Front Porch: Period of time in the video signal which precedes the line sync pulse.

The front porch is a brief (about 1.5 microsecond) period inserted between the end of each transmitted line of picture and the leading edge of the next line sync pulse.

In the previous section I said that the field group is used to trigger the vertical scan of the picture tube and the hor line sync pulses trigger the horizontal scan.

In order that the phase of this locally generated signal can match the transmitted information, a 10 cycle burst of colour subcarrier is added to the video signal shortly after the line sync pulse, but before the picture information, during the so-called back porch.