The SOF signal, physically transmitted over the AXI4-Stream tuser signal, marks the first pixel of a video frame. The SOF pulse is 1 valid transaction wide, and must coincide with the first pixel of the frame. SOF serves as a frame synchronization signal, which allows downstream cores to re-initialize, and detect the first pixel of a frame. 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.