Amplitude Modulation - labAlive experiment

Quadrature phase-shift keying (QPSK) modulator varies the signal's phase to transmit information.

START

In this experiment it can be explored, how echos influence an audio signal. Here a room with four ways signal propagation is simulated. As it's pictured below, there is a direct path and three echos.

By starting the simulation the echo of a speech is perceptible. To dimension the delays, the length of the echo paths were divided by the acoustic velocity. As the absolute delay is irrelevant, the length of the direct path can be subtracted from every path length. In the result the relative paths got a length of 0m, 24m or 48m. These path lengths are used in this experiment.

t = s / v with the acoustic velocity of 340 m/s

Path 1: t = 0m/340 m/s = 0,00000s
Path 2: t = 24m/340 m/s = 0,06993s
Path 3: t = 24m/340 m/s = 0,06993s
Path 4: t = 48m/340 m/s = 0,13986s

The spectrums of the sent and received signal are shown.

Speech signal sent

Received speech signal with echos

Through the superposition of the signal and the three echos destructive and constructive interference arises. As a consequence in the spectrum of the received signal can be seen, that the acoustic channel is frequency-selective.

EXPERIMENT

If the source signal is a sine, the difference between destructive and constructive interference can be heard by modifiing the frequence. As an example 243 Hz for constructive and 250 Hz for destructive interference can be used.