Diagnosing digital cable’s problems

Mar 26, 2001  •  Post A Comment

Many cable subscribers are not happy with the digital TV revolution. They dislike being pressured into either going digital or losing premium movie channels. They complain about customer service-spending 40 to 50 minutes per call getting through to service providers and waiting days for the necessary crew and downloads. They are put out about the need to rent a special box and a remote control. They often complain about cumbersome, advertisement-heavy electronic program guides.

And then there are the technical problems. One cable tech insider took his best shot at explaining why viewers are experiencing the problems they complain about the most.

Problem: The picture quality is not as reliable as analog. There’s a constant digital break-up: The picture is in pieces. It comes off in fragments.

Analysis: Black holes or different colored blocks are indications that there is a problem in the transmission path. They are caused by environmental disruptions-something in the space between you and the transmitter-for instance, a plane or a thunderstorm. Another possible cause is an interfering carrier-someone using a vacuum cleaner, a truck driver on a CB radio. If your house or apartment complex has wiring problems that don’t carry the signal at full strength-too many splitters (boxes with one input and two outputs), for instance-this can be more problematic.

Motion artifacts specifically target the part of the picture with the most motion. The requirements to fully send a motion picture takes 300 megabits of bandwidth per second. No transmission system is that efficient. You have to give up something (bits) when you send it. Program providers make a choice about how much compression their programs will have.

The effect is generally the same whether the digital medium is cable, satellite or over-the-air television, though the programmer can provide different compression rates (optimized) to different media, which use different bandwidth. For example, a terrestrial broadcast system, which uses 8VSB, has a capacity of 19 megabits per second in a single channel. A cable channel uses either 28 megabits per second (QAM 64) or 39 megabits per second (QAM 256).

Each service operator decides how many programs to fit on each channel. Some use statistical multiplexing to pack as many programs as possible by allocating extra bandwidth from more static programs (such as news reports) to programs that need more bandwidth (like live sports).

In the odd case that programs require peak bandwidth at the same time, part of a picture will not be transmitted. The picture will have blurred blocks-they would be the same color, but you would lose the detail and motion.

Problem: Sometimes, when you press the next-channel button, there is a delay of two seconds, which is too long-it used to take an instant.

Analysis: In analog TV, you get a complete picture every 30th of a second-about the time it takes to blink your eyes, so you don’t notice. With digital TV, you’re not sending a full picture at a continuous rate. Periodically you’re sending a full picture, but the rest of the time you’re just sending the change from one picture to another (essentially motion).

If you jump in the middle of the stream when you change channels, you have to wait until the full picture is transmitted. In some TV set designs, the viewer gets a black screen. In others, parts of the picture are painted in (in blocks) until the whole picture appears. The wait is typically not more than a second. The duration is partly due to the design of the receiver and partly because of the operating choices of whoever sets up the encoder or encodes the tape from the program source or satellite uplink.

Problem: My PIP (picture-in-picture) and other special features (split screen, audio swap) in my deluxe Sony XBR are disabled by virtue of digital TV. I can only view and record the same channel-with analog I could record a different program from the one I was watching.

Analysis: The analog TV set has two tuners-which can tune two analog signals at the same time. The tuners cannot decode digital signals. The new digital set-top box has only one tuner.