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How to optimize SPDIF output
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As said before, SPDIF mode re synchronize the sound with the video part by dropping or repeating AC3 frames which can be unpleasant to the ear.
What can we do for this? First of all, try to play media files that do not need to be ¡§reclocked¡¨ by a wide margin. For example, forcing a 24 fps file to play at 25 fps, will result in massive drop of 4% of samples which is too much and should be avoided.
Now, please follow these guidelines :
- if you want to play a file at 23,976 fps (NTSC progressive DVD) or 24 fps, put your monitor or projector at 48 hz or 72 hz, or 120 hz
- if you want to play a file at 25 fps (PAL DVD), put your monitor or projector at 50 hz or 75 hz or 100 hz
- if you want to play a file at 29,970 fps or 30 fps, put your monitor or projector at 60 hz, or 120 hz
- if you don¡¦t fall into one of these categories, use the media adaptation called ¡§refresh rate / x¡¨ with ¡§x¡¨ giving the least speed difference for the file. For example if you want to play a progressive NTSC DVD (23,976 fps) on a NTSC TV (60hz), you should choose ¡§refresh rate / 2.5¡¨ that will reclock the movie at 24 fps.
Now you have setup properly you media adaptation, there is something we can do to improve things a little bit. We can fine tune the hardware refresh rate of your video card to make it closer to the clock of you audio card, and this will minimize the number of audio drops and repeats. To do this, you need a tool called Powerstrip which is available on
www.entechtaiwan.com.
The procedure is a bit tricky and should be tried by people that know what this stuff mean ļ
Step 1: setup your hardware properly, and choose the video refresh rate on your video card
Step 2: launch configuration application, and hit ¡§Cleanup manual frame rates database¡¨
Step 3: launch your favourite player and start playback of your media with AC3, set media adaptation as needed. Now wait for the ppm indicator to stabilize to a very low value (under 10 ppm)
Step 4: stop playback, and start it again for 30 minutes. After that, you can see in the properties panel a count of AC3 frames dropped or repeated.
Now make this little computation:
E = ¡§number of sample dropped¡¨ ¡V ¡§number of sample repeated¡¨
RATE = ( (1000 x 60 x 30) + (E x 32) ) / (1000 x 60 x 30)
Step 5: quit your player
Step 6: launch Powerstrip and go to ¡§Display profiles¡¨ -> ¡§Configure¡¨ -> ¡§Advanced timing options¡¨ and check ¡§Ultra fine geometry¡¨. Now have a look on the refresh rate indicated by Powerstrip. Multiply this number with the RATE value you computed before, and modify it with the new value
Step 7: launch configuration application, and hit ¡§Cleanup manual frame rates database¡¨
Step 8: go back to step 3, and repeat the tests, until you find and optimal ¡§refresh rate¡¨ value in Powerstrip
PS: as an alternate method, instead of recomputed each time the refresh rate with the RATE value, you can increase it by little steps if E > 0 or decrease it if E < 0.
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