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PUBLISHED IN LIMA NEWSLETTER FEBRUARY 1989
QUESTIONS ABOUT DIJIT'S AVPC CARD
GEDITOR'S NOTE: H The following text file was prepared by
DIJIT systmes to answer common questions about their AVPC
card. It should, however, be of interest to all TI computer
users as it contains interesting information about Rave 99
keyboards, switching power supplies, and a very interesting
theoretical discussion of TV and monitor resolution.
AVPC QUESTIONS AND ANSWERS
The following questions have been asked of us concerning
the AVPC and other DIJIT products. Since most of the
questions asked tend to be redundant, we have chosen the
bulletin board medium as a means of disseminating the
information while cutting down on our overwhelming
correspondence. If the answer to your question isn't here
then please address it to us in the E-Mail section and we'll
add it to the list.
WHAT IS THE AVPC?
The DIJIT Systems Advanced Video Processor Card is an
advanced accessory for the TI-99/4A. It is installed in the
Peripheral Expansion Box and functionally replaces the
existing Video Display Processor in the console.
IS THAT ALL THAT'S TO THE INSTALLATION?
No. There is a minor change to be made in the console
which effectively puts the existing VDP "to sleep" and allows
addressing video functions in the P-Box. No soldering is
required.
WHAT TYPE OF MONITOR IS NEEDED FOR THE AVPC?
(BB&P Editor's note: I use a high resolution green
monochrome monitor with my AVPC card with no problems at all.
The resolution is more than adequate for 80 column text work.
You can see every pixel of every letter, even in 80 columns.
I paid $75 for the monitor and $20 for the cable to hook the
monitor to my AVPC card. Color is nice, however, and if you
are interested in color read on. The comments about color
monitors and TVs in this article apply equally well the the
AVPC card, the Geneve computer, and the Mechatronics 80
column peripheral.)
It is strongly recommended that an 80 column rated Analog
RGB monitor be used. Only such a monitor can legibly display
80 column text as well as present the 512 colors and advanced
graphics functions the AVPC is capable of generating. See
our text file "Monitors" for what's available on the market
today.
CAN I USE A COMPOSITE MONITOR WITH THE AVPC?
We do NOT recommend it. A composite monitor doesn't have
the video bandwidth nor a picture tube fine enough to display
80 column text. We did make a provision to drive a composite
video monitor with the AVPC. However we had a monochrome
monitor in mind not a color monitor. Frankly, the color
display is terrible, not even Yamaha uses the composite video
output of the V9938 in their own MSX II computer where the
composite video signal is derived external to the V9938 from
the R, G and B signals.
HOW ABOUT A TV?
Definately no. A TV is even worse than a composite
monitor.
WELL THEN CAN I USE THE TTL TYPE RGB MONITOR THAT IS USED
WITH THE IBM-PC?
No. The TTL RGB monitor requires 5 volts of digital
video data to drive it and it can only display 16 saturated
colors. The AVPC generates 1 volt of analog video and up to
512 colors. They are completely incompatible.
WHAT SHOULD I LOOK FOR IN AN 80 COLUMN RGB MONITOR?
There are two major factors affecting the resolution of a
monitor, the bandwidth of the video amplifiers and the
coarseness of the phosphor dots on the display screen.
Most TV sets can adequately display 40 column text and
have a video bandwidth of approximately 3 to 4 Mz. But if
you really examine the text displayed, you will find that the
characters have a very "soft" appearance, that is, the edges
of the characters are not sharp and well defined like those
on a printed page. This softness is due to the electron beam
not switching on and off abruptly. That little bit of time
required to switch the electron beam on or off is referred to
as "rise time". In the ideal world this time would be zero,
but in the real world even the fastest events in nature take
a certain amount of time. The closer we can approach the
ideal, the shorter the on - off time, the crisper the edges
of the characters displayed.
It can be shown mathematically that "rise time" and
bandwidth are inversely related. That is, as the rise time
approaches zero, the bandwidth of the video amplifiers
required approaches infinity. We can rigorously calculate
the characteristics of the video amplifier required by
Fourier analysis, however for a quick estimate we can use a
"rule of thumb":
BW(MHz) ~ 500 / Tr (nsec)
The formula states that the amplifier bandwidth in
Megahertz is approximately equal to 500 divided by the rise
time of the pixel in nanoseconds.
Typical Tr for the TMS9918A, TMS9928A, TMS9929A is 50 ns.
So the video amplifier bandwidth required is approximately 10
MHz. The V9938 requirement is similar.
The other major factor determining resolution of a
monitor is the screen of the cathode ray tube on which the
data is displayed. There is generally no problem with a
monochrome screen, be it amber, green or white. But the
screen of an RGB display is consists of tiny dots of red,
green and blue phosphors which are lit in various
combinations and intensities. The glow is blended by the eye
to give the appearance of the various colors of the spectrum.
The size of these dots in relation to the size of the
screen determines the resolution of the CRT. Two common CRT
sizes used in computer monitors are 12" (320 mm) and 14" (370
mm). These measurements are taken across the outside
diagonal of the tube. The width of the displayable screen
for these tubes is 243 mm and 280 mm respectively.
Many computer systems use 640 pixels horizontally for 80
column display by formatting the characters in an 8 x 8 pixel
block. The V9938, however displays 80 columns using 512
pixels by formatting the characters in a 7H x 8V pattern.
The 512 is exactly double the 256 used by the TI VDP for 40
column display and does not include an allowance for
"backdrop" or overscan on either side of the computer
generated image. Add approximately 90 pixels for backdrop
for a total of 602 pixels displayed.
If we divide 370 mm by 602, we get a dot pitch of .6 mm
which is the minimum spacing between adjacent pixels to
display the 80 column text. Since the average TV picture
tube has a dot pitch of .6 mm, it sounds great. However,
having just enough pixels to light as there are generated by
the computer or TV only works on those huge TV screens used
in stadiums or other outdoor displays. It doesn't work with
a CRT, especially if you sit very close to it and stare at it
for an extended period of time.
You need about 25% more physical pixels on the face of
the CRT to eliminate undesirable things like the "moire
effect", the pattern we're most familiar with when we look
through two window screens, one behind the other. This
occurs because the computer generated image is never in
perfect registration with the phosphor dots on the screen.
On a 14" CRT, it's been found that .5 mm dot pitch makes for
a marginal 80 column display. We recommend .42 mm dot pitch
for comfortable viewing. Of course .31 mm is much better,
but then the price is also much higher.
There you have it then, a 10 MHz minimum video bandwidth
and a .42 mm maximum dot pitch are required for a comfortable
RGB monitor. We recommend the Magnavox 8CM515, which has
these characteristics, as the best value.
WE'VE HAD A NUMBER OF QUESTIONS CONCERNING MONITORS AND
TVs FROM OUR FRIENDS OVERSEAS WHICH I WILL TRY TO CLARIFY.
First of all I'd like to say that everything above
relating to composite video monitors and TVs still applies.
The AVPC uses the Yamaha V9938 video display processor
designed for use in the MSX II computer. (BB&P Editor's
note: The Geneve computer and the Mechatronics 80 column
peripheral use the same video chip.) There have been very few
MSX computers sold in North America. That computer system is
sold almost exclusively outside of the United States.
In North America the the electric utility power system
(mains) is 60 Hz and 115 volts while most overseas countries
have standardized on 50 Hz and 220 volts. The TI-99/4A will
accomodate either power (mains) standard.
It has been determined in the early day of television
that the vertical scan frequency of video display devices
should be directly related to the power (mains) frequency in
order to reduce the effect of external electrically caused
noise and interference on the video. Thus the noise
interference tends to be stationary on the screen instead of
annoyingly moving up or down.
To accomodate the different vertical scans, TI provided
two different VDPs for the TI-99/4A, the TMS 9918A for
domestic use and the TMS 9929A for use in countries with 50
Hz mains frequency. The Yamaha V9938 accomodates the 50 Hz
and 60 Hz vertical scan frequencies through software, a much
more elegant solution.
The video horizontal scan frequency of the V9938 is 15750
Hz, while NTSC is 15734 Hz and PAL is 15625 Hz. Most RGB
monitors should have no problem locking on although the
horizontal frequency adjust may have to be "tweeked" a little
to center the picture.
CAN WE PROVIDE CABLES WITH THE SCART CONNECTION?
SCART is an acronym for Syndicat des Constructeurs
d'Appareils Radiore'cepteurs et Te'le'viseurs, the French
radio and TV manufacturers association. They devised a
connection system by which a TV could be used to display
peripheral equipment such as VCRs, video disk players, games,
etc., including personal computers. The standard makes use
of a special 21 pin connector known in France as a "peri"
connector and in England as a SCART connector. The pinout
and signal levels are also specified. The SCART standard has
not been adopted here in the United States and the connectors
are almost impossible to obtain although if the quantities
justified it, they could be imported. PLEASE NOTE: As far as
we know, the SCART standard covers only the connector type,
signals and signal levels. It says nothing about screen
display concerning resolution or video bandwidths.
Therefore, just because a monitor has a SCART connector
doesn't mean that it is an 80 column monitor.
HOW WILL THE AVPC DISPLAY MY EXISTING PROGRAMS?
The AVPC will display most popular TI-99/4A programs as
usual, but much clearer when they are displayed on a high
resolution RGB monitor.
IS THE AVPC COMPATIBLE WITH ALL EXISTING TI-99/4A
SOFTWARE?
The AVPC is compatible with most popular programs,
however it is less tolerant of poorly written software than
the unmodified TI-99/4A. (BB&P Editor's note: A review of
the APVC to be published in the March 89 issue of BB&P will
include a short list of software that doesn't work properly
with the AVPC.)
WILL EXISTING PROGRAMS BE DISPLAYED IN 80 COLUMN MODE?
No. Existing programs are displayed exactly the same way
as they are on the TI-99/4A. The enhanced features of the
AVPC require specific application programs to invoke them.
(BB&P Editor note: See the description of Funnelweb v4.13 in
80 columns published elsewhere in this newsletter.)
CAN I ACCESS THE ADVANCED FEATURES OF THE AVPC THROUGH
EXTENDED BASIC?
You can access some of the features such as the mouse,
with drivers written by Dodd and Hoddie, also some of the
graphics features with routines such as those written by Jeff
Kittka. See our X-Modem download section. As other drivers
become available we will post them on our BBS.
HOW ELSE CAN I ACCESS THE ADVANCED FEATURES OF THE AVPC?
The advanced features of the AVPC are accessed through
specific application programs such as the 80 column TI-Writer
editor, 80 column Multiplan etc. All the features of the
card are accessible through FORTH and C as well as Assembly
Language. (DIJIT Systems will have a programmers guide for
sale to people who wish to program in the latter lower level
languages). We have found that many of the early programs
written for Myarc 9640 under GPL, especially graphics, will
also run on the AVPC.
IS THE AVPC COMPATIBLE WITH OTHER THIRD PARTY
PERIPHERALS?
The AVPC was designed to work with and be compatible with
Texas Instruments TI-99/4A peripherals. Hardware and
software not conforming to Texas Instruments hardware and
software specifications may not work with the AVPC.
.IF DSK4.C6
WHAT IS A "QI" CONSOLE?
(BB&&P Editor's note: DIJIT Systems specifically states in promotional
material that their APVC card is not compatible with "QI" consoles. However,
use of the below mentioned "switching power supply", which is a feature of "QI"
consoles, does not by itself render a console incompatible with the APVC. I
have retrofitted a "switching power supply" from Radio Shack to the console I
use with my AVPC card with no problems.) "QI" stands for Quality Improvement
and was the designation given the last production model of the TI-99/4A, before
Texas Instruments went out of the home computer business. Internally, it had a
newly designed PC board on which a large gate array consolidated the logic
functions performed by discrete "glue chips". Another feature was an improved
switching power supply made by an offshore contractor. The latter were
surplused out to Radio Shack and others and had the contractor's name "IRIICHI
TSUSHIN KOGYO CO., LTD." on them. Externally, the "QI" is a beige console
recognized by looking at the side accessory port. The shielding "finger stock"
is silver colored, all non-"QI" consoles have gold or brass colored "finger
stock". We cannot endorse the "QI" consoles for use with the AVPC because they
are a rarity and we have not tested them. Besides, there are plenty of
standard consoles on the used market and we know they will work.
IS THE AVPC COMPATIBLE WITH THE HORIZON RAMDISK?
Yes, there was an incompatibility between the AVPC and the Horizon Ramdisk.
The AVPC, being the new kid in the neighborhood has received some "heat" for
incompatibility. The Trade Union argument that "the last card in the box has
the lowest seniority" and so should take the "heat", we found unacceptable in
this case. We knew the AVPC was compatible with TI, CorComp and Myarc disk
controllers and had no problems when saving Text2 (80 column) files or other
advanced files to floppy disk. If you couldn't save them to the HRD, then
clearly the Ramdisk was not emulating a standard floppy disk drive. The problem
was in the HRD Operating System or ROS. Barry Boone came to our rescue by
writing a ROS "patch" for HRD owners who also have V9938 based video systems.
BARRY'S ROS PATCH really consists of two different programs, one for the TI and
CorComp disk controllers and the other for the Myarc disk controller. Both are
"Fairware" and are included in the download section of the bulletin board under
the title: "Horizon ROS ".
I HAVE A RAVE KEYBOARD, WHY CAN'T I GET THE AVPC TO QUIT WITH "[FCTN] ="?
Incorporated in the power up routine of the AVPC is a little feature to
eliminate the bane of the BASIC programmer. Namely, to have keyed in a long
program and requiring a "+" sign, to have your finger slip from the [SHIFT] key
to the [FCTN] key and watching your work disappear. Now you have to make an
effort to QUIT, you must use two hands and press [FCTN], [CTRL] and "=".
The RAVE keyboard only scans two keys at a time and so it can't sense our three
key QUIT feature. However RAVE does sell a cable assembly for their keyboard
which works like the reset button on the "Widget" and accomplishes the same
thing.
WHY DON'T SOME OF THE "FAST-TERM 80" FUNCTIONS WORK WITH THE AVPC?
"Fast-Term 80" started out being an experimental version of Fast-Term called
"Fast-Term 9640" by Paul Charlton. According to the documentation, he admits
that it was a trial version. It was made usable on the AVPC through the
prodigious efforts of some local programmers working without source code and
not asking for remuneration. If it has flaws, sorry about that, maybe Paul
Charlton will fix them if he has enough requests.
DOES TELCO WORK IN 80 COLUMNS ON THE AVPC?
Yes. Charles Earl, the author of TELCO, has or had in his possession an AVPC
via Bob Boone of the Ottawa Users Group. TELCO Version 2.1 was released to
take advantage of the new 80 column capability of the TI-99/4A. In general the
program works beautifully although there are a couple of minor flaws that we're
sure will be taken care of, in time.
TELCO is a complex program and was written in modules or overlays so that it
could be accomodated with the limited memory available in the TI-99/4A. The
basic 99/4A can only hold 3 of these overlay modules in memory at a time and so
as a new function is requested the program must go out to the disk and retrieve
that overlay. Mr. Earl has ingeniously taken advantage of all the video memory
available with the AVPC and stored 27 overlays in it. In effect it acts as a
"RAMDISK". And so the execution of TELCO with the AVPC is substantially faster
than on the basic TI-99/4A. TELCO Version 2.2 is now in the X-MODEM download
section of the DIJIT systems BBS.
DOES THE AVPC HAVE PROBLEMS WITH SOME TERMINAL PROGRAMS?
The AVPC had problems with interrupt driven terminal programs such as TE II,
Fast-Term, Mass Transfer etc. Non interrupt driven programs such as OMEGA work
fine. After a month of self doubt, we discovered the problem wasn't our doing
after all, but that there was a "bug" in the RS-232 card. See below. If the
EPROM in the RS-232 card is replaced, the AVPC works with all terminal programs
that we are aware of.
WHY DOES THE AVPC HAVE PROBLEMS WITH THE RS-232 CARD?
The AVPC was designed to work within the hardware and software constraints of
the TI-99/4A. To do so required sharing the external interrupt bus between the
P-Box and the console. This incidently, is one of the major differences
between the AVPC and the Mechatronics 80 Column Card. The RS-232 card was the
only card made by TI or third party vendors that made use of this external
interrupt bus until the AVPC came along. There is a "bug" in the TI Interrupt
Service Routine that causes the card to shut down when an interrupt is sensed
but there is no data in the incoming buffer. CorComp and Myarc copied the ISR
and the "bug" with the same results. If TI stayed in business and had brought
out any more interrupt driven peripheral cards, they would have had to contend
with the ISR "bug". DIJIT sells code correction EPROMs for the TI, CorComp and
Myarc RS-232 cards.
WHAT IS "FAST VIDEO RAM"?
The AVPC was designed not just to display computer text and graphics on the
screen, but to work with "real time video frame grabbers" and other digitizers.
For this reason Yamaha recommended the more expensive 120 nanosecond fast video
RAMS. Some brands of 150 nsec RAMs will work just fine. However we found that
some, notably the TI 4464-15 does not work.
CAN I USE THE AVPC'S EXTRA RAM FOR OTHER PROGRAMS?
(BB&&P Editor's note: The AVPC card usually comes with 128K of VDP ram and has
sockets for an additional 64K of VDP ram. This replaces the 16K VDP ram
that comes with the console. This extra video ram is partly why the AVPC card
can display up to 512 colors and can easily show text in 80 columns.) The CPU
cannot directly access the AVPC's RAM and so cannot run programs out of it.
Video RAM can be used as a buffer to store data, however. As explained above,
TELCO does just that and stores its modules or overlays in video RAM, in effect
making it work as a RAMDISK. Access to video RAM for data storage can only be
had through Assembler or other low level languages.
WHAT IS GENLOCK?
The term "genlocking" is a contraction of "general locking" and refers to the
complete synchronization of a video accessory with an external video source.
Genlocking is required in order to do superpositioning of graphics or other
video, on the source video also for digitizing. In a TV studio, all cameras
and special effects devices are synchronized by a standard clock signal that is
distributed throughout the studio. However when such a standard signal is not
available, it is necessary to derive the synchronizing information from the
source video by means of a genlocking device. The name "Genlock" has been
applied by Amiga to an accessory capable of superpositioning or overlaying
computer graphics and text onto externally supplied video.
WHAT IS A 'REAL TIME FRAME GRABBER'?
It is a video digitizer that can capture a video image as fast as it is
presented. Some video digitizers in use today, notably on the Amiga, cannot
capture live video. They work by slowly scaning a picture in monochrome through
color separation filters and storing it in memory where it may be manipulated
for the purpose of screen display or dumping to a printer.
A television image consists of 2 'fields' interlaced together to form a
complete picture called a 'frame' which is equivalent one picture of a movie
film. Each field requires 1/60 of a second, the total frame then takes 1/30
of a second to display. I countries using the PAL standard, a field requires
1/50 of a second and a frame is 1/25 of a second. Our DIJIT-EYE-ZER will be
capable of capturing either a field or a frame in 'real time', as quickly as it
is displayed. And we will be able to do it in 256 colors in medium resolution
mode. (BB&&P Editor's note: The DIJIT-EYE-ZER will be demonstrated Feb 18-19
at the TI-FEST WEST 89 in San Diego CA.)
DOES THE AVPC WORK WITH THE BUD MILLS 32K CONSOLE MEMORY EXPANSION KIT?
We have not had any complaints on that from our customers. Barry Boone claims
to have no problems with it and his Mechatronics card. We will maintain a
conservative wariness on this one because it gets its speed increase by
eliminating the 'wait states' that TI had built in to assure timing
compatibility with peripherals.
.PL 1