Analogue Recorders - Bias current

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High-frequencies contain less energy and therefore have a harder task in polarizing the particles on the ribbon. This takes place when the particles are initially immobile, and therefore require, in order to overcome their inertia, a greater quantity of energy than the amount that would be necessary if they were already moving. The solution to this phenomenon consists therefore, in acting upon the particles in order to make sure that the magnetizing signals find them already in motion, thus easing their polarization. To do this, we will add a bias current to the signal we wish to record. This is a current with a very high frequency content (well beyond the human ear's audible band) which is powerful enough to get the particles moving. The following diagram shows sinusoid to which a bias current added, and the resulting signal sent to the recording head.

Analogue recorders - Bias current

Bias current

The following figure gives us a different view of how the bias current "moves" the sound signal and shifts it into the ribbon's linear zone.

Analogue recorders - Bias current and transfer characteristic of a magnetic ribbon

Bias current and transfer characteristic of a magnetic ribbon

A bias current can be efficiently used to erase a magnetic ribbon. If we apply a bias current using the highest possible magnetizing force (avoiding saturation), the particles on the ribbon all get polarized, which eliminates any information pertaining to the previous magnetization.


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