Coaxial Cable - A concentric cable consisting of a center conductor, a dielectric, and a shield. Coax used for most MATV and CATV work has a characteristic impedance of 75 ohms.
Composite Video Signal - The composite video signal including the picture (luminance) signal, the blanking and sync pulses, and the color (chrominance).
dBmV - An absolute signal level where 0dBmV is equal to 1000 µV across 75 ohms.
In the simplest form cable signal is an RF or radio frequency signal each channel is in a 6 MHz band width. These channels or RF carriers are in uV and can be transmitted over a conductor or through the air based on its signal strength. These signals ride on the outside of the center conductor, “like train tracks” it follows the conductor the path of least resistance. This is why the center conductor of cable wire is copper clad, we only need the conductor on the outside.
Signal, RF (Radio Frequency) - Generally refers to data modulated over a carrier for transmission. This can be video or data information.
Now when you start dealing with QAM or Quadrature amplitude modulation As with all modulation schemes, QAM conveys data by changing some aspect of a carrier signal, or the carrier wave, (usually a sinusoid) in response to a data signal. In the case of QAM, the amplitude of two quadrature waves is changed (modulated or keyed) to represent the data signal.
Phase modulation (analogue PM) and phase-shift keying (digital PSK) can be regarded as a special case of QAM, where the amplitude of the modulating signal is constant, with only the phase varying. This can also be extended to frequency modulation (FM) and frequency-shift keying (FSK), as this can be regarded as a special case of phase modulation.
Although analogue QAM is possible, this article focuses on digital QAM. Analogue QAM is used in NTSC and PAL television systems, where the I- and Q-signals carry the components of chroma (colour) information. "Compatible QAM" or C-QUAM is used in AM stereo radio to carry the stereo difference information.
As for many digital modulation schemes, the constellation diagram is a useful representation and is relied upon in this article.
In QAM, the constellation points are usually arranged in a square grid with equal vertical and horizontal spacing, although other configurations are possible (see e.g. Cross-QAM). Since in digital telecommunications the data is usually binary, the number of points in the grid is usually a power of 2 (2,4,8...). Since QAM is usually square, some of these are rare — the most common forms are 16-QAM, 64-QAM, 128-QAM and 256-QAM. By moving to a higher-order constellation, it is possible to transmit more bits per symbol. However, if the mean energy of the constellation is to remain the same (by way of making a fair comparison), the points must be closer together and are thus more susceptible to noise and other corruption; this results in a higher bit error rate and so higher-order QAM can deliver more data less reliably than lower-order QAM.
If data-rates beyond those offered by 8-PSK are required, it is more usual to move to QAM since it achieves a greater distance between adjacent points in the I-Q plane by distributing the points more evenly. The complicating factor is that the points are no longer all the same amplitude and so the demodulator must now correctly detect both phase and amplitude, rather than just phase.
64-QAM and 256-QAM are often used in digital cable television and cable modem applications. In the US, 64-QAM and 256-QAM are the mandated modulation schemes for digital cable, as standardised by the SCTE in the standard ANSI/SCTE 07 2000. Note that many marketing people will refer to these as QAM-64 and QAM-256. In the UK, 16-QAM and 64-QAM are currently used for digital terrestrial television (Freeview and Top Up TV).
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