Analyzing Bits of Voice

Migration to the all-digital world will take many paths, but operators will want to pay attention to the example Charter Communications is setting in Long Beach, Calif. First, why Long Beach? Wayne Davis, Charter senior vice president of engineering and advanced technology, gives three reasons. Being an 860 MHz cable system, Long Beach had the bandwidth that enabled Charter to simulcast digital without disturbing the existing analog customers. The particular mix of Long Beach residents, with its small number of ad zones, was another factor, because those demographics minimally impact digital-into-digital advertising. Finally, Davis says the technical team was up to snuff. Phase one The migration itself is an ongoing, two-step process. Phase one involved encoding 93 channels of analog programming and putting them into seven QAM digitally modulated carriers, Davis says. Here, Charter used Harmonic’s variable bit-rate (VBR) encoders and "closed loop" statistical multiplexing system. The use of VBR encoding necessitates the multiplexer’s special feedback capability, Yaron Simler, president of Harmonic’s convergent systems, says. How does it work? A pool of encoders, each using a compression chip that "looks ahead" two seconds into the future, sends back information on the content’s "complexity factor" to the multiplexer, which dynamically allocates a bit rate to each channel based upon various decision criteria, Simler explains. Sounds fancy, but it’s nothing new. Harmonic originally developed this kind of technology for the satellite industry. "This is what fed us for many years," Simler says. Phase two at Long Beach has begun, but is not yet finished. Its goal it to enable all subscribers to receive content digitally. Davis says that this already has involved "quite a bit of work around what does that (all-digital) environment look like in terms of a low-cost set-top box, (or) a dongle, perhaps." On this front, many are wondering what kind of specifications Comcast, with brainpower imported from Cisco Systems, will cook up. Convinced that the all-digital subscriber piece is nearly there, however, Davis already is looking ahead to the challenge of winning over subscribers who have never wanted a set-top box. A dongle could help, but Charter also plans to leverage a simple lesson it learned through converting low-bandwidth systems to all-digital, uplinked service several years ago. That lesson? "Communicate, communicate, communicate," Davis says. "The customer has to understand the value." Cut costs-deploy Passage? Recasting cable’s value proposition may be a task for cable’s marketers. But all-digital video also gives them the job of addressing an untapped market segment. "Theft of service goes away if you don’t have an analog product," Davis says. Improved picture quality is one reason operators may be able to convert those analog viewers into digital subscribers. The lower production cost of all-digital set-tops also could accelerate overall take rates, as well as improve the operator’s bottom line. Another plus is the ongoing deployment of digital video recorders (DVRs) and HD boxes. They tend to displace digital set-tops, which can be moved into analog homes. "I end up reusing my inventory in a way that moves me toward an all-digital world," Davis says. What about driving costs further by bringing in a new conditional access (CA) system? "Is (Sony’s) Passage part of this?" Davis asks rhetorically, noting that Charter was the first MSO to license it. "You could get aggressive with that, get an entirely new approach with CA, or you could go with your existing CA and just drive the analog costs out of the box," Davis continues. "And we think that it’s got to be a new paradigm." —Jonathan Tombes The cable industry needs new test tools as it ushers in a generation of more demanding applications, especially those involving voice. Noise Com’s BA1500 BitAlyzer appears to be one such device. Bent Hessen-Schmidt, Noise Com executive vice president of marketing, says it is a is a bit error rate (BER) analyzer that offers cable operators a powerful tool for testing voice equipment. Aimed both at modem manufacture and field application, the BA 1500-which has a capacity of 1,500 Mbps-not only detects bit errors, but also determines where the errors occurred. This is important because voice is simply less forgiving than data services and problems must be dealt with proactively. "[Testing for] voice is probably more critical than most data communications," Hessen-Schmidt says. "You don’t want any delays in voice communications. When you need to correct errors [during transmission], you have delays, too. Voice is more demanding, it requires more testing." The BA1500 is useful in determining if there are errors in the serial to parallel conversions in the modem, one reason it would be used in the DOCSIS modem design phase, says Hessen-Schmidt. In field applications, the BA1500 can help determine where errors are occurring. The ability to see sequences of the ones and zeros can be correlated to outside activities to give savvy engineers hints and allow them to troubleshoot more effectively. Hessen-Schmidt offers an example: "When errors occur they may correlate with times that a transmitter turns on. This allows you to simply filter the power system or provide more isolation." One engineer at an MSO says the product could be useful. "Where I see this having particular relevancy is in testing a cable network for applications that require deterministic time, for example TDM over DOCSIS or VoIP," the engineer says. "Any time you need to use UGS (unsolicited grant service), jitter is an issue, and by using the BA 1500 you can accurately measure it to see if your cable system is compliant." Q-Factor Among the device’s optional physical layer test software are provisions for ANSI jitter measurement, fast-eye diagram displays, eye-mask testing, and display of the Q-factor, or electrical signal-to-noise ratio of a digital transmission signal. The Q-factor, which Noise Com says "is to the amplitude domain what jitter is to the time domain," has gained some currency since the ITU-T’s standardization of the Q-factor method last July. A white paper by Acterna engineer Roland Bach, available on the "Technical Library" section of the Acterna Web site, provides additional information on this metric. Bach writes that Q-factor measurements can be obtained through different methods, but that "the Appendix to ITU-TO.201 (IEC 61280-2-8) describes the ‘decision level shifting method’ with ‘dual decision circuit’ as the reference method." —Carl Weinshenk