Not much left to do. I was going to make the output circuit on a piece of veroboard but the all process is very tedious. I decided to go with a PCB as I feel that the circuit could be handy for other aplication VGA or PAL. Besides considering the price of 10x10cm pcb (5 dollars), the answer is obvious.
this board will contain the output color circuit which transforms the 1V signals into the proper 700mV signals needed by the VGA color standard (it seems fine with 1V thought, but the documentation mentions "peak" so I'll play on the safe side). This part will be able to invert the signal as well.
the board also has the envelope follower with three filters options (full bandwith, low pass, high pass).
here a simulation of the front panel . The project will be housed in an used metal tool box.
This log will be heavy on pictures. Sorry for your internet bandwith.
The lzx cadet vco is built and working. Really nice piece of circuitry. I added an inverted output to both waveform (square and triangle) . I had some oscillation at 42Mhz which was the ringing of the 74HC14 inverter at the output of the LM6172. The noise also fedthrough the other waveform (both inverted output share the same package) .
10pF capacitor in the feedback loop of both op amp solved the issue.
This vco was intented for TV usage. Nonetheless I guess it'll work as well with VGA. I built a small test jig for two oscillators:
behin the panel, it's the video splitter board. It split the PAL video signal from its syncronisation and recombine it afterward. It allows to input an external 1v dc signal and it displays it on the screen (using the sync signal from the input pal signal).
On a tv, this setup gives nice black/white patterns, vcos can also be modulated with video signal !
Now that the vcos are good I need to test them with a vga screen (just to be sure) build the last 3 of them, the envelopper follower and the second part of the output circuitry.
The XR2206 VCO gave poor results. Noises on the waveforms and a reduce amplitude above 200Khz. I'ts a pitty because the amplitude modulation amplitude could have been quite handy.
I decided to go to the LZX industries cadet VCO :
I redesign a PCB for 0805 SMD parts and I added a bit of logic with AND gate to have only one switch for the vco to be sync'ed to the vertical or the horizontal sync signal. I also added an inverted output for the triangle and square signals.
Once the VCO is ok the project will move forward quicker.
Ordering and waiting for parts is so boring....
the vco is working fine minus two non-lethal errors.
I wanted to have a sine and triangle outputs. The sine output was supposed to be shaped after the triangle in the regular synth fashion, with a diode clipper.
I worked a bit on the audio modulation circuit. It will make things move on the screen with the audio input signal.
At first I though about controlling the analog switch with three microcontroller pins . Although it'd be a simpler solution, both circuits are not on the same board which will create a need for another ribbon cable. I wanted a bit more fun with digital logic and came up with the switch controler on the right.
Value for the filters will certainly be variable with trimmers.
The input buffer will be able to accommodate low signal (headphone output of mp3 player), line level signal and modular level signal (very hot 10Vpeak-peak). R1 will be made variable to attenuate and control the depth of modulation.
The output will probably be clipped at 1V to not exceed the standard of the synth.
The VCO board are in the house. I wanted them before the celebration of the Chinese new year (Happy New Year!!). I ordered five and received nine. Always good, thanks Elecrow ! The remaining will be for sale I think.
I added a small prototype area for added circuitry if need be. I'm also working on a video signal modifier (PAL) and the oscillator could be used for it as well.
As I'm a bit slow I haven't ordered the parts at the same time which will delay even more the project. I'm not in a rush anyway. I must say that I hate this part of a project, waiting for components to arrive, borrriiiinnngggg.....
I ordered two DIP LM6172 from alixepress, they work fine. The part is 6euros at Farnell !!
Working on the mechanical aspect of the project (box and faceplate) as well.
While I’m trying to figure out the specific vga timing I decided to put down the current progress.
The plan was to add VCAs to the synth. However, without surprises a LM13700 can’t handle 1Mhz (not really designed for it). The only «ready » solution at those frequencies is the LT1228 from Linear. However it costs 8,88 euros. Needless to say, it’s not for me.
Another solution is to build a vca from discrete components. Simulation are being made with what I called the Yusynth VCA although it is a standard VCA design seen a lot in Synth design. A major drawback is the offset of 500mV at the output. I’m wondering if there will any VCA in that project after all, this will give 5 VCOs instead of 4.
This led to another constrain, the bandwith of the op-amps throughout the synth. The bandwith and the slew rate should be sufficient for the minimun 1MHZ frequency. It’s a nightmare to chose an opamp from parametric search because there’s so many. I reduced the selection by taking into consideration only parts that can be bought on aliexpress or ebay. Remained the LM6172 or the MAX4392 that works only with +5/-5V which may lead to redesigning the VCO. The synth will run with +12/-12V, I’ll stick with the LM6172 for now, besides is the same price as the MAXIM part.
The pin 3 of the
XR2206 controls the frequency. It is fixed at 3V and the design
should not pull more than 3mA out of it. The problem is to not to
exceed those specifications while modulation is applied to the input.
The first solution is to clamp the signal when it needs to be in
order to not fry the XR2206. However it requires more components
the range of the manual frequency control whereas the highest and
lowest frequencies are only obtainable with modulation applied. I took this solution.
problem applies to the « fine » frequency control
when, again, modulation is applied. Therefore I removed the « fine »
The PCB has been designed for cost and size. Jacks and switches will be linked with wires. A prototype area will be installed in the bottom empty space of the board
The VCO are an essential piece of the project. They will produce the shapes and contours of the images. The VCOs will be all the same. They will have a large range from 1 or 2 minutes to 1Mhz (despite the fact that the waveforms are less neat around this frequency).
1Mhz is actually a small limit for the VGA scanline. Meaning that once locked the vco will produce less vertical bar that it can produce horizontal bar (vertical sync). This is the result of a scanline twice faster that the one used in PAL or NTSC (about 15Khz). Although test will be run to run the clock generator lower.
The VCO is using the XR2206, obsolete but still available from China. Are they counterfeit? Clone? Rejected part at the factory? I don't really care as long as they work. Some external corrections are needed to constrain the waveforms into what it is expected for the project.
To get stable image we need to lock the vco to the monitor clocks (H sync or V sync) and the XR2206 hasn’t got a dedicated pin for it. I manage to get the VCO to reset but it creates a «tick» in the waveform that should be removed for processing. This is done with a clamp circuit that reduce the negative peak. The two other op-amps are here to make the waveform into the 1VDC signal range.
Approaching the MHZ range, the square output tends to become something else.
I needed to create a kind of « reshaper » circuit to get back a proper
square wave or at least something less ugly.
For the display, two things are mandatory. Video output drivers and blanking.
Each vga color wire is terminated by a 75R resistor, hence the need for special op-amp able to drive such high load. I'm using the LM6181 op amp which seems to be now obsolete. I may switch for the MAX4392 or equivalent if I need to.
The blanking is done by using the horizontal sync signal. Documentations show a particular signal timing for the blanking of colors but my tests gave the expected result on two different monitors. Maybe the monitors create the needed blanking.
Considering the limited space in the case I'll probably use (an old tool box) a switch is present to send a monochrome signal, letting the color's VCA (if there's any) be used for others tasks. Monochrome is achieved by driving the RGB signals with the same signal (all three colors at full brightness give white).
For the prototyping stage a cheap vga signal generator from ebay is used. It gives timing signals and color input. It'll be replaced by a STM32f0 microcontroller.
I've been interested in video creation and modification for a long time, right after I've read about the Sandin project, the synkie project and LZX industries modules.
All use the composite analog video protocol (PAL/NTSC). It's a complicated signal that combined the synchronisation signals needed for the monitor and the color informations into one signal !
This signal contains digital (sync signals) and analog parts(colors).
It requires large bandwith that most op-amps can't handle, at least 3Mhz.
Being an old analog protocol, it tends to disappear. Analog tv is a thing of the past now and monitor with composite input will certainly vanish as well. I don't even need to mention the death
of cathode ray monitor.
Two reasons for the choice of VGA, the obsolescence of the PAL and the simpler protocol of the VGA. The VGA protocol is simpler, the monitor receives all the signal on different wires.
I want to keep the all project simple and as cheap as possible (the most difficult part I guess).
I decided to take the LZX Industries standard for signals (1Vpp DC)