Fullscreen HTML5Med Res (??MB)HTML4

By Chris Schach & Braden Phillips* *Chris Schach & Braden Phillips are the principals of Current Dynamics. A Teletext decoder for your PC If you haven’t looked at it lately, there’s a heap of information from all sorts of sources on Teletext. Now you can display these Teletext pages on your PC screen using this stan­dard card which plugs into your motherboard. The software is Windows 95 and 3.1x compatible. Imagine the scenario: you’re at your computer and suddenly you’d like to know the weather report on the Gold Coast tomorrow or you need to check some current share prices. Perhaps you’d like to know the current Sheffield Shield cricket scores or you are about to phone overseas and you’d like to check the time in Vancouver before you dial. Or maybe, perish the thought, 6  Silicon Chip you’ve had a little flutter on the horses and you’d like to check the results. You get the picture? Now if you are at home and you have Teletext on your TV, you can switch on the set and wait for it to bring up the screen you want. But there’s something a bit deca­dent about having the TV on while you are supposed to beavering away at the your computer, isn’t there? Wouldn’t it be so much more convenient to click on the Teletext icon and select the page you want from the control panel? Well, now you can do just that with this Teletext card for a PC. The Teletext card occupies a standard 8-bit slot in your computer and it comes with Windows software on a floppy disc. The only other hardware you need is a VCR – that feeds the offair video signal to the Teletext card for processing. By using a standard VCR as the source of off-air video, the Teletext card is much cheaper and less complicated since it does not need on-board TV tuners (VHF & UHF) and all the associated video circuitry. You need to set the VCR to the station you want (generally one of the Seven Network stations carrying Teletext) and then connect its video output to the Teletext card via a cable This Teletext decoder card plugs into a spare ISA slot on your PC. An interrupt is not required; instead, it communicates using polling over a small amount of I/O address space. fitted with an RCA phono socket. Your VCR’s video output may use a BNC or RCA socket so you will need a cable to match. By the way, the VCR you use only needs to have its video functions working; if it doesn’t work in playback or record that is unimportant. So you could use a VCR with a dud transport for the job. Noise-free TV signal We should point out that you will need a good TV antenna or at least, a good noise-free source of off-air TV signals other­wise you won’t get to first base. If your present TV reception is snowy, you will not get reliable Teletext reception, regardless of whether you are using this Teletext PC card or your TV itself has built-in Teletext facilities. Note that your PC doesn’t need to be a Pentium-based machine. The software runs under Windows 95 or 3.1x and can be on a 486 or 386 machine. All the Teletext processing is done on the card itself and does not involve the PC. Nor does the Teletext card require a PC hardware interrupt, something that can be hard to find on today’s feature-loaded machines. Instead, the Teletext card communicates using polling over a small amount of I/O address space. Nor does the PC need to store Teletext pages. The Teletext card captures and stores the pages, using an on-board micropro­cessor and static RAM (SRAM). The PC card itself is half-size. It is double-sided with plated-through holes and uses conventional ICs, transistors and passive components. One IC is mounted in a chip carrier socket. No surface-mount components are used so you don’t have to worry on that score. You will need a temperature controlled soldering iron with a small tip. On-screen features The software supplied with the Teletext card runs under Windows, as already mentioned. To install the software you run Setup.exe and then follow the bouncing ball. Actually, you don’t follow a bouncing ball; you follow the on-screen prompts. By default, the software is installed in a folder (directory) called “Teletext” and, for Windows 95, an appropriate entry is added to the Start menu. Alternatively, for Windows 3.1x, the relevant icons are added to the Program Manager. Launching the program brings up the Teletext control panel, as can be seen in one of the screen grabs accompanying this article. The control panel defaults to page 100 and it brings up a Teletext window with the message “The requested page has not yet been captured ...” Meanwhile, the page counter in the control panel ticks over to show its progress. By the way, as each page comes in, it is stored in the buffer which is virtual memory; ie, on the hard disc. Ultimately, all Teletext pages are stored in this way and so May 1997  7 any page can be accessed almost instantaneously. This is a big advance on Teletext in normal TV sets because they don’t have a buffer and you have to wait for the wanted page to be transmitted before you can see it on screen. Once the wanted pages are in the buffer, you can display as many Teletext windows as you want on screen. You can also print them out, on virtually any printer. You can also elect to save the buffer (to a directory on the hard disc) and you can thereby display those pages on screen at any time, long after they have ceased to be transmitted. So there you have it. This Teletext card enables you to access and display more pages than you could with a normal TV and you have the bonus of storing and printing out countless pages, if you wish. 8  Silicon Chip There are two differences to be noted between those Tele­text pages displayed on your PC’s screen and those displayed on a normal TV. First, because of the resolution of a VGA screen, the Teletext pages on your PC will be sharper than those on your TV. Not that there’s a real benefit but they are sharper. Second, while Teletext pages normally have the same 4:3 aspect ratio as a normal TV screen, when displayed on your PC, they are somewhat squarer. That too is immaterial and is an effect of the software. OK, so you now know what the Teletext card does. Let’s have a brief look at the circuit details. Circuit details As you may be aware, Teletext signals are sent during the vertical blanking interval of a normal off-air TV signal. If you roll the picture up, you will see several bright lines of ever-changing data embedded in the vertical blanking bar. The data is mainly text and single graphics. The data is normally decoded by the TV set and reconstitut­ed into pages on the screen. Up to 800 or so pages can be trans­mitted and they are sent in sequence. The time to access a par­ ticular page off air depends on where it is in the sequence and how recently it was sent. It can take several minutes for a page to be captured. The Teletext data signal comes in at high speed, with a serial bit rate of 6.9375MHz. This, coupled with Fig.1 (right): the Teletext decoder extracts ASCII text and graphics from an off-air composite video signal, usually from a VCR. The data is stripped from the video signal using video slicer U6 and then processed in U3 under the control of U1, the PIC microprocessor.  This screen capture shows how the Teletext Control Panel and the Teletext window appear on the Windows 95 desktop. You can open as many Teletext windows as you like and all incoming pages are stored in a buffer for quick access. The buffer can also be saved to the hard disk and the pages printed out. May 1997  9 Open a new teletext window Close the active teletext window Reveal hidden text the need for other high speed digital processing of the Teletext signal, requires specialised hardware so that the microprocessor only needs to take on a coordinating and hence relatively low-speed role. An XILINX FPGA, U3, was chosen to do the job. FPGA stands for Field Programmable Gate Array. U3 is controlled by the PIC16C57 microprocessor and stores its data in an HM62256 static RAM, U4. The interface between the PC and the card consists of a GAL20V8 logic array, U5, and two 74HC373 Tri-state buffers, U2 & U8 (note: GAL stands for Generic Array Logic). Data from the PC is latched into U8 when the PC writes to one of the four I/O addresses stored in the GAL and selected by the jumpers JP1 & JP2. The PIC1657 reads the data by enabling the outputs of U8 and it can write data to the PC by latching it into U2. The PC can read this latch at any time by reading from the correct I/O address. 10  Silicon Chip Display Capture the next page 100 sub page now Load a buffer from file Save the buffer to a file Empty the buffer The PIC16C57 microprocessor contains 2048 words of on-board program space and 72 bytes of on-board RAM. In addition, it contains 20 I/O lines and can operate at cycle times as low as 200ns. It is used as the interface between the data capture hardware and the PC. It accepts commands from the PC and responds appropriately with actions and/or data. For example, if the PC wants a specific page of Teletext, it will issue the appropriate command word to the PIC followed by the page number. The PIC will set the SRAM address to 0 and then initiate a Teletext line capture. When the line capture is com­plete, the PIC will check to see if the line was a valid header row, in which case it will check to see if it is from the re­quired page. If not, it will reset the SRAM address and continue look­ing. When the correct header line is found, the PIC will continue capturing and storing Print teletext page xxx Stop autocapture Quit the teletext viewer lines until it comes across another line 0. Another line 0 means that a full page has been received and the PC can be notified that the page capture is complete. Data slicer A Philips SAA5231 data-slicer, U6, is used to extract the Teletext clock and data signal from the incoming video signal. The clock and data outputs from this IC must be level shifted to produce TTL signals. This is accomplished using transis­tors Q1 & Q2 and a dual high-speed comparator, U7. The comparators use the average of the incoming clock signal as their reference, eliminating drift problems possible with a fixed reference. The level-shifted signals then go directly to the FPGA. The 5231 needs a “sandcastle” input which must stay low for 8.5µs after the start edge of a video sync pulse. This is generated via a counter in the FPGA. Fig.2: the parts are installed on the PC board and tested in stages, as detailed in the article. Make certain that all parts are correctly oriented before soldering their leads, as this is a double-sided board with plated-through holes. The FPGA uses volatile configuration data and must be reconfigured after each power up. The configuration data is sent from the PC to the FPGA via the PIC in a serial data stream. The whole configuration process takes a fraction of a second. The Teletext data signal is fed to an 8-bit shift register whose outputs can be enabled onto the SRAM’s data bus. An 8-bit comparator is also attached to the outputs which produces a sync signal whenever the line sync byte appears. This sync signal is used to reset both the bit counter and the byte counter at the start of a line and also to set the line capture process in motion. When a line capture is in progress, the SYNC REC output will indicate that fact to the PIC. As the bit counter clocks over each time, the byte currently in the shift register is written to the SRAM at the location pointed to by an address counter also contained in the FPGA and both the address counter and byte counter are in­crement­ed by 1. When the byte count reaches 43, the line capture is complete and SYNC REC will go low. The upper eight bits of the 11-bit May 1997  11 address counter can be set by the PIC before a line capture takes place. The PIC can read the contents of the SRAM by asserting _READ which enables both the SRAM output buffers and connects the microprocessor and memory data busses through the FPGA. The PIC increments the address counter by asserting the CLK signal. Construction Begin by inspecting the double sided PC board carefully for short circuits or broken tracks, being especially careful in areas that will be concealed by components. Fix any problems as necessary using solder or a sharp knife. The metal bracket can be mounted at this stage, adjusting the alignment as necessary to suit your computer’s expansion slot. The suggested way of assembly is to progressively populate the board, testing as you proceed. This is made easy though a program called TT_TEST included on the installation discs. Ensure that each of the test options are run in sequence each time the program is restarted to ensure that the Teletext card is properly initialised. The first components to install are the GAL20V8 (U5), C18, R9, R10 and JP1&2 which form a sub-circuit responsible for I/O address decoding. The first two test program options enable the reset line (U5 pin 21) to be toggled between 0V and +5V respec­ tively. Choose these options and use a multimeter to make sure the reset line behaves as expected. When this part of the circuit works, proceed to the next stage. The PIC16C57 microprocessor (U1), Where To Buy A Kit This Teletext decoder was designed by Current Dynamics who own the design and software copyright. The kit will include a high quali­ty double-sided PC board with plated-through holes, screen print­ed component overlay and green solder mask, all components, 3.5-inch 1.44MB installation discs and instructions. The discs will include the full Windows Teletext viewing software, a test pro­gram and some example C source code for those who wish to develop their own software. The complete kit is priced at $150 plus $5 for postage within Australia. For postage to New Zealand, add an extra $7.00 (Aus­tralian dollars). Remittances may be sent by bank cheque, money order, Visa, Bankcard or Mastercard. Current Dynamics can be contacted by phoning (08) 8303 3349 or by fax on (08) 8303 4363; email currentd<at>ozemail.com.au or http://www.ozemail.com. au/~currentd Send mail orders to Current Dynamics, 37 Queen Street, Thebarton, SA 5031. 12  Silicon Chip X1, C8, C22, R8, C17, C19 and the 74HC373s (U2 & U8) are next. Anoth­ er test option enables the function of this section to be verified by a simple command/echo sequence between the PC and the PIC. It also has the effect of ensuring that both the PC and the PIC have synchronised “clock” variables. Now for the XILINX2064-68PC (U3). Be careful to ensure correct device and socket orientation. The bevelled corners on the socket and overlay should be aligned. Also, the pin 1 dot on the IC should be aligned with the white legend dot on the PC board. Be warned that once the IC is inserted, it can be diffi­cult to remove without a special tool. Next, solder in C15, C16, R7 and the RAM (U4). To check this section of the circuit a test routine has been developed which sends a configuration to the FPGA. If this operation is completed successfully we can be fairly sure the FPGA is alive and well and communicating with the microprocessor and the PC. The next task is to test the RAM. A difficulty here is that the FPGA only writes to the RAM when it is receiving Teletext. Therefore, the best we can do is use a test routine to read the entire contents of the RAM and then check to see that it is stable by reading it again. The remaining components can now be inserted. At this point, a suitable video signal is required. For most construc­tors, this will mean access to the video output of a VCR tuned SILICON CHIP This page is blank because it contained advertising which is now out of date and the page has been removed to prevent misunderstandings. to a channel with a Teletext service (Channel 7 or 10 in Australia). To be sure that the video signal is of sufficient quality, it is wise to monitor the picture quality on a TV while your computer is running. A computer can be a significant cause of noise input to an RF television signal, so it is important to keep your antenna and your computer well separated. Picture quality needs to be reason­ably good with little “snow”. Moderate ghosting does not usually cause many errors in Teletext pictures. The final test routine will verify that the Teletext data and clock lines are active. The test will look for sync charac­ters present at the start of each Teletext line. A video signal must be present for this test to pass. Acknowledgement: all Teletext screen grabs in this article SC reproduced courtesy Austext, Channel 7. May 1997  13