SSTV mode information
compiled by G3ZLS
Home ] Principles ] [ RGB Modes ] YUV Modes ] Feedback ]

RGB Modes

In these modes complete lines are transmitted one after another. These are generally very robust modes which are not susceptible to tuning inaccuracies.  Slight frequency errors will cause the image to appear lighter or darker, but the colours are not affected.

Each line has three component parts, one for each of the three primary colours - red, green and blue, which are transmitted one after another. The order in which the three RGB colour components are sent depends on the mode.

Each of the three component parts of the line represents the gray scale values for the respective colour moving progressively from left to right of the image. The same process is then repeated twice more for the other two colours, again scanning the same line from left to right.

In most of these modes the lines are separated by sync pulses.  The three RGB colour components of each line may also be separated by short black porches.  The function of these porches is somewhat obscure but they may be connected with the colour processing which took place in dedicated scan converters.  They also provide a small border which is not normally displayed but which prevents picture detail from one side of the image appearing on the opposite side if there is a slight synchronization error. In modern computer systems the black porches have no real function, but they must be taken into account in the encoding and decoding software to avoid colour mis-registration.

Once the three colour components have been received they can be recombined to reconstruct the line, pixel-by-pixel, reading the red, green and blue values from the appropriate positions in the three line components. If the values are not read accurately from corresponding positions in the three line portions this will result in colour mis-registration, visible as colour fringes around any sharp edges in the image. This is another reason to maintain accurate timing at both the transmitting and receiving ends.

22 January 2008