What is QAM?
QAM stands for quadrature amplitude modulation. It’s a technique used by cable companies to send digital cable signals to your home. To watch the cable companies digital signals you will need a QAM tuner inside your television or cable set top box.
If you have a QAM tuner inside your television it probably won’t do you much good because it can only tune to non-encrypted programming. Since most digital cable signals are encrypted to avoid piracy, you’ll need a digital cable set top box with both tunes and decrypts digital television signals.
So why do you have QAM? With analog cable, cable companies need 6MHZ of spectrum to deliver an analog station to your home. The amount of stations a cable company can send depends on the size of their plant. If a cable company has an 860 MHZ plant in a particular then it can send just over 14o analog channels to your home (Derived by taking 860 MHZ and dividing it by 6 MHZ).
QAM allows a cable provider to replace that 6MHZ used to transmit an analog cable television station with 38.8 Mbps of digital data. We think of this data stream consisting as television signals but it could be any digital data.
When encoded using MPEG2 compression that 38.8 Mbps of data equates to anywhere between six to twelve standard-definition television channels or two to three HD channels or for digital telephony thousands of voice conversations.
In other words, by using QAM cable companies can fit about up to 12 standard definition television stations into the same bandwidth needed for one analog channel.
Now you see why cable companies want to switch consumers over to digital cable!
Because they contain so much more information high definition signals use up much more bandwidth than standard definition signals therefore cable companies can only fit two or three channels in the same amount of bandwidth used by 6 to 12 standard definition (SD) stations.
So why do we say a cable company can send 2 or 3 HD channels (or 6 to 12 SD stations) in that 6 MHZ? The simple reason is that, if cable companies take the MPEG2 HD signal they receive from a station and send it uncompressed to their customers they will only be able to fit 2 HD channels in place of one analog channel. This is because each channel has a maximum bandwidth of 19.4 Mbps. Multiply that by two and you have the 38.8 Mbps of data which we said earlier was how much data we can send across in place of one analog station.
The compression of signals generally means one thing - poorer picture and sound quality. Viewers are more likely to see digital artifacts (blocks on the screen) and other blemishes on screen.
To reduce these digital artifacts, Cable companies employ a technique called statistical multiplexing (or muxing.) Muxing lets the cable company divide up the 38.8 Mbps across the three stations by letting one â€œburstâ€ above that 12.5 Mbps threshold while restricting the bandwidth on other channels.
Proponents say that muxing helps because lots of time HD signals donâ€™t need the full 19.4 Mbps of bandwidth. Critics agree that muxing can help but they also agree that no matter how good the statistical multiplexing is, there will times when an increased number of digital artifacts will occur.
For example, if a HD channel is showing a fast-motion basketball game, it will need to get up to 19.4 Mbps for reasonable quality. The result is that if the HD channel gets the required bandwidth then the other two channels in the mux will have to split the remaining 19.4 Mbps between them resulting in poor HD quality on the other two stations in the mux.