Teaching Cable Dawgs New Tricks

By Scott Johnston, Comcast

All-digital simulcast (ADS) is a hot topic these days. Your cable company is either designing, building, or already rolling out ADS cable services in your systems. Is it the beginning of the end for analog? Some think so. One thing is certain: Now is the time to start focusing heavily on training your workforce to be able to handle the change.

In cable dawg years, I’m new to the business. I got my start in 1998. Digital cable was relatively new in my system by the time I had been hired on. I’ve always been inquisitive. I would drive many of my service technicians and maintenance people crazy with endless questions about how things worked. But the one question I could never get a clear answer on was how digital cable worked. I wanted to know how it was transmitted. I wanted to know how to measure it. I wanted to know as much as I could about digital modulation. Most of the training I sat in on usually included the classic picture of the “haystack” along with the famous “cliff effect” diagram that we’ve all seen a dozen times. I wanted to know more. It’s been seven years since I started looking for the answers to my digital questions. I haven’t found all of the answers. But I’ve learned enough to introduce digital modulation to others.

Start with analog

Digital modulation is easy to understand once you know the fundamentals. The problem is that for some, the basics aren’t very basic. You’ll want to make sure before you train your technicians on digital modulation that they understand amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM).

Once you’re sure that your trainees understand analog modulation, you should make sure that they know the difference between analog and digital signals. I like to describe analog as being a signal that constantly varies while digital signals have one of two possible states, on and off. One thing that can be confusing is understanding that digitally modulated signals are actually analog signals!

If your audience doesn’t catch on right away, use some analogies. For AM, I like to use the ocean waves and a ship. I have my trainees imagine that they are looking at a ship riding the waves on the ocean. Then I tell them that AM is a lot like the analogy. The ship is the information, and the waves are the carrier. It’s helpful to use a white board so you can draw some waveforms if you need to.

FM is harder to explain, but I like to contrast it with AM. I still haven’t come up with a great analogy for FM, but I’m working on an idea involving a slinky. Phase modulation is something that should be discussed as well since quadrature phase shift keying (QPSK) relies on the concept of phase changes, and quadrature amplitude modulation (QAM) relies on the concept of phase and amplitude changes. For your more experienced employees, you can remind them of color burst and the changes in phase that represent color information. In some ways, digital modulation is a lot like color burst information.

Move to QPSK, QAM

Once the analog concepts are clear, particularly the AM and phase concepts, you can begin talking about QPSK. Even if your plant has retired QPSK for the more advanced 16-QAM, you should start with it. QPSK is simple to explain. Introduce the idea that phase changes can represent information.

Make sure that you are clear that data is not being transmitted, but symbols that represent data are. Many people think that QPSK and QAM are transmitting the actual bits, but they aren’t. The symbols represent the bits that the digital receiver sees as basically a magnitude and phase. The resulting vector (magnitude and phase) represents a stream of bits that vary depending on the modulation method. QPSK can be described as a method of transmitting two bits per symbol with a maximum of four different symbols. Once your trainees understand QPSK, you can move onto the more advanced methods (16-QAM, 64-QAM, and 256-QAM). Once you start talking about 16-QAM, you need to bring up the idea of decision boundaries. Decision boundaries are important to understand. They make discussing modulation error ratio (MER) and bit error rate (BER) so much easier.

MER and BER

Once you have discussed the different types of QAM and their fundamentals, you can discuss MER and BER.

You can describe MER a lot like a signal-to-noise ratio (SNR) for digital carriers. MER is actually a measurement of the deviation from the expected phase and amplitude of the symbol to the actual symbol. Lower MER figures mean that the phase and amplitude of the vector are being skewed by something. Usually, they’re being shifted by noise, distortions, or bad modulating equipment.

BERs are measurements that tell us the actual amount of errors that are seen in the stream of bits that are being transmitted. You’ll want to discuss Reed-Solomon error correction as well as pre- and post-forward error correction (FEC) testing and its significance. I like to teach people that pre- and post- BER measurements are very important. While some may feel that BER measurements are more important than MER measurements, in practice both are equally important. When the pre- and post- BER measurements don’t match, there is an impairment somewhere that is forcing the receiver to use error correction. We want to avoid the use of error correction if at all possible.

There are specifications for acceptable limits concerning MER and BER. Make sure you consult your local engineering team or tech ops folks for those specifications. And remember that in most cases, in clean plant with new rebuilds, you can do much better than the minimum specifications. You’ll want to teach your technicians to aim high when looking at MER and BER figures. If you can maintain excellent MER and low BER in your plants, you are sure to keep your customers happy by providing excellent service to the home.

Happy ending

Once your technicians understand QAM fundamentals, MER, BER and your local specifications for acceptable performance at the customer premises, they will be equipped with the basics they need to ensure success whether at a new install, an upgrade or a service call.

Scott Johnston is a CommTech 4 for Comcast. Reach him at bryan_johnston@cable.comcast.com.