Introduction Of QAM

QAM stands for Quadrature Amplitude Modulator. QAM is the essential device of Digital Headend System. It is modulating device. It is used to modulate the signal for the transmission at long distance in cable tv network field. In other words it can be called as heart of digital headend.

QAM is standard for DVB-C and DOCSIS. DOCSIS stands for Data Over Cable Service Interface Specification while DVB-C stands for Digital Video Broadcasting Cable.

Working Of QAM

As discussed above QAM works for to modulate the signal from the digital headend. QAM have the IP input signal and gives the RF output signal. It have two power supply one is primary power supply and second one is secondary power supply. It works as to convert the IP signal into the RF signal.

QAM does changes in both PSK and ASK. PSK is called the Phase Shift Keying and ASK is called the Amplitude Shift Keying. It is done for to increase the number of states per symbol. QAM modulation is the ideal modulation for cable tv network.

Comparison Of 4 QAM 8 QAM 16 QAM 32 QAM Etc.

There are different types of QAM modulation like 4 QAM 8 QAM 16 QAM 32 QAM 64 QAM etc. Difference between them are mentioned below in diagram,

We can also find Multilevel Quadrature Amplitude Modulation Constellation below,

The dotted line shows the different QAM in single diagram.

For cable industry we mostly use the 64 QAM and 256 QAM. If a digital headend is using 64 QAM then it is easy to maintain the signal level in the field rather than a Digital Headend using 256 QAM. Even 256 QAM provides more bandwidth than 64 QAM. A 64 QAM provides 38 Mbps bandwidth for stream while 256 QAM provides more than that.

QAM is higher order form of modulation so it can carry more information.

MODULATION BITS PER SYMBOL SYMBOL RATE 16QAM 4 1/4 bit rate 32QAM 5 1/5 bit rate 64QAM 6 1/6 bit rate

QAM Applications

64 QAM and 256 QAM are generally used in cable modem applications and CATV. While UK uses 16 QAM and 64 QAM for DTT (Digital Terrestrial Television). In US 64 QAM and 256 QAM are for Digital Cable Television. It is also used for wireless technology. Telephone modem uses 16 QAM.

This is all about Comparison Of 4 QAM 8 QAM 16 QAM 32 QAM 64 QAM 128 QAM 256 QAM.

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