Leaving A at zero, setting B at the first keyframe after the end of the leading commercials and hitting "Del" should work (try this first), but might fail to produce a cleanly playable video due to a not yet understood bug, which results in the first keyframe after the cut being lost. To avoid this, set the marker A to the first keyframe of the video instead, which is often not equal zero.
I assume you use at least the latest release 2.6.15.
Thank goodness someone has finally stated this is a bug, I have experienced sections of black video from the first I-Frame to the second I-Frame, where the video then begins playing. There is also a case (which may or may not be regarded as nit picking
) with DVB-T HD where, as far as I can see, moving to an I-Frame, goes to a frame two frames after the real I-Frame. This can easily be seen by moving to an I-Frame closest to a scene change, then moving three frames back and the scene will then change. It could maybe be argued that maybe the I-frame is sometimes not on the scene change but I think in that case, it is impossible that every I-Frame would be exactly two frames after a scene change, every time.
I have also noticed that some mp4's previously edited a while ago with Avidemux, now play as all black. A quick fix I have found is to import them into 'Machete'
and export them without any changes. This is not a quick plug for 'Machete by the way, it is a much less responsive editor than Avidemux and will drive you mad because of this, if you have already used Avidemux. It does fix this annoying problem though.
I have yet to try Avidemux v2.6.16 and as the changes include, "Fixed keyframe detection", this may be already old news as I type.