Click here to purchase the entire book in PDF format. A / B / X Sometimes, you want to run a test to see if the differences between two stimuli are perceptible. For example, you've just invented a new codec and you think that its output sounds just as good as the original signal off a CD. So, you want to test if people can hear the difference between the original and the coded signal. One way to do this is with an A/B/X test. In this test, we still have two stimuli, in this example, the original and the output of the codec. You present the subject with three things to which to listen, stimulus A might be the codec output, stimulus B will be the other (the original sound) and stimulus X will be the same as one of them. The subject is asked ``X is identical to either A or B - which one is it?'' The subject knows in advance that one of them (A or B) will definitely be identical to the reference signal (X) - they have to just figure out which one it is. If you do this test properly, randomizing the presentation of the stimuli, you can use this test to determine how often people will recognize the difference between the two stimuli. For example, if everyone gets 100% on your test, then everyone will recognize the difference. If everyone gets 50% on the test, then they are all just guessing (they would have gotten 50% if they never heard the stimuli at all...), and there is no noticeable difference. Of course, the thing to be careful of here is the question of the training of your subjects. If you've never heard a sound in MP3 format before, then you probably will not score very well on a original vs. MP3 detection test done as an A/B/X test. If you've been trained to hear MP3 artifacts, then you'll score much better. This is how the original marketing was done for things like DCC and Mini-Disc. The advertising said that these formats provided ``CD quality'' because the people that were tested weren't trained on how to hear the codec's. The A/B/X test was valid, but only for the group tested (naive listeners as opposed to trained, experienced or expert ones...).