home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Atari Mega Archive 1
/
Atari Mega Archive - Volume 1.iso
/
misc
/
rgbtocom.arc
/
CONV.DOC
next >
Wrap
Text File
|
1986-01-02
|
18KB
|
727 lines
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 ca