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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