If both quality and bitrate are set, libtheora will try to meet both constraints, causing it to prefer emitting a smaller number of good frames, to emitting the full number of frames that would not meet the requested quality. This causes a slideshow effect when the bitrate is low and the quality is high. And the default theoraenc is high (48/63). So only set quality when it is requested, and leave it unset otherwise. https://bugzilla.gnome.org/show_bug.cgi?id=658443