WEBVTT 1 00:00:02.272 --> 00:00:07.052 The modulator is an element in the chain of transmission of information 2 00:00:07.053 --> 00:00:10.604 it is located on the emitter side of that chain. 3 00:00:10.605 --> 00:00:15.541 Amplitude modulation is a phenomenon involving mixing of or interference 4 00:00:15.542 --> 00:00:17.586 between two electrical signals. 5 00:00:17.587 --> 00:00:22.385 The first is the low frequency signal that contains the useful information 6 00:00:22.386 --> 00:00:26.591 that one wants to transmit, this is the modulating signal. 7 00:00:26.592 --> 00:00:28.185 The second is the carrier. 8 00:00:28.186 --> 00:00:32.181 It has a high frequency which will be modulated, then emitted. 9 00:00:32.182 --> 00:00:38.660 To simplify matters, let's consider a purely sinusoidal signal is to be transmitted. 10 00:00:38.661 --> 00:00:43.310 It possesses the following characteristics: a maximum amplitude, Am 11 00:00:43.311 --> 00:00:48.874 a continuous component or offset V0 and a frequency, Fm. 12 00:00:48.875 --> 00:00:52.191 The carrier signal oscillates at a frequency Fc 13 00:00:52.192 --> 00:00:55.352 much larger than that of the modulating signal. 14 00:00:55.353 --> 00:00:59.114 The mixing of the two signals is carried out by an electronic component 15 00:00:59.115 --> 00:01:02.875 that is here represented by the symbol of a multiplier. 16 00:01:02.876 --> 00:01:07.995 The output signal generated by this component is the amplitude modulated signal. 17 00:01:07.996 --> 00:01:10.921 It displays beats in the interior of an envelope 18 00:01:10.922 --> 00:01:14.683 whose amplitude varies between two fixed values. 19 00:01:14.684 --> 00:01:21.840 The form of the modulated signal depends on the offset voltage V0 of the modulating signal. 20 00:01:21.841 --> 00:01:27.456 The rate of modulation, corresponding to the ratio between maximum amplitude, Am 21 00:01:27.457 --> 00:01:32.707 and offset V0, determines the appearance of the modulated signal. 22 00:01:32.708 --> 00:01:35.136 A modulation rate less than 1 23 00:01:35.137 --> 00:01:38.846 produces an envelope that reproduces the modulating signal. 24 00:01:38.847 --> 00:01:42.686 This oscillates at the frequency Fm. 25 00:01:42.687 --> 00:01:49.896 In this case, detection of the useful signal, or demodulation, is simple to achieve. 26 00:01:49.897 --> 00:01:57.210 A modulation rate greater than 1 corresponds to a phenomenon called overmodulation. 27 00:01:57.211 --> 00:02:02.199 The alternating positives and negatives of the envelope overlap. 28 00:02:02.200 --> 00:02:06.379 The detection of the useful signal is difficult to achieve. 29 00:02:06.380 --> 00:02:09.932 If the modulating signal doesn't involve an offset 30 00:02:09.933 --> 00:02:13.380 the modulation is said to not have a carrier.