Playback frame rate oddities?

BillyBoy wrote on 5/17/2003, 8:33 PM
Maybe I just forgot, or I never noticed but...

Just doing some more housecleaning. Took a FINISHED DV NTSC file created in Vegas, was already rendered as MPEG-2, weeks ago, popped it on the timeline changed NOTHING, played it on an external monitor and Vegas shows the red warning in the preview window that it is recompressing frames and sure enough, the frame rate is well below the 29.97 rate.

Played the same project that was rendered using the DV NTSC template, but saved as AVI again using the NTSC DV template. Played it back on the external monitor, no red warning message in red saying frames are getting recomressed, just solid 29.97 frames per second. The question is why the difference in frame rate? Aside from the fact one is MPEG-2 and other AVI, the files are identical.

Remember these are FINISHED projects, changed nothing, the original VEG files is gone, did nothing but drop them on the timline, then played them. Did I forget about something or what?

Comments

Paul_Holmes wrote on 5/17/2003, 9:40 PM
From my understanding of mpg files there are mysterious things called I frames, P frames etc. Vegas has to find the first "I?" frame (I really don't know) and then each succeeding so many frames contains the information that is different from that first frame. So Vegas or the decoder used has to do a lot of overhead to convert the mpeg file to a series of complete frames.
SonyDennis wrote on 5/20/2003, 10:43 AM
"Aside from the fact one is MPEG-2 and other AVI, the files are identical. "

Yea, sort of like how a schoolbus is identical to a BMW, except for the seating capacity.

That little red "frame recompressed" flag means a lot.

Here's what happens when AVI / DV is played to external monitor, per frame: Just over 100K of data is read from disk and pushed out the Firewire port.

Here's what happens when MPEG-2 is played to external monitor, per frame: Some amount of data is read from disk (an average of 33K for 8 mpbs MPEG-2). The frame is decompressed into an image over 1 megabyte in size. This image is compressed using the Vegas DV codec into about 100K of data, which is then pushed out the Firewire port.

Over ten times as much data is processed for MPEG-2 versus AVI / DV, including costly decompression and recompression steps.

MPEG-2 is typically not considered an efficient source format, although with cameras like the JVC HD1 and Sony's microMV on the market, this is changing.

///d@
kameronj wrote on 5/20/2003, 10:49 AM
Yea, sort of like how a schoolbus is identical to a BMW, except for the seating capacity.

I'm sooooo diggin that!! :-)