Decoding Dolby

by David Adamson, Audio Preservation Engineer/Quality Control Specialist, Media Digitization and Preservation Initiative, Indiana University. This is an archival version of a post originally published on the MDPI blog, which ran from 2017-2020.

Audiocassettes encoded with Dolby noise reduction are notoriously difficult to digitize. Even if metadata makes clear the presence or absence of Dolby or the type used when it is present, it is often impossible to make it sound as intended during playback. This may be due to a number of factors, including years of degradation. In this post, MDPI audio preservation engineer David Adamson discusses MDPI’s strategy for accurate playback and long-term preservation.

—Mike Casey, Director of Technical Operations, Audio/Video, Media Digitization and Preservation Initiative, Indiana University

In November of 2018 MDPI began using a revised procedure for making preservation copies of Dolby-encoded audiocassettes. Up until that time, our attempts at Dolby-decoded transfers had been underwhelming (often with too much high frequency content missing) and A/B comparison seemed to suggest that in certain cases an undecoded transfer would be preferable to a Dolby decoded transfer. Was that really true? A combination of circumstances had to be deciphered, those of the conditions of the source material, and those of the engineers of the Dolby process itself

Even before November, MDPI’s first step was to design a standard setup and workflow. This workflow was put together by MDPI audio engineers Melissa Widzinski and Dan Figurelli and director of technical operations Mike Casey. They created a Dolby workflow that used the Nakamichi NR-200, an external Dolby B/C noise reduction processor, for decoding. The earlier attempts at Dolby decoded transfers had relied on the internal Dolby circuits in the cassette decks. I was hired on, in part, for the purpose of integrating the new workflow and creating some good Dolby-decoded transfers. I began by working off of the MDPI audio staff’s workflow instructions. These instructions mentioned magnetic tape expert Richard Hess’ technique of using an “azimuth style” adjustment; to dial in the gain before the signal is sent to the Dolby circuit.  This proved to be a key element of creating a good Dolby transfer. I was also forwarded some email conversations between audio engineers with experience doing Dolby conversions for audiocassettes. In these emails it was expressed that the reason Dolby decoding can make something sound worse is because the Dolby encoding process tended to boost high frequencies and add compression that can actually sound pleasant in some cases, depending on the content. The emails went on to describe how Dolby as applied to audiocassettes was a simplified version of the Dolby technology (it was originally developed for open reel tape, with more detailed encoding for finer frequency bands). Dolby B in particular was designed to be less aggressive and even less noticeable if it happened to be played without decoding.

The Dolby picture came into even clearer focus when I reviewed the NR-200 manual. It includes graphs that illustrate when exactly the Dolby circuit is engaged, and confirmed that louder sections may not be affected at all. This reinforced the importance of selecting a correct gain setting on the Nakamichi processor, and suggested that a fixed setting on an internal Dolby circuit would not be ideal, particularly for digitizing large collections with disparate origins.

When working on our initial collection of Dolby tapes, I found that the best-sounding method for decoding was to use Richard Hess’ azimuth style adjustment to find the amount of gain that kept all intended high frequency information in the signal, while still removing a subtle layer of tape hiss. “Azimuth style” refers to making fine adjustments to the playback equipment to ensure that all of the original frequency information is captured in the transfer.To do this I would be physically raising and lowering the gain pot on the back of the NR-200 while riding the master level fader on the front to keep the overall level even in my headphones. While I do this I listen to the high frequency information in the audio signal. Increasing the gain causes the signal to hit the Dolby circuit’s threshold more frequently, and I’m looking for a “sweet spot” that mirrors how the original operator set their equipment. This first collection featured flutes as the main instruments; it was obvious when the NR-200 was removing too much high frequency information.

MDPI decided to create both a Dolby decoded preservation master-intermediate file and an undecoded preservation master, because when setting the gain a judgment call has to be made for each tape based on the sound quality of the content. Since we capture and preserve a corresponding “flat”, or undecoded, version of the tape, decisions like these can be revisited if need-be, further down the road. The two file types are created at the same time during one playback of the source cassette.

For me this process highlighted the necessity of continuing to tackle these specialty formats as soon as possible, and to make good use of the knowledge and perspectives of the audio preservation community to arrive at as accurate a transfer as possible.