http://www.theatertek.com/Forums/showthread.php?t=8399
Why is it even important to have the frequency being set to an exact value?
First of all lets just all agree that whatever values you find in reclock the control panel or anywhere else these are not actually measured frequencies since a PC is lacking any calibrated time base. Whether you derive these numbers from the internal registers of the greaphics processor or the high precisin time in the CPU these are all nubers relative to the frequency of the respective crystals.
Know we know that these numbers are pretty much estimates. There must be a better reason for reclock to exist!
The reason is the synchronization between the video and audio playback. Since the crystals of the video card and the audio card are normally running at their own independent speeds you need to pick one to be the master clock for the rendering pipeline. Normally Windows picks the audio renders clock. In this mode the rendering pipeline sends all the sound samples to the sound card where they play back at the rate that the crystal of the sound card dictates. The algorithm also renders video frames but in order to maintain synchronization between the audio and video it now must either duplicate or drop frames to compensate for the uncorrelated crystals on the audio and video card. Depending on how much these crystals are actually apart you will experience this as visible stutter in the video.
Reclock is tricking the system into believing that it is playing a constant audio stream but in fact it switches the master clock to the video card. In locked mode (green) all the frames on a DVD will get played at multiples of whatever the crystal on the video card dictates and the audio needs to be adapted.
Here is where the rub is. You can't change the crystal on the audio card. If you use analog playback reclock comes with an internal asynchronous sample rate converter that recalculates the samples as clocked by the video card into a different number of samples that are played back at the actual rate of the sound card. This algorithm introduces audible distortion!
For S/PDIF the problem is worse. Since the data is not being decoded on the PC all reclock can do is either duplicate or drop 32ms AC3 frames. That introduces clicks in the audio track!
Why did I waste your time with all that detail? Well, since you can't change the rate of playback on the audio card what you really need to do is to change the frequency of the video card to the value that gives you minimal drift from the crystal on the audio card. Whatever the absolute values are is irrelevant, what is important for S/PDIF output is to minimize the number of repeated or dropped audio frames. You can only find this out by trial and error and looking at the the reclock control panel that you can bring up during playback.
It took me quite a while to calibrate the values but if my machine is warmed up I know get only about 1 or 2 drops per hour of movie.
When the next generation of graphics card will ship that have audio and video output combined via HDMI then you will no longer need reclock and powerstrip since both clocks are derived from the same crystal. This will guarantee perfect sync.
If you made to the end of this mail you seem to care a lot about your video/audio quality and yes it is worth the effort to do that calibration especially with larger screens or a projector where the studder becomes even more noticeable.
Cheers
Thomas
P.S. Since these questions come up at regular intervals it would be good to somehow make this sticky under audio/video setup.