Click here to purchase the entire book in PDF format. AC Bias Go to the kitchen and get a bag of flour. Open it up and try to pour some out. You'll tip the bag higher and higher and nothing will happen. Then, suddenly, a big clump of flour will suddenly drop out of the bag and make a huge mess. This is a bad way to pour flour out of a bag. A better way is to hold the bag of flour, and start shaking it gently back and forth sideways to get the flour particles moving against each other. Then, while you're shaking, tip the bag and start pouring. The flour will come out smoothly if you keep shaking. Magnetic tape behaves in a similar way. Take a look at the $M-H$ curve in Figure 6.62. It shows that if you apply a weak magnetic field to the tape and take it away, nothing will be printed on the tape (just like the fact that no flour will come out of the bag...) If you apply a bigger magnetic field and take it away, you'll leave a magnetic field on the tape (big clump of flour...). So, what's the analog magnetic tape equivalent of shaking the bag of flour? What we'll do is make a really high frequency, really high amplitude sine wave - something on the order of 150 kHz - 400 kHz. We'll then use our audio signal (which is a comparatively low frequency) and add it to the sine wave. The result of this is shown in Figure 6.64. The high frequency tone is called AC bias and has been used in all analog tape recorders since about the 1940's. Figure 6.64: AC bias - the top plot is the audio signal, the middle plot is the AC bias signal, the bottom plot is the resulting sum of the two. How and why does this work? Apparently, no one is really sure, but there are some theories. The best one I've heard is that the AC bias signal basically shakes things up a lot, applying a random behaviour to the little magnetic needles with an amplitude up around the area where the tape saturates. As the tape moves away from the record head, the needles move out of that randomizing tone and what's left as it does away is the offset of the bias - which is our original signal that we're trying to record. NB: Special thanks to Peter Cook for the flour analogy.