Week 6, 2003<\/p>\n
Q) We have digital cable. \u00a0 Lately we have noticed a buzzing sound on the lowest channels. \u00a0 When we go to the higher channels, it disappears. \u00a0 The situation is the same for both of our TVs. \u00a0 Why would this happen? \u00a0 It does not make any sense. \u00a0
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\n-Trish, Greentree<\/p>\n
A) A service visit from your cable company is required to know for sure, but the problems you are experiencing suggest interference on your analog band of channels. \u00a0 Most digital cable services are a hybrid of analog and digital video sources, unlike digital satellite service, which is 100% digital. \u00a0 To explain what this means to you, a little lesson in analog and digital cable service is probably in order.<\/p>\n
With analog cable, the television signal is similar to the signal you would receive with an antenna. \u00a0 It is a radio frequency signal, containing picture and sound information. \u00a0 Either your television’s tuner or your cable box tunes in the signal and sends it for display on your television. \u00a0 Radio frequency signals are subject to interference from outside sources, just like your car radio’s reception may change in sound quality as you drive around the city and through tunnels. \u00a0 Because it is carried over a line for cable TV service, it is much less subject to outside interference. \u00a0 It can still happen, though, and you may have a faulty line in your home which is causing the buzzing.<\/p>\n
Digital cable uses digital technology to transfer analog signals to your television set. \u00a0 A good analogy would be the transporter seen on Star Trek television shows. \u00a0 On the Enterprise, the transporter takes apart Captain Kirk, “beams” his particles to the space station, where another transporter reassembles him. \u00a0 Now, for your digital cable, the Enterprise is your local cable company, the transporter is a device called an MPEG-2 encoder, Captain Kirk is an episode of the Sopranos, the space station is your house and the other transporter is your digital cable box.<\/p>\n
Instead of being fed to the analog cable line, he video feed of the Sopranos is sent to the MPEG-2 encoder. \u00a0 The encoder changes the video signal into a digital signal of ones and zeros. These ones and zeros are then sent to your cable line. \u00a0 Your cable box picks up the ones and zeros, turns them back into an episode of the Sopranos, and sends it to your TV set for display,<\/p>\n
Because the digital cable signal is just ones and zeros, it is not subject to the same interference as the analog cable signal. \u00a0 This does not mean you will necessarily receive better picture quality. \u00a0 \u00a0 This is because of “bitrate” which determines how many ones and zeros are produced from the analog signal. \u00a0 With a higher bitrate, you have better picture quality because more information is used to produce the picture. \u00a0 MPEG-2 digital encoding is used for many video sources, from digital cable and satellite to DVD and HDTV. \u00a0 DVDs average around 5.0 megabits per second (Mb\/s). \u00a0 HDTV broadcasts are a minimum of 19.8 Mb\/s. \u00a0 This is why HDTV programs look so much better than a DVD: much more information is used to create the picture. \u00a0 Digital cable and satellite are usually below the 5.0 Mb\/s mark. \u00a0 A cable system can only handle a certain amount of information, which limits channel offerings. \u00a0 By using digital encoding, cable providers can offer more channels at similar or better picture quality as analog. \u00a0 However, if the bitrate is too low or the encoding is poorly done, the results may be worse. \u00a0 Fortunately for Captian Kirk, the Enterprise apparently used a high bitrate as he (usually) showed up intact!<\/p>\n