Analog television#Structure of a video signal

The porch is in front of the analog television and video technology and the section behind the horizontal sync pulse in the composite video or television signal. It serves mainly to gain control of the image signal.

In order to transmit analog television images for spectators, they are divided into lines and transmitted as a text sequentially. In order for the receiver to recognize when a row is over, a sync pulse is transmitted at the end of the line. It is enclosed by the front and back porch. Thus, the electron beam in the picture tube receives enough time to move to the beginning of the next line.

In these areas, the signal is no image information, the signal level is the same as for black. This level is used as a basis for the calculation of the amplification factor for the image signal. The aim is to obtain regardless of the strength of the signal received by the antenna an image with optimum values ​​for brightness and contrast. For this purpose, the entire image signal can be used, because a weak signal can be applied on a weak station, as well as a (willing ) indicate dark image. However, the porch allows the detection of the signal strength independent of the image content. For a so-called keyed control is required which operates only at the time when the back-porch is transmitted.

To the back porch is the PAL signal or addition of the PAL burst, which is used for recovery of the ink carrier. For internal transfers between transmitting stations and the audio signal or other data in this area occasionally transmitted (see sound in sync).

The signal from image sensors also includes black shoulder - shares, which are used in the digitization of the generation of the reference level. The CCD sensor some columns of photocells matrix black are to be covered. Thus, the noise component is reduced by semiconductor effects.

The English name for the front porch is " front porch ". The English name for the back porch is " back porch ".