On 3/10/15 4:14 PM, Ellis Burman wrote:
re: playing backwards flips polarity
> I too have been trying to wrap my head around that one. Isn't it analogous
> to playing a record backwards? The stylus still moves in the same
> direction at the same point in the groove.
> If someone can explain it, I'd love to hear it. It would be super easy to
> verify using a tape machine.
The repro head of an audio tape machine is not sensitive to absolute
magnetic flux level. You can put as strong a magnet as you like against
the head, and if the magnet is not moving, there's no output from the
head. Move the magnet one way, head has positive output. Move the
magnet the other way, head has negative output.
Slightly more technical explanation:
The head is sensitive to the rate and direction of change of magnetic
flux. Consider a recording which leaves alternating south and north
poles on the tape, represented here by S and N:
S N S N S N S N
If we play this recording forward (which, let's say, is moving left to
right across this diagram) then the initial transition is S to N. Let's
say we have connected the repro head so that this produces positive
output voltage. Peak output voltage will occur halfway between S and N,
because that's the point where flux change is most rapid.
If we play this recording backward, the flux change at the left end of
the recording is N to S, and this produces a negative output voltage.
Thus, playing the tape backward does flip the polarity. The same point
on the tape (in this example the point between the leftmost S and N)
produces a positive voltage when played one way, and a negative voltage
when played the other way.
-- John Chester