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Genlock circuit operation description

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Genlock circuit operation description


   One feature that space does not allow for now is the ability to
have software control of pixel switch disable. Now this is done with a
mechanical switch. Pixel switch disable allows the user to completely
ignore background video, yet have the Amiga computer genlocked. This
eliminates the problem of software having to be written with genlock
in mind. It would add $5-$10 to cost of goods. The control bits would
be encoded in the vertical interval, as audio on/off is presently
done. Eight to sixteen functions (bits) could be controlled once this
ability is on the board.

  CHROMA KEY -- since all signals are in RGB format, it would be easy
to selectively insert video based on color level. Additional circuitry
would add about $10, but would not fit in the existing case.

Circuit description

The purpose of the genlock peripheral is to synchronize the video
output of the Amiga computer with another video source such as camera,
broadcast, or VCR. Circuitry inside the peripheral allows for the
overlay of computer graphics on whatever video source is connected.
Also provided are facilities for stereo mixing of computer and source
audio. Input to the genlock peripheral is composite source and analog
RGB computer video. Output video is in the forms of composite and
analog RGB for high-resolution viewing on an RGB monitor. Also output
are a master 28.636363 MHZ computer clock, H/2, and V/2 video resets
required to synchronize the computer's graphic devices. Power for this
device is derived from computer +5, +12, -5 volts D.C. rails.

Circuitry in the peripheral is divided into several main functions
which are: 1) regenerating the horizontal and vertical components of
the original composite source video, (2) phase locking the 28 MHZ
clock to input video horizontal timing, (3) combining source and
computer video, (4) mixing source and computer audio, (5) and encoding
the RGB overlayed video into NTSC or PAL. 


Source composite video enter on J1. Transistors Q16 and Q7 form a
feedback amplifier with a gain of 3. Simple sync tip clamping is
provided by CR3, whose clamp voltage is set by CR4. The net effect is
to clamp sync tips at around 0 volts. Comparator U3 strips the sync
off of the clamped composite video on it's pin 4. Comparator trip
point is set by resistive divider R55 and R49 to be at about the 50%
amplitude point on the sync. On the output of U3 (pin 9) is composite
sync at TTL levels.

One-shot U1 is a digital integrator designed to detect when video
drops out for more than 12 lines. The output of this detector forces
crystal mode operation (Q4 enables power to the crystal oscillator)
and selects computer composite sync (J8-19) to insure a stable monitor
picture. Nand gates in U5 form the sync selector logic.

The composite sync output of selector U5 is decoded into its
horizontal and vertical components. The time constants of
differentiator C21/R56 and one-shot U12 are chosen to trigger only on
the horizontal components of sync. Output on U12 pin 5 is a series of
pulses at a horizontal rate. One-shot U19 forms a negative going
pulse 4.7 microseconds wide buffered by U23 for monitors requiring
seperate horizontal sync (J10-11). The time constants of integrator R77/R76
/C34 and one-shot U12 are chosen to trigger only on the vertical
component of composite sync. The output on U12 pin 4 is a pulse 90
microseconds wide on line 3 every vertical interval. One-shot U19
generates a negative pulse 200 microseconds wide buffered U23 for
monitors requiring seperate vertical sync (J10-10).

The graphic devices in the Amiga computer require reset every other
vertical interval in genlock mode. Dual-D flip flop U15 performs this
task. It is basically wired as a divide by 2 with horizontal sync
clocking the first stage. This causes the V/2 reset pulse to be
synchronous with horizontal, one line wide, retiming its edges. V/2
reset is buffered by R10 because at times the Amiga computer will
output vertical pulses on J9-23, (ie. genlock mode not selected with
peripheral attached).

The sandcastle generator is made up of U2, U7, Q5, and Q6. This
circuitry generates a multi-level pulse, encoding burst and blanking
timing information for U8, the chroma decoder. One-shot U2 time
constants are chosen to generate the blanking portion of the sandcastle
pulse. One-shot U7 time constants are chosen to generate burst timing.
Transistor pair Q5 and Q6 form a low impedance wide-band inverting
summing amplifier. R27 supplies a D.C. offset to give the correct D.C.
levels at Q6 collector for U8.  R26 and R31 sum in the blanking and
burst signals respectively. The gain of any signal to this amplifier
is set by the ratio of the input series resistor (R26, R31, R27) to
feedback resistor R28. The sandcastle pulse at Q6 collector encodes
blanking information from 0-5V dc and burst timing from 5-10V dc, with 
the pulse looking very much like it's name implies.

H/2 reset is generated by U14 and U17. The input to one-shot U14 is
regenerated horizontal from the 28 MHZ phase-locked loop. U14 time
constant is 33 microseconds, making a square wave at horizontal rate on
pin 12. Dual D-type flip flop U17 is wired as a gated divide by 2. The 
H/2 reset output (J9-21) is a negative going pulse 32 microseconds wide,
with the edges retimed to the Amiga computer color clock (J9-6). This
re-timing to color clock is required to guard against metaphysical
states in the Amiga graphical devices.


The circuitry to generate the 28.636363 MHZ clock is comprised of the
voltage controlled/crystal oscillators, phase detector, and divider, the
classic phase-locked loop. The VCO has some unique features.

The genlock peripheral must generate a stable master clock, allowed to
vary only a few percent in genlock mode. When there is no video on the
peripheral input, crystal stability is required for real-time clocks and
counters. To complicate things the Amiga computer cannot tolerate large
timing variations when switching in and out of genlock mode, missing a
clock cycle is catastrophic. Therefore, a circuit was designed so that a
crystal oscillator can "tickle" the voltage controlled oscillator for
completely synchronous mode switching.

Q24 and its associated circuitry is the 28 MHZ Colpitts crystal
oscillator. Q23 is a buffer to prevent loading of Q24. Power for Q23 and
Q24 is controlled by Q4, supplying current only when there is no input
video (xtal mode required).

Q13 and its associated components form a Colpitts voltage controlled
oscillator. The frequency is changed by varying the D.C. control voltage
on CR7, a varactor diode. C60 varies the VCO center frequency. The
connection made by C63 and R128 allows the crystal oscillator to 
"tickle" the VCO. For an in-depth discussion on oscillators, consult 
"Crystal oscillator Circuits" by Robert J. Matthys.

Q12 buffers the 28 MHZ clock to the Amiga, (J9-1) setting the correct
TTL levels with R64 and R65.  L2 and C41 filter the 28 MHZ to reduce

A synchronous divide by 1820 is formed by U6, U10, U13, U16, U4. U16
is a schottky device because of the frequency involved. Operation of
this circuit is straightforward. The output of U6 pin 13 is a stream of
pulses at a horizontal rate. This is called regenerated horizontal and
is never interrupted, an Amiga requirement. This signal is one input to
the phase detector. 

The phase detector used is the analog sample and hold type. Basically,
this detector works by sampling a ramp generated from one comparison
frequency (feedback) with a sample pulse derived from the other 
(reference). The output D.C. is used to control the VCO.

The reference input for this phase detector is the horizontal component
of input video, applied to one-shot U11 pin 1. Delay in U11 is about 1/2
line, with a potentiometer to fine tune horizontal position, R133. U14
generates a short sample pulse (275 nanoseconds), level shifted by Q14.
Q14 collector drives sample gate Q25, with the sample voltage held on C78.

The feedback input goes to the other section of U11, again adding a 1/2
line delay. The output of U11 is used to trigger the ramp generator
formed by Q21 and Q22. Ramp charge time is controlled by C52 and is
designed to accomodate the large timebase errors present in VCR
playback. Ramp charge time is critical in PLL design: steeper ramp means
high gain and less lock range, slower ramp means lower gain but increase
in tracking range. 

The D.C. operating point for this loop is determined by a voltage
divider formed by R106, R156, and R157. It is chosen to give maximum
dynamic range. 

Error signal on C78 is buffered by dual op-amp U20, section 1. Loop time
constants are determined by the R105, C61, C67, R131 around section 2 of 
U20. The output of this section drives the VCO, closing the control loop.


Now that the Amiga computer is synchronous with the source video,
computer and source video is combined. U8 alone performs the overlay

The main function of U8 (TDA3301) is to decode composite video into its
red, green, and blue components. First, source video is split apart into
chrominance and luminance. Network L1, C27 and R52 filters out chroma,
passing luminence only information to U8-37. Network R83, L3, C43, and
C28 passes only chroma information to U8-1. U8 also internally has a
3.579545 MHZ PLL. By utilizing luminance, chrominance, sandcastle pulse,
frame pulse (U28-29) and an internal PLL, video is decoded.

U8 also has inputs for external analog RGB video. Computer RGB is
applied to U8 pins 25, 26, and 24 respectively. The signal that
determines if source on computer video is ultimately output is pixel
switch (J9-4). This is a software generated control line from the Amiga.
One section of gate U5 is used to force pixel switch to show only
computer video when source video is lost. 

Each video output of U8 is D.C. clamped for black level stability. The
following is a description of just the blue channel. Blue video exits U8
on pin 14. The level is divided down by resistors R71 and R72 and
feedback to pin 16 completing the loop. C25 is used to hold the sampled
clamp value (see Motorola data sheet for full details). The blue video
is then amplified X2 by transistor feedback pair Q10 and Q11. Gain is
set by the ratio of R60/R61. R63 gives the characteristic line impedance
of 75 ohms (J10-5). Operation is similar for the green and red channels.
Other components around U8 are best understood by consulting the TDA3301
data sheet. 

A hue control (R134) is provided to allow user color matching of
computer and source video.


The left audio channel mixer is described. Source audio (J3) is
capacitivly coupled by C82 and terminated in an impedance of 47K.
Electronic switch U22 (CD4066BE) is used to disable source audio via
computer control. It operates as follows.

The control to enable/disable source audio is encoded in the vertical
and horizontal blanking interval of the pixel switch line, software
setable on the Amiga computer. U18 samples pixel switch during the
vertical interval, using the frame pulse to latch audio status. The
latched output (U18 pins 6&5) is used to control transmission gates in
U22. Data is latched on or about line number 10. No horizontal sampling
is done. 

Computer audio enter on J5. Passive mixing network R140, 141, 143, 142
and control R132 combines the two sources. Pot R132 allows user control
relative levels. Right and left mixing is controlled by the same shaft. 

Feedback pair Q19 and Q20 provide gain (x3) to make up for losses in the
passive mixer. C83 provides bandlimiting (20KHZ) to reduce noise pick-up
within the box. Audio and video grounds are kept seperate until the
connector (J9).


Analog RGB signals are encoded into NTSC or PAL by a single device (U21)
from Motorola, number MC1377. This IC requires only continuous
subcarrier and composite sync to output composite video. Subcarrier is
obtained from the oscillator in U8, coupled by C45. Sync comes from the
output of selector U5.

The analog RGB signals (0-1v amp.) from U8 are coupled into pins 3,5
and 4 of U21. Inside the MC1377 a resistive matrix and multipliers
transform RGB into an encoded chroma signal, output on pin 13. To reduce
interference with high frequency luminance information, the chroma is
bandpass filtered to 1 MHZ by T1, R11, C72, and C70. Chroma re-enters
the chip on pin 10.

The luminance signal is derived via an internal matrix from the RGB
input?? output inverted on pin 6. TD1 delays the luminance information
by 400 ns making up the time delay caused in the chroma path by the
bandpass filter. Chroma and luma are summed and clamped inside U21,
emerging on pin 9 as composite video. Emitter follower Q15 buffers the
video to J6. 

Color burst is also added to the video inside U21. External components
R95 and C69 determine burst position. Pin 16 is a stable zener derived
8.5 volts.

  If you would like to receive your own hard-copy of this spec,
please leave email in mailbox 'techs'

   IMPORTANT: Please make our job easier by including the word
              'genlock' somewhere in the Subject line!!

Don't forget to leave your name and address.

I didn't leave out much, the table of contents, a short discussion
of possible uses, a diagram that shows the location of each
connector (to be posted next week) and a drawing the shows the
genlock box and its mechanical interface.

Thank you, 

Randy Weiner  <<rweiner>> Commodore Technical Support

The following material is excerpted from the preliminary Genlock

Please excuse any non-obvious typos, it will take me a week
to uncross my eyes.
          Thank you ,  Randy Weiner  'rweiner'

       Excerpts from the Amiga Genlock Peripheral Specification


Female 23-pin "D" type (to computer)
    pin 1 :         28.636360 MHZ clock out
     2 :       external clock enable out
     3 :       red analog video in
     4 :       green analog video in
     5 :       blue analog video in
  6,7,8,9 :         no connection
     10:       composite sync in
     11:       H/2 reset out
     12:       V/2 reset out
     13:       ground
     14:       pixel switch in
     15:       color clock (3.58 MHZ) in
     16,17:         ground
     18,19:         ground
     20:       ground
     21:       -5 volts in
     22:       +12 volts in
     23:       +5 volts in

Male 23-pin "D" type (to monitor)
    pin 1 :         no connection
     2 :       no connection
     3 :       red analog video out
     4 :       green analog video out
     5 :       blue analog video out
  6,7,8,9 :         no connection
     10:       composite sync out
     11:       horizontal sync out
     12:       vertical sync out
     13:       ground
     14,15:         no connection
     16,17:         ground
     18,19:         ground
     20:       ground
  21,22,23:         no connection

RCA-type jacks (8)
     1 :       composite source video in
     2 :       composite video out
     3 :       R-source audio in
     4 :       L-source audio in
     5 :       R-computer audio in
     6 :       L-computer audio in
     7 :       R-mixed audio out
     8 :       L-mixed audio out


     composite      -3db luminance at 8 MHZ
     Analog RGB          -3db at 8 MHZ
     Chroma I&Q          -3db at 0.5 MHZ

Locking Range:
     Horizontal          +/- 2% from 15735 HZ
     Subcarrier          +/- 300 HZ from 3.579545 MHZ
     Vertical       crash lock
     Horizontal phase    +/- 1.5 microseconds
     Subcarrier phase    +/- 45 degrees from burst

     Vertical       reset output is 3 lines late
     Horizontal          reset output is coincident with input
     Clock jitter        <10ns, 5ns typ. in genlock,
                      crystal stable with no video source
     28 MHZ clock        in genlock mode, is phase locked to 
                      input horizontal timing. Automatic
                      switch over to crystal mode occurs in 
                      10 lines of missing source video. No
                      discontinuity in clock cycles occurs.


     -3db at 12 HZ and 500 KHZ, flat with 2db

     0 to -50db dependent on mix control setting

     source audio can be disabled (-50db) by setting a bit in pixel
       switch line during vertical blanking


Female 23-pin "D" type (to computer)
    pin 3 :    analog red -- terminated in 75 ohms, inut level of 
            1 volt p-p nominal level.
     4 :  analog green -- same as analog red
     5 :  analog blue -- same as analog red
     10:  composite sync -- TTL level, 10K load, negative going
     14:  pixel switch -- TTL level, 1K load, low level enables
            external RGB for overlay. During vertical interval, 
            a low level of pixel switch enables external audio,
            high level disables. This level is valid during 
            horizontal and vertical blanking.
     15:  color clock -- TTL level, 4K load, should be synchronous
            with host computer, freq. of 3.579545 MHZ.
     21:  -5 volts, approx. 50ma load.
     22:  +12 volts, approx. 250ma load.
     23:  +5 volts, approx. 300ma load.

RCA type jacks
     composite video -- 75 ohm load, 1 volt p-p, 0.3 volt sync,
          +/- 6db, will accept block vertical
     R-source audio -- 4000 ohm load, 1 volt rms nominal
     L-source audio -- same as above
     R-computer audio -- same as above
     L-computer audio -- same as above


Female 23-pin "D" type (to computer)

    pin 1 :    28.636360 clock -- semi-sinusoid, able to drive two
            schottky TTL loads
     2 :  external clock enable -- low active, direct connect. to gnd.
     11:  horizontal reset out -- low active, 'LS244 driver, low for
            32 microseconds at beginning of a horizontal line.
     12:  vertical reset out -- low active, 'LS244 driver, occurs
            on line 3, 30 microseconds wide.

Male 23-pin "D" type (to monitor)
    pin 3 :    red analog video - 75 ohm impedance, will drive 1 volt p-p
            into 75 ohms load
     4 :  green analog video - same as pin 3
     5 :  blue analog video - same as pin 3
     10:  composite sync -- low active, 'LS244 driver
     11:  horizontal sync -- low active, 'LS244 driver, 
            4.7 microseconds wide
     12:  vertical sync -- low active, 'LS244 driver, 3 horizontal
            lines wide.

RCA jacks
     composite video out - 75 ohm impedance, 1 volt p-p into 75 ohm
       load, 0.3 volts sync.
     R-mixed audio -- 100 ohm impedance, 1 volt RMS into 600 ohms.

     L-mixed audio -- same as above