Notes

Outline

The Evolution of Audio Tape Recorders Dale Manquen

Introduction Development from WWII to present How the science of recorder design interacted with the art of recording Review all aspects of the magnetic recording system

Agenda Heads Tape Transports Electronics Operating versatility Areas of opportunity Comments on “Why don’t the specifications tell us how a tape recorder will sound?”

A Quick Historic Overview Oberlin Smith  1878; same time that Edison invented the phonograph Vlademar Poulsen – 1898 Early attempts lacked electronic amplifiers German development of wire and tape in 1930’s WWII spurred development for various purposes

What was available in 1945? Ring core heads Electronic circuitry for erase, record and playback High-frequency AC bias Magnetic particles and coating technology Tape and wire transport mechanisms

Early Post-War Products Wire recorders based upon Armour Research Institute work Webster Chicago (Webcor) was best known Brush Soundmirror demonstrated January 1946 2500 Soundmirrors ordered in first 3 months at $250

Early Post-War Products EMI introduced their model BTR1 in November 1947 Model K8 Magnetophon production resumes in 1948

Early machines of the U.S. broadcast market Ampex delivered the first Model 200 in April 1948 at $5000 112 built

Early machines of the U.S. broadcast market Magnecord introduced their PT-6 at NAB in May 1948 at $750 170 orders taken by end of June

Early Ampex products Model 200 lasted about one year Model 300 cut tape speed and reel size, improved heads and electronics, A-wind tape – mid 1949 Model 400 was a portable model– pusher capstan and shared spooling motor – Fall 1950

Early Ampex products Model 350  3 motors – April 1953

Stereo Recording on Tape Germans record stereo in 1942 Magnecord demonstrates (staggered head) stereo at 1949 New York Audio Fair Ampex demonstrates 3-channel stereo on ¼” tape

Multitrack Audio Recording Les Paul began using 8-track Ampex Model 300 in l954 3-track & 4-track ½” machines are common in early ’60’s Left/center/right format provided stereo & mono compatibility 8-track machines from several manufacturers by 1966 ‘The Association’ spends $60,000 on an album 16-track 2” in 1968 24-track 2” in 1971

Eduard Schuller’s original ring core head

Quarter-inch Tape Formats Track width Relative S/N Full track mono ¼” 250 mils 0 dB Half track mono ¼” 100 mils -4 dB Two  track stereo ¼” Quarter track stereo 43 mils -7.7 dB 4-track 37 mils Stereo 8-track 21 mils -10.8 dB 3-track NAB cartridge

Half-inch Tape Formats Track Width Relative S/N 2 track 200 mils -1.0 dB 3 track 4 track 70 mils -5.5 dB 8 track

One-inch Tape Formats Track Width Relative S/N 2 track 468 mils +2.7 dB 4 track 8 track 70 mils -5.5 dB 12 track 16 track

Two-inch Tape Formats Track Width Relative S/N

Other Tape Formats ¾” 6-channel 3M Dynatrack 3” 32-track MCI

Improvements in Core Shape Symmetric C-cores

Gap Shunting Effect Maximize efficiency by minimizing pole tip depth Broad back gap

Improvements in Core Shape

Gap and Tape Area

Gap Shunting Effect Use a conductive gap spacer Gauss Focused Gap (requires very high bias frequency) Doesn’t help at normal audio frequencies

Improvements in Core Shape Low loss hammerhead

Reduce losses in the core Eddy currents

Reduce losses in the core Ferrites are ceramics composed of electrically isolated magnetic particles Very low eddy current losses Very hard (but brittle) Lower permeability & saturation flux than metal ‘Glass Bonded’ gap to avoid chipping

Ferrite Pole Tip Saturation-MIG Flux crowding at corners Metal corner doesn’t saturate

Magnetic Tape Development Base films Binder ingredients Back coatings Magnetic particle characteristics Coating methods Particle orientation Calendering

Base Films Stable with Temperature Humidity Stress Age

Base Films Paper PVC (polyvinyl chloride) Acetate (cellulose acetate) Polyester (polyethylene terephthalate PET) Tensilized polyester PEN (Polyethylene 2.6-Naphthalene)

Binder Ingredients Wetting agents Emulsifiers Solvents Lubricants Fungicides Carbon black Glue

Glue Stronger glues = less glue required = higher magnetic particle density High crosslink thermoset polymers Some glues may become unstable with age Urethanes react with humidity Tape baking required to restore tapes

Back Coating Provides static discharge Improves winding and tape packing Removes carbon black from oxide, giving higher magnetic particle density

Magnetic Particle Shape Physical shape determines ability to tightly pack particles Low ‘dendrites’ or horns for cordwood stacking Uniform particle size for optimum recording Phizer 2228 in the mid ’70’s was a major milestone

Magnetic Particle Types Gamma ferric oxide Chromium dioxide Cobalt-doped gamma ferric oxide Some problems with early types Metal particles Evaporated metal film

Coating Methods Knife coating Gravure coating Reverse roll coating

Orientation Rheological flow Magnetization

Hard Calendering Squeezing out any voids in the coating Polishes tape surface for good high-frequency response

Audio Magnetic Tapes

The Audio Tape Conundrum Raise the output, but Keep the bias requirement the same ‘Compatible Bias’ Keep the coating thickness the same Equalization standards are based upon an assumed tape thickness How can you run with the problem when both feet are nailed to the floor?

Ways to Raise Tape Output

Curves

Tape Transports Pull tape across heads Constant speed Constant tension Correct guiding/alignment Fast wind modes Editing modes

A-Wind vs. B-Wind Early machine had oxide facing toward operator Ampex 300 (and kit for 200) flipped the tape over, with better access to the heads for editing and cleaning

Multiple Tape Speeds Head and tape improvements allowed tape speed to be reduced Multi-speed hysteresis synchronous motors

Remote Control Some early transports had manual switches or mechanical linkages that precluded remote control Total relay logic facilitated remote control Solid state logic made anything possible

Tape Lifters Manual lifters when the head cover opened Solenoid-operated lifters Tape path that pushes tape against head in Play or Record (Studer)

Braking Mechanical band or disk brakes Differential force for leading and trailing reels Dynamic braking Reverse torque until tape stops Requires motion sensing of tape halt

Scrape Flutter Reduction FM Instrumentation decks and early video decks required low scrape flutter Adding rollers was a well-known fix Transport topology determines scrape flutter characteristics The worst machines were built in the ’70’s and ’80’s (MM1100 and Saturn ¼”)

The Other Sideband - AM Tape Particle size and uniformity Particle dispersion Imperfections – dirt, voids Slitting Physical damage

The Other Sideband - AM Transport Variation in tape-to-head contact Stray magnetic fields Heads Face contour Residual magnetization Electronics Bias waveform DC leakage into heads

Tape Tension Control Felt drag brake Holdback tension on supply reel Angle or reel speed sensing Tension sensing Self-generating tension 3M Isoloop Dual capstan machines

Base Plate Rigidity Marine plywood Plywood with metal casting for critical area Metal plate Multilayer metal plates Webbed castings

Signal Electronics Record/bias/erase/reproduce functions Power supplies Metering Mode controls Adjustments

Matching vs. Bridging Lines Old telephone standard was 600 ohms out and in Facilities migrated to low impedance out and bridging in Facilitated multing Removed concern for missing or double termination +4 dBu in studios vs. +8 dBu in broadcast

Is Pin 3 Hot or Pin 2 Hot? Ampex standard was Pin 3 hot

Tubes vs. Solid State Size Reliability Serviceability (plug-in PC cards)

SelSync Made Overdub mode practical Switched playback amp to record head Les Paul’s 8-track in 1954 Unions fought Overdubbing Pay the entire orchestra if one instrument is overdubbed

Packaging Started with multiple chasses per channel Self-contained module for each channel Shared resources Bias synchronization Clustered meters Power supplies Remote panels for mode control Mass packaging No access during operation

Module per channel (3M M23)

Functional packaging (3M M56)

M56 Design Team

Adjustment Started with only a few critical adjustments Two speeds More speeds Multiple tape types Multiple operating levels and equalization standards

Operating Versatility Totally blind remote operation Automated events such as punch-ins

Total Remote Control Status and control of all signal functions Status and control of all transport functions Status and control of tape position & speed Failsafe interlocking of all the above Don’t forget automatic slating!

Tape Position Locating Tachometer on rotating idler in tape path Nixie and rotating dot numeric readouts Search-to-zero without anticipation ‘Smart’ trajectory SMPTE timecode Sprocket holes

Synchronization Speed sync using AC mains frequency Magnetech resolver Position control using SMPTE timecode EECO synchronizer Integrated audio/video editing systems

My Conclusions We don’t know how good analog recording can be If you can’t improve the technology, you can always make the tracks wider!! Bury the problems with more signal

Areas of Opportunity Better tapes Better heads Bias field shaping Lower flutter Lower AM Printthrough removal

“Why don’t the specifications tell us how a tape recorder will sound?” We aren’t measuring the important characteristics in a meaningful manner Flutter AM Nonlinearities vs. level vs. frequency Noise characteristics Time distortions (transient response & phase)

"Bibliography" Bibliography Books 1. H. N. Bertram, Theory of Magnetic Recording, Cambridge U.K., Cambridge University Press, 1994. 2. M. Camras, Magnetic Recording Handbook, New York, Van Nostrand Reinhold Co., 1988. 3. A. S. Hoagland and J. E. Monson, Digital Magnetic Recording, 2nd Ed., New York, John Wiley & Sons, 1991. 4. Eric Daniel, Denis Mee & Mark Clark, Magnetic Recording – The First 100 Years, New York, IEEE, 1999. 5. Jorgensen, The Complete Handbook of Magnetic Recording, Blue Ridge Summit, PA: Tab Books, 1980. 6. J. C. Mallinson, The Foundations of Magnetic Recording, 2nd Ed., San Diego, Academic Press, 1993. 7. C. D. Mee, The Physics of Magnetic Recording, New York: John Wiley & Sons, 1964. 8. C. D. Mee and E. D. Daniel, Magnetic Recording Volumes I-III, New York: McGraw-Hill Book Co. 1988. Related Websites EMTEC  http://www.emtec-magnetics.com/ Quantegy  http://www.quantegy.com/ IBM  http://www.almaden.ibm.com/sst/   Highly recommended.  This site has many animations explaining disk drive technology. ReadRite  http://www.readrite.com/html/tech.html