A variety of noise reduction processing was used This processing was a double-ended system where the record processor boosted certain frequencies and portions of the dynamic range while the playback processor provided a complementary reduction of the signal. These systems are generally referred to as companders for compressor-expander. Two different manufacturers of companders achieved high market penetration. Two others did not, but that is not to say that their equipment was not used somewhere. To the best of my knowledge, no one has written a Direct-X-type plug-in for a computer, so you are stuck having to buy the playback processors for each system you wish to reproduce.

• Dolby—The first and probably the widest-used systems. Dolby systems are all additive, only working on the low-level signals. The use of proper lineup tones is critical.
  • Dolby A—The original 4-band compander from approximately 1968 for professional use.
  • Dolby SR—The updated version of Dolby A from the mid 1980’s.
  • Dolby B—This was the consumer single-band compander that has almost universally been applied to cassettes. It was available as an outboard processor and built into later reel-to-reel consumer tape decks.
  • Dolby C—This was widely available as a built-in processor with cassette decks and also used in a few pro-sumer reel-to-reel multi-track tapes.
  • Dolby S—This was available as a built-in processor with cassette decks and also used in a few pro-sumer reel-to-reel multi-track tapes.
  • Dolby HX-Pro—This is a single-ended headroom extension system that was used in very few reel-to-reel recorders, but it does not matter since it is a single-ended system working during the record process. It extends the headroom by decreasing the bias slightly if there is a high-level high-frequency signal present. No processing is needed to play back tapes made with this system and it is included here to avoid confusion should the tape be indicated that it was made with this process.
• dbx—The runner up whose systems were also widely used. dbx systems were linear decibel 2:1 compression and expansion.
  • dbx Type I—the professional standard.
    It may be interesting to note that it took three processors to find one that sounded right. One was obviously just broken. The second sounded dull. The third one seemed transparent. Be careful.
  • dbx Type II—the consumer standard, also used on a few LPs and pre-recorded tapes.
• Burwen made a 3:1 companding system that never was widely used (to the best of my knowledge) in the recording environment. I believe it was used for a while on some radio links.
• Telefunken made the Telcom C4 system that was more widely used in Europe than in North America. A consumer version of this was issued as Nakamichi High-Com II.

The question of noise reduction companders comes up often on discussion boards. I am unaware of any noise reduction (NR) plugins to decode analog signals, it would be a logical item to create.

However, I do not think that we will see this for Dolby due to licensing restrictions and both dbx and Telcom don’t have the market penetration that might warrant this effort.

When I first started undertaking tape restoration, I went through three dbx I decoders before I found one that “sounded” right.

Another caveat is that the high frequency response of cassettes especially might have drifted over time. I find that although I’m using Nakamichi Dragons that appear to be calibrated (i.e., I get the same result on several) there are many tapes, including those I made on what I thought was a calibrated Nakamichi 550 in the 1970s that do not sound right with Dolby B on playback.

The trick that I have used is to put one hand on the Dragon’s output knob and the other hand on my Blue Sky Monitor knob and adjust the two until the Dolby sounds right. You need to rack through the position just like tape azimuth of focusing a camera lens on a ground glass in order to find the centre of the “sweet spot”. The goal is to keep the overall level constant, while adjusting the level to the NR decoder.

While tones are great for alignment, one must also listen. dbx is more forgiving than Dolby in this regard. Also, just because a tape is labelled with a specific noise-reduction system doesn’t mean it was really recorded with that system AND just because a tape isn’t labelled as being recorded with a NR system doesn’t mean it isn’t.