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ThyDntWntMusic 2nd November 2012 04:33 AM

Minimum opamp specs for video?
Whats the minimum specs on an opamp that could be used for video?

I assume the resolution will have an effect on the opamp required? how does one know what opamp will be suitable?

If an underperforming opamp is used, what will the result be? no picture? interference?


sofaspud 2nd November 2012 06:24 AM

1) it depends on the required quality of the video
2) yes, resolution will greatly determine the amount of data that must be processed
3) datasheet info as a first step
4) distortion, or it may refuse to work at all

Do a search of "video op amp" just as you would for "audio op amp." The AD811 is just the handiest I had, but there are many others.

davidsrsb 2nd November 2012 07:24 AM

And load driving, video is usually 75R load impedance, which requires high currents.
In the video world, the common specs are differential gain and phase, not harmonic distortion

ThyDntWntMusic 2nd November 2012 07:28 AM

I know there are opamps that are specifically listed as being suitable for video, but wasn't sure if they were required in all situations ;)

Mooly 2nd November 2012 07:51 AM

If the video is in the analogue domain then you are looking at wide bandwidth. For example the broadcast TV UK standard would have been DC to 6Mhz. VHS quality video took that down to around 3Mhz. With decreasing bandwidth comes loss of HF definition.

So the minimum specs also depend on the bandwidth you expect to see. The term "video" covers a huge spectrum.

sofaspud 2nd November 2012 08:02 AM

Probably not. The lowly 741 may be capable of some type of (very poor quality) video. Generally, video requires a fast op amp with wide bandwidth. Along with compatibility with the 75-ohm standard as mentioned, and probably some other characteristics other more knowledgeable members know of. Which all relates to your 3rd question; I think the video format and quality determines the amount of information that must be processed per time period, then an op amp must be found that can keep up with that.

ThyDntWntMusic 2nd November 2012 08:04 AM

Amiga 500... PAL? 640256 perhaps?

making reference to "bandwidth" doesn't mean a whole lot to me.

sofaspud 2nd November 2012 08:17 AM

Yes, and add eg 720i, 1080i, color vs black & white, etc. There is a lot of information in a video waveform, and it can't all be placed in a narrow bandwidth. That is true of audio, also. If you just want to playback speech, 300-3kHz is adequate; for music there's a ~tenfold difference either side, because there's a lot more information there. Bandwidth here is referring to the op amp's response to a wide range of frequencies. For quality video it's tens of megahertz.

Mooly 2nd November 2012 08:21 AM

1 Attachment(s)
Look at the numbers down the side of gratings on this test card. Oversimplifying somewhat, these are the "frequency" that corresponds to that level of detail. The 5.25Mhz are very fine vertical lines.

Think of audio. If you cut off the audio at 4Khz things would start to sound dull. Cut off the high video frequencies and the fine detail starts to disappear.

Those figures for the Amiga are the resolution. The PAL (phase alternate line) means the signal conforms to the broadcast standard and can be applied to a TV to display, either directly as composite video or via a UHF modulator to enable the signal to be received via an aerial input.

You still need the high bandwidth even if the video is low resolution because the TV signal is made up of various synchronisation pulses that have to be passed with no distortion. I would guess you still need a couple of meg bandwidth even for an ancient computer video signal.

ThyDntWntMusic 2nd November 2012 08:34 AM

I've ordered two of these:

I had tried the circuit with a TL074, because it was there, and the results were rather poor... :p

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