Tape Recorders

Transcript

Tape Tape Recorders 2014 • A tape recorder, tape deck, reel-to-reel tape deck, cassette deck or tape machine is an audio storage device that records and plays back sound, usually using magnetic tape, either wound on a reel or in a cassette, for storage. It records a fluctuating signal by moving the tape across a tape head that polarizes the magnetic domains in the tape in proportion to the audio signal. • Likely the earliest known audio tape recorder was a non-magnetic, non-electric version invented by Alexander Graham Bell's Volta Laboratory and patented in 1886 (U.S. Patent 341,214).[1] It employed a 3/16-inch wide strip of wax-covered paper that was coated by dipping it in a solution of beeswax and paraffin and then had one side scraped clean, with the other side allowed to harden. The machine was built of sturdy wood and metal construction, and hand powered by means of a knob fastened to the flywheel. The wax strip passed from one eight-inch reel around the periphery of a pulley (with guide flanges) mounted above the V-pulleys on the main vertical shaft, where it came in contact with either its recording or playback stylus. The tape was then taken up on the other reel. The sharp recording stylus, actuated by a vibrating mica diaphragm, cut the wax from the strip. In playback mode, a dull, loosely mounted stylus, attached to a rubber diaphragm, carried the reproduced sounds through an ear tube to its listener.[1] • Both recording and reproducing heads, mounted alternately on the same two posts, could be adjusted vertically so that several recordings could be cut on the same 3/16-inch wide strip. While the machine was never developed commercially, it was an interesting ancestor to the modern magnetic tape recorder which it resembled somewhat in design. The tapes and machine created by Bell's Associates, examined at one of the Smithsonian Institution's museums, became brittle, and the heavy paper reels warped. The machine's playback head was also missing. Otherwise, with some reconditioning, they could be placed into working condition.[1] • Other early tape recorders were created by replacing the steel wire of a wire recorder with a thin steel tape. The first of these modified wire recorders was the , created in 1929 or 1930 by the Ludwig Blattner Picture Corporation. The first practical tape recorder from AEG was the Magnetophon K1, demonstrated in Germany in 1935. Friedrich Matthias of IG Farben/BASF developed the recording tape, including the oxide, the binder, and the backing material. Development of magnetic tape recorders in the late 1940s and early 1950s is associated with Ampex; the equally important development of magnetic tape media itself was led by Minnesota Mining and Manufacturing Company (now known as 3M). • Electric current flowing in the coils of the tape head creates a fluctuating magnetic field. This causes the magnetic material on the tape, which is moving past and in contact with the head, to align in a manner proportional to the original signal. The signal can be reproduced by running the tape back across the tape head, where the reverse process occurs – the magnetic imprint on the tape induces a small current in the read head which approximates the original signal and is then amplified for playback. Many tape recorders are capable of recording and playing back at once by means of separate record and playback heads in line or combined in one unit • Modern professional recorders usually use a three-motor scheme. One motor with a constant rotation speed drives the capstan. This, usually combined with a rubber pinch roller, ensures that the tape speed does not fluctuate. Of the other two motors, one applies a very light torque to the supply reel, and the other a greater torque to the take-up reel, to maintain the tape's tension. During fast winding operation the pinch roller is disengaged and the reel motors provide the necessary power. The cheapest models use a single motor for all required functions; the motor drives the capstan directly and the supply and take-up reels are loosely coupled to the capstan motor with slipping belts or clutches. There are also variants with two motors, in which one motor is used for rewinding only. • Since their first introduction, analog tape recorders have experienced a long series of progressive developments resulting in increased sound quality, convenience, and versatility. • Two-track and, later, multi-track heads permitted discrete recording and playback of individual sound sources, such as two stereophonic channels, or different microphones during live recording. The more versatile machines could be switched to record on some tracks while playing back others, permitting additional tracks to be "laid down" to match previously recorded material such as a rhythm track. Limitations • • • Use of separate heads for recording vs. playback (three heads total, counting the erase head) enabled monitoring of the recorded signal a fraction of a second after recording. Mixing the playback signal back into the record input also created a primitive echo generator. Dynamic range compression during recording and expansion during playback expanded the available dynamic range and improved the signal-tonoise ratio. dbx and Dolby Laboratories introduced add-on products in this area, originally for studio use, and later in versions for the consumer market. In particular, "Dolby B" noise reduction became very common in all but the least expensive cassette tape recorders.
 Computer controlled analog tape recorders were introduced by Oscar Bonello in ArgentinaThe mechanical transport used three DC motors and introduced two new advances: automated microprocessor transport control and automatic adjustment of bias and frequency response. In 30 seconds the recorder adjusted its bias for minimum THD and best frequency response to match the brand and batch of magnetic tape used. The microprocessor control of transport allowed fast location of any point on the tape. • The storage of an analogue signal on tape works well, but is not perfect. In particular, the granular nature of the magnetic material adds high-frequency noise to the signal, generally referred to as tape hiss. Also, the magnetic characteristics of tape are not linear. They exhibit a characteristic hysteresis curve, which causes unwanted distortion of the signal. Some of this distortion is overcome by using an inaudible high-frequency AC bias signal when recording, though the amount of bias needs careful adjustment for best results. Different tape material requires differing amounts of bias, which is why most recorders have a switch to select this (or, in a cassette recorder, switch automatically based on cutouts in the cassette shell). Additionally, systems such as Dolby B, Dolby C and Dolby HX-Pro have been devised to ameliorate some of the noise and distortion problems. Variations in tape speed cause flutter, which can be reduced by using dual capstans. Higher speeds used in professional recorders are prone to cause "head bumps," which are fluctuations in low-frequency response. Tape Speeds Reel to Reel Tape Speeds Reel to Reel • In general, the faster the speed the better the sound quality. In addition to faithfully recording higher frequencies and increasing the magnetic signal strength and therefore the signal-to-noise ratio (S/N), higher tape speeds spread the signal longitudinally over more tape area, reducing the effects of defects in or damage to the medium. Slower speeds conserve tape and are useful in applications where sound quality is not critical. • 15/16ths of an inch per second (in/s) or 2.38 cm/s — used for very long-duration recordings (e.g. recording a radio station's entire output in case of complaints, aka "logging") • 1⅞ in/s or 4.76 cm/s — usually the slowest domestic speed, best for long duration speech recordings • 3¾ in/s or 9.52 cm/s — common domestic speed, used on most single-speed domestic machines, reasonable quality for speech and off-air radio recordings • 7½ in/s or 19.05 cm/s — highest domestic speed, also slowest professional; used by most radio stations for "dubs", copies of commercial announcements; Through the early-mid 90's many stations could not handle 15 IPS. • 15 in/s or 38.1 cm/s — professional music recording and radio programming • 30 in/s or 76.2 cm/s — used where the best possible treble response is demanded, e.g., many classical music recordings Editing Wow and Flutter • A recording on magnetic audio tape is linear; unlike today's digital audio, not only was jumping from spot to spot to edit time consuming, editing was destructive— unless the recording was duplicated before editing • Editing was done either with a razor blade—by physically cutting and splicing the tape, in a manner similar to motion picture film editing—or electronically by dubbing segments onto an edit tape. • The former method preserved the full quality of the recording but not the intact original; the latter incurred the same quality loss involved in dubbing a complete copy of the source tape, but preserved the original. • Tape speed is not the only factor affecting the quality of the recording. Other factors affecting quality include track width, tape formulation, and backing material and thickness. The design and quality of the recorder are also important factors, in many ways that are not applicable to digital recording systems (of any kind.) The machine's speed stability (wow-and-flutter), head gap size, head quality, and general head design and technology, and the machine's alignment (mostly a maintenance issue, but also a matter of design—how well and precisely it can be aligned) electro-mechanically affect the quality of the recording. The regulation of tape tension affects contact between the tape and the heads and has a very significant impact on the recording and reproduction of high frequencies. • The track width of the machine, which is a question of format rather than individual machine design, is one of two major machine factors controlling signal-to-noise ratio (assuming the electronics have high enough S/N not to be a factor), the other being tape speed. S/N ratio varies directly with track width, due to the Gaussian nature of tape noise; doubling the track width doubles the S/N ratio (hence, with good electronics and comparable heads, 8-track cartridges should have half the signal-to-noise of quarter-track 1/4" tape at the same speed, 3-3/4 IPS.) Tape formulation affects the retention of the magnetic signal, especially high frequencies, the frequency linearity of the tape, the S/N ratio, print-through, optimum AC bias level (which must be set by a technician aligning the machine to match the tape type used, or more crudely set with a switch to approximate the optimum setting.) • Tape formulation varies between different tape types (ferric oxide [FeO], chromium dioxide [CrO2], etc.) and also in the precise composition of a specific brand and batch of tape. (Studios therefore generally align their machines for one brand and model number of tape and use only that brand and model.) Backing material type and thickness affect the tensile strength and elasticity of the tape, which affect wow-and-flutter and tape stretch; stretched tape will have a pitch error, possibly fluctuating. Backing thickness also effects print-through, the phenomenon of adjacent layers of tape wound on a reel picking up weak copies of the magnetic signal from each other. Print-through on analog tape causes unintended pre- and post-echoes on playback, and is generally not fully reversible once it has occurred. Noise Reduction • Electronic noise reduction techniques were also developed to increase the signal-to-noise ratio and dynamic range of analog sound recordings. Dolby noise reduction includes a suite of standards (designated A, B, C, S and SR) for both professional and consumer recording. The Dolby systems use preemphasis/deemphasis during the recording/playback, respectively. DBX is another noise reduction system that uses a more aggressive companding technique to improve both dynamic range and noise level. However, DBX recordings do not sound acceptable when played on non-DBX equipment. • Dolby B eventually became the most popular system for Compact Cassette noise reduction. Today Dolby SR is in widespread use for professional analog tape recording and is only surpassed in quality (although difference is almost negligible) by digital audio technologies. Multitrack • As studio audio production progressed and became more and more advanced, it became desirable to record the separate instruments and human voices separately and mix them down to one, two, or more speaker channels later, rather than in real time in the studio before recording. In addition to allowing recording engineers and producers to experiment with different mixing arrangements, effects, etc. on the same performance and to produce multiple versions of a recording (without having multiple duplicates of all the studio control room equipment used for mixing), multi-tracking enables the use of non-real-time effects or effects that cannot be produced in the same studio where the musicians perform. Reel-to-reel recorders with eight, sixteen, twenty four, and even thirty two tracks were eventually built, with as many heads recording synchronized parallel linear tracks. Some of these machines were larger than a laundry washing machine and used tape as wide as 2 inches. A single new reel of 1" or wider tape, blank, could easily cost over $100, to $200. Still, in professional studios, most tapes were recorded only once, and all recording was on new tape, to ensure the maximum quality, as studio time and the time of skilled musicians was much higher than the cost of tape, making it not worth the risk of a recording being lost or degraded due to using media that had been previously recorded upon