Amplitude versus time, the science of analog audio tape playback

There are two predominate distortions commonly introduced in the playback of analog audio tapes.

The first is time base corruption (also called wow and flutter and pitch change). This is observed as unwanted frequency modulation and frequency shift.

The second is frequency response error, observed as a non-linearity of signal magnitude plotted across the frequency spectrum.

Both of these distortion types are easily measured with instrumentation.

Both are readily detectable by trained critical listeners.

Minimizing the playback distortions

Exploring the limits of sound quality hidden in analog master tapes requires playing the tapes back on a system that is adding almost no distortion.

Reducing the time base corruption introduced in playback is accomplished by improving the quality of the tape transport mechanism. This turns out to be costly, but improvements made here also lead to improvements in signal amplitude stability. For example, scrape flutter doesn't only contribute unwanted frequency modulation, it also introduces unwanted amplitude modulation.

Modern FFT spectral analysis for quantifying the time base accuracy of analog audio tape transports (suggested by D. Manquen) uses a 12.5 kHz carrier signal that is four times higher in frequency than the wow and flutter measurement standards historically employed. This broadens the measured bandwidth to reveal scrape flutter, once erroneously believed inconsequential to the perceived sound quality of recorded music.

A reference-quality tape transport mechanism

It's been known for fifty years that a tape transport that introduces the lowest time base corruption requires a servoed constant-tension, servoed capstan, precision guidance design fitted with a precision headblock using no forced guidance. Yet due to the prohibitively high cost of manufacture, less than one-half of one percent of all professional audio tape machines ever made were of this design.

Reference-quality playback electronics

Once a reference-quality transport is available, correcting frequency response errors becomes possible. With the transport contributing near-perfect (not forced) tape guidance with low scrape flutter generation and high dynamic azimuth stability, optimal playback quality can be realized with low-noise, low-distortion playback electronics that accurately follow the industry’s established equalization curves.

Meet the reproducer

The machine we're describing is not a recorder. It's optimized for highest-quality playback and it’s known as a reproducer. A professional quality reproducer must also offer accurately adjustable tape tensions and capstan speed trim.

One could attempt to build new, state-of-the-art reproducers from scratch, but the cost of small-scale manufacturing of the precision mechanical components could be prohibitive. So it's more practical to re-manufacture and modify the very best tape transports from the past.

Thank you Dr. Studer

A survey of the historical best-in-breed analog audio recording instruments incorporating the requisite design features will quickly illuminate the famed A80 and A820 recorder-reproducers built by Studer in Switzerland. Hence our exacting re-manufacturing program based exclusively on these two legendary Studer models.

We're finished only when the new transport has time-base accuracy exceeding the original Studer specification by almost an order of magnitude.

Preserving heritage master tapes: First, do no harm!

The work of transferring the content of surviving master tapes continues world-wide. Yet using the wrong tape machine for playback can damage or even destroy irreplaceable masters.

All tapes are slowly deteriorating due to age-related chemical degradation. Large numbers of Mylar (polyethylene terephthalate) film based tapes exhibit an oxide binder chemical degradation called soft binder syndrome that in some cases renders them un-playable without causing irreversible damage to the oxide layer. Acetate film based tapes suffer from another chemical degradation that makes the base film brittle and fragile.

Playing back legacy master tapes on vintage studio recorders is dangerous and irresponsible.

Any tape machine having a forced guidance tape path, fixed-pin lifters, or unnecessary path elements such as erase and record heads will cause undesirable higher running tensions and can damage tapes.

We began this work in 1987 making this our thirty-second year.

ATAE reproducer system pricing starts at $95,000.

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