Secondary Maxima
If the misalignment of the azimuth is continued at the same time as observing the playback of a high
frequency signal, it can be seen that after having passed by a minimum, the signal will increase
again to pass by another secondary maximum.
If the principal maximum corresponding to the correct angle applies equally to all frequencies, the
secondary maximum applies only to one frequency, which is fixed for the particular conditions. If
this frequency changes, the position of the secondary maximum is displaced. It is clear that the
secondary maximum corresponds to an adjustment which can not be used and which should be
avoided. If it is necessary to adjust the azimuth, it should be done by very small degrees, so that no
risk exists of reaching a secondary maximum. On the other hand, if it is necessary to adjust the
azimuth from scratch, it is better to make the adjustment with a fairly low frequency (1, then 3 kHz)
to obtain an approximate azimuth position. At these frequencies, the secondary maxima are outside
the range for adjustment of the heads.
Orientation of the Heads on the Nagra 4.2
The heads of the Nagra 4.2 are pressed down on a cam. On turning this cam, the azimuth is varied.
The outside of the cam is in the form of a gear wheel meshed with a pinion, which is visible in front
of each head. The pinion can be turned by means of 2½ mm "Allen" key. The Allen key should be
demagnetized before using it as a magnetized tool can induce a very low frequency into the
playback head, which could upset the adjustment.
Bias
To record, that is to say, to magnetize a magnetic tape, it is necessary to submit it to a magnetic
field, which passes a certain threshold value. Below this value, no permanent magnetization will be
produced. To reach the threshold, and to pass into the linear part of the magnetization curves, the
audio frequency signal is superimposed upon it. The peaks of the high frequency signal always
make an excursion into the linear region. The low frequency signal determines, in effect, to what
point the excursion will be made. This is called high frequency bias. Its amplitude influences notably
the quality of the recording obtained, and the determination of its level should be made precisely.
Effect of Bias Signal Amplitude on the Recording
If a low frequency signal (400 Hz) is applied to a recording head whose bias level is varied, several
effects can be observed. A low level bias signal will give a weak distorted signal on playback. As the
bias level increases, so the level of the signal increases, rapidly. A maximum will then be reached,
after which the signal level will be very slowly reduced. The maximum can be called the "Point of
Maximum Efficiency". It corresponds also to the point where the distortion is the minimum. The fact
that the signal becomes too great, renders the determination of the optimum point rather difficult. A
high frequency signal (e.g. 10 kHz) will give its maximum level for a bias level noticeably lower and
which corresponds to a point where a low frequency signal would become distorted. This is due to
the fact that the magnetic layer of the tape is not infinitely thin. The point of maximum efficiency for
a low frequency corresponds to an optimum recording throughout the whole of the magnetic layer.
The outside part will in fact be over-biased and, to a certain extent, even partially erased. The
middle of the layer is further away from the heads, hence the loss of high frequencies. It is essential
to remember that in over-biasing, not only is the efficiency of the recording of the high frequencies
diminished, which can be compensated for by an increase in the recording current, but the playback
signal is attenuated, thereby showing saturation of the magnetic tape. On the other hand, an over-
bias will lower the noise level of the tape.
