Click here to purchase the entire book in PDF format. BS.1116 Once upon a time, the International Telecommunications Union was asked to some up with a standard way for testing how much a perceptual codec screwed up an audio signal. So, they put together a group of people who decided on a way to do this test, and the document they published was called Recommendation BS.1116-1: Methods for the subjective assessment of small impairments in audio systems including multichannel sound systems which can be purchased from the ITU http://www.itu.int/rec/recommendation.asp?type=items&lang=e&parent=R-REC-BS.1116-1-199710-Ihere. The BS.1116 test gives the subject three stimuli to hear, just like an A/B/X test. There's the Reference, an A stimulus and a B stimulus. The Reference stimulus is the original sound, and either A or B is identical to the Reference. The other remaining stimulus (either B or A) is the sound that has gone through the codec. The response display shows two sliders as is shown in Figure 5.20. The subject is basically asked two questions: Figure out which of A or B is identical to the Reference, and give that a rating of 5 (Imperceptible). Rate the level of degradation of the remaining stimulus on a 5-point scale (1. Very annoying, 2. Annoying, 3. Slightly annoying, 4. Perceptible, but not annoying and 5. Imperceptible) At least one of the two stimuli should have a rating of 5, otherwise the subject didn't understand the question. Note that some over-educated people might classify this as a ''double-blind triple-stimulus with hidden reference'' test. Figure 5.20: An example of a BS.1116-1 user interface. There are a couple of other minor rules to follow for this test (including some important stuff about training of your subjects), and the instructions on how to analyze your results are described in the document if you understand statistics. If, like me, you don't understand statistical methods, then the document won't help you very much. Unfortunately, neither can I. My apologies. I have a couple of personal problems with this type of test (he said, standing on his soapbox...). Firstly, it assumes that, if you can hear a difference, then the stimulus under test cannot be better than the reference. It can be various levels of annoying, but not preferable, so subjects tend to get confused and use the scales to rate their preference instead of the amount of degradation of the signal. In essence, they do not answer the question they are asked. Secondly, it's a little confusing for the subject to do this test. Whereas A/B/X or paired comparison tests are very easy to perform, the BS.1116 method is more complicated.