B-H Curve for Magnetic Material

Need for Providing Bias for Recording :

We know that during the recording process, the tape will enter the field created by the recording head. The tape on entry in to the field will acquire magnetization. The extent to which the tape is magnetized depends on the magnetic field strength at head gap the instant the tape reached there. This will also depends on the magnetic history of the tape.

The B-H curve of any magnetic material is non linear. So will be that of the core of the head and the magnetic tape. If recording is done, taking the operating point at the origin of the curve the recorded signal will be totally distorted due to the non-linearity of the B-H curve. B-H Curve for Magnetic Material is explained below.

The signal is represented by a,b,c,d and e. The corresponding output signal is represented, by p, q, r, s, t. It can be seen from the wave form p, q, r, s and t, that the magnetization of the medium is nonlinear. That is for equal increments of magnetizing force, the magnetization is not in corresponding equal increments.

This will lead to severe distortion of the signal. To avoid distortion during recording the operating point can be shifted such that the input signal will lie in the linear portion of the B-H curve (this is also termed as transfer characteristic). Biasing in magnetic recording is the process of shifting the operating point to linear portion of the B-H curve.

(i) D.C. Biasing :

The D.C biasing can be done by applying direct current bias to the recording head and superimposing the recording signal over it. The applied direct current (Bias) will initially magnetizes the core , and the tape. Hence the signal variations lie well within the linear portion of the curve and the distortion is reduced.

However the use of direct current through the record head as bias has  limitations. The limitation is that the output signal has limited amplitude, limited by the saturation of the B-H curve, as shown in Figure. The input signal amplitude and hence the output is limited by the saturation of the curve on the top and non-linearity in the bottom.

The signal to noise ratio is low and is of the order of 25 dB with this type of bias. For good quality recording and reproduction the signal to noise ratio required is much higher than this value.

(ii) A.C. Biasing :

A.C. biasing technique uses supersonic frequency as the bias signal frequency. An alternating voltage within 50 kHz to 100 kHz will be used for this technique. The audio signal will be superimposed on the selected frequency. The recording head will have the high frequency bias signal current over which the audio signal is superimposed circulating through it. The audio signal variations, due to this superimposing, will be well within the linear portion of the B-H curve as shown in the given Figure. The magnitude of the bias signal will be many times the magnitude of the audio signal. The A.C. bias signal is also known as High Frequency bias signal or Supersonic Bias signal.

From the figure it can be seen that the AOB is the curve or the transfer characteristic. The bias signal with the superimposed audio signal produce a field that varies between the limits a and b.

The biasing signal will not be heard during reproduction because it is a supersonic signal. A suitable filter can be used in the amplifier to bypass the supersonic signal during playback.

The bias frequency being supersonic is several times greater than the highest audio frequency that is being recorded. Therefore by the time the bias signal completes one cycle, only minute changes will occur in the amplitude of the audio signal.

It can be assumed that the recording is done on the two sides of the transfer characteristic, at a time. From the figure the curve `p' shows the magnetization of the medium (tape) under the positive side of the recording field. The magnetization of the medium under the negative side of the recording field is shown by the curve 'q'. The resultant magnetization of the medium will be naturally, the average of the curves 'p' and 'q', which is shown as curve `f . This wave form is free from any type of distortion. In addition there will be improvement in the magnitude of the output, and reduction of noise and distortion.

The reduction of noise is done because when there is no audio signal, the tape will be demagnetized by the bias signal, eliminating noise.

(iii) Bias Level

In almost all tape recorders excepting some battery operated low cost recorders supersonic biasing will be used. Therefore it is necessary for us to know the optimum level of bias that can be used in this supersonic system of biasing.

The optimum bias level will be slightly more than the level which gives maximum output at 1 kHz. practically it is possible to adjust the bias, by increasing it to get a maximum output at 1 kHz, and further increasing it to get a fall in the output of ldB. It should be clearly noted  that the optimum bias level differs from manufacturer to manufacturer.

(iv) Bias Frequency and Application Methods :

The frequency range for H.F. biasing has been stated as from 50 kHz to 100 kHz. The thumb rule is that the frequency used as bias frequency must be at least four to five times the highest frequency being recorded.

Usually frequencies between 45 kHz to 75 kHz are used for home entertaining type recorders. These recorders have their upper frequency limit to around 8 to 10 kHz.

Higher frequencies for bias are used for professional recorders.

The bias signal is generally applied in parallel with the audio signal to the recording head.

In some systems cross field head will be provided opposite to the recording head. The cross field head and the recording head are arranged in a manner that the magnetic fields produced by the two heads will not influence each other, even under maximum bias conditions.

The recording head will be supplied with the signal and the cross field head will be supplied with the bias.