As I pointed out in a previous post titled “Joost video quality is disapointing” Joost video quality was lacking when compared to other sources (like Stage6).
Since then I have been talking with a few people at Joost and other beta testers and found out that Joost is actually using much less bit rate than I had originally expected. My original expectations where that Joost was encoding their video at 750kbps. I was quite a way off the mark.
Turn out that Joost is more in the range of 390kbps max average bit rate… for both audio and video stream. Based on Joost audio quality I have to guess that they use at least 96kbps AAC audio (and probably 128kbps). This leaves a meager 284kbps (or less) for the video! WOW! No wonder some of the video on Joost are looking blocky…
But there is some light at the end of the tunnel. Apparently Joost is going to encode video as VBR (Variable Bit Rate) instead of the previously used CBR (Constant Bit Rate) method. But this won’t be for a little while since previous clients (0.10.x) do not properly support VBR encoded streams. All users have to migrate 1st to the 0.11.x series.
The results are quite dramatic. I have captured the same scenes I did in my previous post to give you an idea. The screen captured do no give justice to the actual quality difference. It is best compared while watching the live video:
I did some test encodes to see if I could get to the same quality as Joost given such a low bit rate… and let me tell you, it was not easy. What I have learned in the process is that to get to equivalent quality using h264 codec at 294kbps for the video you have to make sure to use a high key frame interval… in the range of 1 key frame per 2000 frames. The standard key frame rate of 1 per 24 frames was making the overall video jerky.
Cheers to the Joost video encoding team! You are certainly doing miracles with such low bit rate!