Memotech Limited MTX 500, Руководство по обслуживанию

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17
TMS 9929 VDP (Video Display Processor)
Introduction
This Preliminary Specification of the TMS 9929 is to be
considered as an "ADD ON" to the basic TMS 9918
specification. The TMS 9929 is effectively identical
to the TMS 9918 functionally and only has the color
video section that is different.
The TMS 9918 provides a composite color video signal
output that if driven by a suitable amplifier can go
directly into any color video monitor. The color burst
frequency is the VDP oscillator input frequency divided
by three. External video mixing with an external
reference source can also be achieved by inputting this
external source directly into the TMS 9918.
In the TMS 9929 the color and luminance/sync
information is provided on three pins rather than a
single pin in the form of two color difference signals
and one luminance signal with all the vertical and
horizontal timing included. So we have R-Y, B-Y, and Y
respectively. The two color difference signals are
used then by an external quadrature modulator video
encoder. It is outside the Video Display Processor
that the composite color video signal is generated into
a PAL or Secam compatible TV signal. The external
video mixing is also done outside and it is the TMS
9929 to decide when this mode is entered. This is
achieved by a special level distinction made by the R-Y
and B-Y VDP outputs. When external video is entered
these two outputs go to the equivalent of the sync
percentage level of the black-white swing in the
luminance output, i.e. the color difference outputs are
normally swinging between the luminance black-white
voltage levels and it is only in the external video
mode that these outputs go to the reserved " sync"
level.
Phase locking of the VDP to the external PAL burst
frequency is desirable if inter-hum or crawl effects
want to be minimized. The TMS 9929 oscillator clock
must still, however, be maintained within its
prescribed limits of oscillator operation. CPUCLK
signal is no longer available in the European TMS 9929.
MTX Service Manual