SCTE 104/35 Codex

An SCTE 35 message that marks a splice point says where a splice should happen, but it does not by itself make the cut clean. Because splicers work in the compressed domain, they can only cut at points that satisfy the MPEG-2 Systems rules and the video coding format: an In Point has to be an intra-coded picture with no dependency on earlier pictures, which means a closed GOP in MPEG-2 or an IDR in AVC. Out Point requirements vary by codec and are set out in SCTE 172. If the stream has no suitable boundary at the marked time, the splicer may move the cut to a nearby valid In or Out Point, depending on its capabilities, so the actual splice can drift from the requested splice_time. The remedy is to condition the encode so structure and signaling line up. An encoder driven by SCTE 104 can reshape the GOP around the splice point to meet In or Out Point rules; scene-change detection tends to start new scenes on a closed GOP or IDR that then serve as natural splice points; and where neither is possible, a short run of black frames around the point limits the visual damage of snapping to the nearest boundary. In time-shifted workflows an Out Point and the following In Point can fall on the same frame, in which case that frame must satisfy both. The takeaway is that frame-accurate splicing needs the content prepared for it, not just a well-formed message.