Briefly explain NTSC coder?

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written 6.7 years ago by

teamques10 ★ 68k

NTSC encoder is shown in diagram below, the encoding process of colour signals at the NTSC transmitter. A suitable matrix is used to get both I and Q signals directly from the three camera outputs.
Since I = 0.60R – 0.28G – 0.32B, the green and blue camera outputs are inverted before feeding them to the appropriate matrix. Similarly for Q = 0.21R – 0.52G + 0.31B, in inverter is placed at the output of green camera before mixing it with the other two camera outputs.

NTSC Coder

The bandwidths of both I and Q are restricted before feeding them to the corresponding balanced modulators.
The subcarrier to the I modulator is phase shifted 57° clockwise with respect to the colour burst. The carrier is shifted by another 90° before applying it to the Q modulator. Thus relative phase shift of 90° between the two subcarriers is maintained for quadrature amplitude modulation.
Thus, with the subcarrier phase angles shifted to the locations of I and Q, the outputs from both the modulators retain full identity of the modulating colour difference signals.
The sideband restricted output from the I modulator combines with the output of Q modulator to form the chrominance signal. It is then combined with the composite Y signal and colour burst in an adder to form composite chrominance signal.
The output from the adder feeds into the main transmitter and modulates the channel picture carrier frequency. Note that colour subcarrier has the same frequency (3.579545 MHz) for all the stations whereas the assigned picture carrier frequency is different for each channel.

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