Tutorial 3: Speed difference

Understanding Spline markers

With Spline markers you define contemporaneous positions in the video and audio data stream by specifying a correction value for the adjustment of picture and sound in each marker. Hereby, the sound is not cut, but its playback speed is adapted like a rubber band in such a way that the marked positions are played at the same time.

The correction values within the regions between the spline markers are calculated by means of a smooth interpolation function (a so-called spline). Such a spline distinguishes itself by the fact that it runs precisely through the defined interpolation points, but has no sharp "corners". Thereby is guaranteed that the playback speed does change suddenly nowhere.

Linear interpolation Spline interpolation

If you use less than three Spline markers, the playback speed remains constantly in the entire project. From this result the following simplified applications for the spline marker kind:

• A single Spline marker: The entire audio track is moved by the given amount. The playback speed of the sound is not changed. This behavior we have used in the first project.

• Two Spline markers: A straight line results as correction curve which is running through both markers. In addition to the global shift specified by a spline point, the playback speed is thus accelerated or slowed down constantly over the entire project. In this way we just have corrected the gradually growing misalignment.

Working with the correction curve

The correction curve is one of the most important assistance elements when you work with Spline markers. Here you have the entire correction process with all markers in the overview, can assess the effect of certain Spline markers, and at first sight recognize critical mistakes.

• The light blue line shows the changes of the playback speed of the sound. As up to now also, it is a horizontal straight line. This means that the playback speed remains constant in the whole videoclip.

• The both points on the light blue line are the positions of your markers.

• The grey curve represents the course of the correction value. The correction value varies over the run length, but this change is regular (therefore the line is straight). In this way the correction value compensates the likewise regular growing misalignment.

In the previous projects, the correction offset was always constant in a section. Here, however, it changes gradually what at last causes a change of the sound's playback speed. In our example, the sound speed is regular slowed down, what is to be seen in the fact that the gray line is straight and falling.

The project analysis function

Besides the correction curve which gives a qualitative overview about the correction course, AV Adjust provides a quantitative project analysis function. You call this function by means of the menu command Project | Analyze project ...:

Here you see among other things that the sound was slowed down to almost 95% of its original speed, but also, that the playback speed is really constant (the "Global sound speed difference" amounts to 0%).

Besides, it is interesting that at the first frame the correction offset is nearly 0. This means that picture and tone start really in sync. If you had known this before, you could have spared yourselves the trouble to adjust the first marker, and instead of this you would have defined right away at frame no. 0 a Spline marker with a correction value of 0... ;o)