The following post was written by Fred Thal and is presented as a guest editorial…enjoy.
Are we squandering the last opportunity to preserve our recorded music heritage in the highest fidelity?
The new transfers being done today could be the very last time that most surviving analog master tapes are ever played back. It’s because all archived audio tapes are undergoing chemical changes with time, and many are becoming increasingly problematic and fragile.
Analog audio tapes can be broadly classified into two main types, according to the composition of their base films: cellulose acetate and Mylar. Each type has its own characteristics and long-term aging profile. Acetate tapes are vulnerable to becoming brittle and weak. Mylar tapes appear to have a more stable base film, but can exhibit chemical instability in the binder, the compound that solidifies and glues the magnetic oxide layer to the base film. Softening of the binder (termed soft binder syndrome) is commonly seen on Mylar tapes and this can develop into sticky shed syndrome. As the name suggests, sticky-shed tapes can shed oxide, leave deposits and stick to a tape machine’s heads and guides, often slowing down and sometimes even stopping the machine.
The mechanism responsible for moving the tape at a perfectly constant velocity over the sound heads of an audio tape machine is known as the tape transport. Transports can be classified according to their design and their construction. They can be simple (constant torque) or complex (constant tension). They can be servo controlled, or not servo controlled. The mechanics of tape guidance in the transport defines the path that the tape will follow. In the case of transports designed for the common, quarter and half-inch tape widths (almost all surviving mono and stereo master tapes are in these two tape widths) there are both force-guided and precision-guided transport designs.
Force-guided designs are inexpensive to build and thus they account for more than 99 percent of all tape machines ever manufactured. Force guidance overcomes problematic zenith-error tape steering by literally forcing the tape to follow from point to point thru the tape path by the imposition of tape edge guides. But the tape edge is where the vulnerable oxide layer to base film junction is exposed, and forced guidance harmfully abrades this boundary, often causing roughening and wrinkling of the tape edge which can lead to problems including shedding and spacing loss.
Cost-no-object quarter and half-inch transport designs employ precision guidance, an architecture that does not force the tape to travel within a plane, but rather allows it to. With a precision guided transport, precision guided headblocks can be fitted to achieve very low levels of scrape flutter. This can make an audible improvement.
Another completely avoidable tape damaging feature found on most tape machines are fixed-pin tape lifters, used in inexpensive transport designs to lift the tape away from wear-prone tape heads during fast winding. For a tape with soft binder syndrome, just one pass over fixed lifter pins can cause irreparable harm to the oxide layer. Viewing tape under a stereo microscope confirms this.
We have been aware of tape chemistry degradation problems for thirty years and the fragility of surviving master tapes should make it unacceptable to employ traditional studio tape machines for their playback. But we have not yet seen the music industry respond responsibly. Instead, popular work-arounds, such as baking sticky shed tapes, are exclusively relied upon, while the work continues using the same old hardware.
It might appear that a solution for working with high-value master tapes is to obtain old tape machines employing precision guidance and not having fixed-pin lifters. Examples of quarter and half-inch machines fitting this description are the Swiss-manufactured Studer A820 and its predecessor, the A80. Another is the German-manufactured AEG M20.
But these machines will not perform properly today, as they are now far beyond their expected operating life before complete overhaul. A major issue is precision bearing lubricant dry-out, which destroys the transport’s guidance and the machine’s once exemplary low flutter performance. Importantly, correct bearing replacement in these machines can be much more involved than what might at first appear. It is not a simple matter of just exchanging the bearings for new. Unfortunately, most tape machine service expertise here in the USA is of little help with the precision Studers and AEGs, as our industry grew up almost entirely around maintaining Ampex recorders. If you don’t know how a precision guidance tape transport is supposed to work, you’re probably not qualified to be taking it apart for re-building. It’s akin to taking a Ferrari to a Ford garage.
About the author: Adolph Friederich Thal is managing director at ATAE, a California firm that manufactures professional analog audio tape playback hardware.