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The built-in filters in Photoshop are not up to this job. I had to write my own using an authoring program called "Filtermeister." The filters are phase-linear FIR filters designed for the proper I, Q, and chroma trap bandwidth. There is also a sharp cutoff lowpass to simulate the IF amplifier.
The modulation and demodulation is done by assuming that the pixels fall exactly on 57 degrees (I), 147 degrees (Q), 237 degrees (-I), and 327 degrees (-Q). This results in a 720x540 4x3 square-pixel image, close enought to the real NTSC sampled values for illustration.
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