Description
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Cat 22 Dolby A noise reduction card
The Dolby 361 was ubiquitous throughout the 1970s, 80s and 90s. It was the standard single-channel Dolby mother unit, compatible with the original 1965 Cat 22 Dolby A card and the later Cat 280 Dolby SR cards. Every studio had a couple of 361s but they are now surprisingly rare on the secondhand market
As a double-ended noise reduction system, Dolby A encoded the signal and applied reciprocal decoding on playback. If the tape machine was aligned correctly the system was pretty transparent and gave a significant noise reduction, particularly at higher frequencies
Post-digital, Dolbys are obsolete except for archiving and for doing the Dolby Trick. Dolby A splits the audio spectrum into four bands, applying varying amounts of compression to each. Compression is level-dependent so, as levels rise, compression backs off. At higher frequencies, where tape hiss is more obvious and average signal levels are relatively low, a fair amount of compression could be used, resulting in a dramatic perceived reduction in tape noise & hiss
