The Dolby 361 is a complete noise reduction system designed to reduce tape hiss when in analog tape machine recordings. The 361 system can also be used as an interesting effects unit.
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. 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 quite transparent and applied significant noise reduction, particularly at higher frequencies. 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.
The 'Dolby Trick'
A rather obscure yet brilliant use for the 361 system is the so-called 'Dolby Trick', which high-frequency compression of Dolby A as an audio effect. By encoding the signal, but not decoding it, it lifts the harmonics & airiness of sounds in an attractive way. It works like a charm on backing vocals but it’s also useful on guitars, drum ambience and much more. Used with standard Cat 22 cards the effect is most pronounced at high frequencies. By modifying the cards to bypass processing on each frequency band it is possible to create different effects.
The Gear Rack