Corey Bailey
Audio Engineering
A-Wind Meaning; ‘Oxide in.’ An audio tape that has been wound in an A-Wind, will have the tape oxide on the inside layers and the tape backing towards the outside. This is how audio tape has been wound and delivered for about 70 years. Acetate tape Acetate Tape refers to the material used as the base for tapes manufactured from the early 50’s through the 1970’s. During the 1970’s, Polyester became the preferred base material for analog tape. It’s worth noting that each material has exhibited problems over time. The acetate base becomes brittle over time. When conditions are very dry, the tape can deform, causing a condition known as ‘cupping’ and the tape will take the shape of a metal Venetian blind. Too much moisture during storage can cause a condition known as ‘vinegar syndrome’ in which the acetate base releases acetic acid and the tape will have a vinegar odor. FIG. 1 to the right, is an illustration of a typical tape recorder head. The vertical line in the center represents the head gap. Azimuth In the world of magnetic tape recorders, azimuth refers to the alignment of the head gap relative to the direction of travel of the tape. This involves mainly the record and playback heads however, on some tape machines, the erase head is also adjustable. The head gap represents the centerline of each head and should be precisely 90 degrees to the tape. Any variation from 90 degrees will result in a loss of high frequency information. Azimuth is one of the adjustable parameters of each head and there are specific methods to check and adjust for the proper alignment. B-Wind Meaning; ‘Oxide out.’ During the dawn of analog tape recording, some tape recorders used audio tape with the oxide facing out. In the early days of tape recording, one could buy audio tape that was wound that way. Nowadays, a B-Wind is sometimes used to reduce ‘Cupping’ on Acetate base tapes. Back Coating Many brands of professional polyester audio tape had a coating on the backside of the tape (opposite the oxide). Many brands of audio tape had a back coating that was electrically conductive. The back coating performs some important functions; 1) The foremost purpose is to help prevent static buildup, which can cause arcing and subsequent audible snaps in the recording. 2) While helping to eliminate static, the same back coating allows for more even packing (winding) of the tape on the reel. 3) The back coating also serves as an insulating layer that helped prevent print through, a condition in which the recorded magnetic field migrates from one layer of the tape to another. Bias In a tape recorder, bias is a high frequency Alternating Current (AC) signal that is applied to the erase and record heads during recording. The reason for the bias signal is to magnetize the ferrous iron particles in the oxide in a manner which provides the best linearity for recording. The frequency of the bias signal is supposed to be at least 10 times the highest audio frequency capable of being recorded by a given recorder (but rarely is). A typical bias frequency for consumer recorders is anywhere between 40 kilohertz (forty thousand cycles per second) and 100 kilohertz (kHz). Professional equipment can have a bias frequency approaching 250 kHz. The fact is, anything that can be magnetized, does not magnetize in a linear fashion. Early attempts to record without a bias signal, on an oxide containing iron particles, resulted in poor frequency response. Applying a DC (Direct Current) bias proved to be of little benefit. The first patent for AC bias for use in electronics was filed by W. L. Carlson and Glenn L. Carpenter in 1921. However, it was Walter Weber who employed the technique of applying AC Bias to magnetic tape recording. Binder Hydrolysis This is a condition where the chemical binder in the oxide has absorbed moisture over time to a point that causes a chemical change in the composition of the oxide itself. This often results in Sticky Shed syndrome. Another variation in the decomposition of the oxide results in a milky-colored stain on the surface of the oxide that dramatically increases the friction during playback, often causing the tape or magnetic film to come to a stop. Coercivity For magnetic tape, Coercivity refers to the strength of a given magnetic field required to reduce the recorded magnetic field (signal) on the tape to zero after the oxide has been driven into saturation. Tapes using an oxide with greater coercivity, generally require a stronger Bias signal. Capstan On a tape recorder, the Capstan is the rotating vertical shaft that pulls the tape past the heads. The tape is usually squeezed between the Capstan and a rotating wheel known as the Pinch Roller. Degausser A device that emits a strong enough Alternating Current magnetic field to completely erase magnetic media or magnetized metal parts and tools. Demagnetization Simply stated: “The process of removing magnetism from any material that can be magnetized.” In tape recording, tapes are demagnetized by being exposed to an AC field strong enough to overcome the coercivity of the tape. On audio tape decks, this process is accomplished by the erase head during recording. Also on audio tape decks, the metal parts that come in constant contact with the tape (particularly, the tape heads themselves) can become magnetized over time and have to be demagnetized using a portable degausser. The process of demagnetizing the heads and various parts of a tape deck has its own learning curve. If done improperly, one can actually magnetize the intended parts and cause harm to tapes that are subsequently played on that machine. Dropout A dropout is a brief loss or sudden decrease of signal level (volume), most often caused by a defect in the tape oxide. Dropouts can also be caused by damage to the tape itself, a temporary clogging of the record head during recording, or likewise, a clogging of the play head during playback. Tape oxide defects can cause frequency-selective dropouts, such as a brief loss of high frequency information. Tape speed can be a factor in the effect of tape-defect related dropouts as the condition is less noticeable at higher tape speeds. Dynamic Range In the world of audio, Dynamic Range is defined simply as the range of volume from the loudest to the softest of sounds. Dynamic Range is expressed in decibels (dB) which is a logarithmic scale. We Recording Engineers, often refer to Dynamic Range in terms of the difference between the loudest undistorted signal that be recorded down to the noise level (floor) of a given medium. Analog tape is capable of a dynamic range of roughly 70dB. A Compact Disk (Audio CD) has a theoretical dynamic range of 96dB. The average human can hear a dynamic range of approximately 140dB. Erase Head The erase head on an analog tape recorder is self explanatory, it erases tape! It does so by being energized by the bias oscillator with enough voltage and current to saturate the tape with the bias frequency. The erase head is usually only energized during recording. The erase head is also the first to come on contact with the tape during recording. If you view the heads of a reel-to-reel tape deck straight on, the erase head will be on the left. Flanging Flanging is an effect that was originally created by using two analog tape decks playing the same material in sync with each other. The operator would put gentle pressure on the supply reel of one of the tape decks, causing the speed to change slightly. The resulting speed change caused random parts of the signal to react with each other and the result was recorded. Because this process is phase related, it was often known as ‘Rubbing.’ It didn’t take long for hardware to appear on the scene, eliminating one of the tape decks. The signal was played through the hardware and all the operator had to do was turn a knob to achieve the desired effect. Now, the effect can be created digitally with software. Flutter Tape recorders are designed to pass the tape across the heads at a very constant speed. Any changes in the tape speed of a pre-recorded signal are perceived as changes in pitch. If these speed changes occur very rapidly, the effect is a ‘fluttering’ sound. Hence, the term. Flutter can be caused by something as simple as a piece of tape that becomes inadvertently wrapped around the capstan or a very worn pinch roller. A type of tape degradation known as Sticky Shed Syndrome can cause flutter as the tape itself tends to chatter across the heads. Guides Tape guides are generally stationary posts that are placed very near the heads to keep the tape in proper vertical alignment. Worn guides can not only loose their ability to keep the tape properly aligned but can actually damage tape as it is passed across them. Heads out Holding a reel of audio tape in front of you, the tape will un-spool from the left. In order to indicate the state of the tape, the loose end is usually folded and fastened to the reel with adhesive paper tape in such a manner that the folded and taped end will face the operator when the reel is placed on the machine (or a table, for that matter). This makes it easy to identify which way to mount a given reel of tape onto the tape deck. IPS (Inches Per Second) The linear speed of the audio tape as it is played on the tape deck. Tape speeds vary from as slow as 15/16 IPS to as much as 30 IPS for analog tape machines. Reel-to-reel digital audio tape machines will typically operate at a greater speed. Generally speaking, for analog tape, the faster the tape speed the better the sound quality. Tape speeds of 3-3/4 & 7.5 IPS were common for consumer tape recorders and professional machines typically operated at 15 & 30 IPS. Leader Audio tape leader is a paper or plastic product cut the same width as the tape. Leader is used to separate specific segments of audio tape, such as each song on music masters. Leader is typically added to the beginning of a reel and often to the end of a reel of tape. Leader that has been applied to both ends of an audio tape will protect the first couple of wraps of the tape at the head and protect the tape from the threading slots on the center of the reel at the other end. Some manufactured tape comes with leader already attached to the tape. Some plastic leader is printed with markers for every second of time and is called: ‘Timing Leader’ Library wind This is a process whereby the tape is wound from one reel to the other at reduced tension and at a speed that is much slower than full rewind speed. Typically, around 45 IPS. This results in a very even wind on the take-up reel, and a tails-out configuration which, is desirable for long term storage. Lifters The tape lifters are vertical posts that usually reside near the heads of a tape recorder and are energized during fast-forward or rewind. During this process, the lifters move the tape away from the heads to prevent wear during the high speed shuttling of the tape. Know that the tape lifters are usually stationary while the precious oxide is whizzing past and in direct contact with them. Noise Reduction Numerous articles and books have been written on this subject, and it is difficult to summarize Noise Reduction in a few sentences. When it comes to analog tape recording, there are two basic approaches: Single-Ended and Dual-Ended. Single-Ended systems generally deal with tape hiss and other anomalies after the fact or after the recording is made. Today’s digital editing software is often designed to be able to improve upon recorded anomalies after the fact and is a good example of a ‘single-ended’ approach. A number of hardware devices were made during the analog era for single-ended noise reduction by companies like Burwen, DBX, Phase Linear, SAE, etc. and can still be found on the vintage hardware market. These hardware devices work in ‘real time,’ meaning that the recorded audio has to be played through them for processing. Dual-Ended noise reduction involves processing the audio signal before and after recording. Ray Dolby first applied this process in 1966 with the introduction of Dolby A type noise reduction intended for use with professional audio tape recording. Dolby Laboratories subsequently introduced noise reduction processes for the consumer market and became a household word. Besides Dolby Laboratories, DBX and Telefunken produced very effective dual- ended systems that were used in both professional and consumer recording. Oxide This is the composition that has been applied to the side of magnetic tape that contains the magnetic recording. Oxide consists of a slurry containing the metal particles, a binder solution which, helps adhere the oxide to the backing, lubricants to make the dried solution both flexible and less physically noisy when passing across the heads, and several other chemical compounds. The actual chemical composition of oxide remains a closely guarded secret by the various manufactures of magnetic tape. Pinch Roller The Pinch Roller (sometimes referred to as a ‘puck’) on a tape deck is that round wheel that presses the tape against the capstan to move the tape across the heads. Pinch rollers are free-wheeling and are usually made of rubber or a semi- soft composite material. Playback Head If you were able to view the construction of a play head, it would look somewhat like a horse shoe with several turns of fine wire wrapped around it. The tape is pulled across the gap of the horse shoe shape. The actual gap is tiny (microns, in fact) in order to concentrate the magnetism that has been imparted onto the tape by the record head. See FIG. 1 for an illustration of a tape recorder head. If you view the heads of a reel-to-reel tape deck straight on, the play head will be on the right. Play Wind This is how the tape winds up on the take-up reel after being played from left to right. A ‘Play Wind’ is considered to be desirable for long term storage. The reason is that if print through occurs, the print-through information will be after the recorded audio and be masked or sound like echo. If the tape is stored heads-out, the print-through information can precede the recorded audio on the tape and is known as ‘pre-echo.’ Polyester tape This refers to the type of base material used for audio and video tape. Polyester, sometimes referred to as PET, is a substance known scientifically as Polyethylene Terephthalate. (I can’t pronounce it well either!) Polyester back tapes are known to have problems when the oxide absorbs moisture over time. This usually causes a condition called Sticky Shed Syndrome (SSS). Layer to layer adhesion problems can sometimes happen. When this happens, the unfortunate result is that the oxide can separate from the base and stick to the preceding layer of tape or literally fall off in some instances. That condition is generally unrepairable. Print Through When audio tapes are tightly wound on a reel and/or stored for extended periods of time, the adjacent layers can sometimes influence each other. That is, one layer can partially magnetize an adjacent layer. This condition can be more predominant if the recorded levels are extremely loud or ‘hot,’ non back coated audio tape or with thinner varieties of audio tape. The use of conductive back coatings on many brands of audio tape helped alleviate this condition. If the tape is wound heads-out, the partial magnetization will sound like pre-echo. This is one of the reasons for storing audio tapes tails-out. If print through occurs in a tails-out wind condition, it is either masked or sounds more like natural echo. It has been recommended by some that stored audio tapes be periodically rewound and stored again, using a play wind or preferably, a library wind. Periodically rewinding audio tape is a very labor intensive process which, in practice, is rarely done. Record Head The main difference between a record head and a playback head is the size of the gap between the poles of the head itself. We’re talking microns here. However, the gap on a record head is typically wider than that of a playback head. See FIG. 1 for an illustration of a tape recorder head. If you view the heads of a reel-to-reel tape deck straight on, the record head will be in the center of a three head tape deck. Round Robin This is an effect that causes a repeating of the signal. It is actually a type of feedback that is created by playing the signal into a mixing console and assigning the signal back unto itself. Nowadays, the effect can be created digitally with software. Saturation This is the state reached with magnetic tape when the oxide has been magnetized to the point where it cannot be magnetized any further. Exceeding this threshold with an alternating current magnetic field (via the record head) will cause the magnetic particles in the oxide to become disoriented, and audible distortion occurs. Signal-to-Noise Ratio In engineering or scientific terms; Signal-to-noise ratio refers to the strength of a given signal in relation to the background noise associated with that signal. Signal-to-Noise Ratio can be applied to everything from astronomy to video to analog & digital audio recording. It is often abbreviated as SNR or S/N. Recording engineers will sometimes increase the signal-to-noise ratio of analog tape by increasing the reference level of the recording. The overall increase is small but often noticeable, depending on the material. Sticky Shed Syndrome Abbreviated SSS, this condition is particularly damaging to the tape being played. It’s a condition whereby the binders and lubricants in the oxide have absorbed enough moisture over time to cause the oxide to become soft. When played on a tape recorder, the characteristic symptom is often a squealing or squeaking sound which modulates the audio being played. The tape will often shed oxide on all of the stationary parts of the tape deck. The accepted but temporary, fix is to carefully bake the tape under controlled conditions. The Digital Audio Tape formats have recently shown signs of SSS so, if you own any of these, now is the time to inspect them. Unfortunately, these formats will typically not show a problem until they are played and, then it can be too late for your prized digital audio recorder. (Particularly the rotary head formats) Sound-on-Sound An effect (sometimes a mistake) whereby a signal is recorded onto an existing recording. When played back, both signals can be heard but not separated. This process is made possible by disabling the erase head when applying the second signal to an existing recording. Tails-out Holding a reel of audio tape in front of you, the tape will un-spool from the right. In order to indicate the state of the tape, the loose end is usually folded and fastened to the reel with adhesive tape in such a manner that the folded and taped end will face the operator when the reel is placed on the machine or any flat surface. This makes it easy to identify which way to mount a given reel of tape onto the tape deck. Tape Baking The process of baking analog tape involves raising the ambient temperature of the tape to 120 - 130 degrees Fahrenheit (48.8 – 54.4 degrees Celsius) in a very low humidity environment for specific amounts of time based on the width of the tape, the thickness of the tape and the size of the reel. The process was first proposed (even patented) by AMPEX Corporation in the early 1990's. This procedure can only be applied to Polyester base tape. Acetate base tape can be damaged by this process. VSO VSO stands for Variable Speed Oscillator. A VSO is used on a an analog tape recorder that has a DC capstan motor to vary the speed of the tape. Some recorders had a speed control built in and others required an external device. Tape recorders that use an AC sync motor as the capstan motor, are generally not speed controlled. Wow The sudden change of pitch of a recorded signal caused by physically altering the tape speed for a very short duration of time. The reasons for the occurrence can be everything from mechanical failure of the tape recorder, to tape damage. Wow can also be created purposely by applying enough pressure to the supply reel during playback to momentarily alter the tape speed. Zenith On an audio tape recorder, Zenith refers to the relationship of the vertical alignment of the face of each head and the tape guides to the deck plate or mainframe in order to provide even contact over the entire surface of the tape. Zenith, like Azimuth, is usually an adjustable parameter of each tape head. References Howard M. Tremane, “Audio Cyclopeia” Second Edition, Howard W. Sams, 1973 Rudolph F. Graf, “Dictionary of Electronics” Howard W. Sams, 1974 Glenn D. White, “The Audio Dictionary” University of Washington Press, 1987 Dr. John W. C. Van Bogart, “Magnetic Tape Storage and Handling” National Media Lab, 1995 Wikipedia, Return to TOP of page © Corey Bailey Audio Engineering
Digital Audio Digital Audio General Terms General Terms Records Records
FIG. 1 This image is from the inside of a Scotch 111 tape box. (Acetate base)
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