Subject :- Television Engineering

Topic :- Fundamental Concepts of Digital Video

Difficulty :- Medium

• In a composite video signal there is one basic hardware signal conductor, usually a impedance controlled coax. The luminance/sync is in the baseband and the chrominance is on a 3.58 MHz subcarrier.
• In a component video signal there are three signal conductors used, three coaxial cables usually. Each individual color (R, G, B) has a line which carries the specific color luminance, so a chominance signal is not required to separate them. The vertical/horizontal sync is carried on one of them.
• The biggest difference is that the composite signal, the luminance signal is limited to less than 3.58 MHz or about 300 horizontal lines or pixel resolution), so its low resolution. HD TV cannot be carried on composite video.

Component Video:

• In component video, the three components necessary to convey color information are transmitted separately.

• The data capacity accorded to the color information in a video signal can be reduced by taking advantage of the relatively poor color acuity of vision, providing full luma bandwidth is maintained.

• It is ubiquitous to base color difference signals on blue minus luma and red minus luma (B’--Y’, R’-Y’). Luma and (B’--Y’, R’--Y’) can be computed from R’, G’, and B’ through a 3×3 matrix multiplication.

• Once luma and color difference – or chroma – components have been formed, the chroma components can be subsampled (filtered).

  Y’CbCr: In component digital video, CB and CR components scaled from (B’--Y’, R’--Y’) are formed.

  Y’PbPr: In component analog video, PB and PR color difference signals scaled from (B’--Y’, R’--Y’) are lowpass filtered to about half the bandwidth of luma.

Composite Video:

• In composite NTSC and PAL video, the color difference signals required to convey color information are combined by the technique of quadrature modulation into a chroma signal using a color subcarrier of about 3.58 MHz in conventional NTSC and about 4.43 MHz in conventional PAL.

• Luma and chroma are then summed into a composite signal for processing, recording, or transmission. Summing combines brightness and color into one signal, at the expense of introducing a certain degree of mutual interference.

• The frequency and phase of the subcarrier are chosen and maintained carefully: The subcarrier frequency is chosen so that luma and chroma, when they are summed, are frequency interleaved.

• Transcoding among different color encoding methods having the same raster standard is accomplished by luma/chroma separation, color demodulation, and color remodulation.

• However, the data rate of a component 4:2:2 signal is roughly twice that of a composite signal. Four-times subcarrier composite digital coding was resurrected to enable a cheap DVTR; this became the D-2 standard.

Features of Rec. 601:

• It uses 4:2:2 sampling: Chroma components are subsampled by a factor of 2 along the horizontal axis.

• Chroma samples are coincident (co-sited) with alternate luma samples.

• In an 8-bit system using 4:2:2 coding, the 2×2 array occupies 8 bytes.

• The aggregate data capacity is 16 bits per pixel.

• For studio digital video, the raw data rate is 27 MB/s.