TELE-TECH & - AmericanRadioHistory.Com

TI MM
PULSE
DELAY
VT ULM
PULSE
WIDTH
RIZONTAL
'IVE
12AU7
BUFFER
AMPLIFIER
12AU7
DELAY
MULTI -
VIBRATOR
12ÁU7
DELAY
MULTI -
VIBRATOR
12AU7
DELAY
MULTI -
VIBRATOR
12ÁT7
PULSE
SHAPER
RED
BALANCE
RED
BLUE
-O
OUTPUT
GREEN
BALANCE
GREEN
OUTPUT
Measuring
By A. ETTLINGER
NARROW
PULSE
WIDTH
NARROW
PULSE
WIDTH
Fig. 3
Block diagram of unibar generator
A. ETTLINGER, TV Engineer,
Engineering Dept.,
CBS -Television, 485 Madison
Ave., New York, N. Y.
Fig.
l: Primary component waveforms
Fig. 5: Correct Y vs. Q delay
Fig. 6: Y delay .05 p.s shorter than Q
Fig.
2: Resultant encoder output
Fig. 4: Shape of narrow (green) pulse
THE encoding and decoding
processes in the NTSC color
television system require the use
of delay compensation to achieve
time coincidence of the color and
monochrome components of the
signal. The bandwidth limitation
requirement for the I and Q
chrominance signals necessitates
their passage through filters
whose inherent delay would, otherwise,
cause serious misregistration
between the brightness and
the color information. Therefore,
lengths of high- impedance coaxial
delay cable are usually employed
in the monochrome (Y) and I
chrominance channels of encoders
and decoders to achieve coincidence
of these signals with the
narrower -band Q signal.
The necessity for verification of
relative delay as a routine step in
encoder and decoder alignment
procedure becomes apparent when
a comparison is made between
permissible I and Q channel transient
responses and the stated
timing tolerance of ±0.05 µsecs,
as defined by FCC specifications.
This tolerance is very narrow
when compared to the rise time of
an I or Q step function signal and
to the relatively loose band -pass
specifications for these signals.
As a result, realignment of the
band -pass filters in the chrominance
channels can, with relative
ease, cause unacceptable changes
in relative delay without violating
band -pass specifications. Proper
receiver performance therefore re-
88
Tele -Tech &
ELECTRONIC INDUSTRIES June 1956