摘自维基：http://en.wikipedia.org/wiki/Back_porch
Back porch refers to the portion in each scan line of a video signal between the end (rising edge) of the horizontal sync pulse and the start of active video. It is used to restore the black level (300 mV.) reference in analog video. In signal processing terms, it compensates for the fall time and settling time following the sync pulse.[1][2]
- In color TV systems such as PAL and NTSC, this period also includes the colorburst signal. In the SECAM system it contains the reference subcarrier for each consecutive color difference signal in order to set the zero-color reference.
- In some professional systems, particularly satellite links between locations, the audio is embedded within the back porch of the video signal, to save the cost of renting a second channel.
附图解释一下Back Porch的具体位置：
