Is frame rate rounded? (29.97)

[Ohayou]

So I captured a bunch of NTSC VHS tapes and am happy with the results. The frame rate for the files is 29.970030. However when I re-mux the files the frame rate changes to 29.970628. Does the decimal rounding matter at all when it comes to burning the files on a disk? A few files are also 29.970088.

[msgohan]

Remux from what to what?

DVDs and BDs have to be exactly 30000/1001 fps. But the authoring program may simply take a "close enough" rate and rewrite the header with the exact one.

The drift will be equivalent to 1 frame every 1/(file_fps - 30000/1001) seconds. Therefore small deviations require extremely long durations to actually produce a visible difference. Positive values mean a speedup and negative values mean a slowdown from the standard rate.

1/(29.970628 - 30000/1001) = ~1672 sec = 27 min 52 sec
1/(29.970088 - 30000/1001) = ~17232 sec = 4 hr 47 min 12 sec

[Ohayou]

So when it comes down to the decimal place, the different versions I listed should all be fine since if I were to burn the files the header would just rewrite the decimal if necessary as you said?

I would also like to ask if anyone knows of any program that will allow me to re-mux my avi files while keeping the original 29.970030 fps? I've tried MANY such as avi-mux gui, mkvtoolnix and a few others. All change the frame rate decimal place and I'm somewhat picky about not altering the original files. But if any of the frame rates I posted in my first post are all commonly accepted, then I will happy.

[msgohan]

Quote:

Originally Posted by Ohayou

the different versions I listed should all be fine

If the 29.970628 fps files are longer than half an hour, you will begin to notice a difference.

Likely, there will be audio desync. Your remux programs are probably trying to account for an audio track that is shorter than the video by speeding the video up.

In VirtualDub go to File -> File Information and compare the Length of the video track to the audio track's.

[Ohayou]

I just checked and both streams are exactly the same length (each file approximately 18mins each) before and after muxing. So I suppose this should be perfectly fine regardless of the decimal place, as long as they are exactly the same length?

What I'm doing is exporting the audio from my avi captured files, removing noise and muxing them back into an avi. I just wish the frame rate didn't change marginally assuming it makes any difference.

[msgohan]

I wouldn't worry about it for an 18-minute video.

Are you aware that you can do the remux you're suggesting using VirtualDub, if your noise-removed files are still in uncompresssed WAV format? (And if they're not, that could be part of the problem.)

Audio -> Audio from other file... You didn't mention trying it, so perhaps that will keep the same frame rate.

[Ohayou]

I literally just realized I could do that the other day and it works perfectly! As you mentioned I'm simply running a single pass of de-click and not altering the duration. The duration and fps are left completely untouched.

Though when I look at the file in MediaInfo there is a new line added titled:

Interleave, preload duration: 500 ms

Not sure what this means. Thank you very much for all your help thus far.

[msgohan]

I don't know the precise technicalities of interleaving, but that preload duration option is found in Audio -> Interleaving in VDub. 500ms is the default there.

[Goldwingfahrer]

In VDub but stands
Interleaved audio every = 1 Frames [40ms for PAL]

In MediaInfo is also 40 ms

see my picture

[Ohayou]

Here is an example of the changes that happens when I replace the audio using VDub:

Original:

Audio
ID                                       : 1
Format                                   : PCM
Format settings, Endianness              : Little
Format settings, Sign                    : Signed
Codec ID                                 : 1
Duration                                 : 18mn 15s
Bit rate mode                            : Constant
Bit rate                                 : 1 536 Kbps
Channel(s)                               : 2 channels
Sampling rate                            : 48.0 KHz
Bit depth                                : 16 bits
Delay relative to video                  : 1ms
Stream size                              : 201 MiB (13%)
Alignment                                : Aligned on interleaves
Interleave, duration                     : 10 ms (0.30 video frame)

Muxed:

Audio
ID                                       : 1
Format                                   : PCM
Format settings, Endianness              : Little
Format settings, Sign                    : Signed
Codec ID                                 : 1
Duration                                 : 18mn 15s
Bit rate mode                            : Constant
Bit rate                                 : 1 536 Kbps
Channel(s)                               : 2 channels
Sampling rate                            : 48.0 KHz
Bit depth                                : 16 bits
Stream size                              : 201 MiB (13%)
Alignment                                : Aligned on interleaves
Interleave, duration                     : 33 ms (1.00 video frame)
Interleave, preload duration             : 500 ms

Is this perfectly fine, or would you recommend an alteration?

[msgohan]

Seems fine to me.

[lordsmurf]

This is often an encoding error or quirks in the source.
The format matters. I'm guessing this is MPEG? Or is it something else? (And from a retail source/disc?)

[Ohayou]

This was an avi/XviD (MPEG based) capture.

Though interestingly enough when I installed my HDD and booted my OS there was some scan done on the drive to check for errors or something. And when looking at the files the frame rate changed on the raw captures from .970030 to .970029. I'm extremely confused as to what just happened.

[msgohan]

The frame rate is stored in the header as a fraction, not a decimal. Some applications will round 29.97002997... to 29.970030 while others may just truncate. Some will let you see the actual fraction.