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