More about digital video post-production than I really wanted to know.  While editing ISSS video, I discovered a small bug in the export to the Openshot profile for HD 720p 29.97 fps. The audio was fine in the master, but had pumping sounds in the downsampled result.  The original audio was AAC, so why does it get converted to AC3, to get converted back to AAC?

I found a helpful forum post on the MP4 container.

The official mp4 standards does not include AC3 as an intended audio stream in the mp4 container. You are creating a non standard file when you include 5.1 AC3 in an mp4 container. The PCH unlike your computer are designed according to official standards which for the mp4 container are:

Video: MPEG-4 Part 10 (or H.264, also known as MPEG-4 AVC), MPEG-4 Part 2, MPEG-2, and MPEG-1.

Audio: AAC (also known as MPEG-2 Part 7), Apple Lossless, MP3 (also known as MPEG-1 Audio Layer 3), MPEG-4 Part 3, MP2 (also known as MPEG-1 Audio Layer 2), MPEG-1 Audio Layer 1.

Handbrake: MP4 – AC3 Passthru = No Audio by “Spawn” at http://www.networkedmediatank.com/showthread.php?tid=12602#pid106960.

I have reported this bug at https://bugs.launchpad.net/openshot/+bug/1032780

Openshot Video Editor