AC Bias

Measure the non-linearity before trying to defeat it.

1. Record a series of steady tones at increasing levels, from very quiet to very loud.
2. Play each back and measure the output level.
3. Plot recorded level against playback level.
4. Examine the shape near the origin.

The curve is S-shaped, and the flat part near zero is the problem. Small signals produce almost no magnetisation at all — the material resists being magnetised until the applied field passes a threshold. Everything quiet either vanishes or emerges grossly distorted, and since music spends most of its time at low levels this ruins the recording.

A tone far above hearing, added to the audio, at substantial amplitude.

1. Build or set an oscillator running at 60 to 100 kHz — well above the audible range.
2. Mix its output with the audio signal feeding the record head.
3. Make the bias level adjustable over a wide range.
4. Confirm on the oscilloscope that the audio waveform is riding on the high-frequency carrier.

The bias must be far above the highest audio frequency you intend to record. If it is too close, the difference between bias and audio frequencies falls into the audible band as a whistle. Professional machines used bias frequencies several times the audio limit for exactly this reason, and the choice interacts with head gap and tape speed.

Same measurement as step 1, with bias applied — the comparison is dramatic.

1. Repeat the level series with bias applied at a moderate setting.
2. Plot the new curve over the first.
3. Note that the flat dead zone near the origin has gone.
4. Record music with bias off, then on, and listen to both.

More bias is better until suddenly it is not, and the crossover is per-tape.

1. Record a mid-frequency tone at several bias levels and plot output against bias.
2. Repeat with a high-frequency tone and plot on the same axes.
3. Note that the two peaks occur at DIFFERENT bias settings.
4. Choose a setting between them and record music to judge.

AC bias was found by accident at least three times. Walter Weber and Hans Joachim von Braunmühl at German radio arrived at it around 1940 while investigating why one Magnetophon sounded far better than the others — an amplifier fault was oscillating and inadvertently biasing the tape. Marvin Camras in the United States and Kenzo Nagai in Japan reached it independently in the same period.

An oscillating amplifier is normally a fault to be fixed. The significant act was not stumbling on it but noticing that the broken machine sounded better and asking why, rather than repairing it and moving on. That is a recurring pattern in this catalogue — the Mannesmann brothers turning a rolling defect into a tube-making process is the same move in a different industry.

Its effect on the medium was decisive. Pre-bias magnetic recording was usable for dictation and little else. Post-bias it had the dynamic range and low distortion to record music seriously, which is why the wartime German broadcasts sounded like live performances and why tape swept through broadcasting and recording within a decade of the war.

Its honest limits: the high-frequency erasure effect in step 4 caps treble response; bias must be re-set whenever tape type changes; and the bias oscillator can leak into other circuits and produce whistles. Digital recording eventually removed the problem entirely by not storing a continuously varying magnetisation at all — a different answer rather than a better version of the same one.