VGA video lower limit bandwidth? - Page 1
Forum section: electronics / projects

author: rfdes
author_rank: Regular Contributor
timestamp: March 02, 2021, 01:47:11 am
content: I am designing a video splitter using OP amps and cannot find what the lower limit bandwidth needs to be. I wanted to use a single power supply approach and AC couple the output but am worried about the lower bandwidth. Should the splitter have a DC coupled output by using a +/- supply?ThanksJim

author: vk6zgo
author_rank: Super Contributor
timestamp: March 02, 2021, 02:01:26 am
content: Quote from: rfdes on March 02, 2021, 01:47:11 amI don't know a lot about VGA, but with composite analog video, the DC component either needs to be maintained, or to be restored.The latter is done so that ac coupling can be used in amplifiers for convenience, rather than the rather more complex DC coupled route.If this is of interest, you should be able to find the required circuitry by Googling.In some cases, it is sufficient to just maintain the low frequency response down to a low figure, then just "live with" the lost DC component, as was done in some cheap BW TVs in the 1950s, but it is generally frowned upon.As I said, I know very little about VGA, & I don't have a convenient source to probe with an Oscilloscope.A search on Google should reveal the system specifications, though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 02, 2021, 04:01:55 am
content: VGA up to what resolution? HD needs over 100MHz bandwidth. Observe good signal integrity, any reflections show up as shadows to the right of sharp graphics.Or did you mean LF cutoff? In which case, see above. DC restore is a common sight in monitors' video path, along with the rows of electrolytic capacitors carrying it.Obviously(?) the time constant needs to be several frames worth.Tim

author: gnuarm
author_rank: Super Contributor
timestamp: March 02, 2021, 05:27:28 am
content: Quote from: T3sl4co1l on March 02, 2021, 04:01:55 amYeah, he's talking about DC blocking. I'm familiar with composite, but not as much VGA. I know it has three analog video signals, one for each RGB. I believe it also has separate horizontal and vertical sync signals. Since the sync is separate, I suppose the DC restore would be done by clamping the black level on retrace?

author: vk6zgo
author_rank: Super Contributor
timestamp: March 02, 2021, 05:37:33 am
content: Quote from: T3sl4co1l on March 02, 2021, 04:01:55 amWith analog TV, it is usual practice to derive narrow pulses from the line syncs then using a "keyed clamper", switch the "back porch" of the video waveform to zero volts each & every line.This allows a much shorter time constant, hence smaller capacitors.I suppose something similar could be done using the VGA sync waveform.The much more basic simple diode "DC restorer" commonly used on early BW TVs did require a fairly long time constant, but they were pretty much extinct in the last years of analog TV, as keyed clamping can be incorporated in specialist ICs.I haven't really thought of "DC restorers" of the simple diode type for years, so only vaguely remember how they were used, without looking up some really old material.

author: james_s
author_rank: Super Contributor
timestamp: March 02, 2021, 06:26:23 am
content: VGA is just three separate analog luminance signals of 0-0.7V on a 75 ohm coax used in conjunction with TTL level H and V sync signals. The original VGA standard supported several different resolutions topping out at 640x480 with a pixel clock of around 25MHz IIRC. Later extensions increased this significantly. The higher the resolution at a given refresh rate, the higher bandwidth it requires.

author: David Hess
author_rank: Super Contributor
timestamp: March 02, 2021, 03:48:29 pm
content: For VGA, the green signal may also include composite sync.I do not understand what the difficulty here is. Commonly a DC coupled amplifier will be configured for a gain of 2 for each signal allowing for a double termination. Bandwidth requirements extend from 14 MHz for 640x480x60 to 87 MHz for 1600x1200x60 but bandwidth can extend to 300+ MHz for higher resolutions and refresh rates.

author: CaptDon
author_rank: Super Contributor
timestamp: March 02, 2021, 04:40:17 pm
content: Without D.C. coupling or restoration you could not hold a steady all white or all black screen. The coupling capacitors would eventually equalize and your picture brightness would just be a medium gray. That equalization time could be milliseconds or seconds depending on your circuit but a steady hi or low will tend to become steady at the mid-point.

author: TimFox
author_rank: Super Contributor
timestamp: March 02, 2021, 04:52:32 pm
content: In traditional analog video, “DC restoration” is used after an AC-coupled amplification chain. This is forcing the output voltage to a reference level (blackish) periodically (between frames/fields) when the input signal is at a fixed level. The best time to do this is the “back porch”, but simple diode circuits can clamp the sync tip (blacker than black) and still obtain a better result than straight AC coupling. A better term than “restoration” might be “DC insertion”.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 02, 2021, 11:25:42 pm
content: Quote from: CaptDon on March 02, 2021, 04:40:17 pmFor normal analog TV, you very seldom maintain a steady all white or all black screen.for that reason, some early BW TV sets "cheaped out" & used ac coupling.Obviously, the frequency response extended down to extremely low frequencies, just not DC.Critical viewing could show up the ruse, but it was quite useable.

author: james_s
author_rank: Super Contributor
timestamp: March 02, 2021, 11:28:29 pm
content: Quote from: CaptDon on March 02, 2021, 04:40:17 pmThere is a blanking period, both horizontal and vertical, otherwise you would see the retrace lines. I don't actually know offhand if VGA is DC coupled all the way to the electron gun but the service manual for any old VGA monitor could answer that. There must be some of them out there, that was the era when a lot of gear had real service manuals.

author: Medved
author_rank: Contributor
timestamp: March 09, 2021, 07:30:38 pm
content: VGA is generally AC coupled with DC restore circuit. Dunno if the DC recovery works on horizontal or only on vertical pulses.If on horizontal, the DC gets recovered every horizontal blanking, so at 30+kHz. With <0.5% brightness error across the line it means low frequency limit of below 150Hz (assume 1'st order circuit).If on vertical, you have to use DC restore within your amplifier, or it has to work down to FPS/200, so fractions of a Hz (to maintain no more than 1LSB error from the 8bit/channel typical resolution).As far as I remember, the coupling capacitors on the monitor inputs were in the 10..100uF ballpark, so I would guess they've used the H-based restoration.