Tech's Files | Back to the PastUNDERSTANDING AND RESTORING ANALOG TAPE 3/01/2011 4:00 AM Eastern
This month, we’re revisiting analog tape and the physical restoration process—as opposed to using the software restoration apps spotlighted in the December 2010 issue of Mix. If you read no further, your most important step is to not attempt to play or fast-wind any tape before evaluation. For seriously problematic tapes, you may want to consult or hire an experienced professional. Precautions are necessary because fewer people have analog tape experience.
For those who are unfamiliar with magnetic tape basics and common practices, a brief recap is in order.
JUST ONE WORD: PLASTICS
In the U.S., analog tape manufacture dates back to the late 1940s, when the more popular plastic tape component—cellulose acetate—dominated ’50s-era recordings and continued to be used into the ’60s. Acetate does not stretch, but it does break “cleanly” and can be easily spliced back together. Over time, it becomes more brittle and should be played on a gentle, well-maintained machine.
Introduced in the ’50s, Mylar (DuPont) and polyester (generic) tapes are essentially the same plastic animal, aka PET. To see the difference between acetate and Mylar, hold a reel of each up to a light source. Acetate is translucent (it passes light) while Mylar is opaque (it does not pass light). Unlike acetate, Mylar can be stretched until it curls into a “tube,” and while obviously not the goal, the increased tensile strength is much more tolerant of tape machine abuse.
Magnetic recording tape comprises three or four key ingredients: the plastic tape (base material), iron oxide (the memory component), binder (the “glue” that secures the oxide to the tape) and, in later variations, a conductive carbon backing (to reduce static electricity). For some tapes, it is common for the binder to absorb moisture and become more like rubber cement, hence the name “Sticky Shed Syndrome,” or SSS. In many cases, dehydration (via low-temperature baking) re-activates the binder, but it’s not for all tapes. SSS will gum up everything stationary in the tape path (lifters, heads and guides), so be sure to inspect and clean often. When the job is done, clean the pinch roller as the chemical components in tapes do the most damage to these “rubber” parts.
All professional tapes should have been stored in a played state, also known as “tails out.” A visible clue to this is a smooth tape pack. This not only minimizes environmental contamination (dust particles and humidity), but it also makes the dominant print-through (layer-to-layer “echo”) occur after the initial recorded sound, a much better alternative to pre-echo. Tapes may be stored on metal or plastic NAB (large center hub) reels. For tape wound on the hub only—without flanges—you’ll have to find a pair or disassemble an existing reel. Care must be taken to place a flange on the tape and then flip so that the tape and the hub are supported. Otherwise, the hub may drop out from the center and tape may unravel in a most dreadful way.
Tapes stored on small plastic reels should be transferred to a larger reel to reduce the potential of the tape being wound too tightly around the hub. High tension can cause mechanical distortion from the first layer to be “transferred’” into subsequent layers. This condition is made worse if the tape is to be baked.
Make a visual inspection and determine the age, manufacturer and tape type. If you’ve only ever edited via a DAW, you should know that analog tapes were edited and sequenced with the help of a white china marker, a demagnetized razor blade and a special, low-ooze adhesive splicing tape. Improvements in the quality of splicing tape have been made—blue tape is better than white tape. Also note that blank (biased) tape or leader tape (plastic or paper) may exist in different segments of each recording. Splicing tape doesn’t typically adhere to paper tape as well as it does to plastic tape. Leader tape is very obvious, and with a little experience, it’s possible to see splices without playing the tape.
Back in the day, you’d simply rewind the tape to the head and get started, but not today! SSS tapes and those with vintage splices are not tolerant of impatience. A ’70s-era Mylar/polyester tape will want to be “processed” before playback. A suspect tape with a bad “pack”—one where the layers are unevenly up and down—should first be “baked” at a low temperature (no more than 120 degrees Fahrenheit). Afterward, play to determine direction and format (number of tracks, speed, etc.), making sure that the tape ends up tails out before baking a second time. Even with a smooth pack, it might be a good idea to follow the same procedure so that the tape can be re-tensioned between the first and second bake.
Once baked to satisfaction, you will want to avoid a fast-wind because no matter how attentive, you will rarely be fast enough to save a tape if a splice comes undone at high speeds. When that happens, the tape will whip against unforgiving stationary objects, either chipping off the oxide or the tape itself. While it will take longer, reverse play is safer and sometimes necessary, allowing time to inspect and replace all splices, if necessary. Splices that play through the first time may not hold up during subsequent shuttling and playback passes.
MODERN VS. VINTAGE DECKS
Early machines had fixed guides that are only useful because they reveal how badly a tape can shed, especially during fast-wind. Studer machines have roller guides where the heads are the only stationary devices. Late-model tape machines like the Studer A-827 (and modified Ampex ATR Series machines) have a library-wind mode that can be preset by the user for a slower, safer journey.
AND NOW, THE BAD NEWS
Place a soft cloth or paper towel over the heads (Fig. 1) to confirm successful baking and clean remaining residue (Fig. 2). And while not often as easy, do the same for the backside of the tape, if coated. If the cloth looks like Fig. 2b, another round of baking will be required. Sometimes the residue is not the color of oxide or back coating, but white-ish. A combination of more baking and more cleaning may eventually resolve the issue. Baking resolves SSS most of the time, yet some tapes require more extreme techniques, like playing below normal speed or well-below room temperature.
Once the tape is playable, it will be necessary to electronically and mechanically calibrate the machine. Professional tapes should come with their own test tones for level, frequency response and azimuth calibration. The tape might also be encoded with noise reduction (Dolby, dbx and Telefunken).
Eddie is awaiting a new pair of food dehydrators for evaluation and is currently developing presets for a dbx-compatible plug-in.