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Submitted by justin on Thu, 30/04/2009 - 10:13
In our development of AVCHD Lite support for all ShedWorx products, we've come across a potentially confusing area in regard to the video format used in the new Panasonic Lumix AVCHD Lite cameras.
The AVCHD Lite format for these cameras is quoted as '50p recording' but with a disclaimer that the CCD records at 25fps.
Some users might be led to believe that the 50p format (60p for NTSC) will give them 50 frames per second. This is not the case. 50p is a signal setting stating that AVCHD Lite from these cameras will play back at 50 fields per second, from a captured video of 25 frames per second.
How does this work? Well, they provide a 50p output signal by repeating each frame's display twice.
Why would they do this? We can only assume that the 50p signal may look better on some displays (this was not that case in our tests). As an example, in non-digital cinemas, the films are provided as 24 frames per second but the projectors run at 48 cycles per second to reduce flicker.
We can tell you that there are 25 frames per second in the H.264 video stream within a AVCHD Lite video.
For now, we are converting AVCHD Lite to 25fps output (both WMV on the PC and AIC on the Mac). We have tested this approach by taking our converted AIC footage, editing in Final Cut Express, then exporting to AppleTV for playback. We've then examined the video quality by:
- playing back direct from our Panasonic DMC-FT1 via HDMI to the HDTV (Sony Bravia 46")
- copying the raw AVCHD Lite files to a PS3 and playing back
- playing back the Final Cut edited movie through an AppleTV
We have seen no noticeable difference between any of these playback options, making us confident that going from 50p to an (effective) 25p signal, still preserving the 25 frames per second that was recorded, has no effect on video quality.