I converted a Famicom to a video output, and now I play with it by watching it on the monitor of the computer via video capture.
When I was observing the video signal one day, I noticed that the waveform was a little strange.
This is part of the video signal waveform, horizontal sync and color burst.
Notice the arrows and the end of the horizontal sync signal. This is a square wave, but it is rounded off.
The following waveform of the color burst signal is supposed to be a square-shaped waveform generated by the PPU, but it is now corrugated.
If this is blurred, the image such as characters and outlines are blurred a little.
Consider a waveform correction circuit
If the "changing part of the waveform" is blurred, I can correct it to emphasize the "changing part".
To be more specific, I will use this circuit.
The left side of the circuit above is the filter, and the right side is the 2 x amplification circuit. This circuit is a filter circuit in which the magnification changes from a certain value to 1 x or 2 x. The frequency characteristics are shown in the graph above.
The filter circuit on the left uses resistors "R1" and "R2" to reduce the amplitude of the waveform to 1/2. The amplifier on the right multiplies this signal by a factor of 2 to restore the original amplitude. Therefore, R1 and R2 alone do not change anything.
Then, capacitor C1 is inserted in parallel with resistor R1. So that only when the frequency is high, that is, the moment the signal changes, the signal will bypass C1 and enter the 2 times amplifier without reducing the amplitude.
This means that when the signal changes, it is 2 times as large as when it does not change, I. e. the changing part is emphasized. The value of C1 sets the frequency to change from 1 times to 2 times.
Famicom PPU---Circuit---Amplifier 6dp→Video output
You don't have to understand the principle. Let's do an experiment anyway. The parameters are as shown above. The resistance value was 4.7k Ω x 2.
Result of waveform correction
Notice the arrows, the end of the horizontal sync signal. 0 pF has rounded corners. 10 pF has been corrected to be approximately right angles. 22 pF has gone up a little too far and is now back to a little emphasis. 44 pF has gone up too far and is overemphasized.
As a result, 10 pf is the best in terms of waveform, and the blur is corrected.
Changes in the image
Then, what kind of change was there in the image?
This is the difference of the image of Dragon Quest II. I don't know well, so I will enlarge a part.
As the capacitor value increases, you can see that the blurred contour becomes more solid, and now let's take a look at Mother.
It is obvious that the clearness is 44 pf> 22 pf> 10 pf> 0 pf. The contour of 44 pf is emphasized too much, and the surrounding of the contour becomes a little brighter. Moreover, the vertical stripe noise is also emphasized a little bit. The slightly emphasized 22 pf looks good.
Famicom PPU—Circuit—Amplifier 6dp→Video output
By attaching the filter circuit described above to the video output pins of the Famicom, I found that the blurred waveform can be corrected and the blurring can be improved.
As for the capacitor value of the filter circuit, 10 pF is the best in terms of the waveform, but 22 pF, which is slightly emphasized in the appearance of the image, is good.
This makes the picture clearer and clearer than before.