Page 194 - Fundamentals of Communications Systems

6.16 Chapter Six

summed with the voiceband signal). Unbeknownst to the inventors a circuit
nonlinearity in this “linear” modulation process was in fact producing LC-AM
signal. Envelope detection of the modulated waveforms was actually easy to
accomplish with a wide variety of nonlinear devices. In fact, an early LC-AM
detector used a crystal (the Dunwoody and Picard crystal detector) in a very
simple circuit to implement to the envelope detector. Many amatuer radio hob-
byists have used crystals to build radio demodulators without a single active
radio frequency device (only an audio amplifier). Again this pre-1910 work was
mostly pushed by technologists and the theory was not completely understood
until much later.

6.2.2 LC-AM Conclusions
The advantage of LC-AM is again it is easy to generate and it has a simple
cheap modulator and demodulator. The disadvantage is that E B = 50%. Later
we will learn that noncoherent detection suffers a greater degradation in the
presence of noise than coherent detection.

6.3 Quadrature Modulations
Both DSB-AM and LC-AM are very simple modulations to generate but they
have E B = 50% so bandwidth sensitive applications need to explore other mod-
ulation options. The spectral efficiency of analog modulations can be improved.
Note that both DSB-AM and LC-AM only use the real part of the complex
envelope. The imaginary component of the complex envelope can be used to
shape the spectral characteristics of the analog transmissions. As an exam-
ple, the bandwidth of analog video signals is approximately 4.5 MHz. DSB-
AM and LC-AM modulation would produce a bandpass bandwidth for video
signals of 9 MHz. Broadcast analog television (TV) signals have a bandwidth
of approximately 6 MHz and this is achieved with a quadrature modulation.
Engineers in 1915 first realized that there were two sidebands (positive fre-
quencies and negative frequencies) in amplitude modulation and theorized
and realized in experiments that only one of the sidebands was needed to re-
construct the message signal at the demodulator. This mathematical obser-
vation led to a great flurry of development in quadrature modulation. Since
only one sideband needs to be transmitted, this makes a transmission scheme
that has E B = 100% possible. This two times improvement in spectral effi-
ciency compared to DSB-AM and LC-AM is what led early telecommunications
engineering groups to adopt SSB-AM for efficient multiplexing of voiceband sig-
nals. This section tries to summarize the current state of the art in quadrature
modulations.
Vestigial sideband amplitude modulation (VSB-AM) is a modulation that
improves the spectral efficiency of analog transmission by specially designed
linear filters at the modulator or transmitter. The goal with VSB-AM is to