Carrierless amplitude phase modulation (CAP) is a variant of quadrature amplitude modulation

Quadrature amplitude modulation

Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...

 (QAM). Instead of modulating the amplitude of two carrier waves, CAP generates QAM signal by combining two PAM

Pulse-amplitude modulation

Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses....

 signals filtered through two filters designed so that their impulse response

Impulse response

In signal processing, the impulse response, or impulse response function , of a dynamic system is its output when presented with a brief input signal, called an impulse. More generally, an impulse response refers to the reaction of any dynamic system in response to some external change...

s form a Hilbert pair

Hilbert transform

In mathematics and in signal processing, the Hilbert transform is a linear operator which takes a function, u, and produces a function, H, with the same domain. The Hilbert transform is named after David Hilbert, who first introduced the operator in order to solve a special case of the...

.

CAP used for ADSL divides the available space into three bands. The range from 0 to 4 kHz is allocated for POTS

Plain old telephone service

Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....

 transmissions. The range of 25 kHz to 160 kHz is allocated for upstream data traffic and the range of 240 kHz to 1.5 MHz is allocated for downstream data traffic.

History

CAP was the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installs. Now it is deprecated in favour of Discrete MultiTone Modulation (DMT), but it is still used for some variants of HDSL.