1.1 Introduction
The microphone and disc pickup generate programme signals and the loudspeaker changes them back to sound so that they can be heard most programme materials starts as sound waves so the first link in the broadcasting chain is generally the microphone. This changes the sound waves to weak electrical signal which are the programme signals. After suitable amplification and processing the signals are fed either to the transmitter for direct broadcasting or to a magnetic tape recorder for storing and subsequent broadcasting or for disc record making the programme information stored in the groove spiral of a disc records in the form of wriggles is changed back to programme signal whenever required by the disc pickup which consists of a pickup cartridge and arm combination.
The cartridge which is equipped with a stylus for tracing the groove spiral is flitted to the end of the arm (usually in a headshell) while the other end of the arm is commonly pivoted such a way as to allows the cartridge to traverse the record with the very minimum of friction.
The microphone disc pickup and loudspeaker are called transducer because they change sound to signal or signal to sound. Several principles can be used for the transuding effect the three most common ones being electromagnetic piezoelectric and electrostatic.
Electromagnetic Principles
The vast majority of modern disc pickup utilize the electromagnetic principles whereby vibration impacted to the stylus as a result of its tracing the groove wall modulation (wriggle) result in magnetic flux changes relative to a coil of wire.
A signal E.M.F is thus induces into the winding and a signal voltage corresponding to the original signal information is developed across it. This of course is the dynamo effect which was discussed in chapter 1The groove of a disc record of a v-shaped cut and for stereo one wall is modulated with the left signal and the other with the right signal. Thus a stereo pickup has two generating systems one yielding the left signal and the other the right signal the design being such that minimal interactions and hence cross talk occur between the two channels.Reader requiring more information on this technique called the 45/45 technique of stereo recording may find the authors the audio handbook of interest also published by Newnes Butter worth's.
Many microphone also adopt the electromagnetic principle. In this case a law-mass diaphragm is coupled to the electromagnetic system and as the diaphragm vibrates in the presence of sound waves so a corresponding signal voltage is developed across the winding. Some stereo microphone employ two such generator in a common housing so that axis of one is at right angle to that of the other.
The electromagnetic principle of microphones is exemplified by the moving coil arrangement where a coil of wire free to more in a strong magnetic field is secured to the center of the diaphragm. As the diaphragm vibrates so the turn of the coil cut the magnetic lines of force and a signal voltage corresponding to the information carried by the sound waves is developed across the winding.
Many loudspeaker use the moving coil principle but in the reciprocal sense that is signal current (after the programme signal has been significantly boosted by power amplification) is caused to flow through the coil. The resulting magnetic field which is changing in accordance with the programme signal reacts against the fixed magnetic in which the coil is suspended. The coil thus vibrates within the fixed field and carrier the diaphragm or cone with it hereby producing sound waves.
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