Sync ... ?

The sync is the part of a TV signal that has all of the timing (synchronising) information. Without a sync signal the picture is not able to be displayed as it cannot be ‘locked’ onto a screen.

There are in fact 2 sync signals a horizontal sync and a vertical sync. A TV picture is drawn by a single dot of light moving very quickly from top to bottom and from left to right. The whole picture is redrawn 50 times a second (see note on interlacing below)  The Horizontal sync is the timing information for each line of the picture and the Vertical sync is the timing information for whole picture i.e. number of picture per second and when each one should start. Fortunately. a combination of the slow response of the human eye and persistence (slow rate of decay) of the CRT image means that instead of seeing a very fast dot flying about the screen we see what appears to us as a steady solid picture. Some people can make out the flicker caused by the 50Hz vertical scan rate and this is why computer monitors generally use a slightly faster refresh rate and why we now have scan doubling 100Hz TVs 

As mentioned in the section on RGB, the sync signal can have a variety of flavours. It is sometimes sent with one of the video signals, in this case it is usually sent as a 0.3V signal going negative of black or no picture signal point. i.e. 0.3v negative sync. This sync is combined in that the horizontal and vertical syncs are sent in the same signal. (Sometimes referred to as "mixed sync")

The syncs can also be sent as 5V TTL level syncs (TTL stand for Transistor Transistor Logic). Here the sync signal can be either combined as in a H/V sync or separate as in H + V sync. The norm again for is the the signal to be normally high i.e. 5V and for the sync pulse to go negative from this point down to 0V or ground. {negative sync}

Some professional equipment and some older consumer equipment require the sync to be normally at the low voltage and for the pulse to go positive {positive syncs}

sync_example

Please see the section on interlaced versus progressive scan for more detail on horizontal or line frequency

The sync on green RGsB option can be a very quick and easy option to get running with an RGB signal if everything works OK with it as only three connections are required. Unfortunately, it is not at all infrequent to find say a DVD player that sends out sync on green and a TV that does not remove it. The result is a very heavy green cast to the picture. The SyncBlaster BlackBox can be used here to tidy up the signal and produce separate to mixed syncs.

There are now a number of schemes to try and make a video signal unrecordable. They often add extra pulses into the sync pulses in order to make the picture unstable when recorded. A typical case would be an additional pulse  half way in between 2 horizontal sync pulses. While in no way do we wish to condone video piracy we do feel that if these signals impair what might be considered normal viewing of the signal then they should not be present and should be removed. Many projectors can not display a clean picture when fed a signal containing deliberately distorted sync pulses. Until recently most RGB outputs were kept reasonable clean but some more recent DVD players do give out some copy inhibit signals on the RGB output and these should be avoided if at all possible.

Detail frequency response ... ?

Generally speaking the better the frequency response the sharper the picture. If a signal loses some of the high frequency information then the picture will start to look soft and out of focus.

Any processing of the picture especially encoding into PAL and back will affect the resolution and high frequency response and so is to be avoided if the highest picture quality is to be maintained.

Connecting directly as RGB often avoids much of the picture processing and certainly avoids the dreaded encoding in to PAL and as such must be the preferred method of connection. That said both the method of connection and the grade of cable used have a part to play, see the cable section for more detail.

If you already have a PAL signal Keene Electronics manufacture a distribution amp with a line drive feature. This is there to overcome any losses involved in longer cable runs especially with s and composite video