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.
Problems of speed stability (flutter) and tape dropout apart, we considered that it would be useful to implement the most popular analogue cassette noise reduction (NR) in software. In so doing, we implemented an algorithm which is based on the Wireless World Dolby Noise Reducer as shown in the image below¹.
One of the problems with hardware NR 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 NR 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.
The Stereo Lab Equaliser may be usefully cascaded with the Cassette NR process. For example, the Play Trim feature which appeared on high-end cassette decks was an HF cut/boost equaliser which preceded the noise-reduction stage. By selecting the equaliser prior to the Cassette NR process, Play Trim functionality may be accomplished (see below).
Cassette decks are getting quite rare now and the models which are available often do not incorporate selectable (70μS/120μS) equalisation for chrome (CrO2) cassette tapes (IEC Type II), as well as ferrichrome (FeCr, IEC Type III) and metal cassettes (IEC Type IV).
Once again, the Stereo Lab Equaliser may be usefully cascaded with the Cassette NR process.to compensate for the over-bright sound of Type II, III and IV tapes when replayed with the standard (IEC Type I), 120μS equalisation. Select -4dB treble cut. This equalisation correction applied prior to the noise-reduction process is exactly as it would have been done in a high-end hardware cassette player.
1. Wireless World Dolby noise reducer, Geoffrey Shorter, Wireless World May, June, July 1975
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