QAM

What is QAM? First let's pick apart the three words QAM stands for, and then I'll explain how they work in the QAM tuner. QAM stands for Quadrature Amplitude Modulation. This is a 'carrier' format for sending information, and applied to your television it means the format your cable TV providers encode their cable channels and transmit them to your TV set. To break down the words:

Quadrature refers to a unique and complex shape of 'waves', the electronic signals that carry information - in the form of TV channels - along cables from the provider to your television set. Quad means 'four', and in QAM's case refers not to a number of different signals, but to the shape of those signals as they are carried through your cable. If you take a circle, like a pie chart, and divide it into four equal pieces, one of those pieces is a quadrature shape, as can be measured by its central 90 degree angle; technically, quadrature refers to this 3-point shape based around that 90 degree angle and the other two outer points, and this square wave pulse shape is applied electronically to part of the set of carrier waves that transmit TV channels through your cable.

Amplitude refers to the magnitude, or amount, of change to an oscillating system; an oscillating system, in the field of communications, is the carrying of any signal that oscillates, or changes with certain repetition; an AC (Alternating Current) power supply is oscillating forward and backward, rather than sending a steady and even stream of power, as compared to the non-oscillating DC (Direct Current) which sends a steady stream of power in one direction; a pendulum oscillates back and forth with a rhythm; if you hold a spring, hang a weight on the bottom and bounce that weigh up and down, that is an oscillation; changing the volume on your television set or your stereo is changing the amplitude of the sound waves up or down. Amplitude refers to the AMOUNT OF CHANGE of an oscillation... how small or short-timed it is, measured against how large or long-timed it is changed to, and vice versa.

Modulation refers to modifying, or varying; in electronics, it refers to taking a modulating signal and using it to change one or more of the properties of the oscillating, or 'periodic', carrier signal. The three main properties of a periodic carrier signal (also called a waveform) are its phase (timing), amplitude (volume), and frequency (pitch), and each or all of these carrier signal properties can be modified by a modulating signal being input.

To put it all together: QAM, or Quadrature Amplitude Modulation, means the use of an input signal to alter and strengthen the digital cable signal. Quadrature is a very techie term referring to the electronic shape of that signal, amplitude refers to the strength (or amount) of change that signal is altered by this method, and modulation refers to changing that amount through the use of an introduced input signal.

In very simple terms: QAM is a way your digital cable company packages and sends digital cable to your TV. ATSC (see the page 'ATSC Tuner') refers to the electronic format for sending TV shows 'through the air', and QAM refers to the electronic format of sending TV shows through your digital cable. You'll also hear QAM referred to as 'Clear QAM', and this refers simply to QAM that receives normal, non-encrypted TV channels in any given area. It has nothing to do with how clear the reception is, it refers to picking up channels that are in the clear to be picked up without de-encrypting them.