Few serious music collectors do not have a small, but irreplaceable part of their collection on analogue cassette tapes. This fragile medium has withstood the ravages of time rather worse than the LP.
The most popular analogue cassette noise reduction (Dolby B) is implemented in the Stereo Lab app'. We implemented an algorithm which is based on the Wireless World Dolby Noise Reducer as shown in the image below.¹
One of the problems with hardware Dolby B decoding is that, in order to work properly, the audio must be decoded at the correct reference level. This is often not the case in hardware tape recorders - especially with old cassettes. This accounts for the poor performance of many hardware implementations. The result is usually an unacceptable degree of treble-loss when the noise-reduction is engaged.
The Wireless World Dolby decoder accomplished this with the inclusion of meters and line-up tones. In Stereo Lab, we take advantage of modern digital signal processing to ensure that the audio is first of all normalised so that it is at a reference level and with therefore be decoded properly.
B-yond noise reduction is designed to decode Dolby C encoded material. Here again, we take advantage of modern digital signal processing to ensure that the audio is first of all normalised so that it is at a reference level and with therefore be decoded properly. If you use TEQ E or TEQ F equalisation correction, be sure to apply these filters prior to B-yond noise reduction because incorrect EQ upsets the Dolby normalisation.
CATHODE stands for CAssette Tape Hiss-Oriented Dynamic Expander – a system developed by Pspatial Audio to enhance digital recordings made from cassette replays. It is a single-ended system, so it may be used as a tape noise reduction system when no noise-reduction encoding has taken place.
1. Wireless World Dolby noise reducer, Geoffrey Shorter, Wireless World May, June, July 1975
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