sync_audio=true with AssumeFPS in AviSynth?

x_rex30 · Post by **x_rex30** » Mon Jun 05, 2006 4:35 am

is this even that effective to use? When I converted a video to 23.976 for DVD and added the sync_audio=true to my script.. it didn't seem to really do anything good but I guess it would be hard to notice anyway.. but also when I used it, when I played back my video the audio didn't seem to fit as right... when I don't use it I seem to be less paranoid that something is wrong. Is this even that important to use? If so please explain.. I don't have much technical info about sync_audio.

Zarxrax · Post by **Zarxrax** » Mon Jun 05, 2006 11:06 am

That parameter will stretch your audio according to how the length of the video stretched. You generally do not want to stretch your audio, because it changes the pitch. If you want to change fps without effecting an audio stream, you should generally use ChangeFPS() which will add and remove frames as necessary to change the framerate without changing the length of the video.

x_rex30 · Post by **x_rex30** » Mon Jun 05, 2006 9:52 pm

That is very helpful Zarxrax. Thanks for the feedback. ^_^

x_rex30 · Post by **x_rex30** » Mon Jun 05, 2006 10:00 pm

So there is no chance in the audio not synching to the video if I use this right? I hope ChangeFPS() is better for making AMVs for DVD than using AssumeFPS. So this shouldn't F things up if I change a 24fps or 29.97fps AMV to 23.976fps for DVD.. I hope not.. more feedback is appeciated if it's needed.

Qyot27 · Post by **Qyot27** » Tue Jun 06, 2006 4:06 am

x_rex30 wrote:So there is no chance in the audio not synching to the video if I use this right? I hope ChangeFPS() is better for making AMVs for DVD than using AssumeFPS. So this shouldn't F things up if I change a 24fps or 29.97fps AMV to 23.976fps for DVD.. I hope not.. more feedback is appeciated if it's needed.

Why would you change a 29.97fps video into 23.976fps? That is definitely going to lead to frames being dropped.

Another way to 'fix' the audio so that the frequency isn't off when using sync_audio with AssumeFPS is to insert an SSRC(desired frequency) command right after. That resamples the frequency to 44100 or 48000 - whichever you choose - so that it won't screw up when you pipe it into an encoding program. For extra precision, you can set it up like:

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AssumeFPS(23.976, sync_audio=true)
SSRC(48000, fast=false)

Since 'fast=false' activates the HQ mode.

Willen · Post by **Willen** » Tue Jun 06, 2006 6:11 am

IMO, there is no reason for converting 29.97fps video to 23.976fps for DVD. The possible space savings isn't worth dropping frames (ChangeFPS) and you definately shouldn't slow down the framerate (AssumeFPS) since it involves slowing down the audio and may result in noticable pitch changes. Just converting the FPS (ConvertFPS) is hardly ideal since it will possibly add unwanted artifacts or blurring.

Now, if you are putting 25fps PAL/SECAM video onto a NTSC DVD (as 23.976fps), then AssumeFPS the way Qyot27 suggested is the way to go. This also applies to the occasional 24fps video you will run into (damned editing programs that don't full support 23.976fps...

) as long as you only convert to 23.976fps. Any slowdown in audio is minor to the point that I don't worry about it.

Now, I feel sorry for the people that need to properly convert 29.97fps material to 25fps PAL/SECAM.

x_rex30 · Post by **x_rex30** » Tue Jun 06, 2006 6:37 am

I just thought 23.976fps is the only thing progressive when it comes to DVD so I thought 29.97fps will have something mess up when put on a DVD if the video doesn't have interlacing.. So that's why I thought it would be necessary to convert it to 23.976. Most of my conversions are for videos that are 24fps since that is almost the framerate to all the AMVs I make.

So I shouldn't convert my 24fps amvs to 23.976 when bringing them to DVD? And if I am to do that my best bet would to put this script in..

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AssumeFPS(23.976, sync_audio=true)
SSRC(48000, fast=false)

?

Just trying to be 100% sure mainly on what I should do with videos that are 24fps when putting them on DVD.. I have a DVD download available for my latest video in the video profile and it seems to look fine for the most part.. maybe I'll have to swap that if there is any problems with that one.. I put a 24fps Lagarith in Vdub with an additional Code when I rendered the file for DVD output witch is..

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AssumeFPS(23.976)

Does that mean the audio doesn't synch exactly the way it did before since I changed the FPS? Maybe I'll have to redo making a DVD for that video.. not sure on that. Any more feedback is appreciated.

Willen · Post by **Willen** » Tue Jun 06, 2006 6:56 am

x_rex30 wrote:I just thought 23.976fps is the only thing progressive when it comes to DVD so I thought 29.97fps will have something mess up when put on a DVD if the video doesn't have interlacing.

DVD supports Progressive Video (29.97fps) and Progressive Film (23.976fps), although technically they are both encoded as interlaced MPEG-2.

http://dvddemystified.com/dvdfaq.html#1.40

Progressive-source video (such as from film) is usually encoded on DVD as interlaced field pairs that can be reinterleaved by a progressive player to recreate the original progressive video.

Basically, framerate has no impact on progressive or interlaced encoding. As long as your final framerate is either 29.97fps or 23.976fps for NTSC (or 25fps for PAL/SECAM), you'll be OK.

So 24fps video should be converted to 23.976fps. I think most DVD authoring programs/encoders will do the conversion, I wouldn't trust them to do it well (I suspect they will drop frames to do it). So using the code listed above in your script would be my recommendation.

x_rex30 · Post by **x_rex30** » Tue Jun 06, 2006 7:04 am

Willen wrote:So using the code listed above in your script would be my recommendation.

Umm.. just to be sure you mean this one..

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AssumeFPS(23.976, sync_audio=true)
SSRC(48000, fast=false)

or this one..

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AssumeFPS(23.976)

?

Or is there a better recommendation..?

or... or.. srry.

Just making 100% sure what your talking about.

Willen · Post by **Willen** » Tue Jun 06, 2006 7:10 am

If you want to be absolutely sure, or as sure as you can about software, this one:

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AssumeFPS(23.976, sync_audio=true)
SSRC(48000, fast=false)