Tape Track Formats
A rundown on the various 'head' formats available on tape recorders and the reasons for their existence.
The numerous track formats found on today's tape recorders are often a source of confusion to many people. In particular, the variety of different ¼ inch tape formats have caused many a headache to those people who have taken a tape from one tape machine to play on another, only to find that it's a totally different format. Magnetic tape, ranging in width from ⅛ inch cassette tape to 2 inch professional multitrack tape, is in daily use throughout both the domestic and commercial recording world. So, in order to acquire an understanding of the various track formats, it's necessary to look at the subject from an historical point of view.
The majority of track formats developed around the ¼ inch tape width, the reason being that this was the only tape type in existence up until a decade or two ago. Originally, you could only record one (mono) track of sound on the whole tape width, but this subsequently developed into what is known as half-track mono where one track is recorded on half the width. The beauty of this format is that after recording in one direction you can turn the tape around then record another track in the opposite direction and so achieve twice the recording time from the same piece of tape.
The logical progression into stereo recording provided two new formats, the half-track stereo head which has become the standard 'mastering' format and the quarter-track stereo head which has now become the standard domestic tape recorder format. The adoption of the quarter-track format has proven to be very popular on domestic machines due to its economic use of tape, however, you do sacrifice a little sound quality as four tracks of magnetic information (sound) have to be recorded onto the same width of tape as two were previously.
Quarter-track stereo should not be confused with the four-track format; Figure 2 shows the difference between the two formats (G, H). As can be seen, there are actually only two separate recording tracks on the quarter-track head (G), and as the name suggests each track occupies a quarter of the tape width. It is therefore necessary as with the half-track mono format, to turn the tape around in order to record on the other two tracks (see Figure 1). With the four-track format however, this is not necessary as the record head is actually made up of four smaller individual heads, one for each track.
One important factor regarding the ¼ inch tape formats is that only certain ones can be used to record in both directions (E, G). These formats are usually only found on domestic tape recorders, conversely all multitrack machines are used in one direction only, and so utilise the whole tape width to provide the maximum number of tracks.
The development of cassette recorder heads has closely followed that of the ¼ inch tape machine, with half-track mono, quarter-track stereo and four-track all being found on cassette recorders.
In the last few years we have seen amazing advances in tape recorder head design, with manufacturers cramming more and more tracks onto the same tape width. The introduction of the Teac 3340 four-track ¼ inch tape recorder, paved the way for multitrack recording at a price many people could afford, and within a few years the Fostex A-8 and, more recently, the Fostex B-16 have made available eight and sixteen-track recording to those who could only previous dream of owning such facilities.
The number of tracks on a given width of tape directly influences the quality of the recorded sound. But why? It is therefore with this thought in mind that we can draw several conclusions from Figure 2, which shows most of the common multitrack formats and tape widths that you are likely to encounter.
There are several reasons why the sound quality changes when you put more tracks onto the same width of tape, however, for the purpose of this article we shall confine the explanation to only one aspect.
One of the main reasons for loss of sound quality is due to a phenomenon known as crosstalk (sound leakage). Manufacturers' brochures generally quote crosstalk as a number in decibels eg. A Fostex X-15 four-track cassette recorder has a crosstalk figure of -40dB. What this actually means is that if you record a sound on track two of the tape, rewind it and then listen back to track one, having turned down the volume of track two, you will hear a certain amount of the sound leaking from track two. The sound on track two is only 40dB quieter than that of track one.
40dB is not a bad figure as it stands, but when you compare it to a 50dB crosstalk figure for a Tascam 38 eight-track, you can appreciate that the difference in quality is quite large. A 10dB difference in sound level is just over double the loudness, therefore the two adjacent tracks on a Tascam 38, which has a 10dB better crosstalk figure, are going to be, theoretically, twice as quiet as those on the Fostex X-15.
So, this shows in simple terms how the crosstalk figure and hence the quality of sound are directly related to the number of tracks on a given width of tape. The four tracks on ⅛ inch (C) cassette tape used on the Fostex X-15, compared to the eight tracks on ½ inch (J) tape of the Tascam 38, are a good example of the number of tracks versus crosstalk phenomenon. If you divide the ½ inch tape width and track number of the Tascam 38 by four, you will see that an ⅛ of an inch of tape actually carries only two tracks as opposed to the four tracks on the Fostex X-15's ⅛ inch cassette tape. Again, if we look at the ¼ inch format heads shown in Figure 2, it's easy to see that the quality of sound produced by head F (half-track stereo), which is the standard 'mastering' format, is going to be better than that of head I (eight-track Fostex A-8) which is a typical multitrack head. We can therefore state a simple rule that is generally true for all analogue tape recorders. The less tracks for a given tape width, the better the sound quality. However, it must be said that although crosstalk is a major contributing factor to the resulting sound quality of a tape recorder, it is not the only factor.
Besides crosstalk there are two other problems relating to heads with a large number of tracks packed onto them. These problems affect the use of the tape machine and not the sound quality, though they are related to recording head design and function. Some tape recorders will not, for example, allow you to record on two adjacent tracks. This is because the electronics have been designed to allow you only to record on alternate tracks, ie. you must always have at least one track which you're not recording on, between the two that you are.
The reason for this 'safety track' is to stop any of the sound already recorded on the adjacent track from being re-recorded onto the new track, because, as already mentioned, the poor cross-talk performance found on this type of machine makes the safety track a necessity. Therefore, on a four-track tape recorder you may only be able to record on tracks 1 and 3 simultaneously while leaving track 2 as the safety track. Fortunately, this type of restriction has been largely overcome by the majority of recorder manufacturers and you shouldn't find many examples of this design being used on today's tape machines. Examples of recorders which do have this limitation are, the original Teac M-144 Portastudio, Fostex X-15 and the early Fostex A-8.
The second limitation is in some way related to the first. On many tape recorders you can't 'bounce down' to an adjacent track eg. having recorded on tracks 1 and 2 it isn't possible to bounce them across to track 3 while in the sync mode. In this case, the problem is internal feedback.
This is due to the sound that is being transferred to the adjacent track leaking back (crosstalk) to the track it originally came from, forming an electrical loop and thus causing feedback. By leaving a safety track between the tracks that are being bounced down and the track that the sound is destined for, you can avoid the feedback problem. On tape recorders with fewer tracks on the same tape width, feedback will not usually occur as each individual track is physically further apart from the other, enhancing electrical separation.
Further conclusions may also be drawn from Figure 2. For instance, you can see that the sound quality of four tracks on ¼ inch tape (H), eight tracks on ½ inch (J) and 16 tracks on 1 inch tape (M), are all going to be very similar. This is fairly obvious as it's just a simple multiplication of the number of tracks and tape widths.
Figure 3 shows a table of tape head formats and lists some of the machines available at the time of writing and some of the more popular machines from a few years ago which utilise such formats. The list is by no means intended to be a comprehensive catalogue of makes and models, but rather an indication of the more popular tape recorders and their head formats. We have chosen not to deal with the 2 inch wide tape format as this is only used on professional 16 and 24-track tape recorders found in many of the world's top recording studios.
Feature by Paul Gilby
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