Audio Restoration
Audio Restoration – DRME – Tambi Studios
The goal of Audio Restoration is to reach through this curtain of unwanted noise and hear the recording like it was when it was first made. Audio Restoration is designed to “clean up” a recording, much like the paintings of the old masters that have been restored to their original quality. The object is to separate the sound from the medium that carries it, so you can hear the music without the clicks, pops, scratches and hiss that often plagued the original recordings. In some cases, the desired sound is so close in character to the noise, that some noise has to be left so as not to sacrifice parts of the sound that we want to keep.
Sound recordings are machine readable artifacts; they are documents for which the integrity of the information they contain is directly related to the artifacts’ physical well-being. Since the majority of sound recordings are made of plastic, conservation must be treated as a plastics degradation problem, requiring a different approach. It is important to understand the basic chemical degenerative processes and the principles of the retention of sound by the various media in order to ensure that proper action is taken to slow the rate of degradation.
THE DEGRADATION MECHANISMS OF SOUND RECORDING
The lifespan of a plastic is largely determined at the manufacturing stage. Variables such as basic resin, the materials added to the basic resin to alter its properties, the lamination of materials with dissimilar properties, and the manufacturing process itself, all directly affect the lifespan of the plastic. Post-manufacture environmental factors such as storage conditions, temperature, humidity, and handling also contribute to the long-term stability of the plastics.
Plastics can be divided into two main classes, thermoplastic and thermosetting. Thermoplastics soften and flow when heated and are normally shaped by heat and pressure. They will soften and flow again when re-heated. Vinyl, used in the manufacturing of LP’s, is a thermoplastic.
Thermosetting plastics are molded under heat and pressure. A chemical reaction occurs so that once molded they do not soften when re-heated and will normally char before melting. Most 78s are made of thermosetting plastics.
ACETATE DISCS
Since the 1930s, most blank acetate discs have been manufactured with a base, usually aluminum (although glass was used during the war years and cardboard for inexpensive home recordings), that was coated with nitrocellulose lacquer plasticized with castor oil. Because of the lacquer’s inherent properties, acetate discs are the least stable type of sound recording. Continuous Shrinkage of the lacquer coating due to the loss of the castor oil plasticizer is the primary destructive force. The gradual loss of plasticizer causes progressive embrittlement and the irreversible loss of sound information. Because the coating is bonded to a core which cannot shrink, internal stresses result, which in turn causes cracking and peeling of the coating.
Nitrocellulose acetate decomposes continuously and over time reacts with water vapour or oxygen to produce acids that act as a catalyst for several other chemical reactions. As with most chemical reactions, these reactions are accelerated with elevated temperature and humidity levels.
VINYL DISCS
Vinyl discs are made of polyvinyl chloride (PVC) and a small percentage (usually less than 25 percent) of “fillers”, stabilizer, pigment, anti-static substances, etc. Internal plasticization, through a copolymerizing of vinyl acetate with vinyl chloride, is needed to achieve the required properties for the desired application.
Polyvinyl chloride degrades chemically when exposed to ultraviolet light or to heat. Phonograph discs are exposed to high temperatures during molding and pressing. Unless stopped, this heat would be a catalyst for ongoing dehydrochlorination, which is the release of hydrochloric acid (HCl) from the PVC as a result of thermo-degradation. Stabilization is therefore achieved by adding a chemical to the resin during manufacture. This does not prevent the degradation but controls it, mainly by consuming the free HCl. Sufficient effective stabilizer remains in a plastic phonograph disc to protect it for several decades after pressing.
MAGNETIC TAPE
Magnetic tape first appeared in North America just after World War II.
It is made up of two layers: a “base” layer, and a thin “binder” layer which is bonded onto the base. The binder contains ferromagnetic particles whose permanent alignment within the binder produce the copy of sound waves.
MAGNETIC TAPE BINDER
Manufacturers are very secretive about the specific chemical makeup of their products. Binder chemical composition, uniformity and smoothness of application all affect audio quality, noise level, tape-to-head contact, and friction. These factors also affect the tape’s aging properties. The most common binder resin used today is polyester polyurethane. The most common ferromagnetic particle used is gamma ferric oxide (Fe3O2).
The most common and serious magnetic tape degradation occurs through hydrolysis, the chemical reaction wherein an ester such as the binder resin “consumes” water drawn from humidity in the air to liberate carboxylic acid and alcohol. Hydrolysis in magnetic tape results in the binder shedding a gummy and tacky material which causes tape layers to stick together and inhibits playback when it is deposited onto the tape recorder heads. The added friction increases tape stress and can cause machines to stop. Hydrolysis also causes a weakening in the bond holding the binder to the backing, which results in shedding or possible detachment
Chromium dioxide (CrO2) is used extensively as the ferromagnetic particles in cassette magnetic tape. It has been found that CrO2 particles interact with the polyester polyurethane to accelerate hydrolytic degradation. Additives are now added to retard this degradation.
Magnetic Backing
Cellulose acetate-backed tapes were manufactured from about 1935 until the early 1960s. These tapes rely heavily on plasticizer additives for suppleness, and these plasticizers are liable, over time, to evaporate and crystallize. These tapes have extremely low tensile strength and are easily broken. Cellulose acetate tapes are very susceptible to linear expansion in humid and/or warm conditions. Because of the different properties of the binder and the base, the absorption of humidity and heat result in tape curling and edge fluttering. These distortions greatly affect the tape-to-head contact, which in turn directly affects audio quality. Repeated dimensional changes due to environmental fluctuations grossly affect winding tension and can promote binder fatigue, cracking, and finally, catastrophic failure (i.e., the irreversible loss of data).
A serious problem affecting acetate tape is that of the “Vinegar Syndrome”. Vinegar Syndrome is exhibited by the release an acetic acid (vinegar) odour from the tape and is a by-product of acetate tape breaking-down. The process is accelerated by the presence of moisture and iron ferromagnetic particles in the tapes. When the acetate is degrading — giving off acetic acid — it will start to take up more moisture. The process of self-destruction is auto catalytic, once it has started it will continue with ever increasing speed; no solution for its interruption has yet been found. Tapes afflicted with the vinegar syndrome will “infect” healthy tapes.
Polyester (“mylar”) came into use in the early 1960s, and quickly replaced cellulose acetate for magnetic tape backing. Accelerated aging tests have found polyester to be a stable material which in fact undergoes hydrolysis degradation at a much slower rate than does the binder, polyester polyurethane, with which it is combined. However, polyester-based tape has a high tensile strength that can cause it to stretch irreparably (instead of breaking cleanly and reparably as does acetate-backed tape).
A third coating is now added to professional, modern tape on the opposite side of the binder. Made of carbon black particles held in a thin binder, it protects the backing from scratches, minimizes static electricity, and provides a more even wind.
Process
1) Determine what type Master Material… laminated, shellac, soft cut – lacquer, Tape 1/8th, 1/4 inch, half inch, one inch, two inch, cassette etc.
2) Select the correct chemicals and method to clean the surface of the disc to remove embedded dirt and/or any residual mold release compounds left over from the pressing process.
3) Clean the disc to avoid making any additional scratches or doing any other damage to what could already be a fragile item.
4) Determine if the disc was centered properly when it was pressed and the center hole was punched… if not, the disc will need to be carefully centered during playback to avoid adding “wow” to the finished transfer.
5) Examine the grooves to determine the correct size and shape of stylus required to get the best transfer of sound from the disc. Sometimes an odd size of stylus will get above or below areas damaged by previously playing with poor equipment or a worn stylus.
6) “Pitch” the recording, to determine the correct playback speed… many “78″‘s were not recorded at 78 rpm. Some varied from 76 rpm to 80 rpm, and an obscure few very old ones could have been recorded as low as 60 rpm or as high as 90 rpm.
7) NOW, you can PLAY the disc to hear the sound as it was originally recorded. A stereo cartridge allows the quietest groove wall to be selected for further digital processing.
Only now can the rest of the Audio Restoration process can continue…
The sound is converted to digital form 44.1 KHz, in a high quality Analog to Digital converter, and some very heavy-duty DSP (Digital Signal Processing) equipment begins to process the sound, removing the clicks, pops, crackle and even some distortions caused by groove damage. Often, hum or floor noise was present in the original recordings, and this is removed also.
Probably the most spectacular result of audio restoration is the surface noise removal, and final equalization and mastering to make the recordings sound as good as possible
Services
Floor Noise reduction processing
Audio restoration from DAT, cassette and all 1/4 inch and 1/2 inch audio tape formats
Phonograph disc transfer, any speed, any size, any groove, vertical or lateral cuts
DAT to CDr direct flat transfers (master quality) to full red book specifications
CD to DAT compilation mastering or HDD.
Tape or cassette to DAT transfers or HDD.
DAT to DAT digital clone transfers.
DAT SCMS copy protection application or deletion.
DAT mastering and level normalizing
Hum and noise removal, dimmer noise, whistles and TV sync buzz
“Baking” and/or transfer of “sticky-shed” syndrome audio tapes
ARCHIVAL with Meta data tagging.