Remember the test patterns that used to be broadcast by over-the-air
broadcast stations that were used to align the electron beam
defection circuitry in CRT-based televisions? That pattern of
squares, circles, parallel and radial lines was generated by
a special tube called a "Monoscope" (RCA
2F21) on the transmitter end. Focus, 4:3 picture aspect
ratio, linearity, frequency response, and contrast and brightness
were all tweaked to optimize the pattern on the TV receiver
circuitry. Of course not all sets were capable of obtaining
a perfect alignment due to inferior design and/or a scheme by
the manufacturer to provide a lower cost model with the tradeoff
being a poorer picture - that it the type of TV we always had
in our household as a kid.
As a side note, can you imagine the heck that would be raised
Indian chief's head were to be used in a test pattern?
The Television Test Pattern
By M. S. Kay
The test pattern has been designed to convey important performance
data. Study it carefully.
Fig. 1 - "Indian Head" test pattern used
by many television stations.
Every television station, prior to its actual broadcasting
period, transmits a test pattern for the purpose of permitting
set owners to adjust their receiver controls for optimum reception.
In addition, many stations set aside 3 to 4 hours during the
day - generally the morning and afternoon - during which they
transmit nothing but a test pattern accompanied either by a
400 cycle audio note or music. The purpose of this transmission
is to enable television servicemen to install and/or repair
television receivers. Without such aid, television work would
be confined only to those relatively few evening hours when
the stations are on the air. While equipment is becoming available
now which will permit adequate television service work without
the need of a transmitter on the air, no such simple solution
is in sight for television receiver installations. Only a signal
received from a station, as normally transmitted under actual
operating conditions, can be of any assistance in properly orienting
the television antenna.
Television test patterns assume many forms, but the one in
most common use today is that shown in Fig. 1. This pattern
is produced by the RCA "Monoscope" tube, 2F21, from a plate
built into the tube. When the station is transmitting this test
pattern, the "Monoscope" supplies the complete signal in place
of the television camera tube.
With the aid of these test patterns, the television serviceman
can check the adjustment of the following items concerning a
television receiver: 1. Focus Setting; 2. Aspect Ratio; 3. Linearity
Adjustment; 4. Frequency Response; 5. Shading and Contrast;
1. Focus: Focusing of the image is probably the simplest
operation of all. It consists merely in rotating the focus potentiometer
until the various sections of the test pattern stand out sharp
and clear. On either side of the correct focus control setting,
the image will become blurred and fuzzy. Some receivers achieve
focusing control by permitting adjustments to be made on the
physical positioning of the focus coil along the neck of the
cathode-ray tube. These adjustments are generally screwdriver
adjustments and must be made from the back of the receiver.
When one man is working alone on a set, the only way that he
can see what is appearing on the screen is through the use of
a mirror, placed facing the screen and tilted slightly upward.
2. Aspect Ratio: The aspect ratio is the ratio of the image
width to the image height and in modern television has been
standardized at 4 to 3. This means that the width is one and
one-third times greater than the height. On the test pattern,
Fig. 1, the large circle has a diameter which represents the
three in the aspect ratio. In other words, it is three-fourths
of the image width. The picture appearing on the television
receiver screen should be adjusted until the upper and lower
portions of the large circle are touching the top and bottom
edges, respectively, of the screen mask. The width of the image
is then increased until the four smaller circles are each in
one corner of the screen. Note that the background of squares
in the image also indicates the aspect ratio. There are eight
horizontal squares across the image to six vertical squares.
8 divided by 6 equals 4 over 3.
3. Linearity: To check the proper setting of the vertical
and horizontal linearity controls, examine the central circles
in the image. If they are egg-shaped, the beam is not traveling
at a uniform speed over the screen. Vertical non-linearity is
indicated by the circle being "egg-shaped" in the vertical direction.
See Fig. 2. By the same token, horizontal non-linearity is indicated
by having the circle "egg-shaped" in the horizontal direction
(Fig. 3). Whichever control is affected should be adjusted until
the circles become circular again. (Note: It frequently happens
that after this adjustment has been satisfactorily made for
one station, that the same test pattern for one or more of the
other local stations will be slightly egg-shaped. This is due
to non-linearity in the scanning equipment at the broadcast
station. The only thing the serviceman can do is to set the
receiver linearity controls to some compromise position. Fortunately,
with transmitted programs, slight non-linearity is seldom noticeable.)
Fig. 2 - An example of vertical non-linearity
in a test pattern. Note how compressed the Indian Head is whereas
the bottom third of the pattern image is "stretched out."
Fig. 3 - Example of horizontal non-linearity.
4. Frequency Response: The frequency response of a television
receiver is one of its most critical features and determines
how well an image will be resolved. Of particular importance
is the extent of the low and high ends of this response and
a careful check of the test pattern will indicate whether the
system is fully compensated at each of these ends.
High-Frequency Response. High-frequency response of the receiver
can be judged by examination of the four wedges at the center
of the test pattern. The numbers close to the wedges (i.e.,
20, 30, 35, and 45) stand, respectively, for 200, 300, 350,
and 450 line resolution. Between each of these numbers is a
line which stands for the middle value. Thus, the line between
20 and 30 stands for 25, and the line between 35 and 45 is for
40. At the center, the number 30 means 300 lines. The word "line"
refers to the number of lines that could be accommodated within
the vertical height of the pattern. The frequency at which the
system cuts off is indicated in the wedges at the point where
the lines blend together and are no longer visible separately
and distinctly. Thus, in most receivers, the lines run together
between 30 and 40. In a poorly designed set, the lines may blend
at 25. The circles in each corner also contain wedges with numerical
line markings. However, the resolution of these circles is made
less than the resolution of the center circles because the beam
is not capable of being as sharply focused at the sides of the
screen as it is in the center.
The wedges are formed with the black and white lines having
the same width if measured along a line normal to the wedge
centerline at that point. It will be noted that the two horizontal
wedges are identical to the two vertical wedges. This permits
us to check both the horizontal and vertical resolutions of
the system. The horizontal wedges are used to determine vertical
resolution and the vertical wedges are used to determine horizontal
resolution. That this should be so is evident from the following
reasoning. Vertical resolution, for example, is dependent upon
the closeness with which lines can be placed above each other.
In the horizontal wedges, the lines are placed one above the
other, with the spacing between them varying at various points
along the horizontal wedges. When the lines are no longer distinguishable
from each other, we have reached the limit of the vertical resolution.
By the same token, the vertical wedges indicate how closely
lines or details can be placed next to each other horizontally.
When the system is no longer able to resolve these pinpoint
white and black lines, they become indistinguishable and the
limiting resolution has been reached.
In a television receiver, the vertical resolution is almost
entirely a function of the number of lines used, in this case,
525. Normally, the performance of the receiver has little effect
upon this vertical resolution unless the interlacing is poor
in which case other indications on the test pattern reveal this.
(To be explained in detail later.) The horizontal resolution,
on the other hand, is dependent upon the response and performance
of the receiver r.f., i.f. and video-frequency stages and this
is important to the serviceman. Hence, the vertical wedges are
examined to determine- where the lines blend and the reading
at this point is recorded as so many lines, say 300. This means
that up to the number 30 in the test pattern the lines are separate
and distinct but beyond this they run together. Through experience,
most servicemen know that a resolution of 300 lines is close
to a 4.0 mc. bandpass in the receiver, but they possess no accurate
method of determining the system response for other values of
line resolution. With the following formula, the conversion
from the number of lines to frequency response can be readily
Freq. (cycles) = 12,500 X N
Where: N = number of lines as read from the test pattern.
For example, if the lines merge at 300 lines, the receiver response
is: Freq. = 12,500 X 300 = 3,750,000 cycles = 3.75 mc.
When the receiver system is over-compensated at, the high
frequencies, so that a definite peak results, then a process
known as "ringing" will take place. We check for "ringing" with
the two columns of single rectangular areas located on either
side of the inner circle. The numbers (50, 300, 325, and 575)
indicate the width of the nearest rectangle, the width being
given as the number of lines which could be accommodated vertically
in the image. If over-compensation or "ringing" is present,
the rectangles will be followed on the screen by multiple rectangles
somewhat similar to ghost images, but evenly spaced from each
other. This can be explained as follows.
When a signal containing high-frequency components is applied
to this system, the effect, if sudden enough, will produce a
series of high-frequency oscillations. These oscillations generally
die out rapidly, but before they do, they place several small
lines on the screen, each evenly spaced from the original sharp
line (or rectangle) that produced them. The lines are produced
by the negative peaks of each oscillatory cycle. Not all the
oscillations produce visible marks on the screen. Generally
only the first two or three are able to do this, depending on
the extent of the over-compensation.
When only slight over-peaking is present, it can be detected
by examining the black horizontal bars at the bottom of the
image. At the left-hand edge of these bars, the margin will
be excessively black whereas at the right-hand edge, there will
be a short excessively white margin following these bars. The
total effect is to outline the bars, and to make them more sharply
defined. Some servicemen are in favor of slight high-frequency
over-compensation because they feel that a sharper, crisper
image is obtained. However, if this procedure is followed, it
should be done cautiously lest excessive over-compensation and
"ringing" be produced.
Fig. 4 - A comprehensive test pattern designed
for checking television transmitter response and operation.
This is the standard RMA chart.
Low-Frequency Response. The eleven black horizontal black
bars located at the bottom of the test pattern are for the purpose
of checking the low-frequency response. When the system has
poor low-frequency response, the reproduced edges of the horizontal
bars will be sharply defined, but will change from black to
an excessive white, with a streamer shading from this white
back to the normal background. The visual effect is one of smearing,
with the smearing going from left to right because that is the
path of travel of the electron beam. Poor low-frequency response
is generally accompanied by excessive phase shift and it is
difficult to distinguish between the two. For the serviceman,
however, the appearance of the smearing means that the low-end
of the system's response curve is at fault and must be corrected.
5. Contrast and Shading: To check the over-all contrast of
the test pattern, we have the Indian Head at the top of the
center portion of the test pattern. The contrast control should
be set so that the head appears pleasantly shaded, being neither
so dark that the various shades of white and grey are obscured
or so light that the entire image has a filmy, washed-out appearance.
As a further aid in checking the receiver contrast range, the
remaining two oblique wedges in the test pattern have four different
tonal ranges. The innermost section of each wedge is black,
with each additional wedge possessing Ii lighter shade of grey.
If the transmitter and receiver are both operating normally,
then each of the four shading steps should be distinctly separable
on the test pattern. If the received pattern does not indicate
these four graduations, either the transmitter or the receiver
is at fault - the fault in this case being non-linear amplitude
distortion of the video amplifiers. Where more than one station
is operating locally, check with each station to determine whether
their patterns show the proper shading. If all stations appear
to have a poor shading in their test patterns then it is safe
to assume that the trouble lies within the receiver since it
is highly improbable that two or more stations, operating independently,
would exhibit the same difficulty. However, if only one station
presents a poor shading pattern, while the other test patterns
are normal, then the defect lies with the transmitter. To the
serviceman, the defect is important only when it arises in the
receiver. The trouble, when it is here, can be traced to an
improperly biased video-frequency amplifier or a gassy tube
in this system.
Fig. 5 - Station test pattern of WBKB, Chicago.
6. Interlacing: There is one final test that can be made
with this test chart and this is to determine whether the odd
numbered lines are falling properly between the even numbered
lines. If, for any reason, the vertical sweep oscillator is
not operating properly, the lines of one field will not accurately
drop into the spaces between the lines of the previous field.
Lines of one field may fall close to or even directly over the
lines of the previous field, in which case there is a loss in
detail due to this partial pairing of lines. In the test pattern,
we check the quality of interlace by examining the narrow diagonal
lines in the center portion of the chart. Partial interlacing,
which means incomplete interlacing, will be evident by the jagged
reproduction of these lines. When there is a complete pairing
of lines, or a complete absence of interlacing, then the test
fails. In this case, however, the pairing is generally quite
easily visible by noting the wide separation between the lines.
Since the defect is due to an improperly functioning vertical
sweep system, this should be checked.
Fig. 6 - Station test pattern of WNBT, New
A distinction should be made here between the reduction in
vertical resolution due to incomplete interlacing and the same
reduction arising from a limited frequency response. A limited
frequency response will not cause the diagonal lines to become
jagged, whereas poor interlacing will.
The foregoing discussion is designed to indicate the usefulness
of test patterns to television receiver servicemen. More extensive
test patterns are employed in the checking of television transmitters
because of the greater number of adjustments that must be made
in this equipment. A standard transmitter pattern developed
by the RMA Committee on Television Transmitters is shown in
Fig. 4. Finally there are individual test patterns developed
by each station bearing a few of the markings (in one form or
another) contained in the RCA "noscope" pattern and in addition,
giving the station call letters. Two such patterns are shown
in Figs. 5 and 6.
A General Electric television camera in action
with its crew at the U. S. Navy Special Devices Center at Sands
Point, Long Island, where the Navy is performing experiments
in mass training of personnel by means of telecast lectures
and equipment demonstrations.
Color and Monochrome (B&W) Television Articles
Posted September 11, 2015