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Bipolar To Unipolar Signal Converter Circuit

IP.com Disclosure Number: IPCOM000121647D
Original Publication Date: 1991-Sep-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 3 page(s) / 95K

Publishing Venue

IBM

Related People

Santitoro, RT: AUTHOR

Abstract

There is a requirement for some CRT displays to operate in different display modes. For example, VGA (video graphics array) displays operate at a fixed horizontal frequency of 31.5 kHz but have three different vertical frequencies. These models are controlled by the polarity of the vertical and horizontal synchronization pulses that drive the electron beam deflection system. Only these two signals are available to decode which vertical frequency (mode) is being used.

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Bipolar To Unipolar Signal Converter Circuit

      There is a requirement for some CRT displays to operate
in different display modes.  For example, VGA (video graphics array)
displays operate at a fixed horizontal frequency of 31.5 kHz but have
three different vertical frequencies. These models are controlled by
the polarity of the vertical and horizontal synchronization pulses
that drive the electron beam deflection system.  Only these two
signals are available to decode which vertical frequency (mode) is
being used.

      Furthermore, there is a problem associated with having sync
signals with different polarities.  Most deflection processors
require the sync signals to be a single polarity because they use a
particular edge to reset their ramp generator circuits.  Also, other
circuits may need to be switched in to vary parameters, such as the
picture height, to compensate for these vertical frequency changes.

      This circuit takes a positive- or negative-going input pulse
and converts it to an output pulse that is either always
positive-going or always negative-going depending upon the state of
the control line.  The frequency of the signal is not altered.  The
control line could be used as an independent input or could be, as in
the case of a CRT display, fixed to accommodate the polarity required
by the deflection processor.  The select line output could be used to
control additional circuitry which is dependent upon the polarity (or
inverse of the polarity) of the input pulse. In the case of a CRT
display, the select line can be used to alter the picture height
(vertical deflection amplitude).

      The input signal is XORed with the control line by XOR gate 1.
The control line determines whether the output signal will always be
positive-going or negative-going. When control = high, the output
signal will always be a negative-going pulse.  Conversely, when
control = low, the output signal will always be a positive-going
pulse.  Refer to Table 1 for a truth table representation of this
function.

      The select line follo...