The Digital Fact Book - Quantel

DDigital keying and chroma keyingDigital keying differs from analog chroma keying in that it can key uniquely from any oneof the billion colors of component digital video. It is then possible to key from relativelysubdued colors, rather than relying on highly saturated colors which can cause color-spillproblems on the foreground.A high quality digital chroma keyer examines each of the three components Y, B-Y, R-Y or R,G, B of the picture and generates a linear key for each. These are then combined into alinear key for the final keying operation. The use of three keys allows much greater subtletyof selection than with a chrominance-only key.See also: Chroma keying, KeyingDigital labA facility where digital intermediate work is carried out.Digital mixingDigital mixing requires ‘scaling’ each of two digital signals and then adding them. A and Brepresent the two TV signals and K the positional coefficient or value at any point of themix between them (i.e. equivalent to the position of the transition arm on a switcher desk).In a digital system, K will also be a number, assumed here as 10-bit resolution to providea smooth mix or dissolve.Mathematically this can be shown as:A x K = (Mix)1B x (1-K) = (Mix)2Result = (Mix)1 + (Mix)2Note that such math also applies to soft edge keys and any transparency created betweentwo images. As such it is a fundamental part of video processing and good quality resultsare essential.When two 10-bit numbers are multiplied together, the result is a 20-bit number (see Binary).When mixing, it is important to add the two 20-bit numbers to obtain an accurate result.This result must then be truncated or rounded to 10 bits for transmission to other partsof the digital system.Truncation by simply dropping the lower bits of the partial result (Mix)1 or (Mix)2, to 12 bits,or even 14 bits, will introduce inaccuracies. Hence it is important that all partial results,e.g. (Mix)1 and (Mix)2, maintain 20-bit resolution. The final rounding of the result to 10 bitscan reveal visible 1-bit artifacts – but these can be avoided with careful rounding techniquessuch as Dynamic Rounding.See also: Binary, Dynamic Rounding62