Promax GV-698, Instruction Manual

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6 PRINCIPLES OF OPERATION
6.1 Circuit description
This section deals with a general explanation of the operation of the GV-698.
The items below refer to the block diagram and to the diagrams of the equipment.
a) µC control
Controls the presentation of the display according to the user's selections
(chart, channel and frequency). It is responsible for the transmission of data to
the synthesized RF unit and for the storage, in a E2PROM memory, of various
configurations selected by the user (up to 32 memories). In the internal
EPROM, besides the control program, the tables corresponding to TV channels
(standard CCIR or FCC) are stored.
b) Display
It is responsible for the alphanumeric presentation of the data transmitted by
the µC (it can present 1 row of 16 characters). In addition, it provides
illumination to enable the data to be more easily seen.
c) Function buttons
There are 11 buttons situated on the lower front part. They serve to activate
and disactivate the basic functions, divided into three groups: video, sound and
teletext/VPS.
d) Synchronisms and logic control generator
The module which generates the basic signals of the video generator.
In accord with the selection signals of the system transmitted by the µC is
responsible for the generation of all the synchronisms signals, the colour carrier
and auxiliary signals employed by other modules of the equipment.
A PLL circuit is responsible for the generation of a 20 MHz signal providing the
maximum resolution per line.
In accord with the chart selection signals transmitted by the µC and internal
signals, the different addresses of an EPROM, which contains the information
on the data to be transmitted to the video output, are sampled.
e) Multiburst and electronic circle generator
The multiburst generator is based on charge and discharge of a capacitor by
means of a constant current source. The switching between the charge and
discharge is carried out in the comparator circuit and the selection of various
frequencies by means of the intensity variation of the current source. After this,
the triangular signal generated is shaped by means of diodes to make it almost
sinusoidal.
The electronic circle generator is based on scanning the data contained in an
EPROM (at 20 MHz) synchronized with the periods of the picture and line and
switching depending on the standard selected.
In addition, this module contains the R, G and B signal output stages.
f) PAL/NTSC/SECAM colour modulator
This is composed of a common part, which generates signals (R-Y) and (B-Y)
or Dr and Db, depending on the system selected:
05/96 GV-698