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Constant frame rate?


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I have version 3.61 for Windows, running on Windows 7.


Everything works fine, but when I export a sequence to file, the dialogue box specifies a MAXIMUM frame rate. My original clips are 1080p, recorded at a fixed frame rate of 25 fps. If I specify 25 fps (PAL) for the output file, the resulting video, regardless of format (e.g. AVI, MP4) or codec (e.g. H.264, mpeg-2), will be written with a VARIABLE frame rate. Mediainfo will report the video as having a frame rate of anywhere between 8 and 25 fps, with an average of 23.8 fps.


Now, upload sites such a Vimeo, have a preference for video files to be recorded with a CONSTANT frame rate. Regardless of this, I wish for my output files to have a constant frame rate, matching the recorded clips. My question is therefore...


How do I export a video file with a specified constant frame rate rather than the variable rate that Videopad seems to produce?



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Thanks for the reply. I will certainly download that program. However, I still want to know if Videopad will export constant frame rate or not, and if not, why not?


Also, if my recorded clips are at a constant rate of 25fps, what maximum rate should I set videopad to export?


Finally, what is the advantage of variable rate? Is it just so it can record fewer frames per second for slowly or non changing segments of video?

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I seem to have found the cause of the variable frame rate. If I add blank clips to my video sequence, Videopad renders the output file with a VFR. No blank clips = CFR. However, this is not ideal as many videos have intro and credit screens. The option to request a constant frame rate is definitely needed.


There is a second, more critical issue though, which I originally thought was due to the VFR issue. Adding any form of video effect to a clip in the sequence (e.g. brighness, hue, etc.) causes multiple frame drops and frame duplication in the rendered file. This is a major problem which needs to be addressed. I have not found any way of stopping this which is making Videopad unusable at the moment. I cannot have a situation where I get three frames repeated in a row, followed by two dropped frames. This causes terrible stuttering in the output file.


Is there any way around this problem??

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Tested two clips with which I am intimately familiar, using a licensed version of 3.70 for Windows.

To the first I added both a hue and brightness effect.

The adjacent clip (same track) was posterized.


After being produced to both Mp4 and WMV files, no artifacts, dupe or dropped frames were seen in either.


Please list the specs for your PC: CPU/GPU/disc space, etc. Latest drivers installed?

Video editing is resource intensive, and it's possible that your machine isn't keeping up. It's wise to shut down all other programs while editing.

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After considerable experimentation, I can confirm that there is definitely an issue with the exported file rendering under certain, specific conditions. This is not due to any limitations of my PC (which should anyway surely affect only the speed of rendering, which is algorithm driven, not the actual output coding).


I used a short, 2 second clip, both 720p and 1080p, mp4, with continuous movement within the clip.


I tested your assertion, and agree that the output file showed no artifacts, duplication or dropped frames ,when the brightness or hue effect was applied across the whole clip, with no use of keyframe edit points within the video effect editor.


However, as soon as a keyframe edit point is introduced within the clip, the issue of dropped/duplicated frames re-emerges. This edit point can exist anywhere within the edited clip. I even introduced an edit point at the start of the clip, with no actual change in brightness/contrast/hue etc., and this produces the same issue. The removal of keyframe edit points removes the problem.


I am not certain of the reason for this, but my investigations indicate that there is an apparent issue with frame synchronisation within the editor. When a clip is selected and put into the video track, it does not match exactly the selected clip in the clip source window. I set my start and end points of the clip exactly at start of a frame, i.e each frame lasts for exactly 40 ms (it is a 25 fps video). However, in the video track (and preview window), the first frame only lasts for 1 ms before it switches to the second frame. Each successive frame lasts for 40 ms, but the last frame is duplicated in order to produce a preview clip equal in length to the original clip. I do not know why this is done this way. However, if the effects editor is opened for the clip on the video track, it shows each frame in the correct place - frame 1: 0-39 ms; frame 2: 40-79 ms and so on). Could it be that setting keyframe edit points in the effects editor is causing confusion because of this lack of frame synchronisation?


Some form of confusion is definitely occurring because the output files are very different, with the minor addition of a brightness keyframe edit point (which does not even change the brightness of the clip). I have included below the extracts from the two output files (with and without edit points) from MediaInfo.


File with edit point included (note switch to variable frame rate, due to frame duplication in output file, plus extra frame due to duplicated final frame in the editor - original clip is exactly 2.00 s long):


Codec ID : avc1

Codec ID/Info : Advanced Video Coding

Duration : 2s 40ms

Bit rate : 18.3 Mbps

Width : 1 280 pixels

Height : 720 pixels

Display aspect ratio : 16:9

Frame rate mode : Variable

Frame rate : 19.608 fps

Minimum frame rate : 12.500 fps

Maximum frame rate : 25.000 fps


File with no keyframe edit point (note constant frame rate, plus correct file length):


Codec ID : avc1

Codec ID/Info : Advanced Video Coding

Duration : 2s 0ms

Bit rate : 20.6 Mbps

Width : 1 280 pixels

Height : 720 pixels

Display aspect ratio : 16:9

Frame rate mode : Constant

Frame rate : 25.000 fps


To summarise, you should be able to replicate this by doing the following:


1. Take your selected video clip

2. Open the effects editor

3. Add a keyframe edit point at any point in the clip

4. Export to mp4, H264 (I have tested mostly with this format and codec, but have observed same issue in others)

5. Open exported file in VP, and step through frame by frame. Compare to frame step through in original file.


Finally, I also noticed that even with a non-problematic video track source, the insertion of a blank section into the video track results in a switch from a constant frame rate to a variable frame rate (lowest frame rate is a 1/3 the requested frame rate). I imagine that the the H264 or mpeg-4 codec implementation is causing this automatic switch to minimise file size. The use of the older mpeg-2 codec forces a constant frame rate.

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In both clip and preview windows...


Original clip: 15:067 at 30ff NTSC

In VP 3.70 it is listed as 15:065


Mp4 (H264) exported to both PAL (25ff) and NTSC (29.97 drop frame) with one brightness effect keyframe inserted...

PAL 15.079

NTSC 15.081


In every instance - original, PAL, NTSC the frames incremented by 040 when brought into VP, including the first frame.

(I don't know what "40" indicates as it doesn't match the frame rate.) There were no glitches throughout.


Also exported another NTSC Mp4 that has hue, scale and rotation effects. http://hevanet.com/hb/frame_test.mp4

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I haven't, as yet, been able to check your findings, as you are using a 30 fps source file, which I do not have (I record at 25 fps, and any attempt at conversion produces incorrect frame rates). Also, I am not sure of your source file parameters: you say 30 fps NTSC, but what is the resolution? Is it 720p or original low definition NTSC TV?


I am not performing any frame rate change at all in my export. My source file is: 25 fps (PAL), 720p, mp4, H264. (This is either an original file from my camera, or a VP produced file. In both cases I have checked in VP for dropped/duplicated frames and there are none.


I am exporting to: 25 fps (PAL), 720p, mp4, H264 (quality setting 15). As you can see, no change in file parameters.


No key frames -> no frame duplication, no frame drop, constant frame rate at 25 fps.

With key frame -> frame duplication/drop, variable frame rate (one duplicated followed by one dropped frame every 4 frames).


I have also repeated this experiment using a 25 fps (PAL), 768x576 (PAL TV), mp4, H264 source file with exactly the same results.


At 25 fps, the increment in VP of 040 refers to an increment of 40 ms between frames, which is correct. I do not know why you get the same increment for a 30 fps recording, as this corresponds to a frame period of 33.3 ms (033 increment).


Can you check file rendering with my video file parameters?

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I'm having the same problem here. I'm using v3.72. I Rendered file in 30fps, but the properties>details section shows it as 21fps and it does not play at the desired speed. I've tried re-exporting it using Qicktime pro, but no luck!


I might need a refund :angry:

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As this is a YouTube conversion of your file, it's not likely to dupe your results. The download lists...

Video: MPEG4 Video (H264) 1280x720 30fps 1033kbps [V: h264 high L3.1, yuv420p, 1280x720, 1033 kb/s]


Loaded it into VP 3.72, split it, added hue effect to first half and posterize to second half.

No discontinuity or repeated frames observed.

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I don't think that HarryG is suggesting that he is having a problem with dropped frames or duplicated frames. He is confirming that when using the H.264 codec export option, that it is impossible to ensure a constant frame rate. Addition of blank clips, or stationary images to a sequence results in the exported file switching to a variable frame rate in an effort to minimise file size. Blank sections, for example, are rendered at a third the requested frame rate (e.g. at a requested rate of 25 fps, the rate is 8.33 fps). On average across the file, the frame rate is then less than that requested. This is the reason, I suspect, that with HarryG's animated film, the stated frame rate is significantly less than the requested frame rate. Since there are fewer frames, when it is played back at the stated rate of 30fps, it plays too fast. One way around this is to export using the MPEG-2 codec (which is industry standard for movie recordings). It isn't an option with the mp4 format, but can be accessed with other formats (e.g. AVI). This does export at the requested constant frame rate. The trouble is, sites like VIMEO prefer the H.264 codec. Alternatively, could use the Handbrake conversion tool.


However, back to the second problematic issue that I have reported, I have found the following, repeatable, issues. Please note that this is with a 25 fps, mp4 source file, NOT a 30 fps source file. This could be why it is not being seen with 30 fps source files.


1. Use of keyframe edit points within the video effects editor causes frame duplication/drop in exported files. This occurs, whatever exported format is used. I confirmed this by stepping through the exported file frame by frame, after reimporting into VP. This problem does not occur when a clip has a video effect applied across the whole clip without the use of keyframe edit points. The problem only occurs within the clip that that has had the keyframes added.


2. If cut points are used within a source clip which do not correspond to a correct frame transition point, frame duplication in the exported video can occur either at the end of the first clip, or at the start of the second clip. E.g. for a 25 fps file, the frame transitions occur at 0.040, 0.080, 0.012 second etc. Since the cursor can be controlled down to a 0.001 s resolution, it is possible to cut a clip at positions not corresponding to the frame transition point (e.g. at 0.095). Until I re-cut my source clips at the correct points, I had this problem. It is due to the fact that short frames, less than the full frame time period, are created at the start and end of the cut clips, creating a lack of synchronisation between the two clips in the sequence (it is easy to see this in VP by zooming right into the cut location in the sequence. VP should force the cut point to correspond to a frame transition point, to avoid this problem. Note that I have seen that this occurs when two different clip segments from different source files are joined in the sequence. It may not be seen if a clip is simply cut in two.


If this could be confirmed as an issue, within the parameters I have outlined above, it would be useful.

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I have uploaded example exported file to demonstrate the issue. It is on YouTube at:



The keyframe duplication/dropped frame issue can be clearly seen in the video which consists of two identical clips. The first clip has a keyframed hue effect added, the second clip is unedited. First clip clearly stutters. Second does not. You will have to download from YouTube to import into VP to check frame by frame.


Interestingly, MediaInfo reports that at certain points in the video, it is exported at a half frame rate of 12.5 fps, rather than the requested 25 fps, resulting in an overall frame rate of 22.5 fps. I guess that that is due to the duplicated frames.

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