Maker Pro
Maker Pro

TV transmitter - picture slides vertically

GhostLoveScore

Nov 27, 2016
71
Joined
Nov 27, 2016
Messages
71
I built this circuit

tvtx.png


But without the audio part. I have also put simple one transistor RF amplifier on the collector of Q2. The range is alright. 30m or so, maybe 50m. But the problem is that the picture slides vertically. I have an older portable TV that has V-Hold trimmer and I can slow it down so I can see the picture. But it never stops. On my other TV, newer CRT TV, sometimes it can "lock" on the signal and sometimes not.

So what is the problem with the composite video signal when TV can't stop the picture from sliding vertically? I mean, it's obvious that the transmitter distorts composite video signal in some way, I want to know what exactly it distorts. Otherwise, picture looks fine.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Joined
Jan 21, 2010
Messages
25,505
If say the sync pulse at the beginning of each frame is being distorted or removed.
 

GhostLoveScore

Nov 27, 2016
71
Joined
Nov 27, 2016
Messages
71
F_23_9.gif


It's strange really, vertical sync pulses are similar as horizontal sync pulses, except it's more of them. If horizontal sync pulses are not distorted vertical sync pulses also shouldn't be. But obviously somehow they are. Anybody wants to guess where the problem lies?
 

GhostLoveScore

Nov 27, 2016
71
Joined
Nov 27, 2016
Messages
71
Yes, it's very simple. There are more complicated plans on the internet, but I was thinking, TV transmitter is essentially an oscillator who's amplitude is modulated according to composite video signal on the input.
Would it be possible to build TV transmitter similar to this -

RF oscillator--------------------->MIXER->
Composite video input------>MIXER-> AM signal output

Both are fed into a mixer and AM signal is on the output.
 

davenn

Moderator
Sep 5, 2009
14,011
Joined
Sep 5, 2009
Messages
14,011
Yes, it's very simple. There are more complicated plans on the internet, but I was thinking, TV transmitter is essentially an oscillator who's amplitude is modulated according to composite video signal on the input.
Would it be possible to build TV transmitter similar to this -

RF oscillator--------------------->MIXER->
Composite video input------>MIXER-> AM signal output

That would be a better way to start :)

A stable RF Osc section is essential, one that isn't swayed all over the place by other variations happening in the circuit
 

davenn

Moderator
Sep 5, 2009
14,011
Joined
Sep 5, 2009
Messages
14,011
trying to find a respectable AM TV transmitter circuit for you is difficult
there's this one, not sure how readily available that IC is for that first one ?

http://www.circuitstoday.com/ic-based-tv-transmitter-circuit

http://circuitsan.blogspot.com.au/2013/09/tv-audio-video-transmitter-schematic.html


All the TV transmitters I do in amateur radio are all FM voice and video. makes things so much easier.
most of us gave up on AM TV a long time ago because of the hassles
it means you can use any video monitor (TV set) that has a composite video and audio input to plug the receiver output into.


Dave
 

GhostLoveScore

Nov 27, 2016
71
Joined
Nov 27, 2016
Messages
71
I agree, FM transmitter is easier to make, but it also means that I need another circuit, FM receiver for the TV. I think I'll try with AM TV for now. Less trouble with it. At least this way I have working receiver.
 

Audioguru

Sep 24, 2016
3,639
Joined
Sep 24, 2016
Messages
3,639
I think the video signal into the transmitter must have a high enough level for the diode "DC restorer" to do its job of keeping the video-plus-sync DC level fairly constant as the video contrast changes. Because video-plus-sync is supposed to be DC coupled.

If black in the video turns grey then the blacker-than-black sync will not be a very negative voltage like it is supposed to be.
 

GhostLoveScore

Nov 27, 2016
71
Joined
Nov 27, 2016
Messages
71
I have one problem. I made another oscillator, just the oscillator. And I'm looking at the TV, I get black image with few white bars on it. That probably means that the oscillator is not stable, it's amplitude is changing. But what really is problem is this - the image on TV changes depending how the power cord for the oscillator is placed. If I move the power cord the image changes. The oscillator is not shielded, but could that be the only reason for this dramatic changes that I see when I move the cord? The oscillator is simple Colpitts oscillator.
 

GhostLoveScore

Nov 27, 2016
71
Joined
Nov 27, 2016
Messages
71
I don't really understand the question. It's composite video taken from small camera. The image locks, but as soon as I change the power supply cord direction or shape, the TV loses vertical lock.
 

Colin Mitchell

Aug 31, 2014
1,416
Joined
Aug 31, 2014
Messages
1,416
By looking at the waveform above you can see the sync pulses are on the bottom edge of the signal. This means the transmitting transistor must be able to accept a wide band and if it only accepts a narrow band, the sync pulses are not transmitted.
It seems you are lacking both horizontal and vertical sync and maybe emitter-injection into the output stage is not delivering the peaks and troughs.
Maybe base-injection is the answer.
It is always very difficult to inject into the emitter because the emitter presents a very low impedance.
Maybe you should amplify the video signal before injecting it.
 

GhostLoveScore

Nov 27, 2016
71
Joined
Nov 27, 2016
Messages
71
OK. I'm going to build JFET mixer. It uses base injection and report how it goes in an hour or so.
 

GhostLoveScore

Nov 27, 2016
71
Joined
Nov 27, 2016
Messages
71
OK, the dual FET mixer works finally. The picture stopped rolling. I still used DC restorer on video input on the mixer. Without it the picture is not good. I'll now build this circuit again on the new board. Right now it's assembled on a pieces leftover copper boards. Thanks guys for helping to figure this out.
 

GhostLoveScore

Nov 27, 2016
71
Joined
Nov 27, 2016
Messages
71
I was looking at the simulation of oscillator in LTSpice. It seems that oscillator produces rail to rail oscillations. Take a look
oscill.png

Voltage is from Q1 collector. I was thinking of putting FET buffer amplifier after that oscillator, but I am confused how it will work. This is 11V peak to peak voltage. That means that on the FET gate there will be voltage from -5.5V to +5.5V. My FET has Vp of only -2.19V. It seems to me that during part of the sine wave FET will be in pinchoff and part of the sine wave FET will be forward biased.

Can somebody help clear this confusion?
 
Top