Interlaced Scanning:

• In television pictures an effective rate of 50 vertical scans per second is utilized to reduce flicker. This is accomplished by increasing the downward rate of travel of the scanning electron beam, so that every alternate line gets scanned instead of every successive line.
  • When the beam reaches the bottom of the picture frame, it quickly returns to the top to scan those lines that were missed in the previous scanning. Thus the total numbers of lines are divided into two groups called ‘fields’. Each field is scanned alternately. This method of scanning is known as interlaced scanning.
  • It reduces flicker to an acceptable level since the area of the screen is covered at twice the rate. This is like reading alternate lines of a page from top to bottom once and then going back to read the remaining lines down to the bottom.
  • In the 625 lime monochrome system, for successful interlaced scanning, the 625 lines of each frame or picture are divided into sets of 312.5 lines and each set is scanned alternately to cover the entire picture area. To achieve this the horizontal sweep oscillator is made to work at a frequency of 15625 Hz (312.5 × 50 = 15625) to scan the same number of lines per frame (15625/25 = 625 lines), but the vertical sweep circuit is run at a frequency of 50 instead of 25Hz.

Diagram:

Interlace error:

• By selecting an odd number of lines, the symmetry in frame blanking pulses is achieved and this enables perfect interlaced scanning. Any error in scanning timings and sequence would leave a large number of picture elements unresolved and thus the quality of the reproduced picture gets impaired.

• For convenience of explanation the retrace time has been assumed to be zero. Interlace error occurs due to the time difference in starting the second field. If it starts early or late interlace error will be there.

Calculation:

• For a 16 µs delay in the start of the second field starting points of the two fields will be 48 µs apart instead of the desired 32 µs. Then the percentage interlace error =(48-32)/32×100=50%

• If the second field starts 16 µs early even then the error would be 50%. For a delay of 32 µs the two fields will overlap and the interlace error would be 100%, i.e., half the picture area will go unscanned.