TM 11-6625-3025-14/ET426-AA-MMA-010/E154 VII210/T.0. 33A1-8-902-1-1
The description of a scan rate will specify the number of scanning lines per frame and the vertical field rate. Most
television systems are "interlaced" to avoid a visible flicker in the display, usually a 2:1 ratio (figure 4-3). Thus, 2:1
interlace ratio means that two entire display "fields" must be developed to generate one "frame", or complete image. For
example, the scan rate of 675/60 indicates that: - There are 675 scanning lines per television frame.
-
The field rate is 60 Hz. That is, one television field is developed in 1/60 second.
-
Interlace ratio is 2:1. The frame (a complete image) will be composed of two fields. Therefore, one frame is
produced in 1/30 second. Each field is composed of half the number of scanning lines per frame, as in this
example 337.5.
NOTE
The format for this instrument: H drive, V drive and blanking is the same for all
rates. The sync waveform is the INDUSTRIAL format without equalizing pulses and
serrations per EIA Standard RS 343A (see figure 4-4).
Horizontal rate pulses are adjustable so the waveform can be matched exactly to the desired operating format.
Vertical pulse proportions must by synchronous with the horizontal scan lines, and therefore are determined by digital
count and adjustable by count selection.
In order to provide interlace, the master oscillator (on the drive/blanking card) operates at a multiple of twice
horizontal (2H) rate. The serrations and equalizing pulses are developed directly from this master timing reference, at the
2H rate. From this point, the master timing pulses are divided by count to generate horizontal (H) rate and vertical (V) rate
pulses. Horizontal rate pulses are adjustable to allow operation at various formats as required for different scan rates.
Vertical rate pulses are established by scan line count.
The 2H timing reference is used to develop the serration pulse, and also a front porch pulse somewhat shorter than
the serration width to act as a timing reference for blanking and drive pulse leading edges.
The 2H front porch pulses are submitted to a 2:1 count to develop H-rate timing reference for generation of blanking;
H drive and H sync; these particular pulses are generated by individual monostable multivibrators on their respective circuit
cards.
These 2H rate pulses are also applied to a counter which develops the vertical (V) rate. This circuit develops a V
reset which acts as the trigger to generate vertical pulses which signify the end of a television field (two fields constitute a
frame which is one complete
4-4
