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Klaas Robers wrote:How is it named?
On the other hand, you are using the circuits of ROBOT, that Steve improoved. He has also circuits for sync detection. Keep that line.
n my electronics there is a tuned cicuit (inductor and switchable capacitors), resonating on 1200 Hz (1000, 1100, 1200, 1300, 1400 Hz). That wave form is rectified (diodes) and low pass filtered (C-L-C low pass filter). This analogue signal is limited (FCL101 = Schmidt trigger level detector), which converts it to a binary (0 or 1) sync signal. The circuits of Steve will also give a binary sync signal. That is more or less interchangable with this sync signal.
The horizontal saw tooth oscillator should sync directly on this sync, the vertical needs an extra R-C low pass, such that it does not react on the short line pulses, but only on the long frame pulse.
Klaas Robers wrote:Harry, in SSTV there are two ways of sync detection.
1. called "the TV-way".This is the method of the clippped video signal. Everything above a level of half way the sync pulse is considered as a "0", not belonging to the sync signal, and everything below that level as a "1", belonging to the sync signal. This is how all TVs work and all NBTV monitors.
2. called "the RADAR-way". In this method not the video signal is the input, but the FM-signal that carries the video signal. Here a resonator (passive L-C circuit or active opamp-circuit) resonates at the frequency of the synchonisation signal (1200 Hz). The amplitude of the resonating signal triggers a sync signal.
I don't know why this is called the RADAR-way, although some forms of radar use a sweeping frequency. Many SSTV monitors in the past used it. I think that when there are lots of interference on short wave, the 2nd method is more insensitive to that interference.
But with clean signals, coming from .wav files, there is no reason to use this more difficult method. The first method needs only a level detector, which you feed with the demodulated video signal. This can be a single opamp, where the - input is connected to the demodulated video signal (that goes to the base of the BD115) and the + input to a potentiometer that adjusts the level voltage. I think this is the better way for you to go.
In NBTV the 2nd method was never used, because NBTV only has a video signal. It is never FM modulated.
Klaas Robers wrote:Very good Harry. This is more or less what I wrote in words. Althoug I would have done the set circuit slightly different:
- potentiometer 10k
- bottom to "ground".
- top with 47k in series to +12V
- middle via 10k to the + input of the opamp.
You will get sync pulses "upside down", i.e. positive pulses.
It is a good idea to add a resistor of 1M from the output of the opamp directly to the + input of the opamp. This makes it a Schmidt trigger. Without the 1M it is a comparator. Works also.
I don't know the polarity of the sync pulses that your sawtooth oscillator needs. I don't know any more how the BRY39 works. Do you know Steve?
Klaas Robers wrote:I have been reading the circuit diagram and I think that you indeed need positive (upside down) sync pulses. And indeed, you need a diode in between, because the opamp goes to -12 volt. Anode to the opamp output, cathode (ring) to the oscillator sync input.
Klaas Robers wrote:Very good Harry!
And now you may adjust the vertical scan to about the correct timing. I see that it runs about twice the optimal speed. The circle is a usefull pattern to see that at once. Also the squares around the circle, they have to be real squares, not vertically standing rectangles. That should be done anyway before the vertical sync can work properly.
he distortion in the "top" of the picture is because the vertical sync is a longer pulse. Then the line oscillator is "held". I think that you need a diode and a resistor in series. A diode only might give this same result. The resistor about 47k.
Do you have a circuit for vertical sync separation?
I just "invented" one for you:
- From the output of the sync separator opamp a resistor of 100k to a "point"
- From this point a capacitor of 150nF to ground
- The base of a PNP-transistor connected to this point.
- Collector of this transistor to -12V
- Emitter of this transistor via a resistor of 10k to +12V
- Diode from emitter to sync-input of V-oscillator.
Klaas Robers wrote:You got it Harry. Good luck. But first adjust the vertical sync.
Synchronisation works in this way:
- the spot on the tube runs in a certain speed,
- you can adjust that speed.
- It is sensible to have that control on your front.
- Suppose the sync input is floating........
- When the spot is at a certain position the spot returns to the beginning of a line and starts a new line.
- This is called the "fly back".
- If you raise the voltage on the sync input the fly back starts earlier,
- so you get shorter lines.
- The sync pulse should come just before the fly back would start on its own.
[/quote]- With a resistor in series with the diode you can ensure that the sync input goes just 1 volt up
- or even less.
- Then a sync pulse works only in the last cm of the line,
- so the length of the sync pulse is of no influence.
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