The SRS technique helps rectify these problems by processing the audio signals so that the spatial cues are restored. SRS first combines or adds together the left and right channels to create a sum signal (L+R). lt then subtracts each one from the other to create I two difference signals (L-R) and (R—L). The signals are then subjected to various forms of processing and equalisation.
Ambience and spatial characteristics are derived from the processed difference signals. Dialogue, vocalist and soloist sounds are derived from the processed sum signal. Once the complex and dynamic processing sequence has taken place, the signalsare revised and reconstructed into new (L+R) and (L-R) signals, which are matrixed back together in the same fashion as used in stereo FM and stereo TV.
That is, using simple algebra, the new (L+R) is added to the new (L—R), yielding a new L signal (actually 2L, but the 2 is simply an amplitude coefficient which can be ignored). The new (R-L) signal is added to the new (L+R) signal to form a new R signal. In a stereophonic signal, sounds coming from ‘in front’ produce equal- amplitude sounds in the left and right channels, and are therefore present in the sum signal (L+R). Ambient, reflected, and side signals produce a complex sound field, andare present primarily in the difference signals (L—R) and (R—L).
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The Hughes SRS circuitry processes the difference signals to bring the missing spatial cues and directional information. The difference signals are then dynamically increased in amplitude ir , order to increase the apparent image width.
However, since the ear has in- creased sensitivity to midrange frequencies, selective emphasis of the difference signals is necessary to produce a realistically wider stereo image without introducing annoying image shifts.
The selective emphasis of certain frequencies in the difference signal accomplishes several things. For the quieter components, it further enhances the stereo image by restoring the ambience of the live performance, which is normally masked out by the louder, direct sounds.
It also provides a much wider listening area. One may no longer need to sit midway between two speakers! Apart from the above described techniques used in the SRS system there are many control circuits that detect the content of music, and dynamically adjust both the sum and difference signals.