alfredocalza wrote: ↑
Sat Jan 18, 2020 3:35 am
I was reading through the LM1881 data sheet and it says it is optional to install a lowpass filter in the composite video input line before it enters the LM1881. For what I could understand, low pass filters filter out any high frequency signals. Would this help eliminate any of the undesired artifacts?
Yes, and all the rest of the video too
In a frequency sense, a spectrogram of video will look something like this:
Code: Select all
| \ | |
| ----|-----\ |
| ------------|\ F
| A B C D E -------
+----------------------------------------> increasing frequency
the things on the graph are
A: Brightness changes vertically
B: Hsync pulse
C: Wider brightness changes horizontally
D: a mix of wider brightness changes and color information
E: Color carrier
F: a mix of narrow brightness changes and color information.
Increasing the lowpass filter knocks out F, E, D, C in order, until you get just the hsync pulses desired by the LM1881.
But it should be clear why this won't help with visual artifacts if you want to keep the video.
According to the data sheet, the low pass filter consists in a 620ohm resistor with a 510pf capacitor. I do not have a 620 ohm resistor nor a 510pf capacitor, however I could use 2 resistors that I do have to get to that value (500 + 120), and use two capacitors in parallel (220pf + 330pf, I know this is not quite 510pf).
The important part is 620Ω · 510pF = 320ns. You should be able to pick other choices of R and C that are comparable.
Also, if I install this low pass filter, would I need to amplify the SYNC signal after it comes out of the LM1881? It makes sense to me that the 620ohm resistor would make the video input signal smaller?
Except that the purpose of the LM1881 is to take the analog input and generate a digital
output. It'll be fine.