# Defect in Chapter 10 Doppler sample

I am playing with the Chapter 10 sampler for doppler effect (UFO). Its intent seems to be to put a sphere in a circular orbit at a distance from the camera, and to implement a doppler effect on the sound by playing with the sound’s velocity.

The UFO does not appear to follow a circular orbit, and the sound does not shift in an expected manner.

Recommendation: remove the call to setReverbEnabled. (a) It has nothing to do with a doppler effect, and (b) it masks the effect.

Defect repair: Update simpleUpdate as noted below:
public void simpleUpdate(float tpf) {
// this formula calculates coordinates of the UFOs orbit
float dx = (float) (Math.sin(angle) * xDist);
float dz = (float) (-Math.cos(angle) * zDist);
//x += dx * tpf; // NO! angle is already based on tpf…
//z += dz * tpf;
// technically x = (float)(Math.cos(angle) * xDist, same for z,
// but we’ve already computed these in dx, dz
// (taking advantage of velocity being pi/2 out of phase with position…)
x = -dz;
z = dx;
angle += tpf * rate;
if(angle>FastMath.TWO_PI) angle=0;
/* Delete this code…
if (angle > FastMath.TWO_PI) {
angle = FastMath.TWO_PI;
rate = -rate;
} else if (angle < -0) {
angle = -0;
rate = -rate;
}
*/

Sorry, don’t know how the monkey got in there. It should be x-D-i-s-t. Guess I should have "code"ed the block.