By now, it should be obvious that we can create a table for the video food chain. We start with RGBHV at the top and proceed down the list to composite video.
Figure 2 — Video food chain table
| Quality |
Signal Format |
Decoding Requirements |
Advantages |
| Highest |
R G B H V |
None |
Original image, highest bandwidth |
| High |
R G B S |
Separation of sync |
Same as above |
| High |
R GS B |
Stripping of sync from green channel plus separating H & V |
Same bandwidth if display strips sync properly |
| Very Good |
Component
(Y, B-Y, R-Y) |
Simple matrix decoder, strip sync from channel |
Nearly as good as RGB but requires good matrix design |
| Good |
S-video |
Synchronous detector plus matrix decoder & sync processing |
Requires only two cable connections |
| Lowest |
Composite |
Complete decoder with Y & C comb filtering, synchronous detector, matrix, and sync processing |
Requires only one cable connection & can be used to modulate an RF transmitter |
Moving down the list, we can see some of the same format points as in Figure 1 with the camera system. Note that RGB video precedes component video; which precedes S-video; which precedes composite video. Decoding requirements are listed for each step in the chain. The NTSC decoder is nearly a mirror image of the camera encoder. While some noise or distortion may be added in the encoding process, the ability to disassemble the composite NTSC signal into its RGB components is much more difficult and prone to error.
Within the RGB formats, the only issue is sync processing. However, don't be misguided by thinking that sync processing is a minor issue. In the television world, sync construction is carefully specified and there are many circuit designs and systems that handle composite sync very well. It's in the computer community where the caveat remains.
There were never any significant standards established for constructing composite sync for the myriad of computer signal formats. While it is possible to construct composite sync (and many computer manufacturers used it for years) for computer graphic outputs, the details of construction can be troublesome for many displays and projectors. Furthermore, removal of sync from the green channel, in the case of RGsB signals, can affect the performance of the green channel or the proper operation of the display's black level controls. While this aspect is not an issue with the design of purely NTSC video distribution systems, it is an issue of which to be aware if the integration of computer graphics is anticipated.
The component video format refers to the intermediate three elements used to construct the composite video signal; namely, Y (luminance) channel, R-Y (red minus Y) channel, and B-Y (blue minus Y) channel. Decoding this format requires only a good video matrix design. A video matrix is, in its simplest form, a resistive interconnection of each signal channel to the other in a combination that yields the algebraic sum of the difference channels (R-Y and B-Y) to produce the missing channel G-Y; plus, a mixture of the three difference channels with the Y channel to yield R, G, and B channels. The advantages that component video holds over just using RGB are:
1) the same information may be transmitted, generated, or stored in less bandwidth than three RGB channels, and
2) the presence of composite sync on the Y channel ensures that any sync processing anomalies will be distributed evenly among the channels upon exiting the matrix.
The S-video format represents color imagery in two channels of information. The Y channel conveys the brightness and detail information. All chroma information is contained within the C channel. The C channel consists of the phase and amplitude encoded subcarrier that represents the color portion of the image. This signal is accompanied by the subcarrier burst sample. The main advantage to the S-video signal is that luminance information is already separated from chrominance information. Any system using S-video must have a good quality synchronous detector to recover the chroma difference signals (R-Y and B-Y) used in the matrix to create the RGB image.
Decoding the composite video signal requires the most complex scheme. The primary difficulty is in recovering, or separating, the luminance information from the chrominance information. In lower cost systems, separation is accomplished by the "notch filter method". In newer systems, varieties of "comb filtering" are used with improved results. The goal is to remove, or "comb" out, the luminance without affecting the quality of the chroma; and, vice versa. Combing is possible as a result of the method in which the chroma information is interleaved with the luminance information during the encoding process. The energy associated with these Y and C components occupies different RF spectrum space by design. Full NTSC decoding is a very demanding task. Finding a high quality decoder is the challenge. The last thing one should do is pass a video signal from one appliance to the next, requiring each appliance to decode and re-encode the signal.
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