slowing 29.97 Hz audio down to 23.976 Hz???

[Dom Silverio]

You don’t need to slow it down. For all intent and purpose 29.97 audio = 23.976 audio.

[Steven Gonzales]

Just to be clear, audio wouldn’t be at 29.97 Hz.

The sample rate for audio (how often the pressure wave changes, converted to varying electrical signal, are turned into numbers, or digitized) must be at least twice the highest frequency (pressure wave vibration) being recorded. Since humans can more or less hear pressure waves which vary up to 20,000 times per second, the sample rate is above 40,000 times per second, or above 40,000 Hz, or 40 KiloHertz.

Usually it is 44,100 or 48,000 samples per second.

29.97 is a video frame rate. Something vibrating at 29.97 Hz would be heard as a low tone.

Usually, the times you need to change the speed of audio is when something is recorded on film, and the audio is recorded separately. Then, when the film is transferred to video, it is also slowed down, usually by 1/10th of 1 percent, or .1% (because 24 frames per second of film has to be converted to 29.97 frames of viceo per second).

For the audio to stay in sync, it also needs to be slowed down by that same amount (or played at 99.9%, which is the same thing).

[David Roth weiss]

Steven,

Thanks!!! After sixteen hours in the edit bay yesterday I couldn’t think straight any more. As you guessed, I am in fact slowing down audio to sync up with film — its from a project I engineered 20 years ago. Original split tracks are on 35 full coat, which I had xfered at 24fps, but of course now I can’t find that, and so I’m trying to sync the audio ripped from my DVD at 30fps (29.97). I’m sure you get the picture…

DRW

[Steven Gonzales]

I helped someone with mismatched audio and video on a long timeline, and this method was good for learning the speed difference:

Put the long piece of video in the timeline. Somewhere near the beginning, find a distinct sync mark (door slam) and mark IN. Near the end of the video in the timeline, find another sync mark (balloon pop) and mark OUT.

Open the audio in the viewer. Mark an IN at the sound of the door slam, and an out at the sound of that balloon pop.

Place the audio into the timeline using “fit to fill”. They should now be in sync. Now you can look at the properties of the audio from the timeline, and see what the speed adjustment (automatically made) was.

[Herb Sevush]

David –

Simple math says it should be slowed down .799 (.8 would probably work):

29.97 * .799 = 23.946 (which should be close enough).

Herb Sevush
Zebra Productions

[David Roth weiss]

Hey there Herb!!! Well, my friend, good to see at least one of us isn’t senile. My brain was so fried at 12:30 in the morning that even my pocket calulator was beyond my level of comprehension. I swear, I whipped it out to do the math and I couldn’t figure out how to add the percentage of the slow down. Sound Forge works in reverse because the plugin it uses, aptly named “Time Stretch,” which, as you might guess, requires one to calculate the increased running time rather than the percentage of the slow down. I simply was not able to get my brain in gear to handle it at that hour…