Teac MT-20D-IO - Page 24

Since the frequency characteristic of the level detect amplifier is
determined so that the level difference between the 1F signal (1001 data
pattern) and 3F signal (1111 data pattern) may be controlled, the even
threshold level is set even during random data pattern. The threshold
detector monitors the output signal of the level detect amplifier by a
comparator to keep it at the constant level or more.
When the signal is applied to the threshold detector at the specified level
or more (threshold level), the following stage retriggerable
monostable multivibrator is retriggered continuously. If the input signal
level of the threshold detector is reduced due to drop-out, etc., the
trigger pulse signal to the retriggerable monostable multivibrator is
interrupted, and the output signal of the retriggerable monos table
multivibrator is inverted, and then read gate is closed.
The timing of this retriggerable monomultivibrator is set to the data length
of 7 - 15 bits.
Therefore, when the readout signal from the read head is dropped out over
the length of 7 - 15 bits or more, the transfer of the RD signal is stopped.
The threshold level is changed over by the threshold control circuit so that
the threshold level becomes 35% during write operation (read after write),
10% during gap read operation (read only), and 0% during data read operation
(read only).
Fig. 310 shows an example of the operational waveforms of the above-
mentioned read circuit.
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