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Atari RGB to Composite Video Converter
RGB TO COMPOSITE VIDEO CONVERTER DOCUMENTATION
December 27, 1986
version 1.0
The following document is placed in the public domain. You may
make as many copies of it as you like and transmit it in any form
you want provided it is not sold commercially, nor any product
derived from it is sold commercially. The author is not responsi-
ble for any damage, physical, mental or otherwise caused by fol-
lowing the instructions given below. Please mail corrections
and/or suggestions for improvement to the address given below.
Anees Munshi
58 York Road
Weston, Ontario
M9R 3E6
Canada.
(416) 246-0670
27 Dec 1986 (C) Anees Munshi 1
Atari RGB to Composite Video Converter
1. _I_N_T_R_O_D_U_C_T_I_O_N
The schematic described in this document converts the analog
RGB video signal that is output by the ST to an NTSC composite
signal which can be displayed on a colour or monochrome composite
monitor, or on a TV set by adding a modulator. Please be careful
when building the circuit. Should anything in the schematics or
this documentation seem suspicious, use your better judgement. A
certain amount of experience at building electronic circuits will
be very helpful. Also, a good oscilloscope and knowledge of the
theory of RGB to composite conversion may be necessary in order
to debug the circuit (should you have to). I have tried to pro-
vide some background in this article.
2. _A_N__O_V_E_R_V_I_E_W
The MC1377 RGB to composite video converter IC used does
most of the work in converting the red, green and blue signals to
composite video. The red, green, blue, horizontal sync and vert-
ical sync signals are taken from the ST and fed into the 1377. A
colour burst carrier (3.579545 Mhz) is fed in from a separate
oscillator [1]. The 1377 generates the R-Y, B-Y and luminance
signals by passing the red, green and blue signals through a
________
1.
The separate oscillator is not really necessary since 1377
contains a common-collector Colpitts oscillator which can be
used to generate the colour-burst on-chip. However, I found
it easier to generate the signal off-chip to make sure the
thing is indeed oscillating.
27 Dec 1986 (C) Anees Munshi 2
Atari RGB to Composite Video Converter
matrix. Then, the B-Y signal is modulated using the colour-burst
frequency carrier, and the R-Y signal is modulated using a 90
deg. phase shifted carrier. This results in the I and Q (in-
phase and quadrature) components of the chroma signal. The two
components of the chroma signal are added and amplified and made
available on pin 13 of the chip. This allows the chroma to be
band-pass filtered [2] externally and then fed back into pin 10.
The band-pass filter should be centered at the colour-burst fre-
quency and should have a bandwidth of about 1.6 Mhz. Not having a
very good colour TV to experment with, I chose to do an el-cheapo
filter since it wouldn't make any difference on my set anyway.
(Besides, I plan to use my board on an old green-screen monitor
to run an occasional colour-only program). A simple second order
LC filter may be used. Set the resonant frequency of the tank to
the colour-burst frequency, and choose an appropriate R so that
you get the 1.6 Mhz bandwidth [3]. Introducing a second (or
higher order) BPF to do chroma-filtering will probably result in
________
2.
If the chroma signal is not band-pass filtered, the low
frequency components it contains (those components having
frequency less than 2Mhz or so) will interfere with the
luminance signal since it is very hard to put the chroma
specral lines exactly in between the luminance spectral lines
without any interference between the two.
3.
Note Filter bandwidth = Wo/Q, where Wo is the resonant
frequency in radians/second (Wo=2*PI*3.15 Mhz) and Q is the
quality factor required. Q=R*sqrt(C/L). A few filters are
sketched in the Motorola Application Notes if you don't want
to design one. A reference to these notes is in the appendix.
27 Dec 1986 (C) Anees Munshi 3
Atari RGB to Composite Video Converter
a visible delay on the chroma-signal. So, to make sure the
colours are not offset from the B&W image (like in sloppily
coloured comic books), the luminance signal must be delayed an
equal amount so that the luminance information is not ahead of
the chrominance information. To allow this, the luminance signal
is looped out from pin-6 to pin-8. An approprate delay line
inserted between pin-6 and pin-8 will create the required delay
[4]. The luminance signal does not need any filtering.
3. _I_N_T_E_R_F_A_C_I_N_G
The horizontal and vertical synchs from the Atari must be
combined and fed into the composite synch input of the 1377
(pin-2). The HSYNC and VSYNC are taken from the ST's monitor out-
put, AND-gated and fed into the comp-sync input. This works fine
since the syncs are active low, TTL level signals and satisfy the
(-0.6V, 0.9V) active and (1.7V, 8.2V) inactive threshold levels.
The colour signals, red, green and blue must be capacitively
coupled through 22uF capacitors and attenuated through 2-4 Kohm
resistors (in series with the input) so as to not interfere with
the chip's bias and satisfy the 1Vp-p signal requirement respec-
tively. All the three colour signals output by the ST have a
________
4.
The Motorola Application notes show how to hook up a TDK
delay line if you need one.
27 Dec 1986 (C) Anees Munshi 4
Atari RGB to Composite Video Converter
1.8Vp-p range with a 1.2V DC bias. 1377 inputs: Pin 3: red input;
Pin 4: green input; pin 5: blue input.
The colour-burst carrier signal is generated as shown in the
attached schematic and coupled to pin-17 through a 2.2Kohm resis-
tor and a 0.1uF capacitor in series.
The colour-burst is added after every sync pulse, (burst is
not suppressed after VSYNC) approximately 5.5us (micro-seconds)
after the sync's leading edge and it lasts for approximately 3us
or 10.7 cycles of the carrier. This timing is done by an R-C
timer in a fashion similar to an LM555 operating in astable mode.
A 0.001uF capacitor is connected between pin 1 and ground, and a
51Kohm resistor is connected between pin 1 and an 8.2V reference
(available at pin 16). When a sync occurs, the capacitor is
unclamped from ground and begins charging through the 51Kohm
resistor from 8.2V DC. When the capacitor voltage reaches 1.0V
(approx 5.5us after the sync), the colour-burst carrier is gated
on. When the capacitor voltage reaches 1.3V, the colour-burst is
gated off. The capacitor continues to charge until the voltage
reaches 5.0V. At this point, the capacitor is discharged to 0V.
Clearly, by changing the RC values (and hence the time-constant),
the burst can be made shorter or longer (8 cycles of colour-burst
is the NTSC spec, but I don't think a slightly longer period will
hurt (gives the PLLs in the receiver plenty of time to lock)). If
the time constant (Time const. = R*C) is increased the burst will
occur later and last for a longer time. The converse is true if
the time constant is decreased. If the time-constant is made too
27 Dec 1986 (C) Anees Munshi 5
Atari RGB to Composite Video Converter
long, the ramp may not reach 5.0V before the next HSYNC, which
will result in the some missing bursts (not too good). If you
feel like it, put a 50Kohm potentiometer instead of the 51Kohm
resistor. You may be able to get some cheap special effects while
calibrating the pot. :-)
A chroma band-pass filter must be introduced between pins 13
and 10 as mentioned before. You may use the simple bandpass
filter shown in the schematic, or to get better results, use one
of the many shown in the Application Notes or design one your-
self. As mentioned before, if you design your own, the filter
should have a bandwidth of about 1.6-2.0 Mhz and a center fre-
quency of 3.58 Mhz. Since most colour TV's have a BW around 3
Mhz, the BP filter will help reduce cross-talk between the lumi-
nance and chroma components (on some TVs, you would never see the
cross-talk in all the noise!). If your TV has a comb filter, you
won't need a fancy BP filter (and the corresponding delay line);
in this case, the simple bandpass filter sketched in the
schematic will do.
Again, as mentioned before, if you do insert a fancy band-
pass filter and see a noticeable shift between the outlines of
objects and their colour fill, you will need to put a delay line
between pin 6 and 8. The Motorola Application Notes should help
in this department.
Pin 20 must be grounded to select NTSC operation [5].
________
5.
27 Dec 1986 (C) Anees Munshi 6
Atari RGB to Composite Video Converter
Pin 19 provides the reference voltage for the voltage-
controlled phase shifter (needed for I-Q phase shifting). It must
be capacitively de-coupled to ground through a 0.01uF capacitor
to provide a stable voltage reference at the pin. By pulling the
pin up through a resistor to 8.2V, or by pulling it down through
a resistor to ground, the axes can be tilted to get some colour
adjustment (about 7 degrees, for a slight effect, according the
application notes).
A 12V power supply is to be connected to pin 14. The power
supply need not be regulated.
The 8.2V DC reference voltage appears at pin 16. A 0.1uF
cap between pin 16 and ground will provide adequate filtering for
this reference voltage.
Pin 15 is the ground connection.
Pins 11 and 12 are coupled through 0.1uF capacitors to
ground. By sourcing or sinking some current through these pins,
whites may be made whiter and the blacks blacker (sounds like a
detergent commercial) by compensating for the balanced modulator
feedthrough thus.
Pin 7 is coupled to ground via a 0.01uF capacitor.
_________________________________________________________________
If pin-20 is connected to the power supply, PAL operation is
selected. For all you PAL hackers: If you replace the 3.58
Mhz crystal with 4.43 Mhz crystal, and if you have a properly
serrated vsync, you should be able to use this circuit to get
PAL composite. If you do hack and get it working, please
post. (PAL is the colour system used in England, India and
some other countries.)
27 Dec 1986 (C) Anees Munshi 7
Atari RGB to Composite Video Converter
Pin 9 is the composite video output. It has an output
impedance of about 50ohms. To drive a 75ohm monitor input termi-
nal, put a 25 ohm resistor in series with pin 9 and a 75 ohm co-
axial cable. Connect the other end of the co-axial cable to the
monitor.
27 Dec 1986 (C) Anees Munshi 8
Atari RGB to Composite Video Converter
4. _C_O_N_S_T_R_U_C_T_I_O_N
You will need to buy a _c_o_l_o_u_r monitor cable from Atari [6].
Start by cutting the cable in half. Then, strip about 2 inches of
insulation off the end of the cable. Now carefully remove the
shielding without cutting the wires underneath. This should
expose 7 wires. Four of the wires are shielded themselves; these
are the red, green, blue and audio out wires. Strip the shielding
off these wires as well. The following is a colour-code chart to
help you find the wires:
_c_o_l_o_u_r _o_f _w_i_r_e _s_i_g_n_a_l _p_i_n#
white RED output 7
red GREEN 6
black BLUE 10
yellow HSYNC 9
blue VSYNC 12
green AUDIO out 1
brown ground 13
________
6.
If this is not available, you may kludge one up as mentioned
in the Abacus _I_n_t_e_r_n_a_l_s book.)
27 Dec 1986 (C) Anees Munshi 9
Atari RGB to Composite Video Converter
The pins are labelled as follows (looking at the monitor
output plug from outside). Please test the cable with an
ohm-meter to see if my colour code is applicable to your
cable.
pin 4 -> 0 0 0 0 <- pin 1
0 0 0 0 <- pin 5
0 0 0 0 <- pin 9
0 <---- pin 13
Solder the cable onto the board you will be using, then
solder or wire-wrap the circuit as shown in the schematic. Try
to keep the construction as clean as possible. Keep as much of
the shielded cables shielded as you can. Try to RF shield the
whole enclosure if you can. All the best.
27 Dec 1986 (C) Anees Munshi 10
Atari RGB to Composite Video Converter
_B_i_b_l_i_o_g_r_a_p_h_y
o+ Motorola Application Note AN-932
o+ Motorola Linear Data Book
o+ _C_o_m_m_u_n_i_c_a_t_i_o_n _S_y_s_t_e_m_s , Simon Haykin, Wiley 1983. pp. 168,
169.
27 Dec 1986 (C) Anees Munshi 11
