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Originally Posted by Ethan Winer  I promise you it is indistinguishable from my test file. The main reason I'm reluctant to post that is people could then null files and see where I turned the dither on and off. If my test is to be at all scientific, I need to ensure that nobody can cheat.  |
Dear Ethan: You are making some very wrong assumptions about how the double-blind test that you propose needs to be carried out. Au contraire: Your test is extremely UNSCIENTIFIC when judged by psychoacousticians and experts in double-blind testing. It violates all the rules of how to produce a sensitive double-blind listening tests.
For example, when the switching is unknown to the listener, it produces EXTREMELY skewed and unreproduceable results. In ALL valid double blind tests, the listener is told that a switch has occurred and the switch may be to the original material, or to the altered material, but the listener is NOT asked to try to guess where in the material the splice has occurred! This is entirely possible ONLY for EXTREMELY obvious material, not for subtle differences such as dither, which require an EXTREMELY sensitive test modality.
The test should be constructed to ensure success, not failure! The test you have constructed is destined to come up negative, even for the most sensitive of listeners on their own system in perfect acoustics!
1) Please read ITU-R Recommendation BS.1116, it outlines the procedures required for a sensitive double blind test
2) As per BS.1116, the test you will need to construct will have to consist of short segments of identical musical material repeated for the listener. In some types of tests, the first play is the "reference" and the second play is either a duplicate of the reference or the processed version. The listener is asked whether the sound is identical or different. As you can see, this is a fair test modality and can be scored quite accurately.
3) TRAINING is required. You have eliminated listener TRAINING, which is absolutely necessary for such a test to succeed. The short original sample is played (e.g the 24 bit original), and the result (either truncated or dithered to 16). The listener is allowed to spend time training his ears. This is not deceit, this is a REQUIREMENT for any test of sensitive material such as you propose.
4) The musical material has to be carefully chosen by listeners who claim to know the sound character that they are trying to identify (e.g. dithered vs. undithered), not be randomly chosen by the skeptic (you)!
And there are far more requirements which once you study them should become straightforward. Such a test is EXTREMELY expensive and time-consuming to construct and generally should be conducted by a trained psychoacoustician who needs to make sure that all factors are done properly.
I can almost guarantee that if you conduct this test in this manner, properly done, that you will find that dither does matter. I'd be happy to invite you to my studio any time to take (or give) a test in this manner.
Best wishes,
Bob