13 articles about "Magnetic Recording" - RCA (1964)
MAGNETIC TAPE TESTING (1964)
G. P. HUMFELD, Mgr. and R. D. BROWNING, Admin. - Compound and Tape Formulations Tape Products Performance - RCA Victor Record Division, Indianapolis, Ind.
ROBERT D. BROWNING
ROBERT D. BROWNING graduated in 1948 from Auburn University with the BS degree in Electrical Engineering. He joined RCA as a Recording Engineer in the New York Studios in early 1949. In 1955 he was transferred to the RCA Chicago Studios and was Manager, Recording, there until 1957. He then joined Ampex Corporation and was Manager, Quality Control Division, at their Magnetic Tape Plant in Alabama until he rejoined RCA in 1962. He is presently Administrator, Product Performance, at the Magnetic Products Plant in Indianapolis, Indiana. He is a member of the Audio Engineering Society and the American Society for Quality Control.
GEORGE P. HUMFELD
GEORGE P. HUMFELD graduated from Purdue University in 1937 with a BS in Chemical Engineering. He worked as a chemist in a non-ferrous foundry for five years after graduation and then joined the U. S. Rubber Company as a rubber compounder during the war years. He joined RCA in 1946 as an engineer in the Record Compound Group and was appointed Manager of that group in 1956. This activity was expanded to include formulation development work on magnetic tape products when RCA entered the tape manufacturing field. He is a member of the American Chemical Society and the Society of Plastics Engineers where he has served as a National Councilman.
The RCA tape test center at Indianapolis
A tape test center has been created at Indianapolis where evaluation is carried out on alf tape products to assure customers that our products meet all magnetic specifications. A physical test laboratory, complementing the work done in the test center, carries out the physical tests on all tapes.
Magnetic tape finds many applications in home, industry, education and government. Technically, we are interested in discussing tape applications in computer machine operation, instrumentation recording, and audio recording. Within these technical divisions there exist two general marketing areas - commercial, and the U. S. Government.
General-purpose audio tapes
General-purpose audio tapes probably have the least-critical magnetic and physical specifications because of the subjective nature of the recording. Here, the importance of individual bits of information is minimized. In order to meet Government specifications audio (and instrumentation) tapes must duplicate within extremely close tolerances the sensitivity, output level, distortion, and bias requirements of standard reference tapes supplied by the Bureau of Ships. Also, they must meet all the physical property tests as outlined later in this article.
For computer use, tapes must have completely uniform surfaces free from pinholes and nodules or bumps. These defects invariably result in errors during recording and read-out. The oxide surface must not abraid away during tape usage. The oxide coating must withstand temperatures well above 100°F. without the surface becoming soft or sticky.
The magnetic pigment must be well dispersed in the lacquer binder to permit magnetic recording at high packing densities. Tape tensile strength and elongation properties must pass certain minimum and maximum values to assure ruggedness of performance at high tape speeds. Width and thickness tolerances must be carefully measured and closely controlled in order to meet the mechanical and magnetic limitations of any given computer machine design.
MAGNETIC TAPE TEST CENTER
The test center is in a specially designed white room which is maintained at 50% relative humidity and 70 °F.
In cooperation with RCA Electronic Data Processing, test stations were built using the same type of transport upon which the tape would ultimately be used (Fig. 1). An automatic test program was incorporated using a read-after-write head assembly to check for dropouts, noise errors, and skew. Each tape tester also checks for the presence of begin tape and end tape markers, and indicates the total length of tape on a reel.
Computer Tape Testing
The tape is first threaded on the tape transport. The programming device then records a signal simultaneously on all tracks at a pulse-packing density at least 25% above that ultimately used in computer operations.
All recorded tracks are read out while writing and must reproduce each bit at an amplitude at least double that required to register in a computer tape station logic. If any recorded bit fails to produce the necessary output voltage, it is automatically reread to determine whether it is a transient or permanent error.
Tape surface defects, such as pinholes, nodules, and dirt produce a loss in signal. The machine is designed to stop with the defect positioned so that it can be examined and repaired if possible.
Nobody produces perfect tapes
Perfect tapes are not being produced by manufacturers today, hence all tapes ultimately used must be repaired. A nodule or a piece of lint or dirt stuck to the tape surface can be removed by a light wipe with a special tool. If the defect is repaired properly the machine will pass this point on retest and continue testing.
Dropout tests and noise pulse tests
After the dropout test, the tape is then subjected to a noise test during rewind. For computer tape, noise refers to any spurious voltage pulse generated by a flux change while running a DC saturated tape over the reproduce head.
Any noise pulse due to loss of tape-to-head contact or lack of oxide on the tape which is greater than 10% of the average signal amplitude is cause for rejection of the tape. A pinhole is an example of a defect which may generate such a noise pulse.
Calibrated test stations
Equipment maintenance is very important to reliability in testing, so all test stations are calibrated each shift.
In addition, more thorough preventive maintenance checks are made on a weekly and monthly schedule. Wear characteristics are determined on a statistical sampling basis on wear test simulators. These simulators are mechanical duplicates of the computer transport, as shown in Fig. 2.
No loss of a bit of information
In the short-length wear or abrasion-resistance test a tape is recorded with 20 discrete messages each approximately 1 inch long with a 0.5-inch inter-message gap. The transport is then programmed to read in both directions with the pressure roller impact occurring in the message area. A tape must run for at least 5,000 passes in each direction without the loss of a bit of information.
For the oxide rub-off test, the tape is written continuously throughout its entire length and then programmed to read back and forth, checking for read errors. An acceptable tape must make 12 passes over the heads without error or evidence of coating buildup on the heads or guides.
Quality is further assured by environmental testing to observe the effect of humidity and elevated temperatures upon coating adhesion, layer to layer adhesion and cupping. Such tests compress years of service into a few short test hours. Finally, measurements are made on overall tape length, width, tensile strength and coating opacity to assure perfect mechanical operation on the machines.
Audio Tape - Magnetic Tests
A pictorial representation of the quality tests on audio tape is shown in Fig. 3. Oxide slurry is drawn from each mixing machine and coated on a miniature coating device in the "Control Laboratory" before being released for production use (Fig. 4). This test tape is placed in a 1,000-oersted, 60-cycle magnetic field and the resulting hysteresis loop projected on an oscilloscope indicates the magnetic characteristics of the coating (Fig. 5).
From this loop, measurements are made of the coercivity Hc, retentivity Br, and loop squareness or degree of orientation of the magnetic particles in the coating.
General-purpose audio tapes
General-purpose audio tapes normally have an intrinsic coercivity of about 250 oersteds, a retentivity of approximately 900 gauss, and a squareness of about 0.8.
Production coatings are checked on the same hysteresis loop tracer by testing short lengths of tape slit from the web as it emerges from the drying ovens (Fig. 6).
As production slitting progresses, sample reels are tested for sensitivity, output, long and short wavelength response, bias, distortion, noise, print-through, and output at various distortion levels. A custom-built test station (Fig. 7) enables technicians to perform all these tests rapidly so that any deviation from RCA standards is detected immediately, and only high quality tape will pass on to the visual inspection and packaging operation.
PHYSICAL TEST LABORATORY
Exhaustive physical and environmental tests, requiring up to 24 hours in some cases, are performed in the "Control Laboratory" shown in Fig. 8. These tests are described in detail in the following paragraphs and include tests for evaluation of both "Government Services Administration" (G.S.A.) and regular commercial tapes.
Tape surface quality can be evaluated by microscopic examination. These tests are aimed to guard against poor surface texture, a corrugated coating surface, surface nodules, or foreign particles of dirt imbedded in the coating. These defects can have a direct bearing upon output and short wavelength response. Other visual tests involve observations on quality of slitting and uniformity of wind-up of reels and hubs.
Coating thickness must be accurately measured and carefully controlled to produce a high quality tape product. An electronic gage (Fig. 9) converts vertical motion of a sensing probe into a signal which is amplified and read out on a sensitive microammeter graduated in increments of an inch. The coating thickness of general use audio tape approximates 0.0004 inch. However, coatings as thin as 0.0001 inch are becoming practical for applications in computers.
Tape width is closely controlled by the precise setting of the slitting machine. An optical comparator (Fig. 10) is used to determine width accurately.
Standard measurements for a 1/4-inch tape are 0.246 ± 0.002 inch. For 1/2", 3/4", and 1" wide tapes, the limits are +0.000, -0.003 inch.
Fig. 11 shows the tensile machine used to stretch 1/4-inch tape at a constant rate of 12 inches per minute until the specimen reaches the yield point. Minimum acceptable values for cellulose acetate are 4.7 pounds for 1.5-mil film and 3.2 pounds for 1-mil film. For polyester (Mylar) film, minimum acceptable values are 5.5 pounds and 3.7 pounds, respectively.
Shock Tensile Strength:
A pendulum-type impact tester as shown in Fig. 12 subjects 1/4" tape to the striking force of a free-swinging pendulum. Impact strength is determined by calculating the difference between the original energy value of the pendulum at the start of the test and the pendulum energy remaining after break. This difference is a function of the travel (in an upward arc) by the pendulum following impact with the sample. For cellulose acetate, the minimium values are 0.35 footpounds for 1.5-mil film and 0.25 footpounds for 1-mil-thick, 1/4"-wide tapes. Comparable minimum polyester values are 0.58 for either thickness of 1/4" tape.
Elongation Under Stress:
This consists of applying a specified load to a 20" length of 1/4" tape for a period of 3 hours at room temperature and observing the amount of permanent stretch the tape maintains 3 hours after the load is removed. The maximum values for cellulose acetate are 1% for both 1.0-mil and 1.5-mil film; maximum values for polyester are 0.30% for 1.5-mil and 0.50% for 1.0-mil-thick polyester.
Humidity Stability (Cupping) Test:
A two-chamber box is used for this test. One chamber is maintained at 90% relative humidity and the other at 15% relative humidity. Both are maintained at 90°F (Fig. 13). Duplicate samples are mounted in a horizontal clamping device and stored in each environmental condition for 16 hours. The environment causes the tape to curl, i.e., the edges of the tape either rise or fall. A low-power telescope is used to view the tape ends and measure the angle between the horizontal and a line tangent to the edge of the tape. The arithmetic difference in degrees between the angle measured on the desiccated tape and the angle measured in the same manner on the humidified tape gives the value for this test.
Layer-to-Layer Adhesion (Blocking):
A 3-foot length of 1/4" tape is wound onto a 1/2" diameter mandrel under the tension of a 1,000-gram weight clamped to the lower end of the tape during windup. After the weight is removed the loose end is fastened with pressure-sensitive tape. This assembly is then stored in an environmental chamber, first for 18 hours at 130°F and 85% relative humidity, and then for four hours at 130°F and 5% relative humidity. A good tape at the end of this test will spring free when the tape fastener is removed. There will be no sticking of adjacent layers and no separation of the oxide coating from the coated surface.
Flammability of tape is determined by conducting burning tests in a carbon dioxide atmosphere. Tape coatings containing materials which will support combustion in such an inert atmosphere are not accepted by the industry.
Fungus growth is a problem in tropical sections of the world and coatings must not support spore growth. Four fungi used in these studies are: aspergillus niger, aspergillus flavus, penicillum luteum, and trichoderma T-1.
Test samples are inoculated with a composite spore suspension of these types and incubated for a period of 21 days at 28° to 30°C. Tapes showing no growth or only a slight trace of fungus such as might develop from an unusual mass of spores in the original inoculum are considered acceptable.
Quality must first be built into a product before it can be accepted in the field. This is being done regularly in the 158 various products currently in production. Test effort such as described in this article assures the producer and the customer of a continuing level of high-quality product.
Fig. 1 - Indianapolis computer tape test center.
Fig. 2 - Computer tape wear test simulator.
Fig. 3 - Quality tests in tape manufacture.
Fig. 4 - Control laboratory coating machine.
Fig. 5 - Hysteresis curve of a magnetic tape.
Fig. 6 - Production sampling.
Fig. 7 - Audio production test equipment.
Fig. 8 - Process control test laboratory.
Fig. 9 - Electronic coating thickness indicator.
Fig. 10 - Optical comparator.
Fig. 11 - Tape tensile strength test.
Fig. 12 - Pendulum impact tester.
Fig. 13 - Humidity stability test equipment.