Case examples
If the difference between picture and sound increases gradually, this
does often have its cause in the fact, that (analogous) picture and sound
were recorded into the computer using different hardware components. Better
cards for the videodigitization as well as good external video grabbers
have except the video input also an audio socket (usually named "line
in"). Only if the sound is recorded using this input, a "running
away" should be really impossible, because then picture and sound
are read in using the same time base. If your video grabber does not have
an audio socket, you must record the sound instead of this by means of
an additional sound card. So the video is digitized using the clock generator
on the grabber, the sound, however, using that on the sound card. Both
clock generators should run crystal stabilized, but are however not accurately
calibrated one on the other and can therefore work with slightly different
frequencies. Therefore the sound is a little bit faster or slower than
the picture, what results in a constantly growing misalignment over the
running time.
Another application is the conversion of a video between different television
standards. For example, in the USA motion pictures run in the television
with effectively 23.976 frames per second (fps), in Central Europe with
25 fps. After all, this is a difference of more than 4 percent. If you
want to convert a movie on a US video cassette as cleanly as possible
into the European CCIR standard (often called as "NTSC-to-PAL-conversion"),
you have indeed the most work with the decartelizing of the 60 interlaced
TV image fields into 24 film pictures per second, however also the sound
is to be accelerated by the 4,271% mentioned.
Tutorial files
For this project you will need the following files (from the package
Tutorial 01 or
by these links):
| |
Name |
Description |
Size |
Download |
 |
Tutorial 01 Video |
the sample video |
1616 KB |
nagel.avi |
| |
Tutorial 01 Sound 3 |
the sound track to be adapted |
87 KB |
step03.wav |
|
