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Attenuation, Dispersion, Latency, Distortion and Noise

Attenuation

Attenuation is the weakening of the signal with distance from the transmitter.

Dispersion

• Dispersion is the spreading out in time of the transmitted pulse.
  • If the pulses spread too much, they merge into each other and the receiver can no longer separate them.
  • Different radio or light wavelengths travel at slightly different speeds so the pulses of radio or light spread out as they travel.
  • Square pulses contain multiple frequencies which travel at different speeds causing the pulse to spread out.
  • Using monochromatic ( single frequency LASER ) light helps.
  • Modulating the light beam creates sidebands on different frequencies so the problem is not completely solved.
  • In optical fibres, the light can zig-zag down the fibe at different angles. Shallow angles arrive sooner than more zig-zaggy ones.
  • Single mode fibre is so thin that the light can't zig-zag. This reduces dispersion.

Latency

• This is how long it takes for a signal to travel from the sender to the receiver.
  • On TV interviews, the latency or delay becomes noticeable if satellite or slow internet links are being used.
  • Even at the speed of light, the trip to a satellite and back takes a noticeable time.
  • Encoding and decoding signals takes time. This adds to the latency.
  • Time interleaving improves the reliability of a link but it increases the latency.
  • If you have TV playing in the lounge and another in the kitchen, you might get an echo effect due to the different latencies of the TV receivers.
  • I'd like to patent a device to buffer digital data so all TVs and similar appliences have the same latency. No more echoes!

Distortion

Amplifiers should be linear. The output should be exactly proportional to the input. In real life this is hard to achieve so the output is distorted. Two common types of distortion are cross-over distortion with small signal inputs and clipping or limiting with large signal inputs. Dont confuse distortion with noise.

Digital on/off signals are much less likely to get distorted.

Noise

Unwanted signals. It can be a background hiss or cracks and bangs caused by lightning or the switching of industrial machinery. This is impulsive noise. Radio signals often suffer from noise due to unwanted signals leaking into the receiver. FM demodulators ignore the signal amplitude and therefore have better noise immunity then AM receivers. Don't confuse noise with distortion.