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Wednesday, 4 September 2019

Methods of Recording in Instrumentation

(1) Direct Recording

(2) FM. Recording
(3) Pulse Duration Modulation Recording

(i) Direct Recording: 

The principle of direct recording has been explained earlier in this blog. The following are the advantages of direct recording:

1. It offers wide frequency response in the frequency range 50 Hz to 2 MHz with tape speed of 3.05 m/s. 
2. The electronic circuitry required for this type of recording is simple.
3. Offers good dynamic response, giving signal to noise ratio of over 30 dB at 1% total harmonic distortion.

(ii) Frequency Modulation Recording :

When D.C. voltages are inputs or the D.C. component of the input signal is to be restored and when high degree of accuracy in response is desired, the Frequency Modulation recording is preferred. The input signal is modulated over a carrier frequency. The deviation in the carrier frequency represents the amplitude of the input signal. In F.M. recording the carrier frequency or the center frequency or the mid frequency is selected based on the tape speed. Higher speeds require larger carrier frequencies. The frequency deviation is selected to be ± 40% over the carrier frequency. A typical high speed carrier is 108 kHz for a tape speed of 30.84 m/s. With such a speed and carrier frequency, the frequency response can be flat over a frequency range of 0 Hz to 20 kHz. This selection offers a signal to noise ratio of 50. Every time a change of tape speed is preferred to effect economy of the tape, the carrier frequency also needs related variation. It is to be clearly noted here that irrespective of the speed of the tape the D.C. component of the input signal remains the same. The reason is that the frequency deviation used is 40%. As the input circuit sensitivities of the tape recorders are of the order of I V, magnitude, the input signal needs amplification before application to the modulator.

(iii) Pulse Duration Modulation (P.D.M.):

The pulse duration modulation is also known as Pulse Width Modulation. This is extensively used in digital data communication. In pulse width modulation the amplitude and the starting time of every pulse is maintained fixed. The pulse width will be varied proportional to the amplitude of the signal at the characteristic instant. Pulse duration modulation or puke modulation has its application in Digital Recorders.

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