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Signal Strength Meters and BER All digital televisions and receivers have a "signal" meter built into the tuner. This is commonly, but mistakenly referred to as a "signal strength meter". Actually, the value represented on the meter is a ratio of signal strength to system noise, resulting in a signal quality reading or signal-to-noise ratio. This describes the meter that is used in home satellite systems. Some digital tuners also have a separate signal strength meter. We all know what higher quality means. If that scale measures the quality of your signal, then the quality of your signal matters. This is a data signal that the meter measures. When we input high quality data into a digital system, we receive a high quality result. When we input low quality data and you get a low quality result. What is there about digital receivers that would allow them to function outside the digital understanding that we've acquired so far through computers? Nothing. This is digital science. If Digital TV were not a digital system, it would have been named something other than "digital".
A signal strength meter that you find in a TV or receiver, has a scale. Some from 0 to 100, and some 0 to 125 (Dish, old scale). Every system is different, but in general terms, here's how it breaks down. From 0, signal must be increased to a high enough point on the signal meter to produce a picture. Until that point, there is not enough information for the digital-to-analog converter (DAC) to produce a picture at all. The result is a "loss of signal" message. You haven't actually lost the signal, there just isn't enough signal, or enough information for the DAC to do its work. This is described as the failure point on the signal meter scale, where the digital equipment will get, or lose signal lock. At this point there is just barely enough information to provide a picture. The picture will be unreliable and loss of signal may occur frequently. The picture will "pixelate" or show a "macro-blocks" or square blocks of color. Just above the failure point (FP), you will see a relatively stable, but slightly degraded picture. At the threshold of visibility (ATSC) there is still a relatively low signal-to-noise ratio and high bit error rate (BER) that compromises the picture quality. (BER, for the threshold of visibility system failure point, is 3x10-6) Increasing signal gives better signal-to-noise ratio and reduces BER, which are both signal-dependant measurements. When you increase the signal on the signal meter a little more, there comes a point where signal is strong enough to overcome the noise in the system and produces a low enough BER to present the digital picture , uncompromised. This is referred to as the quasi error-free (QEF) condition. (Quasi Error Free (QEF)' condition - a BER of the order of 10-11 ) The QEF is a level of data performance described by the errors in the bit stream (BER) and represents the minimum acceptable picture quality for a digital system. Further increase in signal will continues to lower the BER and increase picture quality, though visual perception of the difference would be determined by the amount of increase and the perceptions of the viewer. The system now has enough information to provide the complete picture in full resolution and perform up to standards. This signal level reading is a relatively small increase from the threshold point, yet still far from the top of the signal meter scale. The amount of signal in excess of the QEF condition, is the signal power margin. The signal power margin is the amount of signal you have "left over" to compensate for signal drops that occur from atmospheric disturbances - like storms and sunspots. This "excess signal" also is used to counteract the effects of variable noise sources like electromagnetic interference (storms), RF ingress (unwanted RF signal), sunspots, electric appliances, and such. The higher your signal is, the lower your downtime will be. The higher the signal strength you have, the better the system will perform during rain and storms. The higher signal strength you have, the lower the BER and the higher picture quality you will have. When digital systems get signal lock, this amount of signal provides enough uncorrupted and correctable data to begin function and produce a picture, but doesn't provide enough signal to be deemed the "acceptable" digital picture. The prescribed threshold for "acceptable" digital picture begins at the QEF condition. (On the Digital Stream 9900 converter box, for example, the signal quality meter has this printed across the screen: "You need to have 70 for clear picture." Odd wording if it weren't designed by an engineer who understands what is going on. These get lock with as little as 25 in the red scale, There is NOT always a discernable difference in picture quality on low or poor signal. What is important to know is that there CAN BE.) This is not the top of the scale, it is the bit error rate that is associated with the minimum acceptable picture quality. Incrementally, there are two more "benchmarks" of picture quality increase. These benchmarks are not like "flipping a switch" when you reach them, they are points on a steady scale. It is a scale of picture quality determined by BER. Any increase in signal reduces BER, which results in a picture that more closely resembles the original content. QEF isn't the "top of the line", it's where the line starts for viewing with an "acceptable" amount of errors. QEF is what the "average" person considers to be "acceptably noise-free" according to the powers that be. QEF is the minimum acceptable picture quality. References: Digital Communication
Basics The basic signal processing for any digital TV or wireless application is baseband source coding/formatting, channel coding, modulation, multiplexing, signal spreading/scrambling, and timing synchronization" http://www.videsignline.com/showArticle.jhtml?articleID=206600019 "For operational purposes, the monitoring of analog video signal properties is still key;" and "The reason signal quality measurements work with analog and full-bandwidth digital systems is uncompressed systems are linear." And, "Signal quality of the uncompressed video remains critical in systems that use compression for several reasons: 1. The input to a video
compression codec must be accurate, in compliance with
appropriate standards, and of as high a quality as possible
to provide for efficient encoding. This leads to a strong requirement for testing of the analog and full bandwidth digital portions as well as the sophisticated compression and transmission systems." http://www.tek.com/Measurement/cgi-bin/framed.pl? BER vs. signal http://happy.emu.id.au/lab/rep/rep/9801/9801_317.htm#s3p17 "Performance Comparison of ATSC 8-VSB and DVB-T COFDM Transmission Systems for Digital Television Terrestrial Broadcasting" http://paginas.fe.up.pt/~mandrade/tvd/2003/docs/DVB/ATSC/ICCE99paper.pdf"Lab Report 98/01 Section 1: Australian Laboratory Testing of Modulation Systems" http://happy.emu.id.au/lab/rep/rep/9801/9801_001.htm#index Note: The Dish Network 622 and 722, among others, have built in 8VSB and 8PSK technology. |
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