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Signal Science - A Rebirth Digital picture quality is far superior to analog picture quality. The high level of technology that has been introduced is all about picture quality. So much work has gone into improving the picture and removing the artifacts, that the general consensus is that the new digital "super" picture is immune to corruption. This is a misconception. Technical papers that describe digital signal processing refer to the digital medium as: more robust to co-channel interference more robust to multi-path interference more robust to all of the previous analog downsides... Digital picture isn't immune to the foes of the analog system, it is much more robust versus them. The same issues are being dealt with as before. Yes, it's true! The difference is that digital systems use algorithms and coding (FEC-forward error correction) to vastly improve the picture. But the performance of that systems depends on the bit error ratio (BER) and BER is a signal dependent quantity. There is science behind signal collection and processing. The science behind digital signal includes measurements of signal quality. Why? Because just having enough signal to get the picture isn't enough. Some of the terms to express signal quality are signal-to-noise ratio, noise figure, and their digital cousins bit rate, and bit error rate or BER. In the most basic of terms: The amount of signal that you have in ratio, to how much noise you have from intrinsic and extrinsic sources, determines the signal-to-noise ratio. Signal strength is biggest variable in the hands of an installer, since much of the noise in a system is fixed. A very small increase in signal will produce a large reduction in BER. Signal-to-noise ratio determines the bit error rate of the data stream. Increase in signal strength results in lower bit error rate. Bit error rate measures the quality of a data stream. The lower the bit error rate, the better the data stream. Better data "in", means better data "out". When we get into digital signal processing (DSP) we are digital, right? Now we're talking bits and bytes. Computers. Who's not familiar with "Garbage-in, Garbage-out"? The fewer errors there are in a data stream, the more accurate, more correct, the end result will be. Conversely, the more corrupted the data stream (higher BER), the lower the quality of the reconstruction. If you read On Shannon and "Shannon's Formula" you will see that the basics of Forward error correction: The essential elements of “Shannon’s formula” are: 1. Proportionality to bandwidth W 2. Signal power S 3. Noise power P 4. A logarithmic function And further, The channel bandwidth sets a limit to how fast symbols can be transmitted over the channel. The signal to noise ratio (P/N) determines how much information each symbol can represent. The signal and noise power levels are, of course, expected to be measured at the receiver end of the channel. http://www.iet.ntnu.no/projects/beats/Documents/LarsTelektronikk02.pdfFrom January 2003 High Frequency Electronics Copyright © 2003 Summit TechnicalMedia, LLC: "One of the changes that modern digital communications systems has brought to radio engineering is the need for end-to-end performance measurements. The measure of that performance is usually bit-error rate (BER), which quantifies the reliability of the entire radio system from “bits in” to “bits out,” including the electronics, antennas and signal path in between." Further, "With a strong signal and an unperturbed signal path, this number so small as to be insignificant. It becomes significant when we wish to maintain a sufficient signal-to-noise ratio in the presence of imperfect transmission through electronic circuitry (amplifiers, filters, mixers, and digital/analog converters) and the propagation medium (e.g. the radio path or optical fiber)." http://www.highfrequencyelectronics.com/Archives/Jan03/HFE0103_Tutorial.pdfNote that with a "strong signal" we overcome BER problems. BER is a signal dependent value. From Encyclopedia of Laser Physics and Technology: "Even a digital data transmission system is not totally error-free – statistical fluctuations related to noise influences cause a small percentage of the transmitted bits to be corrupted. The average fraction of incorrectly transmitted bits is called the bit error rate." And "The maximum capacity of a reliable data transmission system is not reached by keeping the bit error rate at an extremely low level (basically avoiding any bit errors), but by pushing the data rate to a level where some tolerable bit error rate of e.g. 10-12 can be maintained and by detecting and correcting most of the remaining bit errors." http://www.rp-photonics.com/bit_error_rate.html "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?
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