Dolby system. If this is truly the case, and not just a line-up problem, then good luck to you. Switch it off with my blessing. For me, I can do without all that hiss.

MANGLED TAPES



A dog is allowed one bite. And as far as cassette decks go, they should be allowed to mangle one tape, then it's off to the vet to put them out of their misery.

Why do cassette decks always chew up your favourite cassette? Probably for the same reason that you always drop toast butter-side down. It seems strange to me that I've had more problems with audio cassettes than with video cassettes in this direction, but no matter how sophisticated the tape transport - and I've used cassette decks costing over £1000 - the odd cassette still gets the treatment.

Sometimes it's the fault of the cassette, sometimes the player. My advice is to make things as easy for the machine as possible and give it a thorough cleaning regularly. Not with one of those 'head cleaning' cassettes either. What I recommend is isopropyl alcohol (isopropanol), which you can get from most chemists, provided you don't look the type to abuse it. Isopropyl alcohol is not drinking alcohol and therefore carries no duty, but it's a lot better for cleaning tape heads and guides. Applied with a cotton bud, there is no reason why your cassette deck should not be spotless at all times.

Don't forget that a demagnetiser is also a good investment which prevents stray magnetic fields, which build up on the deck's metal parts, from partially erasing your tapes.

ODDITIES



Although Philips were pretty strict, in the early years, on what a cassette deck ought to be like, there are now several versions of the format.

The standard tape speed is 1⅞ inches per second. Needless to say, the sound quality is nowhere near good enough for serious recordings. Better than that is the 3¾ips speed pioneered by Tascam in their Portastudio format. Other manufacturers such as Fostex and Yamaha now make multitrack cassette machines which run at this higher tape speed. The advantage is not so much in the extended frequency response, but in the smoothness of the sound you get from them. If you look at a cassette tape closely you will see, even with the naked eye, that it is not perfectly smooth. This roughness makes your recordings sound uneven. Running the tape at a higher speed overcomes this, although I still await the cassette deck that runs at 7½ or even 15ips. Is anyone working on it? It could be good, but you would have to stick to short pieces of music!

Extensions and alterations to the original Compact Cassette format may be useful in some circumstances, however the overriding reason for most people's use of the cassette is its universality. Everyone has one. The sound quality may be high or low, but at least you can give someone a cassette and say: 'Here, this is what I sound like'. Predictions of the demise of the format are many. Digital cassettes in the form of R-DAT may eventually take over, but the standard cassette has many years of life yet. It's certainly far from perfect, but if we all do our best to keep our heads clean and look after our machinery, then we will be able to exchange cassettes to our heart's content.

BIAS & EQ

Although Bias and EQ are normally combined on one switch on most cassette decks, they are quite separate functions. Bias is a high frequency signal which helps the signal you are recording get onto the tape without distortion. EQ, or equalisation, is used because some tapes can take more high frequency level than others.

Some tapes need more bias than others. 'High bias' does not necessarily mean that a tape is better, it's just that it often works that way. The bias level has to be set just right for the make and type of tape in use. There are fine adjusters inside the cassette deck, the external switch just lets you choose between Ferric, Chrome and Metal cassettes. Most cassette tapes, these days, are made so that they have similar bias requirements to other tapes of the same type, so providing you set the switch correctly, you should get good sound quality.

Chrome and Metal tapes can accept more level at high frequencies than Ferric can, so the EQ is there to put more on the tape when recording, then cut it back on replay. This increases the signal-to-noise ratio. The 70ps (microsecond) and 120ps settings refer to the frequencies at which the EQ takes place. 120ps is for Ferric tape, 70ps is for Chrome and Metal. If your cassette deck has no Metal position on the bias switch, then you cannot record on Metal tapes successfully. They can be replayed correctly, however, using the 70ps (Chrome) setting.

Some sophisticated cassette decks have an 'auto-alignment' feature. This means that they have some way of actually measuring the parameters of the tape in use and adjusting bias and EQ accordingly. Other cassette decks have probes which can tell from cutouts on the rear of the cassette shell whether it is a Ferric, Chrome or Metal tape. This should not be confused with auto-alignment proper.


Series

Read the next part in this series:
How It Works: the Noise Gate (Part 10)



Previous Article in this issue

Big Guns

Next article in this issue

Yamaha DX11 synthesizer


Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Jun 1988

Topic:

Recording

Tape, Vinyl, CD, DAT


Series:

How It Works

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12


Feature by David Mellor

Previous article in this issue:

> Big Guns

Next article in this issue:

> Yamaha DX11 synthesizer


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