Four different pictures were considered, one at a time. Each subject was shown a series of images arranged in pairs - one being a full quality image and the other, an image that was gradually degraded and then improved in quality throughout the series (The quality of an image reduces as JPEG compression increased). They were asked to say if the two images appeared the same or different in quality.
The process of determining JPD for each image involves two phases in a technique which approaches the subjects limit from both positive and negative directions. Phase one comprises presenting two identical images and asking the subject if they are the same or different in quality. The presentation is repeated as the JPEG index of one of the presented images is reduced until the subject indicates that they notice a difference in the quality of the images.
Phase two differs from phase one, in that one of the images is of a very low quality (has a very low JPEG index). The presentation is repeated as the JPEG index of the low quality image is increased until the subject indicates that they notice no difference in the quality of the images.
Each presentation pair is organised in the following way: the first image is presented for 1 second and then removed from the screen. After a gap of 1 second the second image is displayed for 1 second and then removed from the screen. The high quality image and altered quality image for subject comparison were presented in a random order. After the second image has been removed from the screen, two buttons appear and the subject can indicate with a mouse click that the images appear the ``THE SAME'' or ``NOT THE SAME'' in quality.
Phases one and two are repeated for each picture to complete a set. A set comprises determining the JPD for all four images. Each Subject completed the set twice.
As well as recording the subjects' scores, age and sex were noted.
Statistically a conservative cut off point is one where no percentage of the population see a difference between the image they thought that they were downloading and the image that they receive via the lowband proxy server. We have defined this as the mean plus twice the standard deviation of a sample of JPDs. However, a less conservative figure might be the mean plus one standard deviation; these issues are discussed below in section 4.2.