There are two major problems with audio either it is stretched
or displaced. This can be illustrated thusly:
When the synch is perfect both the audio and video will
match, as seen in the first image above. Video frame1 matches to Audio
frame 1 as does frames 2, 3 and so on. In the second picture it shows
the audio as displaced, which means that both audio and video are correct
length, but either the video or audio will start at different times
causing displacement. In the third sinerio we have an audio and video
sequence that are different lengths because one of them has become stretched
somehow.
To fix these problems you must check out the film very
carefully. If the audio is equally out of synch at the start of the
movie as it is the end of the film, then you have an Audio Displacement
problem (see Fixing Audio Displacement below). If the audio is very
close to correct at the beginning of the movie, but is quite badly wrong
at the end of the movie (or vise versa) you probably have a Stretched
Video problem (see Fixing Stretched Audio below). It is possible to
have both problems of course ;-P. So if you are sure you have both I
suggest you fix the Audio Displacement at the very start of the film
first and then fix the Stretched Video problem once you know somewhere
in the film is absolutely correct!
Stretched audio is the most common reason a video will
be out of synch with a video. Luckly the first release of the Terabit
AVI Info tool is now ready :). Designed by yours truly and programmed
by my own brother - all credit goes to him, though, for all his hard
work. Terabits AVI Info tool is designed to make manual synching much
easier and also to make easy some of those little things you'd like
to do but do not know how. I will explain these other features at the
end of this synching article though, so we can stay on track.
Before we do anything I'd always recommend that you keep
a back up of your original AVI file in case something goes wrong. Once
installed you do not run AVI Info like most programs, it is more like
an extension to Windows. Just find the AVI file you wish to synch and
right-click with the mouse. Up will pop an option saying AVI Information
- select that.
Note: For Windows ME or
2000 users the right-click option currently does not work. As
a temporary work around for this you can use the Launcher.exe
application bundled with this download to open your AVI files.
Alternatively, you can use this replacement clickme.reg file to try and solve this problem under Windows ME or 2000. Special thanks to Jimbo for submitting this tip.
Then the following box will appear. It is a convenient
way of finding out all those little bits of information that neither
Windows Media Player nor VirtualDub are tell us about our AVI files.
Click on the Synchronize Audio
tab and we get a preview of the video we want to correct, and the important
part, a control pannel designed to correct the synching.
Examine your movie carefully using the AVI Player.
Preferably very close to the end of the movie to better see how
far the audio is out. And also be sure to find a section where
their mouths are large enough to see clearly. Locate the frame
number of a part just before they are going to speak and put it
in the 'From' box below (1).
Then take note of the frame number from after they stop talking
and enter that into the 'To' box
(1). Great, now the movie
will loop between these two ponts. This make life much easier
because feedback is instant and we do not need to keep opening
the file, finding the right place, guessing how much it is out,
changing it, and then starting all over again like we need to
do with all other synching methods! Oh, by the way you have to
press the apply button or it wont
start looping :).
Take note of if the audio comes before their mouths
move or if it comes after their mouths move.
1. If the audio comes after they speak we must decrease the frame rate a little!
2. If the audio comes before they speak we must increase
the frame rate a little!
It doesn't sound logical at first, so just to clarify:
to make the video longer we decrease the framerate. To make the
video shorter we increase the framerate. Imagine it like this:
if you had a pack of cards and threw 5 cards onto the floor every second,
it would take longer for you to finish the pack of cards than it would
if you were to throw 10 cards onto the floor every second. In the same
way, a movie will finish earlier if it shows more frames per second.
This means a higher framerate makes a shorter movie.
Now the synching is quite straight forward. Play your
movie, if the audio comes after the mouth moves hit the -0.001
button (4) once, and try the movie
again. If it is not correct, press the -0.001
button again, and again, and again etc., until it is almost perfect.
Then use the fine tuning arrows (2)
to get the final perfect audio. Conversly, if the audio comes before
the mouth moves we must use the +0.001
button in the same way as already described.
If you want to make really huge changes you can
use the slider bar (2) or the increase
by a whole frame per second with the +1 and -1 Frame buttons (3).
There is also the option to enter any framerate you like with the 'Enter
Desired Frame Rate' option.
But usually we will only change the framerate by avery
small amount! I usually only use 0.001. So if my movie was 25.000 fps
and I wanted to make it shorter I would change the framerate to 25.001.
If that was too long I'd go to 25.002 then 25.003 and so on. If you
want to make the movie longer I would try 24.998 then 24.997 and so
on. Once it is almost perfect you can use even smaller amounts such
as 0.0001. So imagine 25.007 is the almost perfect, but just needed
slightly more. I would try 25.0071 or something like that. It shouldn't
take long for you to become an expert.
If you think you are totally out you can reset any section
back to how it originally was by using the 'reset' button.
To apply it or not to apply it, that is the question?
Whether it be nobler to suffer the slings and arrows of
outrageous framerates or not. If we apply it now, the movie will play
correctly in Media Player and, in fact, on all PC players. But what
if we were synching our AVI file not because it was a DivX movie but
because we wanted to convert the AVI to Mpeg or SVCD etc? It is unlikely
that the VCD we created would be in synch! This is because all VCD,
SVCD and DVD files must be exactly 25 or 29.970 fps to play in a DVD
player or a stand alone player! A framerate such as 25.998 will not
work!
Other synching tools offer to change the audio samplerate
instead. Changing the audio samplerate may be able to put the movie
in synch just as effectively as changing the framerate. But again this
causes conversion problems. A VCD, DVD or SVCD will only take 44100Hz
or 48000Hz respectively. Feeding an encoder 11025Hz will almost always
end up with synch problems!
For this problem Terabits AVI Info tool is the only utility
so far that can offer a solution. Notice that, as you change the framerate
to get the movie in synch, it offers a streach (stretch) amount (5).
So we use the framerate controls as usual to get the movie
in synch but this time wedon't save the framerate changes!
Instead write down (or copy and paste) the 'streach' number exactly.
Now we will use this number to stretch the audio in Cool Edit. But before
we do that we must extract the audio.
Change to the 'Utilities tab'.
Select a folder to save the wave, enter a name for your extracted wave
file, and hit the 'Extract Audio To Wave File'
button. Alternatively you can use VirtualDub to extract the Wave, but
I see no need because this is basically the same thing.
Open the extracted wave in Cool Edit and choose Time/Pitch
> Stretch...
Paste the stretch amount into the ratio box (encircled
in red). I suggest you use 'Low Precision' option unless you wish to
spend many more hours stretching the audio.
Note: The 'High Precision' uses interpolation
to keep the audio higher quality, but after we have recompressed it,
it will not make any audiable difference so choosing higher quality
will probably just waste time.
Cool Edit 2000 automatically converts the audio to PCM
Wave for editing. So be careful because this takes over a gigabyte of
space! If your are using another audio editing program then you may
have to convert the wave to PCM first. Using Winamp is probably the
easiest way so check out my 'Converting Audio' section for more
info on this subject.
Reintergrating the Audio
Probably the most reliable method now is to save our newly
stretched audio as PCM Wave and multiplex it back to our video file.
Go to my Multiplexing section for details on this.
PART 2: Fixing Audio Displacement
Open your video in (you guessed it) VirtualDub. Choose
a piece of the movie where people are talking. Their mouths should be
clear so you can see easily when the synch is correct or not.
Go to Audio > Interleaving
Up pops the box below. Leave everything as it is except
the 'Delay audio track by' part. Look at
the movie and see if the sound comes before or after the video. If the
sound becomes before they speak we will use normal numbers to
slide the audio forward. If the audio comes after they speak
we will use minus numbers (-) to slide the audio backwards.
1000 represents one second of time, 500 represents half
a second, 250 represents a quarter of a second etc. Lets say the sound
appears just after the person speaks. To solve this we must slide the
audio to the left a bit. So we can put in minus 250 (ie. -250). Press
OK and play the movie in VirtualDub. Is it correct? If not, does the
sound come before the person speaks or after? Lets say it is much closer
but still comes after the persons mouth moves, we could try increasing
the delay by putting in -300. Press OK and check the movie again. Keep
doing this until the audio slides into the perfect place!
Now lets assume the persons mouth moves and then the audio
comes after they speak. We do exactly the same thing except we no longer
use the minus sign (-). So we start off small with 250 and press OK
to see the results. Then, if its not right, try some more like 300 then
400 or 500 etc. Usually a movie will only be out by about half a second
(i.e 500), but I have sometimes found movies to go out by six seconds
(i.e. 6000) or more! But usually this is becase I made a mistake in
the encoding by setting the wrong samplerate or something, so its easier
to grab the audio again.
When the audio is correct save the movie by choosing:
Video > Direct stream copy
Audio > Direct stream copy
File > Save Avi
Fixing Audio Displacement and Stretching
If you are really unlucky you will need to fix both problems!
If this happens fix the audio displacement first and then the
audio stretch. Just repeat steps 1 to 3 and you should be fine:
1. Go to the very very start of the movie and find
the first part where they speak. Use the displacement methods already
described to get the audio starting in the correct place.
2. Then go to the very very end of the movie and
use the audio stretch fix method already described to put it back in
place.
3. Check the movie. It will probably still not
be correct becase the audio stretch moved the start displacement to
the wrong place again! So repeat steps 1 and 2 again :).
PART 3: The Quick Fix Audio Synch Method
You could also try the "quick fix method" for
audio synch by opening your movie in VirtualDub and choosing Video
> Framerate
Then choose the 'change so video
and audio durations match'
This method sounds amazing but doesn't usually work =(.
More about using the Terabit AVI Info Tool
Finally this looks like a good place to add a few more
details on the AVI Info tool we are designing. It isn't completely finished
(as you probably guessed) and there are many thing on the cards for
this tool. You already know how to extract a wave from an AVI with it
and how to use its synching functions.
Delay Audio
We have put in a delay audio function (6)
but it is only partially working so use it at your own risk. It can
only delay the audio's start time and will not slide it to the left
or right like VirtualDub. Therefore I suggest you use VirtualDub for
audio displacement problems.
Change FourCC codes
If we go back to the 'Utilities
tab' we are given the option to change the Header code of the
AVI file (A). As you probably realise
Windows knows what codec to decode the AVI with by its FourCC codes.
If, for example we wanted to join a High Motion DivX movie
clip with a Low Motion DivX movie clip, it would be possible to do this
(provided they were the same resolution) by changing one of them to
the FourCC code of the other! So a Low Motion video which has:
DIV3
DIV3
Can be changed into a Fast Motion video clip by changing
it to:
DIV4
DIV3
Once this change has been done you should be able to
join them together in VirtualDub. Check out my 'Cutting & Joining'
AVI files section for details on how to do this.
Author Details
Since our DivX method doesn't allow us to add author details
to our movies like ASF files we wanted to make a tool for doing this.
This tool doesn't work yet BUT you should be able to open a previously
authored movie and change it. So you could take someone elses latest
ASF movies and change their author details to 'made by joe bloggs' or
something crazy like that =o).
Again this will only work with a video file that already
has someone else's information in it.
Duplication of links or content is strictly
prohibited. (C) NICKY PAGE 2000
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