Q. Explain about Simplex communication?

Simplex communication: Microphone and the earphone are transducer elements of the telephone communication system. Microphone converts speech signal to electrical signals and earphone converts the electrical signals into audio signals. Most commonly used microphone is carbon microphone. Carbon microphones don't produce high fidelity signals however give out strong electrical signals at acceptable quality levels for telephone conversation. In carbon microphones, a certain quantity of small carbon granules is placed in a box. Carbon granules conduct electrically and resistance offered by them is dependent upon the density with which they are packed. One side the box is covered is flexible and is mechanically attached to a diaphragm. When sound waves impinge on the diaphragm, it vibrates, causing carbon granules to compress or expand so changing the resistivity offered by the granules. If a voltage is applied to the microphone, current in the circuit varies according to vibration of the diaphragm.

When sound waves impinge on the diaphragm, instantaneous resistance of microphone is given by r,

i= r₀- r sin ωt

Where

r₀= Quiescent resistance of the microphone when there is no speech signal.

r = Maximum variation in resistance offered by the carbon granules, r 0 .

Negative sign indicates that when carbon granules are compressed the resistance decreases and vice versa. Ignoring impedances external to microphone in the circuit given in figure. without loss of generality, instantaneous current in the microphone is given by

i = V/(r₀- r sin ωt) = I₀(1 - m sin ωt)^-1

Where

 I₀= V/r₀= Quiescent current in the microphone.

 m = r/r₀, m < 1

By binomial theorem, the Eq. may be expanded as

i = I₀(1+ msinωt + m²sin²ωt + ....)

If value of m is sufficient small that is typically the case in practice, higher order terms can be ignored in the above Eq. Giving thereby

  i = I₀ (1+ m sin ωt)

Therefore  carbon  granule  microphone  acts  as  a  modulator  of  direct  current  I which is analogous to carrier wave in AM system. Quantity m is equivalent to the modulation index in AM.