Restoration Tips & Notes Working with audio media (mostly tape) restoration


Noise Reduction

Filed under: — Richard L. Hess @ 16:15

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.

Unless noted otherwise We have at least some capability with all of these formats.

  • 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 1967 for professional use.
    • Dolby SR—The updated version of Dolby A from 1986.
    • 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. Introduced in 1971.
    • 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. Introduced in 1983.
    • 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. Introduced in 1990
    • 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 for playback 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. It was  developed by Bang and Olufsen but licensed by Dolby.
  • dbx—The runner up whose systems were also widely used. dbx systems were linear decibel 2:1 compression and expansion and both Type I and Type II were introduced about 1971.
    • 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. (I don’t have this format)
  • Telefunken made the Telcom C4 system that was more widely used in Europe than in North America. It was later sold by ANT. A consumer derivative of this was issued in Europe as Telefunken High-Com and a probably slightly different derivative was sold worldwide as Nakamichi High-Com II. This system became available about 1977.
  • Sanyo Super D circa 1981-1982
  • Toshiba ADRES (1981-1982 according to the late Peter Copeland) (for the Aurex–thanks O.J.Rancans, I don’t have this format)
  • JVC ANRS and SUPER ANRS (1976 according to the late Peter Copeland) (thanks O.J.Rancans, I don’t have this format)[section updated with dates 2010-05-01]

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 level and/or 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.

Tim Gillett, a tape restorer from Australia, and I have had a prolonged discussion about this and he feels that all attempts should be made to repair any HF loss prior to Dolby decoding. He claims success with this technique, but indicates that it is extremely difficult to do by ear. While I have had success with merely adjusting level, I do think Tim has a good point that the correct approach would be to try and fix any EQ changes prior to adjusting overall level. In actuality, this subject alone–especially as we move into more complex noise reduction systems–could warrant its own research paper. The loss of highs with time has not been well-researched, and it is my understanding that it can happen with the tapes in a drawer away from damaging magnetic fields. Fortunately, most of this high-frequency loss-in-the-drawer has been found on cassette tapes and not on reel tapes. Magnetostriction has been floated as a possible cause considering the much smaller radii guides that cassette tapes are wrapped around.

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.


My friend Grant W wrote me telling me of the saga of the JVC DD-99 which was an early machine alleged to have Dolby C, but the actual processing was done with something like two Dolby B stages. So, when Dolby C is selected on that deck, it is neither Dolby B nor Dolby C. Fortunately, Grant still has the machine and he’d be happy to copy tapes for anyone with tapes made on this one particular machine. Please contact me and I’ll put the two of you in touch. This comment has generated some controversy. All I know was what Grant told me.

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