Hi, thanks for the quick response. The playback problem is very repeatable (on files with the issue – not all have it). Taking e.g. “F1 Singapore Grand Prix Live_20170917_1233.ts” the first advert bookmarked from 14:18 to 18:02 (copying the cropped recording to my PC and playing with VLC, the timecode skips from 14:13 to 17:58).
Very repeatable is (1) Select program, (2) Play from start, (3) Fast forward to just before the 1st cropped ad-break, let it play through it and on for about another 5 minutes (i.e. longer than the ad-break length). Pressing the instant replay button |<< (mine is set to 7 seconds) the program will rewind to a point which was at 21:05 in the original file (so ~17 min 20s into cropped). Now letting it play for 1, 2 3, up to about 8 minutes, the instant reply button always returns to exactly the same place. Also, whilst playing, the front panels stays on “PLAY 00:17” and the green progress bar also sticks on 17 minutes, until about 8 minutes later it skips forward in one big jump to the mid-20s (just the timecode I mean, actual program playback is fine).
I renamed the .nts file and rebuilt with sidecar but this failed (the resulting .nts was much smaller than the original, it only took 5 mins to do (program is 3 hours so should take ~30 mins, and it does estimate that's how long it will take), and now playback stops at the first cropped adbreak. So sidecar seems unable to rebuild the .nts from this cropped .ts (I tried twice).
For fun I recorded and cropped a “Frasier” (just happened to be the next prog on 4HD) to try all this on another file, again VLC skips the timecode but sidecar was able to rebuilt the .nts successfully (also this recording doesn’t suffer the skipping issue - not sure if that's related), so it seems sidecar has an issue rebuilding the F1 but not the Frasier (so not a generic issue with cropped .ts).
Happy to try more things if you need. I think the F1 was recorded with the "chaserun" option and CPU limited to 50%.