Com3: A Composite-Compatible Component coding system.
This Report describes a method for conveying a component video signal on a single cable in such a way that it may be passed through any existing composite studio equipment that has a bandwidth of approximately 9 MHz. The system was originally termed Extended Studio PAL, although it is now referred to as Composite-Compatible Component (Com3, Com cubed) in order to emphasise the quality difference it offers over PAL. It is particularly suitable for use with digital composite equipment operating at a sampling rate of four times the colour subcarrier frequency, such as D2 and D3 video recorders. The coding method yields a picture quality approaching that of CCIR Recommendation 601, without any crosstalk between luminance and chrominance. The absence of such atrefacts and the additional horizontal resolution it offers compared with conventional PAL are particularly useful for working with widescreen 16:9 pictures. The coded signal is highly compatible with PAL, allowing mixing with conventional PAL and the use of existing studio monitors. The principle of Weston, or phase-segregated, PAL is used to form a signal in which the components are separated by phase. No temporal processing is used and therefore no motion artefacts are introduced. The propagation delay of a codec is about 8 lines, so compensating sound delays are not required. The higher frequencies of the coded signal are used to carry high-frequency luminance and chrominance detail, so the signal is tolerant to high-frequency losses. The principles are equally applicable to a system compatible with NTSC. Experimental equipment has been constructed to implement the coding system, which has been used in field trials in a studio and an outside broadcast vehicle. These trials verified that existing PAL equipment can convey the coded signal with little or no impairment.
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