illrrrrrrW

~
~
Installation of the switch and jack as sho ,wn
in Fig. 4 fu rther enhances th e versat ility of
the patch box, allowing the operat or to
choose easily between video an d live oral
signals.
Fig. 3. Generalized diagram -
T-pad attenuator.
Circ uit Details
Though the logic behind the des ign an d
o peration of the patch box is largely selfevident,
a numb er of co mments are warranted.
The output of the autho r' s vidicon SST V
camera is sligh tly over 3 volts R MS. This is
considerably larger t han th at required to
drive the monitor' s FET input amplifier.
T hus, aT-pad attenuator (F ig. 3) is inserted
between the camera and monit or, here
designed to re duce the input volta ge to the
output.
monitor by a fac to r of 6. To pro tect the
ta pe recorder's input circuits , all recorder
inputs are attenuate d by a facto r of 20. As
T-pad s other th an those used he re may be
required, design criteria for the T-pad s are
given below :
Let k = Ein/Eout, and define Z to be, the
characteristic line imp edan ce. Then :
R _ Z(k -I)
1 - (HI)
R - 2Zk
2 - (k 2-I)
Phone patches use d in SSTV co mmun icatio
ns rarely need be more complicate d th an
that shown In Fig. I. Two 1 I1Fd high-quality
(Mylar) capac itors provide for de isolation
an d lin e balance. The transformer provides a
single-ended line to the patch box, and a
ba lanced line to the telephone. A cc nventional
audio T-pad (IRC TP SOOA , Mallory
T500 or T600) is used to set the telep hone
line level, t his level monitored on t he
inexpensive VU me ter provided .
As show n in Fig. 1, SST V signals are fed
directly to the transmitter's microphone
inpu t. Examinat ion of Fig. 2, however, will
sho w that pan el space is available (lo wer
left ) for a switch and microphone jack.
Fig. 4. Alternate SS TV ·m icrophone input circuit.
The switch an d m icrop hone jack may be mounted
on the lower left portion of the fron t panel.
To protect the tape rec order's output
transistors, an 8.2 ohm resistor is autom a­
tically swit ched in to th e un used channel' s
Additional protect ion of the tape
recorder's output circuitry is assured by
using the mini mum playb ack gain req uired .
Subcarrier FM SST V is a co nstant-level
mode, an d un less pla yback gain is minimized,
output transistors an d t ransformers
will overheat and be destroyed . To protect
the tape record er during SST V recording
sessions, place therecorder selector switch in
the PAUSE or RECORD position on ly when
yo u are ab out to rec or d. Further, the individual
recor ded segments sho uld be kept
sho rt . For practical reasons, an SST V picture
is usually not re peate d more t han 10 times
(mos t SSTV'ers get bored after viewing the
same picture twice). Thus, with the 8-second
frame period employed, a typical record ed
segment should be less than 1 Y.z minutes
long. Most tape record ers will han dle a
cons tant-level input signal for this period of
time.
Conclusions
The patch box described ha s been in use
at the author's sta t ion for over two ye ars.
During that period, it has be en foun d to
greatly facilitate tap e record and playback
sessions. Furt her, it has proven indispensible
for local SST V operations where the ca mera,
monitor , tap e record er, and te lephone are
frequently employed in various co mbinations.
Those desiring further information on
slow-scan television are referr ed to the lit er-
28 .
73 MA GAZINE