Click here to purchase the entire book in PDF format. Playback Back in Section 6.3.1 I oversimplified a bit and said that the magnetic signal on the tape produces a field that temporarily magnetizes the playback head. If the magnetic field changes (by moving the tape, for example) then the magnetic field around the coil around the head changes and we get a current output. This is essentially true, but we have to look at things in a little more detail. The magnetic field that is ``read'' by the playback head is a measurement of the difference in magnetic field across the head gap length. Take a look at Figure 6.65. If you have a very low frequency, and therefore a very long wavelength on the tape, there is very little difference in magnetic field across the gap (because the gap length is small compared to the wavelength). Another way to think of this is that there is very little phase difference across the gap length. Consequently, there is a very small magnetic field generated in the head, and we don't get much output. The lower the frequency, the longer the wavelength and the smaller the output. As the frequency is increased, the difference in the signal across the gap length increases (it becomes more and more different) so we get more output from the playback head. In fact, we get 6 dB more output for every increase in frequency of 1 octave. Eventually, we get to a point where the wavelength is two times the gap length. This is where we have the maximum possible output from the head, because we have the maximum possible difference in magnetic field across the gap length. What happens when the frequency goes higher than this? Now, we have more than half a wavelength across the gap, and the output starts to drop again. This will continue to drop until we reach a point where the wavelength is equal to the gap length and we get no output because the difference in magnetic field across the gap is nothing. Figure 6.65: A conceptual diagram of the playback head and the signal recorded on the magnetic tape for different frequencies. Notice that the difference in magnetic field across the gap is different for different wavelengths on the tape, and therefore different frequencies. So what? Well, we know a couple of things: The output of the playback head is dependent on the relationship between the wavelength of the signal on the tape and the gap length. The wavelength of the signal on the tape is dependent on the frequency and the tape speed. (Double your tape speed and you double your wavelength for the same frequency.) When the wavelength of the signal on the tape equals the gap length, there is no output. Therefore we can conclude a couple of things: The higher the frequency, the higher the output. (Up to a point where the wavelength on the tape is two times the gap length. To increase the highest possible playback frequency, we have to either reduce the gap length, or increase the tape speed. The smaller the gap, the lower the output (think of low frequencies, for example...) The higher the tape speed, the better the signal playback. So, we can conclude that we should run the tape at a high speed - the higher the better. This is true, however there is one minor problem. The higher the tape speed, the more tape you use and the more money you spend. So, if you want a better signal from your tape, you have to spend more money. Sorry. Just to give you an idea of typical gap lengths, a playback head will have a gap length between 1.5 and 6 $\mu$m whereas a record head doesn't need to be as small - typically between 2.5 and 12 $\mu$m[Woram, 1989].