5.2.2
Using
the 14 MHz
clock
the ULA
derives
line
and
field timing
compatible with the external TV receiver. Video is derived by
accessing the memory mapped display area in the RAM in a set seguence
at set times throughout the picture frame. The addresses are
necessarily independent of the CPU and appear on the ULA address lines
A6
through
AO as two
separate bytes timed
by the
RAS/CAS
row/column
address select lines.
5.2.3
The net
result
is
three
separate video
waveforms
outputs
from
the ULA
on pins 15, 16 and 17. These carry the luminance signal Y,
incorporating the line and field sync, and two unmodulated colour-
difference signals U and Y making the Spectrum compatible with both
colour and monochrome receivers.
5.2.4
From
the ULA the
colour difference
signals
are
applied
to the
colour
modulator IC14 via two level shifting networks. These match the ULA
output levels with those reguired at the B-Y and R-Y inputs to the
modulator. In the Issue 2 Spectrum the level shifting network is
passive, incorporating two potentiometers VR1, VR2. These are
reguired
to
set-up
the
chroma
bias
level
on
IC14
pin 3
such
that
the
voltage difference measured between pin 3 and the colour difference
signals
on
pins
2 and 3
respectively
is
nominally
0V
d.c.
In the
Issue 3 Spectrum two active networks incorporating IRS and TR9
eliminate the potentiometers, greatly improving colour stability.
5.2.5 The level shifted colour difference signals, input to IC14, are then
encoded, by guadrature modulating two 4.43 MHz chroma sub-carriers.
The sub-carriers are generated with the assistance of an external
crystal X2 and a CR lead/lag network introducing a 90° phase shift
between pins 1 and 18. (A further difference between the Issue 2 and
3
Spectrums
lies
in the
bias
oscillator.
The
early
issues
incorporate
a trimmer TC2 allowing the chroma sub-carrier freguency to be
adjusted;
on the
later
issues
the
freguency
is
fixed).
The
resultant
modulated colour difference signals are finally mixed producing a
composite chroma sub-carrier at IC14 pin 13.
5.2.6
At
this
point
the
chroma
signal
is
a.c. coupled
to the
base
of TR2 and
added to the inverted luminence signal on TR1 collector. The
resultant composite video is then buffered and applied to an
encapsulated UHF modulator operating on European standard channel 36.
5.3 Keyboard Scanning
5.3.1 Every 20 ms (i.e. once per maskable interrupt), the CPU systematically
scans the keyboard recording which 'keys '(if any) have been pressed.
The scanning method is described below with the aid of Figure 1.3. As
the
figure
clearly
illustrates
the
keyboard
consists
of an 8 x 5
matrix, the inter-section of each row and column bridged by a normally
open switch contact. The row 'outputs' and column 'inputs' are shown
connected by separate ribbon cables KB1 and KB2, one to the ULA and
the
other
to the
high order address
lines
A15-A8.
Pull-up
resistors
R64
through
R68
ensure
that
when
the
address
bus is in the
high
Z
state,
or
none
of the
key-switches
is
closed,
row
outputs
KBO to KB4
remain high.
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