Canon A-1 Repair Manual page 38

difference leads to an error.
The A-l is equipped with a device to prevent
variations due to A.C. flicker.
c) A high degree of noise-resistance
About 80 to 90 percent of digital voltmeters
employ the dual ramp integration analog-digital
converter.
(3) Input Information
With the exception of brightness, photographic informa-
tion such as shutter speed, aperture, ASA film speed
are all input in the form of digital numerals by the
digital switch composed of a code plate and a
pre~i­
ous metal brush. The digital switch needs four to
Six bits and in order to incorporate these bits in
a limited space, the most primary concern is to
reduce the interval between conductors. (The AV/TV
information switch has an int erval of 0.15 mm between
conductors and a conductor width of 0.15
Mm.
This is the limit of present technology.)
Erroneous information must not be input when the
wire brush moves somewhere between click-stops.
Otherwise the preciSion of the camera will be
jeopardized.
The A-I employs a Gray code (or a cyclic binary code)
instead of a natural binary code. .Let us mention
the difference between these two codes. The ordinary
type of natural binary code has 2" as its base.
(As the signal is transmitted by the switch being
turned on/off, everything is processed by the binary
code.) Supposing the code has three bits, the
natural binary code will be as follows:
1
0 x 22
+
0 x 21 + 1 x 2 0
*
001
2
0 x 22
+
1 x 21 + 0 x 2
0
-
010
3
0 x 22 + 1 x 21 + 1 x 2 0
-
011
4
1 x 22 + 0 x 21 + 0 x 2 0
~
100
The problem arises between
3
and 4. (The diagram
is idealized to facilitate your understanding.)
39