DP Tool Club 17/CD_ASCQ_17_101194.iso/vrac/vester_e.zip/ARTICLE.TXT

Text File | 1993-08-14 | 24KB | 762 lines

               A Software-Based Color-SSTV/FAX480/Wefax System

          John  Langner's  great article on Slow-Scan TV  (reference  1)

     really  peaked  up my interest in creating such a system.  And  his

     statement  "..be sure to use an external crystal-controlled  timing

     source....any  attempt  to use software timing loops is  doomed  to

     failure"  was just too much of a challenge to ignore. Since we  had

     managed  to make some respectable 64 color (or gray  shades)  Wefax

     systems  (references 2 and 3) for both Commodore and  IBM  machines

     using  only  a simple clipper interface between  the  receiver  and

     computer, it didn't appear that demodulating the SSTV signal  would

     be  that  much  more  difficult. What  has  resulted  is  a  Color-

     SSTV/FAX480/Wefax system for IBM and clone PC's that is essentially

     built with software.

          We  will not re-plow ground that John has already  covered  so

     well. If this article prompt's your interest, you should definitely

     read his article (reference 1) to fill in the gaps and learn  some-

     thing about the history of SSTV.



     HARDWARE

          Figure  1  is the simple clipper circuit. It is  identical  to

     that  used in reference  3 with the addition of an  extra  coupling

     circuit used in Transmit mode to connect the computer to the trans-

     mitter audio input. This should go into the input where the  phone-

     patch normally connects (often labelled Line input). If you already

     have a phone-patch, you can eliminate the isolation transformer, T2

     in  figure 1, and go directly into the phone line terminals of  the

     patch.  All patches I'm aware of have transformer isolation, but  a







     simple  ohmmeter  check  will verify that is true  in  your  patch.

     Resistor  R-x  is chosen to set the proper level for the  fax  audio

     going  into  the transmitter.  Fax is a 100% duty cycle  signal  so

     don't  set the level higher than your rig can handle without  over-

     heating. R-x ended up being 43K in my system where I went  directly

     into the phone-patch.

          In the interest of hardware minimization, we have not included

     any  low  pass filtering on the audio coming out of  the  computer.

     This is a square-wave signal rich in harmonics, not unlike what  is

     generated  in  the  speech clippers in many  SSB  transceivers.  No

     additional filtering is required with mechanical or crystal  filter

     type  SSB transmitters, but if you wanted to use this on an old  AM

     or  phasing type SSB rig, or on VHF/UHF FM type transmitters,  spe-

     cial audio filtering to give the required spectral purity would  be

     required. Telephone company type approval is required to send  such

     signals  over  the phone lines and undoubtedly they  would  require

     very   restrictive  spectrum control filters. Each of  these  cases

     would require a unique design which we have not addressed.

          There  is  nothing critical about the circuit  values  in  the

     schematic nor is the construction critical. Do use a socket for the

     IC.  Normal  perf board construction with short leads  works  fine.

     Hundreds  of  these have been built with the  most  common  problem

     being wiring errors. For some reason the pin numbers on the IC  get

     confused with the pin numbers on the serial port connector.

          The  most  important  piece of hardware is  the  computer.  It

     should  have an 80286 or better microprocessor. I have used an  old

     12  mhz 286 laptop with the system but it definitely  gives  poorer

     results  than the 386 machine at 16 mhz or 33 mhz. You need  a  VGA







     color monitor which can provide 640x480 at 256 colors without inter-

     lace and a VGA (usually labelled as SVGA) video card that offers  a

     640x480x256  mode.  There are many, many different types  of  cards

     offering this mode. Our software is configured to directly  address

     six  of  the most common SVGA chip types and also includes  a  VESA

     standard  choice. If your card does not match one of the 6,  you'll

     need a VESA driver program (a TSR program) for your specific  card.

     These are available from the card manufacturer or you may find  one

     in recent programs you have purchased. I'm told that  BBS:(416)729-

     4609 has many of these if all else fails.



     SOFTWARE

          As  in  previous Wefax programs, we have used GWBASIC  as  the

     programming  tool. While most of the real guts of the  program  are

     contained in assembly language code (.ASM files), it is  interfaced

     to the user through BASIC and all of the modifications to the  core

     program  (.ASM  files) to adapt them to the multitude  of  SSTV/FAX

     modes  is accomplished by POKE's in the BASIC program. This  allows

     the  experimenters with only a limited knowledge of BASIC  program-

     ming to modify the program to add other modes, etc. In deference to

     a few of my friends who complained about learning any BASIC, I have

     included  a SYSTEM CONFIGURATION list in the programs which can  be

     filled  out  by anyone; the program uses this to  figure  out  what

     POKE's  to make. My work is aimed at the  experimentally  inclined,

     however,  so  be prepared to learn a little BASIC if  you  want  to

     maximise the utility of the system..

          One  other difference from most recent programs is the  system







     is  strictly  keyboard  controlled. I find  Mouse  controlled  menu

     selection  most awkward and since many laptop computers  only  have

     one  serial port (where all the FAX action is) they would  be  pre-

     cluded from using the program.



     PROGRAM CONFIGURATION CHOICES

           Like  Langner   does in Pasokon our previous  Wefax  programs

     have  counted clock pulses between zero crossings (reference 2)  of

     the  incoming fax signal to measure the audio frequency. The  clock

     frequency affects the accuracy with which you can measure the audio

     frequency, but in general all of the clocks in computers are easily

     fast enough to give far more resolution in the measurement than you

     can display. The 1.193 mhz clock used by IBM PC's to run the  time-

     of-day  display is a natural choice since it is common to  all  ma-

     chines.  It also keys a programmable timer which the PC has  avail-

     able  for generating audio tones. My Wefax programs have used  this

     tone  timer to set up the sample-data period over which you make  a

     measurement  of the incoming audio frequency. The  high  resolution

     programs  used  a 1024 hz square wave tone with a  different  sample

     being  gathered each 1/2 cycle. This 2048 samples per  second  just

     about  matches the optimum for the typical Ham receiver  bandwidth,

     so we decided to retain it for the SSTV program. Also we found that

     the unmodified Wefax would copy the AVT-90 mode directly. Of course

     it showed up as four (instead of three) successive black and  white

     frames  on the 1024 x768 Wefax screen, but perfectly  synchronized.

     Further,  this  sampled-data or pixel rate gave exactly  320  pixel

     wide  AVT-94 pictures which just matches the screen on a  PC.  This

     suggests that the AVT mode formats may have evolved  from  computer







     based Wefax formats.

          Once I had decided to use the Wefax pixel rate, I just  copied

     SSTV picture data tagged with the mode used and then made a  series

     of  guesses as to how many pixels were in each line until I  got  a

     coherent set of pictures---typically three RGB frames. The  result-

     ing image allowed me to then measure the length of the various 1200

     hz  synchronizing  pulses, the pixels per color frame, and  make  a

     close  estimate  of the exact timing for each line.  These  numbers

     were poked back into the Wefax program and then the line timing for

     that  mode was tweaked into place by copying pictures in that  same

     mode on the air. Fortunately I didn't spend too many weeks on  this

     torturous  process before I ran into Bert, W5ZR, who just  happened

     to  have that data all collected for the most popular  modes.  That

     along  with the wealth of information in Langner's article  greatly

     increased my rate of progress.

          I started with 16 shades (ie,4 bits) per RGB color which  gave

     me  the potential of 4096 (16x16x16) different colors  but  finally

     abandoned  that  for  64 shades (6 bits)  per  color  which  allows

     262,144  different colors. This fully uses the capability  inherent

     in  the most common SVGA video cards. While it takes  more  memory,

     you can always reduce the files to the lower resolution with   some

     fairly  simple manipulation if the compressed file sizes  become  a

     problem. Langner chose 32 shades (5 bits) per color for his Pasokon

     color  resolution. With Windows driving the computer hardware  mar-

     ket,  memory  costs have had the fastest decrease per year  of  any

     hardware element, so at least my choice was consistent with  market

     trends.







          It  is worth noting that changing from 4 bits to 6 bits  would

     have been a major effort on some hardware based systems, but was  a

     minor change here in the software.

          When it came to displaying the resulting color pictures on the

     screen,  I  opted for filling the screen with the picture  (like  a

     TV).  Having no need for Mouse selected menus the full  screen  was

     available.  Since the color modes had a maximum of 320 pixels  wide

     and  240  pixels down in the actual picture frames,  just  doubling

     each  pixel  in both directions gave maximum picture  size  without

     overscan when using a 640x480 VGA screen. This larger display was a

     two edged sword,however, since it made defects in the picture proc-

     essing more obvious. It forced me to make improvements in both  the

     fax  demodulation  software and in the  color  registration.  These

     improvements  fed over to the Wefax to give much  better  satellite

     photos--ground features are much more visible now.

          Since  all  of the displays I had seen of attempts to  show  a

     variety  of  color pictures on a 256 color VGA board  had  left  me

     unsatisfied, I decided to create my own technique. While describing

     how I accomplished it is beyond the scope of this paper, I leave it

     for  you to judge the results. Figure 2 shows a typical  Scottie  1

     picture.  It fills the screen vertically but not horizontally.  The

     AVT94 mode in figure 3 fills it horizontally but not vertically.

          All  of the modes except the AVT modes include a 1200 hz  line

     sync  pulse of some sort to re-sync each line. I chose to  use  this

     only  to sync the beginning of each picture and adjusted the  crys-

     tal-controlled  line timing in each mode to eliminate slanted  pic-

     tures.  This is much more immune to interference and after  copying

     hundreds of pictures on the air I have found this to be the optimum







     choice  for  all but the Robot modes. Here there are a  variety  of

     line  timings  that show up on the air, so I had to add  line  sync

     capability  for  the two Robot modes. Actually you can  copy  in  a

     synchronous fashion if the station  you are working has the  latest

     Robot modifications so we kept both capabilities.

          Ralph  Taggart introduced his FAX480 mode (reference 4)  while

     we were in the midst of developing this program. With the flexibil-

     ity built into the BASIC/.ASM structure described above I was  able

     to  add  FAX480 to the available modes  within a couple  of  hours.

     Since my processing was setup for 64 shades of gray, I kept it  for

     this  mode.  Ralph  was limited to 16 shades  (presumably)  by  his

     hardware base. Once  you have the higher pixel resolution, however,

     I expect people  will also want the real photographic quality of 64

     shades.  After  all, it takes up no more r-f spectrum  or  time  to

     transmit than 16 shades.

          Finally,  we had to make choices on the Transmit program.  One

     of  the common practices in use on SSTV is to retransmit a  picture

     you  just received so people out of the skip zone of  the  original

     transmitter  can  see  it.  We  included  the  capability  in   the

     Receive/Transmit  program to parrot back any picture just  received

     in  the  mode it was originally sent in since that  was  relatively

     simple.  While  you can transmit any picture you receive  and  have

     saved,  we  chose  to exclude the Robot modes when it came  to  the

     program used to create new picture files. This was both to conserve

     our  time to more important tasks and because I believe  the  Robot

     color  modes  will  fade with time due to  the  extremely  accurate

     tuning  required  to  maintain color fidelity  and  the  fact  that







     chroma  information is shortchanged to maintain compatibility  with

     the old black and white systems. This is not to denigrate the Robot

     systems since they still give some of the best pictures going,  but

     I have noted that many people with the Robot hardware are now using

     Scottie and Martin  modes most of the time---and their pictures are

     gorgeous!



     DESCRIPTION OF PROGRAMS

          RT.BAS---This  is  the receiving and  retransmit  program.  On

     receive,  you only need to choose the mode from the menu, and  wait

     for the picture to be completed. As of this writing Robot 36 and 72

     modes are available in either a synchronous or a line-synced  mode.

     Other modes (all synchronous) are Scottie 1 and 2, Martin 1, AVT90,

     AVT94,  Wraase  48, FAX480 and Wefax. I have the  info  to  include

     AVT125 but it is so inferior to FAX480 that I didn't. AVT188 can be

     copied  as AVT94 but you won't get the whole picture. I  definitely

     want to include the Scottie DX mode but haven't yet heard it on the

     air.  The receiving does not yet include automatic  recognition  of

     modes  from the VIS  header, but the transmit does include the  VIS

     headers and the AVT picture sync count down sequence, so others can

     use auto recognition on your transmitted pictures.

          When receiving, if you fail to get the mode selection made  in

     time to catch the frame sync you can bypass directly to copying  by

     hitting the Spacebar. On all but the AVT modes, the next line  sync

     is  picked up and starts the picture. The AVT modes will just  copy

     out of sync. But since the program allows you to scroll horizontal-

     ly across the RGB color frames you can re-sync after the picture is

     copied. There is a barely detectable loss in color registration  on







     AVT modes when you miss the sync, but that's better than losing  the

     whole  picture. Having noted that a few of the systems I've  copied

     have  a nonstandard color registration I also included the  ability

     to  adjust color registration after the picture is  received.  This

     shouldn't be needed very often, but is handy to have.

          You also can SAVE the picture--usually after you have scrolled

     the  picture  so the CRT screen frames just the part  you  want  to

     keep.

          TX.BAS---You  can  transmit any picture file with  this.  When

     queried,  you  give the mode and the file name and  after  a  brief

     pause  while  it loads, hit G(o) to transmit. To  avoid  additional

     switching complexity, VOX is used to turn on the transmitter. Since

     all  pictures start with a long 1900 hz calibration tone,  any  VOX

     delay has no impact.

          VU.BAS---As  it's name infers, this program allows  viewing  a

     file picture. It has the same adjustments available as the  RT.BAS.

     One  that  I haven't mentioned yet, applicable only  to  the  Robot

     modes,  is the ability to "re-tune" (in 10 hz increments) the  pic-

     ture as you view it's color balance.

          QRM  seen in the pictures is of two types, some of  it  giving

     signals  inside  the 1500-2300 hz range where the fax  is  and  the

     other giving signals that measure outside this range. Allowing  for

     some  tuning tolerance, the program identifies color bytes  outside

     the  range  as  being "ringers" and substitutes the  value  of  the

     previous byte for the damaged one. I could have gotten more sophis-

     ticated  and taken  the average of all four surrounding  bytes  but

     when I tried the simpler version it worked so well we kept it. Note







     that we are replacing only one color byte, not the whole pixel when

     a  ringer  occurs.(Note to Paul Pagel--we can  supply   Before  and

     After pictures to illustrate the improvement if you have room.)

          SLIDESHO.BAS---Once  you  collect a bunch  of  pictures,  this

     program  gives you the vehicle to display them in slide-show  fash-

     ion. It is best  placed in your DOS or TOOLS directory where it can

     be  called up from anywhere in the directory tree. My pictures  are

     collected  by  mode into compressed archive files. Also,  I  use  a

     RAMDISK to access the extra memory above 640k. A simple batch file,

     SSARC.BAT,  clears  the RAMDISK, decompresses the  chosen  archived

     file  into many picture files in  the RAMDISK, and loads in  SLIDE-

     SHO. All files are then viewed sequentially wihout having to  refer

     to their name.

          TIFCONV.BAS---This  converts pictures which are in a  640x480,

     24 bit color, TIFF format into the proper format to be  transmitted

     in  any  of the modes except Robot. TIFF is the most common  format

     used  to  transfer higher resolution pictures between  programs.  I

     have used this program with both the Computer Eyes/RT and SSC frame

     grabbers.  The picture which outputs from this can be  viewed  with

     VU.BAS and of course is bounded by 320x240 with 18 bit color.  Four

     TIFF  bytes  are  averaged   to get each  output  byte  (except  in

     FAX480).  This gives some desirable smoothing to  the  (affordable)

     real  time  grabber images. In FAX480, the three  color  bytes  per

     pixel are combined to give a gray shade pixel.

          LABEL.BAS---To  add  call  signs and other text  to  the  SSTV

     formatted pictures, this program was generated. It takes any TIFF 1

     bit (ie;black and white) file and creates a mask cut out where  the

     black is. You can superimpose over the SSTV picture where the  mask







     is  cut out either any color you want or transfer a cut out of  any

     background  file. Typical background files might be a picture of  a

     shiny brass plate, a highly polished piece of mahogany,or an orange

     peel---anything with an interesting pattern. The letters look  like

     they were cut out of the background picture. Obviously you can  use

     squares or circles in addition to fonts to transfer a piece of  one

     file  onto another one. We use a cheap hand scanner (which  I  have

     never found any previous use for) to capture any interesting  fonts

     that show up. You can get a three dimensional effect by painting  a

     color  through  the  mask, then moving the mask a  few  pixels  and

     rerunning through LABEL with a background file or another color. Or

     run  several  different  masks through LABEL  in  sequence  to  get

     different colors or patterns on different letters.



     FILE FORMATS

          When I started copying pictures, the data bytes were saved  in

     exactly the sequence received, with formats labelled as S1, S2, M1,

     etc. This ends up using 3 bytes per pixel, which seems a bit  prof-

     ligate. One of the oldtimers in SSTV looked at my pictures which he

     thought looked OK but complained that 6 bits per color looked  like

     too much memory. The Pasokon uses 5 bits in the Targa format  which

     squeezes  the colors for one pixel into 2 bytes. A 50% increase  in

     bytes  did  seem to be a high price to pay. After  looking  at  the

     Targa format, however, it occurred to me that it might not compress

     too well. I took several typical pictures taken in the hamshack and

     ran  them  through  to form Targa, TIFF, and  Scottie  1  formatted

     pictures.  Then  I  compressed  all the  files  using  PKARC  which







     doesn't  remove  any information.  To my surprise,  the  Scottie  1

     files  (10 samples) averaged a squash factor of 49% while both  the

     Targa  and  TIFF  files  only  squashed by 10  to  11%.  So  the  3

     byte/pixel S1 format used less memory in compressed form than the 2

     byte/pixel  Targa  format! Being a little suspicious of  the  large

     difference, I examined the large collection of archived SSTV  files

     copied on the air that I have. Here the squash factor averaged only

     34% with the lowest squash factors being heavily QRMed pictures. My

     discrimination  in  deciding  what to save has  increased  as  more

     pictures  were collected and that shows up in the data as  improved

     squash factors over time. Interesting! There may be other  compres-

     sion algorithms designed for Targa which do better.



     SUMMARY

          We  have  presented  a brief description  of  a  basic  Color-

     SSTV/FAX480/Wefax system which is constructed almost entirely  with

     software. It is still being added to. The software is free, is  not

     copy  protected and can be obtained from the ARRL BBS at  (888)888-

     8888.  Since I expect to be cruising on my sailboat LADY when  this

     is published, mail won't be answered until late spring (except  for

     Christmas  holiday). You may be able to locate me on the  Slow-Scan

     frequency, 14,230 khz., if you need information sooner.



                                                  Ben Vester, K3BC

     8/14/93



     References:









     (1)  J. Langner, "Slow-Scan TV--It isn't expensive anymore!",  QST,

     Jan 1993, pp 20-30.



     (2)   B. Vester, "C-64 Wefax Improvements",  Technical  Correspond-

     ence, QST, Jan 1988, pp 47-49



     (3)   B. Vester, "Improved HF Weather Facsimile Programs",  Techni-

     cal Correspondence, QST, Sept 1991, pp 40-41



     (4)   R. Taggart, "A  New Standard for Amateur Radio Analog Facsim-

     ile", QST, Feb 1993, pp 31-36