[sdiy] Brain dump on analog video synthesizers

Grant Richter grichter at asapnet.net
Thu Jun 6 20:03:10 CEST 2002

Here is a quick video primer, as it relates to analog synthesis.
Also how to interface analog audio modules to video.
A monitor is a video monitor with a composite video input.
An RGB monitor can be thought of like 3 black and white inputs. The green
signal usually carries the sync info.

Signal levels, bandwidth and impedance's:
(not written in stone)

Analog Audio Synthesizers:
10 volt peak to peak signal levels
50-100K input impedance
1K output impedance
Bandwidth of DC-20KHz or better

Analog Video Synthesizers:
1 volt peak to peak signal levels
75 Ohm input impedance
75 Ohm output impedance
Bandwidth of DC-4Mhz or better

Analog video is composed of four primary components. When all are present,
it is called "composite video".

Zero volts DC level (ground)
This tells the monitor to shut off the beam so you do not see it during
"retrace" which is when it goes back to the beginning of the line, or top of
screen (during the reset portion of the sweep sawtooth).

Minus 300 millivolts
A short negative pulse resets the monitors horizontal sawtooth oscillator. A
longer pulse resets the vertical sawtooth oscillator. The period of zero
volts before H sync is called the "front porch" and the period after is
called the "back porch".

Positive 70 to 700 millivolts
Varies the brightness of the beam during the sweep portion of the monitor
horizontal sawtooth oscillator. The same as a Z input on an oscilloscope.

3.58 Mhz phase modulated sine wave riding on top of the luminance signal
Thank WinDOS is a kludge? Here's the original "must be backwards compatible"
horrorshow. A burst of 8-10 cycles on the back porch (color burst) is gated
to the monitor local 3.58Mhz oscillator to synchronize the phase. Then the
phase of the 3.58Mhz riding on the luminance is compared to the local
oscillator by I and Q (inphase and quadrature) detectors. The combination of
3.58Mhz amplitude and phase forms a 2 dimensional color surface which is
mapped to the RGB color guns by a resistor matrix. Ow, brain hurt.

Generator Locking, this is when the internal sync generator is synchronized
with and external sync generator. In order to mix camera signals, all the
cameras must be synchronized to the same source. In studios, there was a
master sync generator (house sync) which synchronized all the cameras,
graphics generators, etc.

Inside an analog video synthesizer, all color signals are treated as black
and white luminance signals. The Red Green Blue to NTSC encoder combines
these luminance signals with sync and generates the chroma signals sent to
tape and monitor.

The best design for an RGB to NTSC encoder would do the following.

1. Provide precision half wave rectifiers on the RGB inputs to keep them
from going negative (sync area).
2. Shut off the inputs during blanking
3. Clip the inputs over 1 volt ("specula highlights" i.e. sunlight on metal)

At which point ANY signal will be properly formatted to show up on a video
screen (even white noise).

A last bit of math: Your typical audio synth module has a 1K output resistor
and a 10 volt range. What happens if we connect this to the 75 ohm input
resistor of a video module?

10 volt / 1075 = 9.3ma x 75 ohms = 697 millivolts (full scale is 700 mv.).

In other words, your ARP 2600 already interfaces perfectly with video
synthesizers, as far as voltage levels are concerned. The oscillators just
need to have synchronization and go to a higher range.

> From: Grant Richter <grichter at asapnet.net>
> Date: Wed, 29 May 2002 13:18:15 -0500
> To: eric b <metasonix at earthlink.net>, Gavin Russom <elmystico at earthlink.net>
> Cc: <synth-diy at dropmix.xs4all.nl>
> Subject: Re: [sdiy] [AH] analog video synthesizers
>>> Any leads on an NTSC Video Interface Module?
>> I am trying to design one now. Stick around.
> Without bragging, I did this 20 years ago. I still don't think there is
> enough of a market for a commercial product. The Wiard modules are all
> roughed in for video work. With a cap change the Classic VCO will run up
> into the megahertz range, with synchronization.
> I checked on parts availability for a minimum parts count Sandin Image
> processor.
> If you go to Mouser and search for CD22402, it will provide the NTE cross
> for $18 or so. This is the full sync generator. You can gen lock it to the
> LM1881 using a CD4046 to provide the high frequency clock.
> The LM1881 sync stripper is still in full production according to the
> National web site.
> The Motorola MC1372 RGB to NTSC encoder can be had from BGMicro for $.50
> each.
> The 300ns delay lines for time aligning the luminance signal can be still
> gotten from Digikey, TOKO makes them, but they are $10 each.
> The original image processor design is at
> http://artscilab.org/archive/Artists6/Sandin/Feb28,1976.pdf
> that should provide the theoretical background.
> A video "color corrector" will contain all the circuits needed for video
> synthesis. i.e. sync strip & regen, NTSC to RGB and RGB to NTSC. The easiest
> method is is probably to get a $120 color corrector like this one
> http://www.bgsales.com/electronics/scc.asp
> and patch it out with buffer amps. You should use CD4066s or the like to
> enforce the blanking period on inputs to the RGB to NTSC encoder.

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