An operator at Sony’s Recording Media Restoration Centre in Dax, France, supervises playback and restoration of a U-matic tape.
Magnetic memories fade. Around the world, broadcasters have vast tape archives that represent a cultural history of the latter half of the twentieth century. According to UNESCO, there are 200 million hours of audio-visual archives on this planet. This archive is of interest as entertainment and as a source of documentary material. Unfortunately, magnetic recordings have a finite life, so if nothing is done to restore the archives, this valuable material could be lost.
After about 20 years, the tape medium starts to deteriorate. By 30 years, the reels of content can become unplayable. Unlike paper documents or movie film, videotape playback is inextricably linked to specific hardware. The problem lies with the VTR; formats are forever changing, and it is becoming more difficult to keep machines from the earlier formats operating. There are fewer and fewer quad machines still working, and other formats such as B and C will inevitably go the same way. At least it is relatively easy to turn up new parts for a quad machine with a lathe and a mill. The manufacture of one-off parts for more recent cassette formats will present quite a challenge to future generations.
Broadcasters find that it expensive to maintain a large tape archive. Maintaining videotape under optimum conditions means that it should be at around 15°C at a relative humidity between 20 percent and 40 percent. A staff is required to index tapes and to perform all the regular librarian's duties.
Industrial scale restoration
If an archive is to be restored, there is the cost of copying older formats to current formats. It is difficult to make a financial case for preservation or restoration for a great deal of material. The tape archivist always has to struggle for funds to maintain old libraries. The cost of manually transferring old analog tapes to digital formats (that can easily be cloned in the future) often means that the program tapes are trashed. It is just not economically viable to run the warehouses or to transfer them to a modern format.
Several years ago, Sony identified an opportunity to restore audio and videotape on an industrial scale. Many broadcasters have restored archive material on an ad-hoc basis. The possibility of lowering the cost of restoration means that more material, either valuable or of historic record, can be saved for future generations.
Archive tapes are mostly open-reel 2in quad and 1in helical scan, although early cassette formats such as U-matic and the original Betacam are beginning to show signs of decay. It has been estimated that 84 percent of the European archive is in analog format, with 3 percent on quad and 15 percent on 1in.
Sony used its tape manufacturing plant in Dax, southwest France, as the base for the Recording Media Restoration Centre (Sony RMRC). The Dax facility manufactures videotape for both consumer and professional markets. The factory staff has accumulated 20 years of knowledge about recording tape formulations, and it has a well-equipped laboratory for the chemical and microscopic evaluation of recording tape. These assets are becoming useful when dealing with historic tapes and for judging the best ways to deal with the inevitable surface contamination of archive media.
The principle of preservation is to save the content, not the medium. The target should be the needs of future audiences; it is just not feasible to rescue every reel or cassette. If the future worth of a tape can be estimated, then it is easier to make the decision to restore or destroy.
The preservation process involves recovering the analog recording, processing it and then recording it to a new digital media. This recovery process could result in another videotape, or it could be a data file; the choice is up to the client. Ideally, a file-based format allows the content to be abstracted from the media. Files can be stored on data tape and migrated to new media as necessary. The file formats should be future-proof. That means playback or rendering in the future should not require esoteric proprietary compression codecs or specific computer operating systems that have become obsolete.
Figure 1. Shown here is the physical structure of magnetic tape, which consists of a layer of magnetic particles attached to a base film. Click here to see an enlarged diagram.
Magnetic tape consists of a layer of magnetic particles attached to a base film. The magnetic layer is about 5 microns thick. The magnetic particles are held in a binder that adheres to the base film. (See Figure 1)
Problems of archive videotape
The binder also allows the magnetic layer to be calendared to a high polish to give good head contact and low friction. The binder is a mix of several constituents, including lubricants and cleaning agents. The magnetic material makes up about 40 percent of the volume. Typical base film thicknesses are about 38 microns thick for quad tape and 28 microns for C-format. (Human hair ranges in diameter from 20 to 180 microns.)
Early tape used iron oxide for the magnetic particles, but later formulations changed to metal particles or evaporated metal. Evaporation lays the magnetic layer directly on the base film and does not need a binder.
The tape carries a thin top layer of lubricant to reduce friction over drums, guides and heads. Operations such as fast wind could cause a build-up of static electricity, so the tapes often have a back coating loaded with a conductor such as carbon black that can dissipate any build-up of electrical charge. This layer also balances the magnetic layers to prevent curling of the tape.
The primary process that causes deterioration of videotape is chemical breakdown of the binder, typically composed of polyester and polyurethane. Over time, water in the atmosphere hydrolyzes the polymers to acid and alcohol end groups, which causes the long polymer strands to break. The net result is that the binder can shed particles from the substrate. In severe cases, the oxide layer can delaminate from the base.
There were particular problems around the 1970s to 1980s with binder formulations. The binder would sometimes break down in a few years, becoming soft and gummy. This is called “sticky shed.” When these tapes are played, the oxide and binder clogs the heads, causing low RF levels. Plus, loose particles cause dropouts.
Old tapes also can suffer from mechanical damage from poor tension control during wind. The layers can slip, causing steps in the tape pack (pack slip). If the tape is loosely wound, layers can rotate, causing cinching or bucking of the tape. Another problem is blocking, where layers can stick to one another.
Some early tapes used an acetate base film. This decays to give off acetic acid — the vinegar syndrome. Once this stage is reached, the tape must be rescued immediately. Improper handling can cause the tape's surface to be contaminated with dirt. The tape spool linings also can disintegrate, further contaminating the tape with adhesive.
Older analog recordings have high noise levels. Plus, if the tapes have been played many times, the wear and tear can cause problems with tracking and control track playback.
Although binder deterioration is the biggest problem, all these factors combine to make satisfactory playback difficult.
There are two aspects of playback. One is to create the best conditions for the mechanical transport of the tape; the second is to recover the maximum electrical information from the magnetic coating.
Before the actual playback can take place, the tape should first be sent for special treatment. Here, it undergoes a four-step preparation process:
- Dry cleaning (physical).
- Wet cleaning.
- Coating with lubricant.
Building new-old VTRs
First, the tape is examined to determine the treatment that will be necessary. The tape is physically cleaned and, if necessary, washed in a solvent to remove surface contaminants. RMRC uses cleaning machinery from RTI plus some custom equipment. Any contaminants are analyzed for chemical content, and the best solvent is selected to suit the tape formulation.
If the binder is in poor condition, a baking process can be used to stabilize it. The chemical cross-links that hold the binder together break down by hydrolysis. Baking at an elevated temperature can restore the links sufficiently to allow playback. Baking does not reverse the damage caused by hydrolysis; it is a temporary fix allowing the tape to be copied.
If the tape lubrication layer has degraded, playback will result in head clogs with attendant fluctuation of RF level, plus heavy jitter. Audio engineers will recognize this problem as a squeal, which is clearly obvious when a tape with this problem is played. If necessary, a top lubricant can be applied using an evaporation process similar to that used during the manufacture of DVCAM tape.
The VTRs used in the process must be carefully restored. A local workshop can make new parts if necessary. Not that an operator would make a mistake, but these machines usually have the erase electronics removed just to prevent any accidents with precious tapes. The tape machines are carefully aligned to optimize RF levels and to maintain the integrity of the video signal.
Some tapes undergo surface cleaning to avoid oxide shedding during playback. Only now is a tape ready to be loaded on the VTR. The tape is played and converted to digital video. The video output is recorded to a videocassette, perhaps Digital Betacam, or can be stored as a file. The media specification is up to the client.
Restore it or lose it
A range of conventional video processing is used to maximize the image quality. This can include time base correction, noise reduction, scratch removal and frame retouching. Once in a digital format, the material can easily be cloned in the future without further degradation. The final operation is disposal. The old decaying masters are scrapped to avoid storage costs. European law dictates that modern environmental regulations are followed in the disposal.
Some clients are reluctant for media to leave their vaults. In such a case, machinery can be set up at the broadcaster's premises. It is very much a bespoke business, tailored to meet the budget available for the project.
Each archive treatment program is tailored to the special needs of the individual broadcaster. The RMRC opened last year with an audio restoration project for INA, the French National audio-visual repository. The current capacity is 20,000 hours of content per year, with the ability to ramp up to 250,000 hours by 2007.
Although archiving is often neglected when compared with new program production, future generations will not forgive us if we lose our recorded legacy of cultural heritage. Archive restoration will not wait because tape binder decays, and old VTRs can't run forever.
Unfortunately, restoration requires money and, therefore, is typically not a high priority for the broadcaster focused on today's productions. Isn't it ironic that correctly processed black-and-white photographs will outlive videotape by a factor of 10 times or more?