ITU 775 stands for International Telecommunications Union, Operational Bulletin No. 775 in which recommendations are given for a multi-channel surround standard for “5.1” speaker positions.
The ITU-775 setup is sometimes referred to as “3/2 format,” indicating a division between a 3-speaker frontal sound stage and a 2-speaker rear “surround”. To arrive at this standard, 20 speakers were placed in an anechoic room to find the critical angles for the best speaker placement. For the surround (rear) speakers, a compromise had to be found between 90 degrees and 135 degrees. Whereas 90 degrees (directly on either side of the sweet spot) was the best placing for ambience or “envelopment”, 135 degrees turned out to be best for “surround-placement” or localization in the rear, hence a compromise at 110 degrees.
In the reference loudspeaker arrangement for mode 3/2, it is recommended that the loudspeakers be placed on the arc of a circle. In those cases where the front speakers must be placed on a straight line, for example, when the center speaker cannot be placed behind the screen, it was recommended that the sound signals be appropriately delayed so that all signals reach the listener’s ears simultaneously. It was further recommended that all the front loudspeakers be driven by discrete audio signals. In those cases where the center loudspeaker cannot be placed behind the screen, it must be placed above or below the screen.