Tutorial 3: Speed difference

1. Introduction

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