What Effects Does Frame Rate Conversion Have On Video?

[moviebuff2]

Hi everyone:

I have several AVI files that I want to append. However, these files have different fps (frame-per-second) rates. I plan to covert their frame rates to 29.97, using VirtualDubMod, set at "direct stream".

I wonder if the frame rate conversion will alter the video quality even if I use "direct stream".

Thanks

[jagabo]

VirtualDub won't change the frame rate in Direct Stream Copy mode.

You can use AVIFrate to change the frame rate (without reencoding) but that will cause the frames to flip by faster or slower. Audio will get out of sync.

[Baldrick]

Hi everyone:

I wonder if the frame rate conversion will alter the video quality even if I use "direct stream".

Thanks

If you change the fps a lot it could look strange and the audio will be out of sync.

[jagabo]

Oh wait, if you change the frame rate in the top part of the dialog (source frame rate) it will do the smae thing as AVIFrate -- simply change the frame rate in the AVI header resulting in a file that plays faster or slower with audio out of sync. If you change the frame rate in the middle part of the dialog (conversion) it will do nothing to the frame rate in Direct Stream Copy mode.

[moviebuff2]

Hi jagabo:

VirtualDubMod does.

I selected "direct stream" and the "Frame Rate" was still available whilte other choices, such as "Filters" and "Color Depth" were turnedd off.

I clicked on "Frame Rate", the Video Frame Rate Control" window opened with "Frame Rate Conversion" available.

I did a conversion test, changinf the fps from 25 to 29.97 fps. My eyes could not tell the video difference between the original and converted files. But I don't think I can rely on my eyes only, because what looks good to me on a 20-inch TV monitor may not look good on someone else's plasma TV.

And Balrick, in my test, the audio was still in sync.

What I am trying to find is some concrete measurements.

[moviebuff2]

jagabo, I think you just answered my reply.

Thanks

[jagabo]

And if you recompress the video, changing the frame rate in the middle part of the frame rate dialog will either duplicate occasional frames (to speed up the video) or drop occasional frames (to slow down the video). This will make playback jerky but audio will still be in sync.

[moviebuff2]

OK. I just appended successfully 2 small AVI files. These files had different fps rates (25 and +23). Using VirtualDubMod, set at "direct stream", I changed both fps rates to 29.97 and appended them.

The result is: I have one single AVI file, at 29.97 fps rates.

The audio is still in sync.

The video looks as good as the original, according to my own eyes (and that is very subjective :0).

I wish I could get some concrete numbers.

[jagabo]

In Direct Stream Copy mode frames are not decompressed and recompressed, the compressed data is simply copied from the source to the destination. The contents/quality of individual frames will always be exactly the same.

I repeated some experiments with frame rate conversions in VirtualDub 1.7.2 in Direct Stream Copy mode. Using the middle section of the frame rate dialog, the Convert To FPS item selected and a new frame rate entered, and using Divx and Xvid files with and without B-VOPs, here's what I found:

When converting from a lower frame rate to a higher frame rate (eg, 25 fps to 29.97 fps) VirtualDub occasionally inserted duplicate frames. This creates slightly jerky video. The jerks are most noticeable on smooth scrolling scenes. For example, 25 to 29.97 fps conversion gives 5 small jerks every second.

When converting from a higher frame rate to a lower frame rate VirtualDub appeared to cut GOPs short. For example, a 300 frame GOP might have the last 50 frames cut off to make it into 250 frames (30 fps to 25 fps). If you watch the video closely you'll see the audio starts out in sync. Over the duration of the GOP it slowly drifts out of sync. At the start of the next GOP sync is restored. This is repeated for each GOP. This can create big jerks at the start of each GOP.

Results may differ with different codecs, especially those with all I frames (MJPEG and HuffYUV for example).

[moviebuff2]

Hi jagabo:

Here are some information I could get from VirtualDubMod's File Information. It looks like nothing was changed in the video part, except the number of total frames.

[jagabo]

Here's a sample of a 25 fps video converted to 30 fps with VirtualDub in Direct Stream Copy mode. Note the duplicate frames (all of the original frames were numbered sequentially).

https://forum.videohelp.com/images/guides/p1733599/25to30.avi

Here's a 30 fps video converted to 25 fps in Direct Stream Copy mode. Note the sudden jump in frame number between the end of one GOP and the start of the next:

https://forum.videohelp.com/images/guides/p1733599/30to25.avi