Disc NR

The great majority of analogue noise-reduction systems rely on dynamic processors (compressors and expanders) in paired applications. In these systems, the processing of the encoder, (compression) is to some degree “un-done” by a decoder or an expander.

In certain cases, the compression and expansion characteristics are precisely matched; the idea being to "squeeze" wide dynamic range material into a restricted dynamic-range channel by complementary compression and expansion over a wide dynamic range. Sometimes referred to as compander (COMpression - exPANDER) systems, these clever designs more closely resemble the sophisticated data compression systems of digital-audio than they do analogue noise-reduction circuits.

Several compander systems of this type were developed for studio tape-recorders. One such was known as dbx (of dbx Inc.) and the concept was adapted for the stereo television audio service in America (dbx-TV). A design was also conceived for gramophone (phonograph) records known as dbx discs and survived for over 10 years amongst some enthusiasts from the early 1970s into the 1980s and a considerable discography was accumulated of these discs which are now only playable if you can source hardware decoders from 40 to 50 years ago!

Implementation in Stereo Lab

All these systems relied on two, ingenious circuit blocks: a RMS detector circuit and a voltage-controlled amplifier which were developed by David Blackmer; the founder of the company dbx Inc.1

......... a considerable discography was accumulated of these discs which are now only playable if you can source hardware decoders from 40 to 50 years ago!

The Disc NR system in Stereo Lab is based on an accurate simulation of Blackmer's circuits in software. In common with the other dbx applications, the dbx discs system added high-frequency pre-emphasis to the recording encoder prior to the compression and complementary de-emphasis in the decoder. This too is emulated in the Stereo Lab Disc-NR processor.

Disc-NR is engaged as a downstream process when modern equalisations (RIAA) is selected as the primary process. Be sure that needle-drops are made with the rumble-filter engaged when processing with Disc-NR to avoid low-frequency "clutter" getting into the expander level-detector2.

Notes and references

1. Every device that used Blackmer's circuits required the payment of royalties; first to dbx, Inc. and subsequently to THAT Corporation which was spun off from dbx in 1989 and acquired the relevant patents in 1994. These patents expired worldwide in 2004.

2. In the hardware units rumble is prevented from entering the RMS level-detector by a simple AC-coupling at rather a high-frequency. This was a reasonable compromise at the time, but it does mean there is some incompatibility between the encode and decode processes. We have chosen therefore NOT to implement this HPF in Disc-NR; hence the injunction to be sure to check the rumble filter in the Phonograph preferences dialogue.


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