248 BELL SYSTEM TECHNICAL JOURNAL 



spondence between the caller's actual position on the pick-up stage 

 and his apparent position on the virtual stage. Apparent positions to 

 the right or left correspond with actual positions to the right or left, 

 and apparent front and rear positions correspond with actual front 

 and rear positions. Thus the system afforded lateral or "angular" 

 localization as well as fore and aft or "depth" localization. For 

 comparison, there is shown in the last diagram the localization afforded 

 by direct listening. The crosses indicate a caller's position in back 

 of the gauze curtain and the circles indicate his apparent position as 

 judged by the observers listening to his speech directly. In both 

 cases, as the caller moved back in a straight line on the left or right side 

 of the stage, he appeared to follow a curved path pulling in toward the 

 rear center; e.g., compare the caller positions 1, 2, 3, with the apparent 

 positions 1, 2, 3. This distortion was somewhat greater for 3-channel 

 reproduction than for direct listening. 



The results obtained with the 2-channel system show two marked 

 differences from those obtained with 3-channel reproduction. Posi- 

 tions on the center line of the pick-up stage (i.e., 4, 5, 6) all appear in 

 the rear center of the virtual stage, and the virtual stage depth for all 

 positions is reduced. The virtual stage width, however, is somewhat 

 greater than that obtained with 3-channel reproduction. 



Bridging a third microphone across the 2-channel system had the 

 effect of pulling the center line positions 4, 5, 6, forward, but the 

 virtual stage depth remained substantially that afforded by 2-channel 

 reproduction, while the virtual stage width was decreased somewhat. 

 In this and the other bridged arrangements the bridging circuits 

 employed amplifiers, as represented by the arrows in Fig. 1, in such 

 a way that there was a path for speech current only in the indicated 

 direction. 



Bridging a third loud speaker across the 2-channel system had the 

 effect of increasing the virtual stage depth and decreasing the virtual 

 stage width, but positions on the center line of the pick-up stage 

 appeared in the rear center of the virtual stage as in 2-channel repro- 

 duction. 



Bridging both a third microphone and a third loud speaker across the 

 2-channel system had the effect of reducing greatly the virtual stage 

 width. The width could be restored by reducing the bridging gains, 

 but fading the bridged microphone out caused the front line of the 

 virtual stage to recede at the center, whereas fading the bridged loud 

 speaker out reduced the virtual stage depth. No fixed set of bridging 

 gains was found that would enable the arrangement to create the 

 virtual stage created by three independent channels. The gains used in 



