Monday, 5 May 2014

Positive and Negative Modulation

Positive and Negative Modulation:

 We use AM for video signal. Generally, the amplitude of the carrier increases with increase in amplitude of the signal and vice versa. This is called “positive modulation”. AM is effected by noise.
Generally noise, when occurs tends to increases the amplitude of modulated signal. This increase causes bright blobs (spots) on the screen, when ever noise occurs. To over come this, negative modulation is followed.

In negative modulation, the instantaneous amplitude of the carrier decreases with increase in instantaneous amplitude of the signal. The resulting modulated wave has white level at lower side, black levels at higher level. So, whenever, noise occurs, due to increase in amplitude, the level goes towards less brightness level i.e., gray or black. Thus, the effect of noise is minimized.
In addition to reduced effect of noise in negative modulation, there are many other advantages.

Advantages of negative modulation over positive modulation:

(a) RF noise pulses will not cause adverse effect on picture.
(b) Greater peak transmitter power can be possible.
(c) AGC circuit will have stable level as reference.

Thus, negative modulation is preferred in TV broadcasting. Output frame camera tube (video signal) when it reaches the modulator, the polarity reversal is done so that black level is at 72.5 % and white level is at 12.5 %. Thus, negative modulation takes place. However, strong noise pulses may cause false triggering of deflection oscillator being mistaken as sync pulses.

Differences between Positive and Negative Modulation:

No:
Positive Modulation
Negative Modulation
1
Affected by Noise.
Effect of Noise is minimized.
2
Bright blobs (spots) appear on the screen due to noise.
No bright spots appear due to noise.
3
The instantaneous amplitude of carrier increases with that of modulating signal.
The instantaneous amplitude of carrier decreases with that of modulating signal.
4
RF noise pulses cause adverse affects on picture.
RF noise pulses will not cause any adverse affects on picture.
5
Peak Transmitter power is less.
Peak Transmitter power is more.
6
AGC circuit does not have stable reference level.
AGC circuit has stable reference level.

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