There are many methods used to store and interconnect vision signals.   The method used depends upon the desired resolution or picture quality and many other considerations such as whether it is an "end-product" or to be edited further, how it is to be displayed, how it is to be stored, how far it has to travel, etc., etc.

For the video enthusiast, the most common video signal formats at this time are Composite video, S-Video and Digital video.  To further confuse the uninitiated they also have alternative names which may be favoured in certain markets. Composite video may be known as "video", "baseband video" or "CVBS"; S-Video is also known as "Y/C" or "Super Video"; Digital Video is also referred to as "DV", "Firewire" and "iLink" to mention some of the most common names you are likely to see.

To explain S-Video it is necessary to describe Composite video first.   The European name of CVBS actually stands for Chroma (the colour information), Video (the monochrome information), Blanking (the parts of the signal forced to be Black to form a concealing frame around the picture) and Sync (the synchronising pulses which keep the picture straight, horizontally and vertically).  A normal composite video signal is composed of these various parts and is therefore aptly named. Composite video was originally devised as the means by which a single radio frequency carrier could be used to transmit all the information for a TV picture into our homes.  Incidentally, the TV (or RF) transmission also contains the audio which is actually added on seperately, so don't confuse this with composite video which contains no audio.

Originally TV was Black and White (or monochrome) and contained no colour information.  The resolution was quite high and contained frequencies from 25 Hertz (30Hz in the USA) out to about 5MHz.  When colour TV was being invented a means had to be found to retain compatibility with all the millions of monochrome sets.   Fortunately, there was a small portion of the signal which could carry the colour synchronisation and the actual colour information was able to be added onto the monochrome video in the form of a modulated sub-carrier by sacrificing some resolution.  The net effect on monochrome TV's of the added colour information was largely nil, though it was possible to see the colour signal as a fine dotty pattern where saturated colours existed (due to the high signal amplitude of saturated colours).

In the PAL system of colour video, the colour information is all contained in a single frequency sub-carrier of 4.43MHz (3.58MHz for NTSC). By shifting the phase of this sub-carrier relative to the colour synchronisation signal (the "colour burst") the range of hues is obtained. By varying the amplitude, so the saturation is defined.  There are some restrictions on the range of hues and their maximum saturation values, and the resolution of the colour signal is very roughly about one-tenth of the video signal's resolution, but on the whole the system works very well and has done so for many many years. In the previous paragraph I mentioned that some resolution was sacrificed from the monochrome video signal, so now I will explain.

A rule-of-thumb figure for monochrome video's upper frequency limit is 5MHz.  Of course, this could be extended if the source and the transmission system had sufficiently high resolution. As I have already mentioned, the colour sub-carrier is 4.43MHz (for PAL) and it should be obvious that this frequency is well within the 5MHz limit. It is therefore necessary to filter the video signal in such a way as to eliminate any content at that frequency or all sorts of bizarre colours will be seen when the signal is displayed.  It is the nature of low pass filters that the "rolling off" must commence much lower than the target frequency, for practical filters usually causing significant loss at least an octave below the target frequency.

The high frequency limit of the monochrome portion of a composite video signal is more likely to be 2.5MHz to 3MHz because of this effect.  The loss of resolution is approximately 50% (5MHz down to 2.5MHz) and quite noticeable when a comparison is possible, but it has to be the way it is to allow the colour sub-carrier to be mixed onto the video without causing unwanted sum and difference frequency artefacts to be produced.

In recent times the performance of professional and domestic video cameras has improved by leaps and bounds, but the TV system is stuck in an old rut because no improvement in resolution can be gained without changing the system radically. This is why you are seeing mention of High Definition TV, and Digital TV, etc. in the media.   High-end S-VHS, Hi 8 and consumer DV Camcorders are capable of capturing very high quality pictures, but this is of no use if you can't edit it or view it without keeping the high resolution quality. Enter S-Video!

By keeping the monochrome video signal (the Y, Luma or Luminance portion) seperate from the colour information (the C, Chroma or Chrominance portion) the high frequencies responsible for fine deatil in the Y signal can be allowed to remain intact. Because the Y and C are not going to be mixed together there is no need to restrict the high frequency limit of the Y and it can be as high as the particular video system components (CCD sensor/s, tape storage, video electronics, etc.) will pass.  So now you know what S-Video is.  It's a means of maintaining high resolution by keeping the Chroma seperate from the Luma.

A few pointers for those wanting to employ S-Video.  The Y and the C have to be kept seperated at every stage of the process.  If there is as much as a single composite video connection in line with the signals, the resolution will be lost permanently. It cannot be restored. There is equipment that will take a composite video signal and convert it to S-Video, which is great for compatibility and convenience, but you can't get back the high frequency information once it has been lost.  Digitising an S-Video signal with a computer-based video capture card is highly successful.  All DV Camcorders have an S-Video output for the purpose and  SVHS, Hi8 and Digital 8 camcorders can also be used this way.  Don't use composite if you have S-Video facilities! There are not many TV's / video monitors with S-Video inputs. If the screen is small there would be little advantage, so S-Video inputs usually only appear on big-screen monitors, video projectors and professional monitors.

S-Video cables.  Almost without exception, pre-made S-Video cables are made from pretty ordinary audio or data cable.  This is no good at all if you want to retain the quality of your signals. The Y and the C are meant to be interconnected by 75 ohm co-axial cables, just like any other video signal.  Proper, high quality 75 ohm S-Video cables can be as long as you need them to be without compromising their quality (we suggest keeping them to less than 50Mtrs unless you are using an S-Video distribution amplifier such as our DVS5s).  At QUESTRONIX we have proper high-quality 75ohm S-Video cable and we can make high quality assemblies to any length you require. Have a look at our CABLES page.