ISP Information:
The technology that allows computers to "read" the text from physical objects. It requires a graphical representation of text to interpret. This usually comes from a scanned image.
ISP Glossary:
OCR - edbl@mweb.co.za wrote:floyd@barrow.com said:Two different equations have been adopted, with A-Lawcompandering used in Europe and Mu-Law compandering used inNorth American and Japan. They are similar, and I'm going toalmost totally ignore A-Law.All of which raises the question .. Mu-law versus A-law. What featureshave determined the global choices where A-law is seemingly used inmore countries than Mu-law? Insofar as the "quantization noise " isconcerned presumably there is no difference?Why CCITT chose to use A-Law seems to be a mystery.Occasionally, in books written by Americans, there are verynearly snide remarks made about that... (they did it just to bedifferent!)I can't remember which it is, but one of the two gets a slightlybetter dynamic range, and the other gets a slightly better SNR atlower levels. In neither case is it significant.So, there are the details of why 37 dB is what you'll see forSNR end to end over a PCM quasi analog channel. It is theresult of logarithmic quantizing instead of linear quantizing.And the max SNR over a PCM digital channel is .. ?Well, lets define "a PCM digital channel" first. That would bea channel where the input and output are both digital, soinstead of SNR the operative term is "bit error rate".T1 digital channels operate at 1.544 Mbps and are specified tohave at least a 1 x 10e-7 bit error rate at least 97% of thetime. Generally for anything other than fiber or satellitecircuits the actual rates will probably be more than 1000 timesas good and simply aren't measured by the equipment available.Even satellite circuits are going to be operating somewherebetween 10e-5 and 10e-8 _before_ error correction, which willimprove BER by 1000 times or so. (The customer is probablygoing to measure BER as 1x10e-9 or better on most T1 circuits.That is 1 bit error in every 1,000,000,000 bits.)At any of those bit rates, an analog PCM channel will not seenoise from bit errors that is significantly even close to thequantization distortion. But at a bit error rate of 1 x 10e-3(1 error every 1000 bits) the channel will be measurablyimpaired, and at higher errors becomes unusable for voicetransmission. Just exactly how the channel looks at points inbetween depends on what equipment is being used (there are vastdifferences in equipment used for a wire line T1 circuit, afiber circuit, or a satellite circuit).--Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
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