What Is QAM?

QAM stands for quadrature amplitude modulation. It is a way of sending digital data over a radio-frequency signal, and it is the method cable systems in the United States use to carry digital TV channels, along with the downstream data for cable modems. When people talk about a "QAM tuner" they mean a tuner that can receive these digital cable signals.

How quadrature amplitude modulation works

A QAM transmitter starts with two carrier waves at the same frequency, shifted a quarter of a cycle (90 degrees) apart. One is called the in-phase (I) carrier and the other the quadrature (Q) carrier. The transmitter changes the strength, or amplitude, of each carrier and then adds them together. Because the two carriers are a quarter cycle apart, the receiver can separate them again.

Each combination of I and Q amplitudes is one symbol. Plotted on a graph, with I across and Q up and down, the allowed symbols form a grid of dots called a constellation. The number in the name tells you how many dots there are:

• 64-QAM has 64 possible symbols, so each symbol carries 6 bits.
• 256-QAM has 256 possible symbols, so each symbol carries 8 bits.

More dots means more bits per symbol, but the dots sit closer together. Noise, reflections and other impairments push a received symbol away from its ideal spot, and with a dense constellation it takes less of a push to land on the wrong dot. That is why 256-QAM needs a cleaner signal than 64-QAM.

QAM on US cable systems

Digital cable in North America follows ITU-T Recommendation J.83 Annex B, which the cable industry also publishes as ANSI/SCTE 07. Each digital channel occupies the same 6 MHz slot that one analog channel used to fill. The standard defines two modes:

The usable rate is what is left after error correction. J.83 Annex B uses Reed-Solomon coding, interleaving and trellis coding so that the receiver can fix many errors on its own. The payload is an MPEG-2 transport stream, the same container used for broadcast digital TV, and one 6 MHz QAM channel usually carries several TV programs at once. Your tuner picks out the one you want by its program number.

Clear QAM and encrypted QAM

QAM describes how the signal is modulated, not whether you are allowed to watch it. A cable operator can send a QAM channel unencrypted, which is called clear QAM, or scramble it so that only an authorized box or CableCARD can decode it. Any tuner that supports QAM can receive clear channels. Encrypted channels need the operator's security system. See Clear QAM and Cable Encryption and Clear QAM.

Where else QAM is used

QAM is a general technique, not just a TV standard. Cable modems receive data on QAM channels, and many other wired and wireless systems use QAM constellations. Over-the-air TV in the United States does not use QAM: ATSC 1.0 uses 8VSB and ATSC 3.0 uses OFDM. See QAM vs ATSC 1.0 and ATSC 3.0.

Sources

• ITU-T Recommendation J.83, Digital multi-programme systems for television, sound and data services for cable distribution
• ANSI/SCTE 07, Digital Transmission Standard for Cable Television
• ATSC A/53 Part 2, RF/Transmission System Characteristics