Black & Burst Sync Buffer

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Betreff: Black & Burst Sync Buffer
Von: g8mny@gb7cip
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Datum: 11. Sep 2019, 09:54:00
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By G8MNY                        (CQTV 2001, new to Packet Feb 05)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

When genlocking video sources together sometimes there is cable or camera
crosstalk from the genlock source feed when distributed with ordinary video as
the genlock source. This circuit will solve that problem while still allowing
the use of a non genlock camera as the genlock source master rather than the
use of a PAL colour SPG.

This 6 transistor circuit (4 PNP & 2 NPN) is a precision vision buffer amp, or
sync black & burst e~ a  Ĵ  Phase2           ===
        R2       /e              __    VC2          /e  __
        1M   Ĵ   C2           /_\  100p/\      ĴT6
            T2 \  u1               ĴĿ       \ +    R18
                  ĴĴ             /             Ĵ68(o
Video             +                 R14    /     1500
In &     /       Ĵ)))4k7ĴT5    uF 68(o
 o)ĴĴT1    470 C3                          \e        R19
 o)Ĵ u1 \e     R7 4u7                   /e          68(o
 out       Ĵ         )ĴT4                 R20
    75  R3  100     R8       2k2  10k        \      R16
    R1  M39  R5    330    R9  R10  R11    Ĵ     180 1/2W
SW1 o    /\ VC1  /\   1M       /\ R13     +    
Term/'     ===100 100    n27   VR2   2k7    ===C6  
           \pF /VR1    C5===  /22k         4u7  
           Phase1  Gain         Sync SW2                      __
The video input can be looped through or terminated with R1 (this can be made
from 2x 150 in parallel if needed). Vision is fed to a high input impedance
(>200k) transistor amp T1 & T2 with a gain of just over 2. The low value of
pull up resistor R4 & the use of the PNP emitter resistor R6, ensure very good
colour linearity. Presets VR1 and VC1 set the gain & colour phase (delay) of
the amp.

The 2V P-P video feeds a lightly biased T3 PNP transistor to make a sync
stripper stage. Its collector is high during syncs & VR2, R11 & C5 form a sync
& colour burst timing window that will operate the blanking stage T4 PNP
transistor during picture time. Resistor R10 in series with C5 lets fast sync
rise time edges through. Mode switch SW2 will allows composite or black & burst
modes by disabling the blanking stage. Capacitor C5 can be omitted if the
colour burst is not wanted.

The amplified video also feeds through C3, then it is clamped at 0.6V below
from the half supply rail with D1. Using a 2V P-P video level this point sets
the black video level to approximately the bias rail with standard sync
amplitude of the genlock source without the need for a proper black level
clamp. When the collector of T3 is 0.5V more negative than the half rail
(during picture time) T4 is turned on, shorting out the picture video that has
gone through R14 & VC2 to the half rail.

Transistors T5 & T6 make up a unity gain video buffer amp, with a low value of
pull down resistor R16 to provide good syncs while driving 3 loads. Again a low
value of pull up resistor R15 & the use of the PNP emitter resistor R17, ensure
very good video & colour linearity. VC2 trimmer sets the HF gain & colour
phase. Good LF performance is ensured with a large value output C7 & each of
the 3 output source impedances will be very close to the ideal 75 with the use
of 68 as the feeding resistors.

              __  __MaĿ
      Colour     __  __RdĿ
       Burst        __  __Bl
Line    Ŀ Video input__  __Bk     BurstĿ       No Picture
Sync   _ _  100% Colour __  __         _ ________________________
     .         Bars    __              Video Output     
     .                                 Sync, burst & Black
   .        .                       .
+12V__      .                       .     __
     \     . T3 Sync Seperator     .            T4 Blanking current
      `.   .     Waveform          .   ...____........................
.........`........................... Clamp Diode         ___...---'''
---'       `------------------------'   Current  |...---'''
           T4 Blanking Transistor on

The circuit should draw about 35mA from 12-14V DC anything wildly different
should be investigated. Using an oscilloscope check the 2x amplifier is working
OK. Then in black & burst mode, check the sync separator output, it should have
inverted line syncs from 12V to 6V with a lazy back edge. Check at frame rate
that there is no LF problems with the syncs.

Scoping the output buffer (T6 Collector) you should have just syncs & burst
with no picture content at about 6V DC. Adjust the burst window preset VR2 so
that blanking starts just after the burst. In composite mode there should be
around 2V P-P. Terminate all the outputs into 75 & check that the buffer amp
output stays nearly constant (showing that the buffer amp has nearly zero
output impedance). Scope the output on one of the terminations, & adjust gain
preset VR1 for unity overall gain of the circuit. Also check at frame rate that
there are no LF droop problems.

Colour phase is best set up with a colour bar source & a vectorscope using
identical leads. Adjust the 2 HF phase presets for best phase & amplitude match
on the vector display. It will be seen that the two presets do have slightly
different effects & by alternate tweeks the correct phase & amplitude can be
obtained. Switching to black & burst mode the phase of the bursts should not

But for those without a vectorscope & colour bars, adequate phase setup can be
done by subtraction comparison using an evenly lit brightly coloured card in
front of a colour camera & a dual beam scope with channel subtraction.
Calibrate the scope by probing the video input with both probes & adjusting the
timebase & gains to show a single colourful line. Put the scope into subtract
mode, & tweek channel gains & scope probes adjustments to produce the best
nulled out blank line. Now move a probe to one of the terminated outputs &
adjust the 2 HF phase presets for best burst & colour picture carrier null.

In composite mode the buffer amp works as well as can be expected. In the back
& burst mode the video blanking is only approx at the black level & will depend
on source sync levels. The exact amount of picture vision left after blanking
may depend on the component layout. Both of these should not have any affect on
the kit being genlocked.

Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP

Datum Thema#  Autor
11.09. o Black & Burst Sync Buffer1g8mny

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