In SX Microcontrollers, SX/B Compiler and SX-Key Tool, PJ Allen wrote:

"The serrated vertical pulse consists of six individual pulses, each of approximately 27us."[sup]1[/sup]

These pulses get integrated after the Sync Separator, the resulting output is a triangle wave, at or near 60Hz (in North America.)  

"Generally, the vertical hold control sets the oscillator at 50-59Hz, to be locked in sync at 60Hz at the time of the third or fourth division (serration) of the vertical sync.  The main requirement for timing is that the triggering occur at exactly the same time of the vertical sync pulse for every field." [i]Ibid.[/i]

[sup]1[/sup] "Basic Television Principles and Servicing", Bernard Grob, 4[sup]th[/sup] Ed., 1975

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