• All QAM is not the same

    QAM. Most likely you think it's either an unpronounceable random set of letters or an Australian airline. In both cases you would be wrong. It stands for Quadrature Amplitude Modulation. That's a fancy way of saying that unlike AM or FM which have one waveform, QAM has two and that makes it much more efficient. It means that HD channels can fit into much smaller spaces.

    QAM is used by cablesystems and also by DIRECTV in their COM2000 system. There's a dirty little secret to QAM, though... it's not all the same. A single QAM channel carries 38.8 megabits of information, but that full amount is never used for just one channel. If it was, you could carry Blu-ray-quality video and audio over cable, but then you would probably get maybe 20 channels.

    Cable companies will try to fit as many as 8 HD channels on one QAM channel. That means the quality is generally worse than Netflix or Vudu quality. They do it through techniques like bitstarving and gamut compression that strip quality out of the signal while still letting the cable company call it "HD." The more channels they want to have, the more channels they shove into a single QAM.

    On the other hand, DIRECTV's COM2000 system puts only two HD channels on one QAM, which gives each channel the same quality that regular DIRECTV boxes put out. It can also use a QAM to carry a single 4K channel, but this capability is really still in the lab. This means clearer sharper images and none of the shenanigans that the cable companies use. It's a little more expensive to do it that way but the results speak for themselves.

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    Comments 2 Comments
    1. dsiebert's Avatar
      dsiebert -
      Are there different formats for clear QAM channels? My local cable company sends out about 80 clear QAM SD channels, which I am able to receive on my various TVs. However, the two oldest ones, a couple of Panasonic plasmas purchased in 2005, model TH-42PD50U, can't tune certain channels. When I do a channel search they seem to find about 3/4 of the clear QAM channels the others do, and can't tune the channels the others see even if I type in the channel number manually. Its not a big problem, but I am curious why this is, and wondering if they eventually update all clear QAM channels to do whatever these problems channels are doing, those TVs may eventually be unable to tune any of them. I guess having a QAM tuner isn't enough, you have to have one newer than this? Maybe a firmware upgrade would help, when I googled I found talk of one but the link is dead.
    1. Stuart Sweet's Avatar
      Stuart Sweet -
      There are several QAM modulation schemes, and they keep changing to try to take into account the channel capacity that customers request. I suspect that your older TV only supports QAM16 and QAM64, while your cable system uses QAM256 or QAM512. Each succeeding scheme allows for a higher bitrate on a single QAM channel, allowing for more total channels to be added as the QAM channel is carved up into pieces.