After the unit was properly installed and aligned, a clean reel of Scotch 226 tape was threaded onto an Otari MTR-10 with the speed set to 15ips. A simple voice track was recorded with the SR "out" then again with the SR "in." An unprocessed Neumann U87 mike was used. Following the voice track, two "silent" segments were recorded, the first without SR and the second with SR. The unit was then set to the playback mode, fingers were placed on the bypass switches, and the test began. The result was quite impressive!
We set our monitors to normal listening levels and rolled the tape. The first segment without Dolby offered a clean, crisp voice track accompanied by the usual amount of tape hiss we're all used to. When the segment with Dolby encoding came up, the unit was switched in line and the hiss literally disappeared! The voice track, on the other hand, seemed untouched by any processing whatsoever. It remained as clean and crisp as before. The following "silent" segments provided even more dramatic results since the mike was not open and noises from within the studio itself were not recorded.
The monitor was then cranked to "stun" level and the test tape rewound and played back again. First came the non-Dolby voice segment. The unit was bypassed and the hiss was definitely there. When the Dolby encoded segment came up, the unit was again switched in-line. With monitors at nearly full tilt, tape hiss was not distinguishable behind the voice. When the "silent" segments played back, the effect of the SR was most noticeable. It was at this time that each of us in the studio agreed that the reduction in noise was indeed around the touted 25db level. While there was some audible noise during the silent segments, it was questionable whether it was from the tape or from the console. Considering how high the monitor level was, this noise was deemed insignificant. The monitor was returned to normal listening level and upon playback once again of the Dolby encoded voice segments, it was indeed a strain on the ears to distinguish this analog recording from a digital one.
We considered putting the unit on the bench and hooking some test gear up to it to measure noise reduction, but later reconsidered this. After all, how many production studios have you worked in where EQ is set with a spectrum analyzer or compression is set with an oscilloscope? Our ears remained the only test equipment used.
The unit was instead installed on one of the ITC-99B cart machines in the studio. Again, hookup was no more difficult than using a patch bay. We recorded some classical music to cart, bypassing the Dolby encoding every ten seconds for A-B comparison purposes. The music was recorded at a low level to simulate what happens to more contemporary music on cart when low passages in the music appear. The result was once again VERY noticeable. This test brought to mind the many stations whose "on air" processing is very compressed. You've heard what happens on these stations during low music passages. Think about how many times you've heard Stairway To Heaven on a highly processed station. During the vocal solo at the end of the song, the audio gets sucked up by the processing chain, and along with Robert Plant, there's a world of noise -- most of it tape noise in the case of a station playing the song from cart. Using Dolby SR on carted music for airplay would result in a reduction of noise even more apparent on highly compressed signals (since noise levels are increased greatly during low passages to kick that modulation up to 100%).
The model used for this Test Drive was the Dolby SR/A model 363. The SR/A offers both SR and A type noise reduction. This particular model might be used most in a recording studio that uses both types of noise reduction. It is easy to switch from one to the other with the flip of a switch. For broadcast purposes, the model 363-SR with two No. 350 modules is all that is necessary for installation on one stereo recorder.