It's the week of yellow school buses and book bags for most young people, which serves as a good enough reason to venture back to school on one of cable's more enigmatic and pervasive technologies: modulation.
Modulation is as essential to industries that move information for a living as oxygen is to humans. It is present in every analog and digital service ever delivered over a communications network — cable, telephone, wireless or otherwise. Without modulation, signals wouldn't move, or at least they wouldn't get very far.
From the earliest days of cable, through the rise of digital, and as far out as the next-next version of the Data Over Cable Service Interface Specification cable-modem standard (beyond 1.1), modulation sets the pace for what MSOs can and can't deliver.
To modulate is to imprint information — voice, video, data or whatever; analog or digital — onto the spine of a carrier signal, so it can get from one place to another. Maybe it's from the studio to the uplink, or from the uplink to the satellite, or from the headend to the home.
A carrier signal is an electromagnetic wave, which is a form of energy — just as heat, light and sound are forms of energy. Electromagnetic waves are invisible and ubiquitous. One of their more interesting scientific properties is their ability to propagate through space, or over a conductor (such as a cable), essentially at the speed of light. Like most other forms of energy, the strength of the electromagnetic wave dissipates in a known and predictable manner.
The best way to envision a carrier signal is to picture the letter "S" on its side. One transit from the beginning to the end of the sidelong S-shape is one cycle. One cycle per second is 1 hertz, or Hz. One million cycles per second is 1 megahertz, or MHz. The total number of cycles per second is the frequency
of the carrier signal.
Now draw a horizontal line through the center of the sideways "S." The distance from that line to the peak, and to the valley, is the amplitude
of the carrier signal. Amplitude, translated, is power. The shape of the curves is known as the phase
of the signal.
You see how this gets pretty dense, pretty fast. Hang on. We're almost there.
As it turns out, changing the frequency, phase
or amplitude
of a carrier signal is what modulation is all about. Changing those core electromagnetic ingredients is how one impresses information onto a carrier. It's what's happening when you hear of "amplitude modulation" (AM) and "frequency modulation" (FM). The former manipulates a signal's power; the latter, the number of cycles per second it transmits.
Then there are digital-modulation types, such as quadrature amplitude modulation (QAM, pronounced as a word that some rhyme with "Sam," and others rhyme with "Tom.") and quadrature phase shift key, or QPSK (spelled out, conversationally). Both are hopelessly technical terms, but not impossible to grasp.
In a very condensed translation, QAM works by grouping digital bits into symbols, which can be imprinted on a carrier by adjusting two things: Amplitude (power) and
phase (shape). The "quadrature" reference, in this case, means "at right angles." There are four right angles in a 360-degree totality. The "quadrature" part of QAM, then, has to do with shifting the phase of the digital signal by 90 degrees, in order to imprint the digital information.
The number that usually sits ahead of QAM refers to the number of symbols — or grouped bits — impressed onto the carrier. In 64 QAM, eight symbols are sent. In 256-QAM, 16 symbols are sent.
QPSK is a subset of QAM that manipulates only the phase of the carrier to move bit groups. As such, it is roughly equivalent to 4-QAM, or four symbols sent per second.
The constant in all modulation types is this: They walk a fine line between transmission speed and noise immunity. QPSK is slower, but sturdier against noise. That's why it's mainly used in upstream, home-to-headend transmissions, where noise is common. QAM is faster, but less rugged in big noise. It mostly gets used in downstream, headend-to-home transmissions.
For all its venerability, the modulator itself isn't much to look at. It's one of those slender panels with blinking lights, stacked up one over the other on a headend rack. There's generally one modulator for each channel on a cable system. In a signal-path sense, the modulator generally sits between the signal source (the satellite receiver, for example, or digital multiplexer), and the combiner (the thing that smooshes all the channels together to be squirted into the fiber or coax plant.)
If you've ever toured a headend, you've seen modulators. Maybe you nod appreciatively, secretly wondering. Or, if you're technologically gracious, maybe you step around the back to admire the tidy wiring.
The subject of modulation is dense and old, but knowing its rudiments is worthwhile. For one thing, it's not going away. For another, it will continue to evolve — but, thanks to the laws of physics, its basic tenets will remain fairly stable.
