The Start-Of-Frame (SOF) signal, physically transmitted over the AXI4-Stream TUSER signal, marks the first pixel of first incoming video field at slave/input side and first pixel of every outgoing video frame at the master/output side. Every incoming interlaced Video Frame represented by two video fields, which are odd line and even line video field. The SOF pulse is 1 valid transaction wide, and must coincide with the first pixel of the frame. Refer to This Figure.
SOF serves as a frame synchronization signal, which allows downstream cores to reinitialize, and detect the first pixel of a frame/first field. The SOF signal can be asserted an arbitrary number of ACLK cycles before the first pixel value is presented on TDATA, as long as a TVALID is not asserted.
