Simple video fader (mixer) and amplifier

Hello,
I would like to build a very simple discrete video fader (to be used as a video mixer) and video amplifier, as an accompanying circuit to my transistorized UHF video/audio modulator.
I have found these two circuits.

I need your help of how to combine them to build a video mixer and amplifier.

Shall I connect the outputs of two faders at the input of the amplifier?
Shall I connect one fader in series with one amplifier, and connect it's output to an identical fader-amplifier pair?
Shall I connect the faders after the video amplifier?

Also is it better for the 1n4148 in the amplifier circuit to be bat82 as well? (can I use 1n5711 or 1n5712 instead of bat82?)


Please give me some hints.

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I have also found a third circuit, which seems better, as it combines a video amplifier ans enhancer. Can I connect the outputs of two such amplifiers in parallel, or through video faders in parallel?

Those old circuits were designed for old fairly blurry video, not for modern very clear high definition video.
Video amplifiers are never paralleled. Why do you need to parallel them? Each video amp drives its own 75 ohm cable and feeds one 75 ohm video monitor.

Audioguru said:

Those old circuits were designed for old fairly blurry video, not for modern very clear high definition video.
Video amplifiers are never paralleled. Why do you need to parallel them? Each video amp drives its own 75 ohm cable and feeds one 75 ohm video monitor.

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My intention is to build a simple video mixer (fade in one channel, fade out the other) and amplifier. I have made an analogue AV to RF modulator made out of a few transistors **broken link removed** and I need a simple mixer for it.

How do you propose to connect or combine the circuits I attached (or any other discrete), so a useful video mixer is made?

23 years ago I made video amplifiers like the Elektor circuits that were printed 23 years ago. Make them now if you want.
The levels for the video fader must be accurate so that the sync pulses are not attenuated. How will you synchronize the video signals when fading?

Audioguru said:

23 years ago I made video amplifiers like the Elektor circuits that were printed 23 years ago. Make them now if you want.
The levels for the video fader must be accurate so that the sync pulses are not attenuated. How will you synchronize the video signals when fading?

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I think that the job for this specific video fader is this one. to fade out the video but let the sync pulses come out.
I do not see a problem, unless the video signal (and sync) are at very low levels.

Now that I am thinking it, for the video fader I shall better add a sync separator http://www.tradeofic.com/Circuit/7719-SYNC_SEPARATOR.html and then fade out only the video with a simple pot. Then add the sync at the faded out output ass well.

Adding the sync is only a matter of connecting the sync circuit output and the faded video output together?

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But then, how should I know which of the two sync signals to add at the output, the sync from fideo source 1 or 2?

If the two video signals do not have their sync's synchronized then fading from one to the other will result in a loss of sync and the picture will be a mess.
Instead of fading you can abruptly switch from one video and its sync to the other.

Audioguru said:

If the two video signals do not have their sync's synchronized then fading from one to the other will result in a loss of sync and the picture will be a mess.
Instead of fading you can abruptly switch from one video and its sync to the other.

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Thank you.
Is there any way I can use this circuit (and make sume changes) so I can synchronize the two syncs?
I think it might be difficult.

The other way would be to completely fade out one signal and after that fade in the other. But this is not mixong of course

You spotted the problem. Video fading isn't like sound fading because only the active video part of the signal is controlled, the sync pulses have to stay at the same level. The Elektor fader design is terrible and should be avoided, all it does is rely on the diode voltage drop to let the syncs pass through and bypass it with a resistance to let the rest through. It isn't constant impedance, may not accurately fade to black level and will be highly dependant on temperature and any DC on the input or output.

The usual way to do it is to use a sync separator IC and an analog change-over switch. The LM1881 (and equivalents) is cheap and automatically adapts to different signal levels, it also splits the syncs into horizontal and vertical types although you would only need the combined syncs for a fader. What you do is split the video three ways:
1. to the LM1881
2. direct to one input of the analog switch
3. to the other analog input through a potentiometer (like a volume control).

The syncs operate the changeover switch so that during the picture part of the signal the input through the potentiometer is used and hence the level can be adjusted and in the sync periods, the direct input is used so the level remains the same. It can be done in less than 10 components and they are all inexpensive.

Cross fading is far more complicated unless you can synchronize the video sources. Pro equipment has 'genlock' inputs so everything can be run from one central sync generator unit, it makes fading and effects very simple. If you can't genlock, your only option is to pick one source as 'master', take a copy of it's syncs and use them to lock the 'slave' picture. This involves digitizing the picture at slave rate and storing it in a memory then playing it back at master rate. It isn't easy! You need dual-ported memory and ADC's that can digitize at faster than 10MHz rate.

You can do a 'dirty' cross fade by fading one source to black, switching to the second source, including it's syncs, while it is also black then bringing the second level up. It doesn't synchronize the video sources and although both will be faded right down during the switch, the monitor/recorder might still sees a discontinuity in the sync patterns and cause a visible picture tear or roll.

Brian.

Are you using old PAL video signals or newer HD video signals?

You have to genlock both the vertical/horizontal sync and the chroma burst if you want to ensure a smooth fadein/fadeout.

Genlock means replacing the two different video sources with a single master clock, which will ensure no jarring and no color shifting

betwixt said:

You can do a 'dirty' cross fade by fading one source to black, switching to the second source, including it's syncs, while it is also black then bringing the second level up. It doesn't synchronize the video sources and although both will be faded right down during the switch, the monitor/recorder might still sees a discontinuity in the sync patterns and cause a visible picture tear or roll.

Brian.

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Yes that is what I have thought as a very simple solution. However this is not a cross fading effect so it is useless.

I like the elektor videoenhancer circuit posted on post 1. Shall I just connect in parallel with it this circuit http://www.tradeofic.com/Circuit/7719-SYNC_SEPARATOR.html
will that work as a simple fader without sync lost even at lowest fading levels?


However the output of the enhancer is about 1Vpp and the input of the separator should be about 1Vpp so I guess they have to be connected in series, with the separator and "fade out potentiometer" in parallel to themselves?

The video enhancer is only a high pass filter that will allow a higher portion of the high-frequency video content, which occurs mostly during sharp luminance transitions. Think that of a simple "tone control" as found on simpler radios.

But similar to the radio analogy above, it will also enhance noise.

There is an additional issue. Since I assume you will be attempting to enhance composite video (luminance plus chrominance), sharp color transitions may induce a color "halo".
For best results, this has to be applied only to the luminance of component video (Y, Cb, Cr).

You cannot simply connect the output of the video enhancer in series with a fader pot and in parallel with the output of a sync separator:

1) The fader pot destroys the 75 ohm output impedance of the enhancer.
2) The high output impedance of the sync separator circuit will have its signal squashed by the enhancer and fader circuit.

You also cannot connect the enhancer in series with the sync separator.

Instead you must separate the video from the composite signal and fade it. Also separate the sync from the composite. Then mix them in a mixer circuit that has a 75 ohm output impedance.

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If I remember video correctly, high frequencies are black and white because high frequencies in color will cause interference with the 3.58MHz (NTSC) or 4.4MHz (PAL) color modulation.

I think you are over complicating things.

Use an LM1881, it is a far better sync extractor than the transistor design and it only needs two capacitors and one resistor to function. Use a CD4066 as the change-over switch, one section for direct, one for the controlled level and one wired as an inverter so the first two work alternately. That's 7 components in total, including the level control!

If you want the 'enhancer', which is is just a HF boost control with P1/C8 as the top of a voltage divider and R6 at the bottom, use just those components then restore the video level with a single stage amplifier. Ideally, you would incorporate it after the sync pick off to the LM1881 and before the analog switch as it will distort the sync pulses and might upset the slicing level.

Brian