Difference Between QAM and ATSC

The digital modulation techniques QAM (Quadrature Amplitude Modulation) and ATSC (Advanced Television Systems Committee) are used in communication systems.

QAM and ATSC are both used in digital television transmission, but in different contexts. ATSC is used for over-the-air broadcast, whereas QAM is used for cable television transmission. QAM can also be used in other applications, such as satellite and wireless communication networks, whereas ATSC is mainly used for television broadcasting.

Read this article to find out more about QAM and ATSC and how they are different from each other.

What is QAM?

Quadrature Amplitude Modulation (QAM) is a digital modulation scheme used to send digital data over analogue channels in communication systems. It is a modulation method that combines amplitude and phase modulation (AM and PM) to transmit two different digital bit streams over a single channel. The two digital bit streams are combined in such a way that they are represented by two different out-of-phase carriers.

QAM works by modulating two carriers that are 90 degrees out of phase with each other. These carriers, also known as quadrature carriers, are typically sine and cosine waves. To indicate a digital signal, the amplitude of each carrier is varied. In general, amplitude variations take the shape of discrete levels or amplitudes that can be represented as bits.

The QAM signal can be represented as a constellation diagram, in which the amplitude and phase of the modulated signal are displayed on a two-dimensional graph. Each point on the graph represents an individual combination of amplitude and phase that correlates to a specific binary symbol or sequence.

The size of the constellation is determined by the number of levels or amplitudes used to represent the digital signal. In a 16-QAM constellation, there are 16 possible amplitude and phase combinations, each of which can indicate 4 bits of data. (2 bits per carrier).

There are two kinds of QAM: coherent and non-coherent. Coherent QAM requires a reference signal for synchronising the recipient with the transmitted signal, whereas non-coherent QAM does not.

What is ATSC?

ATSC stands for Advanced Television Systems Committee, an organisation that establishes digital television standards in North America. The ATSC standard specifies the technical requirements for digital television transmission over-the- air, via cable, or via satellite.

The ATSC standard uses MPEG-2 video compression to send digital video signals with resolutions of up to 1080i. It additionally allows 5.1-channel digital audio and is highly resistant to noise and interference.

The ATSC standard uses 8VSB (Vestigial Sideband) modulation to send digital signals. 8VSB is a type of amplitude modulation that sends digital data in a sequence of discrete amplitude levels. The VSB signal is an analogue signal that has been designed to carry digital information. The vestigial sideband is a frequency band that has been partially filtered out to reduce the transmission bandwidth requirements.

The ATSC standard also contains the PSIP (Programme and System Information Protocol) system, which gives programme guide information, closed captioning, and other broadcast-related data. PSIP is used to send programme schedules, programme descriptions, and other broadcast information. This data is delivered alongside the video and audio signals and can be decoded by a digital television receiver or set-top box.

One of the ATSC standard's main advantages is its ability to deliver high-quality digital television signals over the air. This has allowed the United States and Canada to make the shift from analogue to digital television broadcasting. In addition, the ATSC standard is used for digital video signal transmission over cable and satellite networks.

Difference between QAM and ATSC

The following table highlights the major differences between QAM and ATSC −





Quadrature Amplitude Modulation (QAM) is a digital modulation scheme used to send digital data over analogue channels in communication systems.

ATSC stands for Advanced Television Systems Committee, an organisation that establishes digital television standards in North America.


It is used for modulation of digital signals.

It is used for broadcasting digital television signals.

Signal Constellation

It uses a two-dimensional signal constellation.

It uses 2D and 3D signal constellations.

Transmission of signal

QAM is used in cable modems, Wi-Fi, digital television, and cellular communication systems.

ATSC is used in over-the- air, cable, and satellite transmission of digital television signals.


It doesn't include video compression.

It uses MPEG-2 video compression.


It doesn't include audio support.

It supports 5.1-channel digital audio.

Program Guide

It doesn't include programme guide information.

It includes the PSIP system for programme guide and closed captioning information.


In conclusion, QAM and ATSC are two digital modulation techniques that are used in communication systems.

QAM is a modulation scheme that combines two amplitude-modulated signals into a single channel, whereas ATSC is a digital television standard used in North America for digital television broadcast over-the-air transmission. QAM is widely used in cable television systems, whereas ATSC is used for digital television signal transmission over the air.

While they share some similarities, such as their use in digital television transmission, they are used in different contexts and for different purposes.

Updated on: 17-Apr-2023


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