TechNote

Enhanced Seek appears not to work with Flash Communication Server 1.5 and VP6

Issue

The enhanced seek option in Flash Communication Server MX 1.5 appears to not work with FLVs encoded with Flash 8 / VP6 video codec.

Reason

Like most video files encoded for the web, FLV video is composed of key frames (I) and Prediction frames (P), arranged in a sequential order such as this:

I - P - P - P - P - P “ I - P - P - P - P - P “ I - P - P - P - P - P “ I - P - P

Keyframes are a complete video frame encoded without reference to other frames. Prediction frames, which are bookmarked between Keyframes, contain much smaller amounts of data and are encoded as a difference from the previous I or P frame.

When a user seeks into a video by moving the playhead, there's a good chance that the playhead will land on a P frame. Because P frames do not contain all the information needed to display the video, in traditional video systems the user must wait a short period of time until the next keyframe is available before the video can start being displayed. Do you ever wonder why when you switch the channel on your cable TV that you get a black screen for a small amount of time before the next channel starts? Thats because you landed on a P frame.

FCS contains a unique feature called Enhance Seek that enables you to instantly play the video even if the user seeks to a P frame. If the user lands on a P frame, FCS will very quickly and in real-time encode and deliver a new I frame at that point so the user can instantly start displaying the video.

In FCS 1.5 this feature works great for FLVs encoded with the Spark codec. However, if you play an FLV encoded with the VP6 codec with FCS 1.5, enhanced seek will not work because FCS 1.5 did not have the VP6 encoder built into it when it was released. While enhanced seek doesn't work in this case, it doesn't mean that video playback will fail or even that the end-user will have a bad result. The end result is that video will start playing at the next available I frame.