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Wednesday, 4 September 2019

Parameters in Magnetic Recording on Tape

Important Parameters in Magnetic Recording: 

The parameters to be considered in designing a good quality magnetic recording system are:

(i) Recorded Wavelength:

It is the length of tape magnetized by one cycle of input frequency signal.
l = S/f
l is recorded wavelength in cm
S is speed of tape in cm/s
f is frequency of audio signal in Hz

(ii) Gap Width and its Effect on Reproduction:

The playback head will have a finite gap width. Therefore the magnetic flux on the tape will not be picked up point by point, but will be picked up with full width of the gap. At low frequencies, the recorded wavelengths are larger than the width of the gap. As a very small part of the wavelength will be scanned at a time the gap width has little effect at low frequencies.

At high frequencies, this is different as the gap width and the wavelength recorded on the tape are comparable. Larger part of wavelength progressively will be scanned as the frequency increases. At one frequency that is dependent on the tape speed, the gap width will equal half the wavelength.

The losses due to the gap width will be low up to this frequency. Further the frequency increases, more than half the wavelength will be scanned at a time leading to reduction in the flux linking with the head. This reduces the output. When the wavelength becomes equal to the gap width, identical poles will be under the two tips of the head. So flux will link with the head. There will be no output at this frequency.

Hence if higher signal frequencies are to be played back, or in other words: if the band width of the reproduced signal is to be more, the width of gap of the head must be very small. If the gap width is reduced excessively, the available signal output will also be reduced. Furthermore the design and construction of the head will be difficult if gap width is very less.

(iii) Relation Between Tape Speed, Frequency and Gap Width :

 The tape speed, the highest usable frequency and the gap width are related by the following equation:

f = S/2G

(iv) Speed of the Tape : 

Higher speeds record program of smaller duration on longer length of tape. This gives a better quality.

For long play tapes low speed is recommended. If speed is too low distortion will be produced.
The tape speed gap width and frequency are inter related factors as per equation:
Practically a compromise is chosen and the following speeds are standardized for tapes :
38 cm/s, 19 cm/s, 9.5 cm/s and 4.75 cm/s.

The entertainment type of recorders use tape speeds of 4.75 cm/s and 9.5 cm/s. Professional tape recorders used with radio, TV. broadcasts, and recording studios use high speed of 19 cm/s & 38 cads.

 (v) Losses in Record Head :

The recording head will be made from a magnetic material. It has its core a magnetic material. Therefore the hysteresis and eddy current losses are present with the head. The losses increase at high frequencies. Hence the strength of magnetization decreases as the frequency of recorded signal increases.

 (vi) Demagnetization Effect of Bias :

 Demagnetizing effect at high frequencies occurs whenever high frequency or A.C. bias is given to the recording head. This results in reduction of the strength of magnetization of the tape at higher and higher audio frequencies during recording.
Therefore it is necessary to drive more current through the recording head at high frequencies to compensate for the loss. Also the cross field head produces least demagnetization effect.

 (vii) Erasing :

In order to completely remove the contents of tape already recorded or to normalize the tape for fresh recording we need erasing of the tape.

Erasing can be done either by demagnetizing the tape or by magnetizing the tape to saturation. To demagnetize the tape alternating current is used. To saturate the tape direct current or strong permanent magnet will be used.

Generally in all types of tape recorders a special head called the erase head will be incorporated. The tape passes first the erase head before it reaches the record head. Usually the high frequency bias or AC. bias voltage will be applied to the erase head.

As the tape passes the erase head it will be magnetized by the bias frequency, erasing the previous contents of the tape. The gap of the erase head is selected such that several cycles of AC. are completed within the time any point of the tape passes the gap.

Some recorders use direct current through the erase head. The erase head becomes a strong electromagnet when direct current flows through it. Hence the tape will be magnetized to saturation by the field produced at the gap of the erase head, leading to erasing of the previous contents from the tape.

Permanent magnets are also used for erasing. Here the permanent magnet takes the place of the erase head.

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