Mark Waldrep, aka Dr. AIX, has been producing and engineering music for over 40 years. He learned electronics as a teenager from his HAM radio father while learning to play the guitar. Mark received the first doctorate in music composition from UCLA in 1986 for a "binaural" electronic music composition. Other advanced degrees include an MS in computer science, an MFA/MA in music, BM in music and a BA in art. As an engineer and producer, Mark has worked on projects for the Rolling Stones, 311, Tool, KISS, Blink 182, Blues Traveler, Britney Spears, the San Francisco Symphony, The Dover Quartet, Willie Nelson, Paul Williams, The Allman Brothers, Bad Company and many more. Dr. Waldrep has been an innovator when it comes to multimedia and music. He created the first enhanced CDs in the 90s, the first DVD-Videos released in the U.S., the first web-connected DVD, the first DVD-Audio title, the first music Blu-ray disc and the first 3D Music Album. Additionally, he launched the first High Definition Music Download site in 2007 called iTrax.com. A frequency speaker at audio events, author of numerous articles, Dr. Waldrep is currently writing a book on the production and reproduction of high-end music called, "High-End Audio: A Practical Guide to Production and Playback". The book should be completed in the fall of 2013.

4 thoughts on “Let’s Talk Noise Reduction

  • Howard Kaufman

    Hi Mark,

    I’m usually right with you on most things but this sounds a little off:

    “I can remember listening to an undecoded SR tape. The sound was almost unintelligible‚Ķnot merely dull or muffled like undecoded Dolby A.”

    When I used to listen to undecoded Dolby tapes they sounded very bright, not dull or muffled. Of course as I get older I wonder if I’m remembering things the way I should!

    Otherwise a nice article!


    • It depends on the type of Dolby encoding that was used. Dolby A was listenable although dull…but Dolby SR was completely different.

  • The beauty of the Dolby systems was that above a certain level (I wanna say -20, but it’s foggy), they didn’t process and were linear. Below that threshold the compression ration increased gradually. That meant high level mistracking just didn’t happen, it only happened on lower level material. They were also multi-band devices, so the action of one band’s compander was partially masked by the others.

    The big calibration bug was the reference level. Get that wrong, and things weren’t good. Frequency response was an issue too, but we always tried for flat anyway. Changing to a different reel of the same tape, but different batch would slip your cal level enough to be a problem. The Dolby Tone was distinctive, ubiquitous, and required for proper level cal.

    By contrast, dbx suffered from mistracking all the time, as it was a 2:1 compression and 1:2 expansion over the entire dynamic range, single band, but with additional pre-emphasis. When the mistracked at the top end, which was common, the entire decoded level modulated. There was no cal tone, or reference level. It worked well some of the time, despite.

    A funny tidbit, the Dolby A type card used in most processors was designated Cat. 22. Someone made a dbx card that fit in Dolby processor frames, so you could swap between the two without adding a whole bunch of gear. Their designation? Of course…had to be…K9-22.

    I still say a master with 30dB of dynamic range (that’s like S/N of 24dB with 6dB of peak headroom) wouldn’t be acceptable, but then I never mastered a rock record. Add at least 10dB, I’m on board.

    • I’m with on this Jim…I remember very well the days of Dolby tones and the K9-22 card. As for the dynamic range of most pop and rock records…sometimes 30 dB would be generous. The new Tom Petty record has less!


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