Following recent discussions we have introduce Matroska v4. It is backward and forward compatible with previous versions. New elements: - CueDuration and CueRelativePosition so that matching subtitle blocks can be retrieved easily when seeking - TimecodeScaleDenominator which gives the Segment timecode in seconds with the fraction TimecodeScale/TimecodeScaleDenominator. The value 1,000,000,000 ensures that the regular TimecodeScale values have the same meaning as before. Deprecated element: - TrackTimecodeScale which was meant to combine tracks from different timecode clocks (like PAL audio with NTSC video) but is bogus because the Block timecodes have to be stored in sync with matching timecodes. It shouldn't be up to the player to stretch audio or video time so that they match. In the case of stretching audio that's quite tricky. CueBlockNumber may be another candidate for deprecation now that we have CueRelativePosition which is more convenient and faster. I'm not sure it's used at all when writing or by any player.