Black shoulder

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In analog television and video technology, the black shoulder is the section in front of and behind the horizontal sync pulse in the FBAS or television signal . It is primarily used to regulate the gain of the image signal.

FBAS standard bar sequence: the image signal of a line. 3 shows the front porch,
6 the rear. 4 is the sync pulse. More information in the article TV signal .

In order to be able to transmit analog television images to the viewer, these are divided into lines and transmitted one after the other like a text. A sync pulse is transmitted at the end of a line so that the receiver can recognize when a line has ended. This is enclosed by the front and rear black shoulder. This gives the electron beam enough time in the picture tube to move to the beginning of the next line.

There is no image information in these areas of the signal; the signal level corresponds to that for black. This level serves as the basis for calculating the gain factor for the image signal. The aim is to get an image with optimal values ​​for brightness and contrast regardless of the strength of the signal received by the antenna. The entire picture signal cannot be used for this, because a weak signal can indicate both a weak transmitter and an (intentionally) dark picture. The porch, however, enables the signal strength to be recognized regardless of the image content. For this, a so-called keyed control is required, which only works at the point in time when the black shoulder is transferred.

With the PAL signal, there is also the PAL burst on the back porch , which is used to recover the color carrier. In the case of internal transmissions between broadcasting stations, the audio signal or other data is occasionally transmitted in this area (see Sound in Sync ).

The signal from image sensors also contains porch parts, which are used to generate the reference level during digitization . In the CCD sensor , some columns of the black photocell matrix are covered. This can reduce the noise component due to semiconductor effects.

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

literature

  • SRT training manual for audiovisual media professions, pp. 278–280, Hüthig, 4th edition, ISBN 978-3778528532
  • The basics of television technology: system theory and technology of image transmission, Springer, Berlin, 1st edition