Analog tape came into commercial use in the US just before 1950 when major record companies began switching to the tape format for recording. Tape formulations have changed over the years, not always for the better. Most of this section deals with 1/4" analog tape.
Paper Tape
Some of the earliest tape in the U.S. was made of oxide coated paper which was very dark, almost black. Paper tape is dimensionally stable, which means it usually survives without curling, shrinking or twisting. Audio recorded on paper tape sounds rough because the surface texture of the paper itself prevents close, continuous contact with the playback head.
In addition, audio recorded on paper tape was typically not recorded to the edges of the tape, and so it is often described as "center track," although the term has nothing to do with the much later center track time code format.
Therefore one of the challenges in playing back paper tape is track format compatibility. Quarter track and half track playback heads miss much of the audio. The best solution is to use the two inside tracks of a 4 channel playback head, or a full track playback head. The use of proper playback heads is important to minimize the textural problems of paper tape.
back to top
Acetate Tape
Acetate tape does not always age well. To determine if a tape is acetate-based, hold the reel up to the light. If it appears translucent when looked at from the edges, then it is acetate.
Acetate tape was popular from about 1950 through the mid 1960's. As it decomposes over time, the tape curls, shrinks and becomes lighter in mass. Deteriorated acetate tape puts off an odor of acetic acid (vinegar), and feels substantially lighter in weight in comparison to non-deteriorated acetate tape. Deterioration is thought to be storage related, but even properly stored tape will have problems on occasion. Tape ends that are not secured during storage will suffer damage first on the ends that are left loose.
Playback can be especially problematic, and the only way to play severely deteriorated acetate tape is to increase tape tension in order to maintain proper tape to head contact. Acetate tape in an advanced state of decomposition can be curled so badly that it will jump out of the tape guides entirely; but one of the features of acetate tape is that it breaks cleanly and does not deform readily, so tape tension can be made quite high without stretching the tape. The method to achieve this increased pressure is limited only by your imagination, based on your own recorder/player. Increased tape tension will cause accelerated tape head wear and playback speed fluctuations, but there is often no other way to play such tape.
Sometimes badly decomposed acetate tape cannot be returned to its original reel after playing because the tape deformation prevents a proper tape pack on the original reel. Sometimes the solution is to increase tape tension during rewinding or fast forwarding, or to simply store the tape on two reels.
back to top
Polyester Tape + Baking
Polyester or mylar based tape is opaque when held to the light and viewed from the side. Polyester tape will deform permanently before it breaks, so be careful not to stretch it.
Generally, polyester tape has a superior tape base, and oxide/binder formulations have steadily improved over the years to prevent oxide from flaking off, with one big exception: serious problems occurred on certain tapes manufactured from roughly 1975 to 1990 (and even later). The problem is known as "sticky shed syndrome."
Tapes with "sticky shed syndrome" leave a waxy residue on rollers, heads and guides, which destroys high frequency response and can eventually cause a tape player to stop entirely.
If a tape is suspected of having sticky shed syndrome, it should be treated before playing. Playing untreated tapes damages them, because it smears part of the binder containing the oxide and reduces high frequency response. In severe cases the binder will totally separate from the base.
Backcoated tapes are flat black on the side that does not make contact with the playback head, and any backcoated tape manufactured after 1975 should be checked for sticky shed syndrome. To diagnose the tape, create a roller only path, and rewind the tape slowly. Good tape comes off the supply reel at a near perfect tangent, but sticky shed syndrome will cause tape to stick slightly to the pack as it is pulled away. You can also hold a finger against the oxide side during rewind (be careful - it's easy to cut your finger), and if you detect any friction, or if your finger picks up any residue, then the tape has "sticky shed syndrome." Stop rewinding.
Generic Instructions for Treating Tape with Sticky Shed Problems
(as provided to me by an engineer from Quantegy Inc. on February 18, 1999)
The process for treating magnetic tape with sticky shed syndrome is relatively simple, provided you have proper equipment and adhere to the following instructions:
Acquire a convection oven. I use a Farberware T-4850. A radiant heat oven will not work, and can damage the tape as temperatures fluctuate more, similar to the way the temperature differs depending on whether an object is in sunlight or shade.
Calibrate the oven temperature with a verified accurate thermometer. Oven dials are not accurate enough for this purpose.
After the oven is calibrated, place as many tapes in the oven as possible provided you have stacked them with spacers so that there is a good inch of air space between them. I use pieces of broken duplicator hubs as spacers.
Bake tapes for 8 hours.
After 8 hours remove tapes from the oven and allow them to cool to the control room environment for 24 hours prior to working with the tapes. This allows the tapes to cool, relieves pack stresses, gives the binders time to re-adhere to the base film, and allows residual lubricants deep in the layers of the tape to exude to the surface to make the tapes runnable.
A temperature between 120°F and 130°F was first recommended by most authorities. Ampex recommended 50°C (129°F) in their process. When 3M made audio tape, they suggested 125°F for 4 hours for 1/4", and 7 hours for 2" tape. Some have recently (in 2008) recommended far longer times for 2" tape, reportedly as long as 120 hours or more in difficult cases.
A scientific convection oven is far better for supplying air at a proper and constant temperature when a larger mass is present, more so than when using a food drier, for example.
On June 5, 1996 Roger Nichols made a presentation to the New Orleans chapter of the Audio Engineering Society in which he described a vacuum device which produces good results, apparently because of the effects of vacuum drying. However, baking is recommended by most authorities, and scientific ovens are preferred.
Recently we have been increasing time and temperature settings for difficult cases.
NOTE: The information on baking tape is provided as is, without warranties of any kind. Baking tape is normally a routine operation, but problems can and do occur. Proceed at your own risk.
Further information on baking tape can be found here:
Binder Breakdown in Back-Coated Tapes by Scott Kent, Recording Engineer/Producer July 1988.
Recovering From Sticky Tape and Getting a Better Picture of Photo CD by Barry Fox, Studio Sound (date unavailable).
Sticky Shed Syndrome by Philip De Lancie, Mix May 1990.
back to top
Track Formats (1/4")
There are many different track width formats (e.g., full, half, quarter, etc.) in 1/4" analog tape.
Among popular 2 track formats for 1/4" tape, there are at least three track widths Ampex (1.9mm), NAB (2.1mm) and IEC stereo (2.8mm). Since lower frequencies are boosted by playing back with narrower playback head track widths (i.e., the "fringing effect"), this should be taken into consideration.
There are sprays available which can be applied to tape to reveal whether a tape is full track, half track, quarter track, etc. But these sprays are messy and don't provide a method of continually inspecting a tape whose format may change at any time. For this reason, the use of a professional 4 track 1/4" tape player is highly useful. By setting all channels at level gain, it is possible to see if the tape was full track, half track, quarter track, or even the paper based center track format.
back to top
Tape Speeds
Tape speeds for professional applications are normally 7.5, 15 or 30 ips, although tapes are also recorded at 15/16, 1-7/8 and 3.75 ips.
back to top
Equalization
Playback equalization is usually either American (NAB) or European (CCIR), and these standards are sometimes now known as IEC1 or IEC2. Another standard, AES, is for 30 ips, while Nagra had a different standard altogether. It is essential to play back tapes using the proper equalization.
back to top
Azimuth Adjustment
Unless the playback head is perfectly aligned in the same way as the record head was during record, the audio will have phasing problems, which will become apparent first as a loss of higher frequencies. If there is a high frequency test tone at the head of the tape, it is an easy matter to adjust the playback head for minimum phasing problems as shown by a vectorscope or oscilloscope. Without a test tone, adjustment is slightly more difficult - you adjust for the maximum high frequency response when summing to mono. There are also computer programs that will display phase vs. frequency.
back to top
Noise Reduction
The two most common reduction schemes used with open reel analog tape are Dolby and dbx. Actually, there are many more, but these are the most common.
Professional Dolby encoding for open reel tape is either A or SR. Both work by dynamically changing signal levels during record. Processing during playback is required, otherwise high frequency material will sound too bright.
Dolby has also B, C, and S type noise reduction schemes for consumer uses, namely audio cassette tapes.
Dbx works by compressing audio during record, and expanding it during playback. The limitation of dbx is that during playback the decode process must track the dynamics of the original record process, and this is not always easy to do, especially when there is large low frequency content.
Usually, neither Dolby nor dbx will be found on tapes recorded at 30 ips.
Dolby HX Pro is a single ended scheme and does not require decoding during playback.
back to top
Cassette Tape
Analog cassette tape presents special problems because the tape is only 3.81mm (0.150 inch) wide and is recorded at 1-7/8 ips.
Mildew on cassette tape will cause dropouts, and old splices at the leader will often break. When a splice breaks at the leader tape, it's usually practical to repair the splice and then put the repaired tape and hubs in a new shell.
Adjusting for proper azimuth alignment is essential when playing back cassette tape, because tape speed is slow compared to professional tape formats. Even a small azimuth misalignment will really compromise audio playback from cassettes.
Cassette recorder/players vary considerably in speed, and it's not uncommon for a machine's speed to be off considerably. Normally when a cassette tape is played on the same machine that also recorded it, then this problem will not occur.
back to top
Handling and Storage
The Audio Engineering Society publishes standards on tape handling and storage, and the Library of Congress (US) has more information here.
We repaired the broken pack on the left without resorting to cutting and splicing the tape.
back to top
Compact Disc