Communications Technology: Archive

Countdown to Digital
Tips for Smooth Launch in Your System
By Joseph B. Waltrich

Digital technology is rapidly being deployed throughout North America. Five hundred digital systems were launched in 1997, and twice as many digital headends were expected to be in operation by the end of 1998.

Digital technologys rapid growth has created operator concerns about optimizing its introduction into their systems. This article explores some issues operators typically encounter when adding digital technology to an existing cable system and provides solutions for minimizing them.

With larger service offerings, digital systems are more complex than their analog counterparts. For example, a 550 MHz analog system provides about 70 video services. In contrast, a 750 MHz mixed-signal system, with analog channels in the lower 550 MHz and digital channels in the 550-750 MHz range, increases the total number of video services to more than 200.

With a mixed-signal system, program information becomes more complex, making electronic program guides (EPGs) necessary. An EPG must be downloaded to consumers digital set-tops.

Digital headend equipment configuration also becomes more complicated, requiring equipment control to move from manual operation to computer operation.

The increased number of services to be authorized makes access control more complex.

Preparation, training and systems integration are the keys to a successful digital launch. By understanding all of the differences between analog and digital technology and by properly preparing for digital introduction, cable operators can make a more efficient transition from analog to digital.

Figure 1 provides an example of a typical digital system. It illustrates a pass-through system incorporating local access control. Its system components include:

The digital system also contains a headend configuration tool (HCT). As its name implies, the HCT is a laptop computer that performs the initial configuration of each piece of headend equipment. The HCT also handles some system monitoring functions and reconfigures the system if a new component (such as a new IRT) is added to the system.

Properly preparing for digital equipment installation before the equipment arrives at the headend helps avoid unnecessary delays during the installation phase. Begin by completing a thorough site survey. Like a site survey for analog system expansion, it should consider such factors as power, cooling and floor space. In addition, satellite dishes should be erected, and satellite signals should be checked for proper levels and carrier-to-noise ratios (C/Ns). An integrated receiver decoder (IRD) should be used to check the received satellite signals video and audio quality.

Because a digital system is so software-intensive, it is important to train system personnel on the software tools they will be using for configuration and control of the headend equipment. Technicians must have a thorough knowledge of the proper configuration of all headend equipment. They also need to learn how to set digital signal levels and measure digital C/N ratios. Unlike analog signals, digital signals require a bandwidth correction when measuring signal power but do not require a bandwidth correction to the C/N measurement.

Figure 2 illustrates the technique for measuring power level and C/N of a digital signal. The following paragraphs describe the technique.

Set the spectrum analyzer resolution bandwidth to 300 kHz. Set the analyzers video bandwidth to 30 kHz or lower. If using an analyzer with a video filter, turn the video filter on.

If the analyzer has a signal-averaging mode, turn on the averaging and measure the signal level at the center frequency. If the analyzer does not have a signal-averaging mode, measure the signal level midway between the top and bottom of the "grass" on the signal.

Add a 12.2 dB correction factor to the measured level for a 64-QAM signal to obtain the correct signal level. Add 7 dB to the measured value of the out-of-band (OOB) signal to obtain its correct value.

The bandwidth corrections for digital signal levels are necessary because unlike an analog signal, which has most of its energy concentrated in the visual carrier, the digital signal power is distributed uniformly over the entire signal bandwidth. This distribution pattern also accounts for why the digital C/N does not require a bandwidth correction.

To measure digital C/N, set up the spectrum analyzer and measure the average power of the digital signal as previously described. Do not apply a bandwidth correction.

Measure the noise floor of the system in an empty channel. Subtract this value from the measured value of the digital signal to find the C/N.

When making this measurement, make certain that the system noise floor is above the analyzer noise floor. If there is less than 10 dB between the two noise floors, apply a correction as specified by the spectrum analyzer manufacturer.

(Editors note: This method is recommended only for "flat-top" digitally modulated signals. Measurement inaccuracy will result with "haystack" or other shaped signals. When in doubt, use test equipment that has a digital carrier power measurement capability.)

Before putting digital signals on the system, make certain that the system meets Federal Communications Commission Part 76 specifications. This inspection will assure that sufficient margin exists so that the digital signal will remain unaffected by random noise and/or distortions. Also check that composite second order (CSO) and composite triple beat (CTB) levels meet specifications in empty channels, where the digital signals will be located. This examination should be made at a few receive sites near the end of the system.

The return path will become increasingly important as more digital systems are deployed and interactive services are added. Proper setup and maintenance of the return path is just as important as it is for the forward path.

System maintenance for a mixed analog/digital system is no different than for an all-analog system. Fortunately, cable is a fairly benign system, and if good maintenance practices are followed, the digital portion of the system should encounter no problems. However, some precautions to take can minimize potential service calls.

Make certain that the digital signal levels are set up correctly relative to the analog carriers. Use the procedure described previously. In addition, make sure that the digital RFUs are set up properly. The upconverter output should be adjusted with an unmodulated analog carrier at the IF input and the automatic gain control (AGC) on. AGC should be disabled once the digital signal is connected, since the digital signal has no carrier on which the AGC can operate.

Avoid operation in the roll-off area. Although good results may be obtained at some receive sites, this success is not guaranteed for all receive sites. The roll-off area is a gamble, and the odds are against the operator.

Make sure that digital signals are not located next to trapped channels. The group delay in the trap may be more than the digital set-tops adaptive equalizer can correct.

Be careful about sweeping in the digital channels. Use a noninterfering sweep, and program the sweep generator to skip the digital portion of the spectrum. A low-level sweep may place an excessive load on the digital set-tops error-correction budget, leaving less capability for correcting errors from transmission system impairments. Check with your sweep equipment manufacturer for specific recommended procedures.

Finally, when adding digital signals to a fiber-optic link, the additional signals may overdrive the laser, thereby generating unwanted intermodulation products. To avoid this condition, readjust the overall levels into the fiber-optic transmitter.

The operator has little control over the subscribers location or circumstances. However, an installer can take several steps to minimize potential problems.

When installing the digital set-top, check the integrity of connections within the home. In particular, inspect the quality of splitters that the customer may have installed. Poor-quality splitters lack enough port-to-port isolation to prevent transient reflections (which are caused by switching another device) from generating errors in the digital picture. If a splitter is suspect, work out an agreement with the subscriber to replace it.

After installing the digital set-top, use the units on-screen diagnostics (OSD) to check digital signal quality. The OSD screen will display a signal-to-noise ratio (S/N) estimate in dB. This estimate is actually an equivalent S/N, taking all of the system impairments (such as noise, CSO, CTB and multipath) into consideration.

This figure will be lower than the actual S/N that would be measured if a spectrum analyzer were used. However, the S/N estimate likely will be on the order of 30 dB or more in a well-maintained system.

Several test-equipment manufacturers now offer digital signal level meters (SLMs) that are capable of measuring bit error rate (BER) and adaptive equalizer tap values, as well as signal level and C/N. These are useful tools for service technicians. In normal operation, short-term BER readings should be zero. Any other value is cause for concern.

The adaptive equalizer tap display indicates possible system problems such as bad connections and faulty cables. Figure 3 provides examples of equalizer tap displays under normal and problem conditions. Normally, the equalizer taps should decrease gradually in amplitude as the distance from the reference tap increases. A significant increase in one or more of the outlying taps indicates a problem within the system. Figure 3 shows a reflection at approximately 1.2 µsec.

Although it is more complex than analog technology, digital technology need not be intimidating. It is vital that a cable systems technical personnel understand the differences between the two technologies as well as how to handle these differences. This understanding, combined with proper site preparation and system maintenance, is the key to a successful digital launch in an increasingly digital world. - CT

Bottom Line

Digital Blasts Off

With larger service offerings, digital systems are more complex than their analog counterparts. Preparation, training and systems integration are the keys to a successful digital launch. By understanding all of the differences between analog and digital technology and by properly preparing for digital introduction, cable operators can make a more efficient transition from analog to digital.

Joseph Waltrich is manager of special digital projects for General Instruments Digital Network Systems. He may be reached via e-mail at .