22-03-2018, 10:35 AM
Hi
The filtering on the video output is just a 9 MHz low pass filter which is part of the video amplifier. Before the DAC it is just over sampled at four times pixel rate.
By its nature the 9MHz filter produces over and under shoot, I have the coefficients of the over sampler adjusted to give minimum over and under shoot. This may not be the correct way to have them set.
The test card is a bit modern looking for a vintage set and is harsher than one generated from an image but after all my antics with PIC test cards I just couldn't resist
.
I should point out that the frequency gratings are 1, 1.5, 2, 2.5 and 3 MHz. They are correct on 4:3 output but on 5:4 because the same card is cropped and stretched the frequency's will be lower. I don't have access to the VHDL files right now so cant say exactly how much lower they are.
Frank
The filtering on the video output is just a 9 MHz low pass filter which is part of the video amplifier. Before the DAC it is just over sampled at four times pixel rate.
By its nature the 9MHz filter produces over and under shoot, I have the coefficients of the over sampler adjusted to give minimum over and under shoot. This may not be the correct way to have them set.
The test card is a bit modern looking for a vintage set and is harsher than one generated from an image but after all my antics with PIC test cards I just couldn't resist
.I should point out that the frequency gratings are 1, 1.5, 2, 2.5 and 3 MHz. They are correct on 4:3 output but on 5:4 because the same card is cropped and stretched the frequency's will be lower. I don't have access to the VHDL files right now so cant say exactly how much lower they are.
Frank
