groepaz wrote:thing is, it doesnt just magically change. magnetism is very very stable once something has been magnetized.
At a temperature near 0 K this is very much true, but we're at 300 K room temperature, and this disturbs the elementary magnets over time. The effect may be small, but accumulates over time and becomes faster with an exponential law near the Curie-Point.
I'd like to bring another point into the discussion here regarding the datasette schematic: what is recorded by the head is *not* what is being read back by the head! It is the reason for the analog amplifier of the read electronic to *reconstruct* the signal that was used to write the tape
I'm referring to schematic CAS-12R
First, the write circuit:
There we have one gate of IC3 as buffered inverter, and then the signal is split and one branch is inverted once, the other one inverted twice. Means these two signals are inverses of each other. They're biased against two pull-ups. In write mode, the head coil is connected in series with a 10K resistor to this bridge: when a 0 is on the write signal from the VIC-20, the coil is polarised in one direction; with a 1, the coil is polarised in the other direction. These polarisations are reflected in two possible magnetic flux orientations recorded on tape.
For ease of the following explanation, let's say a 1 writes a upward flux (U), and 0 writes a downward flux (D).
However, when read back, a constant flux doesn't do anything on the head. Regardless whether it is up or down. The only thing that effects is a magnetic flux change
. In case of down to up, the head produces a small positive voltage spike; in the other case (up to down) it produces a small negative voltage spike. Now what's the function of the 4 op-amps?
IC1-B simply pre-amplifies the spike. The next stages are coupled to the pre-amplifier with C1 to remove any DC bias.
IC1-A and IC2-A act as 'leaky' integrators. They transform the spikes first into a step, and then into a ramp. C5 once again removes the DC bias going into the last stage.
There, IC2-B is driven as comparator. A small positive voltage on pin 6 will drive the output of the op-amp into negative output (which is returned to pin 5 with R17) and 'clamps' the op-amp there. With a small negative voltage, a clamped positive output results. The result is an analog flip-flop!
Two TTL inverters of IC3 complete the pulse-forming circuit. The op-amps are also all inverting, i.e. the positive spike is formed into a high level TTL signal, the negative spike is formed into a low level TTL signal.
The whole reason behind the read circuit, I repeat, is to reconstruct the original two-level write signal with as sharp edges as possible, without it being sensitive to much noise. That requires the circuit to be tuned to the speed of the actual flux change on write.
As long as the flux changes on tape don't overlap, it is always possible to reconstruct the original write signal.
That also explains why the azimuth of the head is critical: when the azimuth is wrong (or different from the recording azimuth), the spikes are broadened when read back and won't register any more good to the pulse-forming circuit. But the record amplitude itself is much less important - the pre-amplifier and the comparator-flipflop in the last stage take care of that.