QAM stands for Quadrature amplitude modulation. It's a modulation scheme that transmits data by changing the amplitude of two carrier waves. The two carrier waves are out of phase with each other by 90 degrees. Digital Cable uses QAM to transmit the majority of their signals. Two major QAM schemes are 64QAM and 256QAM. 64QAM contains data throughput around 28 mbps, where 256QAM has data throughput of 38.8 mbps.
If your TV has a QAM tuner, it has the ability to read this signal and display it. The cable company can encrypt the digital video carried in a QAM signal and this is commonly referred to as Encrypted QAM. To receive Encrypted QAM, you would require a cable card or an official Comcast digital receiver. A QAM tuner on a TV or DVD recorder does not 'map' the channels to the same location on the Comcast channel lineup card. A QAM tuner will display unencrypted channels on their physical location in relation to the frequency they are carried on.
Some area's are able to pass PSIP data along with local broadcasts. PSIP data is embedded in the digital information that tells your TV what channel number to display. For example, Channel 3 in HD may embed 3.1 in it's video stream. Your TV would be able to tune this by typing in 3-1. Each Comcast area is different, and some may not pass PSIP, so look for HD locals on odd numbers, such as 113.1, etc..
It's also recommend to force your TV or DVD recorder to rescan the QAM channel lineup from time to time. Additional channels may show up unencrypted or may of changed frequencies. Comcast does not guarantee channel availability through a free QAM tuner.
If your looking for a QAM channel lineup, there is no official one from Comcast. You could try the SiliconDust HD-Home Run page. Enter your zip code at this following website:
»www.silicondust.com/hdhomerun/channels
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by cypherstream 
last modified: 2009-01-31 16:06:53 |
