Dr. AIX's POSTS — 01 December 2014

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There’s more to the story of analog tape than what I’ve revealed in recent posts. I’ve written a lot about the failings of analog tape when it comes to its inherent noise floor, which hangs about 60-72 dB below the signal. That’s the figure for a first generation tape recording done on a professional machine using quarter inch tape running at 15 inches per second in stereo. This tape format was used for tens of thousands of masters made during the heyday of analog tape in the 60s and 70s.

My first professional multitrack machine was a 3M 56 16-track 2-inch machine. It was huge, heavy, and ran like a truck. It was the first multitrack machine to have all of the electronics located in the belly of the machine and the meters above the transport. The 2-inch format allowed for the same track widths that were available to the 2-track quarter inch format mentioned above. In the land of analog tape, the wider the tracks the more signal can be recorded and thus the noise floor diminishes. That’s why some esoteric machines run only 2-tracks on a 2-inch tape…I think it’s ridiculous but there are custom machines out there that are built to stretch the limits.

Then along came the ubiquitous 24-track multitrack machine. This format gave producers, engineers, and artists 8 more tracks to play with…but the fidelity of each track suffered because they got thinner. The difference is not huge but it is noticeable. The level of hiss on an analog tape is manageable on the master but grows with each transfer…by about 6 dB. You can mute tracks or use gates during mixdown but a copy is a copy and the increasing hiss level can’t be denied. The final master delivered to the guy cutting the lacquer can be as much as 3 or 4 generations down from the original tracks. That’s one very compelling reason why digital rules…you don’t have any generation loss.

How do you improve the dynamic range of analog tape? If you’re Ray Dolby, you design and build equipment that compresses source signals prior to recording them and then expands them back to their original dynamics on playback. He introduced a 4-band noise reduction method called Dolby A for professionals back in 1966. His approach allowed the mixdown machines another 10-15 dB of noise reduction depending on the frequency in question. His invention was a major step forward for professional studios using analog tape.

But the improvement came with a downside. The equipment required very careful calibration to ensure that all of the compressing and expanding of the signals were matched at every frequency and level. The process was challenging but not impossible. I can remember aligning countless machines during my stint at Mama Jo’s studio in North Hollywood.

The use of noise reduction systems like Dolby A and its competitor dbx were widespread in professional studios back in the 70s and 80s. They were matched with the Dolby B consumer format used on cassette tapes to bring maximize high frequency response.

It wasn’t until 1986 that Dolby introduced its SR noise reduction system for multitrack machines. Called Spectral Recording, Dolby SR was a very aggressive scheme that used a very large number of filters that responded in realtime to the source signals in order to maximize the level of the signal and minimize the noise of the tape. The complexity of Dolby SR was many times greater than Dolby A. It was used in professional studios but was very expensive and cumbersome. By the mid 80s digital systems were gaining a foothold in studios.

I can remember listening to an undecoded SR tape. The sound was almost unintelligible…not merely dull or muffled like undecoded Dolby A. To think that every bit of the source signal is restored without distortion is a stretch. Dolby SR worked but wasn’t embraced by audiophile labels.

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About Author

Dr. AIX

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) Readers Comments

  1. 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…not 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!

    Howard

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

  2. 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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