Editing Amplitude modulation (section)

==Modulation index==
The AM modulation index is a measure based on the ratio of the modulation excursions of the RF signal to the level of the unmodulated carrier. It is thus defined as:
:<math>m = \frac{\mathrm{peak\ value\ of\ } m(t)}{A} = \frac{M}{A}  </math>

where <math>M\,</math> and <math>A\,</math> are the modulation amplitude and carrier amplitude, respectively; the modulation amplitude is the peak (positive or negative) change in the RF amplitude from its unmodulated value. Modulation index is normally expressed as a percentage, and may be displayed on a meter connected to an AM transmitter.

So if <math>m=0.5</math>, carrier amplitude varies by 50% above (and below) its unmodulated level, as is shown in the first waveform, below. For <math>m=1.0</math>, it varies by 100% as shown in the illustration below it. With 100% modulation the wave amplitude sometimes reaches zero, and this represents full modulation using standard AM and is often a target (in order to obtain the highest possible [[signal-to-noise ratio]]) but mustn't be exceeded. Increasing the modulating signal beyond that point, known as [[overmodulation]], causes a standard AM modulator (see below) to fail, as the negative excursions of the wave envelope cannot become less than zero, resulting in [[distortion]] ("clipping") of the received modulation. Transmitters typically incorporate a [[limiter]] circuit to avoid overmodulation, and/or a [[Dynamic range compression|compressor]] circuit (especially for voice communications) in order to still approach 100% modulation for maximum intelligibility above the noise. Such circuits are sometimes referred to as a [[vogad]].

However it is possible to talk about a modulation index exceeding 100%, without introducing distortion, in the case of [[double-sideband reduced-carrier transmission]]. In that case, negative excursions beyond zero entail a reversal of the carrier phase, as shown in the third waveform below. This cannot be produced using the efficient high-level (output stage) modulation techniques (see below) which are widely used especially in high power [[broadcast]] transmitters. Rather, a special modulator produces such a waveform at a low level followed by a [[linear amplifier]]. What's more, a standard AM receiver using an [[envelope detector]] is incapable of properly demodulating such a signal. Rather, synchronous detection is required. Thus double-sideband transmission is generally ''not'' referred to as "AM" even though it generates an identical RF waveform as standard AM as long as the modulation index is below 100%. Such systems more often attempt a radical reduction of the carrier level compared to the sidebands (where the useful information is present) to the point of [[double-sideband suppressed-carrier transmission]] where the carrier is (ideally) reduced to zero. In all such cases the term "modulation index" loses its value as it refers to the ratio of the modulation amplitude to a rather small (or zero) remaining carrier amplitude.

[[File:Amplitude Modulated Wave-hm-64.svg|thumb|400px|center|Figure 4: Modulation depth. In the diagram, the unmodulated carrier has an amplitude of 1.|alt=Graphs illustrating how signal intelligibility increases with modulation index, but only up to 100% using standard AM.]]