That scared me between a little and a lot. A cable being 75 ohm means its characteristic impedance is 75 ohms. Means if you had advanced equipment you could solve 75 = sqrt(L / C), where L is the inductance and C is the capacitance. These values are small and increase linearly with cable length. Thus the (L / C) ratio is constant and you get 75 ohms with 10 cm or 10 meters. These are the reactive components as maxtherabbit mentioned. The resistive component, R, is also small and increases linearly with cable length. Can be ignored at, say, 2 meters or 6 feet. L and C can be ignored, except in 2 cases:vol.2 wrote:...I ran across this web page about matching video signals that clearly demonstrates that the author has absolutely no understanding of impedance at all; in one of the bullet point, they actually say that the cables aren't 75 ohms because they don't measure any resistance. (insert emoji of guy slapping his forehead) https://chipnetics.com/tutorials/unders ... o-signals/...
-Impedance mismatch (AC only). The actual use of a 75 ohm terminator is to match the cable impedance to prevent reflection and reduce RF interference.
(and/or)
-Too long a cable length or too high a frequency. In AC, the textbook definition is characteristic L and C matter when cable length > (wavelength / 4). In our case of 6 MHz video, matters when cable length > ~9 meters or 30 feet. Too high a C will kill DC, which is one reason why transmission lines are AC.
Since we have an impedance mismatch, the actual correct way to model in a circuit is with a transmission line model. I was taught the so-called Pi model. Can more easily calculate the reflection coefficient and standing wave ratio with 50 and 75. I found a chart that shows 1.5 swr (75 / 50) loses 4% in reflected power. That is not much and we might be able to ignore except...luminance (brightness) and color saturation are amplitude modulated in analog television. A reflected wave will add or subtract those values, with more deviance from a greater mismatch.