Magnetic Tape Recording

The head is a magnetic circuit deliberately broken at one point.

1. Build a small ring core from ferrite or laminated iron.
2. Cut the ring at one point to leave a gap of a few hundredths of a millimetre, filled with a shim of brass or paper.
3. Wind 200 turns of fine enamelled wire around the opposite side of the ring.
4. Lap the gap face flat and smooth so tape can slide across it.

The gap is where the field escapes, and its width sets the shortest wavelength you can record. A narrow gap can write finer detail — higher frequencies at a given tape speed — but passes less flux, so the signal is weaker. Every tape format's specification is a choice about that trade, and it is why professional machines run tape fast and cassettes needed decades of refinement to sound good slow.

Speed variation is audible as pitch variation, and the ear is unforgiving about it.

1. Make a capstan — a precisely round driven shaft — with a rubber pinch roller pressing tape against it.
2. Drive the capstan through a heavy flywheel to smooth out motor irregularities.
3. Add a supply reel with light back-tension and a take-up reel with light forward tension.
4. Ensure the tape's speed is set by the CAPSTAN alone, not by either reel.

The reels must not determine speed. A reel's effective diameter changes as tape winds on and off it, so tape driven by a reel speeds up continuously through the recording. The capstan and pinch roller pull tape at one fixed speed regardless of what the reels do — the reels merely supply and collect slack.

Establish the problem now — the next blueprint solves it.

1. Feed an audio signal to the head and run tape past it.
2. Play back through a second head and amplifier.
3. Listen: the sound is present but distorted and thin, especially at low levels.
4. Record a quiet passage and a loud one and compare how each is degraded.

This is what wire could never do, and it is the reason tape won.

1. Record a spoken sentence.
2. Play it back and mark the tape with a chinagraph pencil at the head gap when a particular word begins and ends.
3. Cut both marks at 45 degrees using a splicing block.
4. Rejoin the two outer pieces with splicing tape and play back — the word is gone.
5. Now reverse a section and splice it back in.

AEG demonstrated the Magnetophon at the Berlin Radio Show in 1935, using tape developed by BASF — paper, then plastic, coated with iron oxide. Fritz Pfleumer had patented the coated-tape idea in 1928, having previously worked on coating paper with metal for cigarette tips, which is an unusually direct route from one industry to another.

How it reached the rest of the world is a good story. German broadcasts during the war were noticed to be of impossibly high quality at hours when no orchestra would be playing live. After the war Jack Mullin, an American signals officer, shipped captured Magnetophons home, demonstrated them, and Bing Crosby — who hated performing live twice for different time zones — invested in Ampex to build American machines so he could pre-record his shows. Tape entered American broadcasting through a singer's dislike of repeating himself.

Against its predecessor: Poulsen's wire recorder stored sound magnetically and worked, but wire cannot be edited, twists so the recorded track rotates away from the head, and a break means a knot that bumps through every subsequent playback. Coated tape solves all three by being flat and cuttable. The physics is identical; the handling is transformed.

Its honest limits: print-through, where a strongly recorded layer magnetises the layer next to it on the reel and produces a faint pre-echo; oxide shedding as tape ages; the non-linearity heard in step 3; and physical fragility. Every one of those became a specialism, and the next blueprint tackles the most important.