What is the need of equalization pulses? Explain pre and post equalization pulses in brief.
- Equalization pulses are unrelated to the process of equalization that is used to compensate poor frequency response of coaxial cable, or poor frequency or phase response of a filter.
- Analog sync shown in diagram illustrates the development of the combined (vertical and horizontal) sync waveform.
- Every line outside the vertical interval starts with a normal sync pulse having a duration of 4.7 µs. Vertical sync is identified by a sequence of broad pulses, each having a duration of half the line time less a full sync width.
- The broad pulses are serrated so that a receiver can maintain horizontal sync during the vertical interval.
- When analog sync separators comprised just a few resistors and capacitors, imperfect vertical sync separation was prone to exhibit line pairing, where scan lines from the second field were not laid exactly halfway between lines of the first field.
- Line pairs were prone to be visible. Achieving good interlace required interposing narrow equalization pulses, each having half the duration of normal sync, between line syncs.
- There are three lines of pre-equalization pulses, three lines of broad pulses, and three lines of post-equalization pulses.
- In 480i, line 1 and 0V are defined by first equalization pulse of a field. (In other scanning standards, including HDTV, line 1 and 0V are defined by the first broad pulse of a field, or frame.)
- The first field in 480i was historically denoted odd, and the second field was historically denoted even. Historically, 0V was defined for each field, and lines were numbered from 1 in each field.
- Nowadays, 0V for the second field is largely irrelevant, and lines are numbered through the frame. 0V for the 480i frame is defined by first equalization pulse coincident with 0H.
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