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Text File | 1992-02-08 | 26.5 KB | 557 lines

Dirt Cheap Frame Grabber V2.03 by Michael Day as of 8 Feb 1992 Public Domain This device is free and released to the public domain. If you actually find a use for it, all the more power to ya! The purpose of the DCFG is to simply provide a very simple method to grab video pictures and display them on a computer monitor. Version 1.01 of the DCFG only provided 4 levels of gray scale. Version 2.01 makes a modification which allows for 12 gray scales by reading the port four times and interpolating the results. Version 2.02 fixes a schematic error (DO3 is pin 5, not pin 4). Version 2.03 fixes the lack of CLDs before doing string instructions. The DCFG requires at least a 80286 or better and a VGA display to run. While it is possible to change the program to allow operation on a 8086 and/or a different display, I'm not doing it. If you want to, you can. My intent was to just prove that it was possible. The frame grabber program at this point is very simple, and as a result has some problems. The heart of the program relies on a very fast IO port to memory instruction. It can operate as fast as the computer can shove data from the port to the memory. This is as fast as DMAing the data could do. This method is sensitive to computer speed. As such, you may not get a stable picture at first. If you don't, use the left or right arrow keys to adjust the horizontal sample size. The up/down arrows adjust the vertical sample size, but this normally doesn't need to be done. You will probably notice some sparkles (pixels shifting left and right one pixel position). This is caused by sample jitter and is normal. All sampling systems have this problem. Some less than others. Even your TV has the problem, but it is normally good enough that you don't see it. The jitter is caused by the fact that the sampler is not perfectly synchronized to the incoming video signal. Your TV has a lot of circuitry in it to achieve the synchronization. Being that this is a dirt cheap frame grabber, there is only limited synchronization of the picture, and no synchronization of the sampler. If you want to try to improve on it, please do. To use the Dirt Cheap Frame Grabber, plug it into a PC printer port, and attach the other end to the video output from a VCR or TV Monitor and run the program. Use the left or right arrow keys to adjust the horizontal sample size to obtain a stable picture. Do not attach the Dirt Cheap Frame Grabber to the computer with a long printer cable. We are dealing with high speed signals here (up to 4MHz). It requires as short a connection to the computer as you can give it. I built my circuit inside the back shell of the connector that plugs into the printer port on the computer. Dirt Cheap Frame Grabber (DB25P) DO0 ---330ohm-------------*--->|--*-->|---* IC1 = LM339 pin 2 Vr | red 1N914 | quad (2.4v) | led | comparator DO1 ---330ohm-------------* | pin 3 | | * = connection 330ohm | 1N914 | | ) = no connect DO6 ---|<---*---1Kohm-----* Vc (2.21v) | (jump over) pin 8 | | 1N914 | | Vr = ref voltage DO7 ---|<---*---510ohm----*------* | 2.4 volts pin 9 | | | (trimmer) 5Kohm<--* | Vc = 00 = 1.65v | | 01 = 1.80v DO2 ----------*-----------)------| |------* 10 = 1.95v pin 4 | | .1uf | 11 = 2.21v 3| | | DC3 |/ |6 | | Vz = 00 = 0.74v (slct) 1 / -|--------* Vz (1.0v) | 01 = 0.81v DS6 --------|IC1a| | | 10 = 0.88v (ack) \ +|----* | | 11 = 1.00v pins 10,17 \ |7 | 360ohm | | | | Vy = 00 = 0.45v DC2 / |4 | | | 01 = 0.54v (init) 2 / -|----)---* Vy (0.66v) | 10 = 0.59v DS5 --------|IC1b| | | | 11 = 0.66v (pe) \ +|----* | | pins 12,16 \ |5 | 220ohm | Vx = 00 = 0.34v | | | 01 = 0.37v DC1 / |10 | | | 10 = 0.40v (afd) 13 / -|----)---* Vx (0.45v) | 11 = 0.45v DS4 --------|IC1c| | | | (slct) \ +|----* | | Vs = 00 = 0.086v pins 13,14 \ |11 | 360ohm | 01 = 0.093v | | | 10 = 0.101v DC0 / |8 | | | 11 = 0.115v (stb) 14 / -|----)---* Vs (0.12v) | Assumes Vz set DS7 --------|IC1d| | | | for 1.0v using (busy) \ +|----* | | 5Kohm trimmer pins 11,1 12|\ |9 | 120ohm | with DO6, DO7 | | | | set high. Gnd ----------*-------)---*---------------*------*--------* pins | | | | | 18,19,20,21, | 100Kohm | 75ohm | 22,23,24,25 | | .1uf | | | *---*-------| |-----)------*------o ) 2N2222 | | RCA *---------C E | phono | B | jack pin 5 | | 1N914 | DO3 ----*--4.7Kohm--*-------->|-----------* ----------------------------------------------------------------- Parts List 1ea DB25P connector 1ea RCA phono jack 1ea 5K ohm trimmer pot 1ea 510 ohm resistor 1ea 1K ohm resistor 2ea 330 ohm resistor 2ea 220 ohm resistor 2ea 360 ohm resistor 1ea 120 ohm resistor 1ea 100K ohm resistor 1ea 75 ohm resistor 1ea 4.7K ohm resistor 2ea 0.1uF capacitor 1ea red LED 4ea 1N914 diode (or 1N4148) 1ea 2N2222 transistor (NPN) 1ea LM339 quad comparator ----------------------------------------------------------------- Dirt Cheap Frame Grabber Circuit Description It is assumed that the video signal will be a NTSC type signal complying to the RS170A video standard with negative going sync and a signal level between 0.5Vpp to 2Vpp (1Vpp desired). This is the typical signal that is available on the video output jack of a VCR or TV monitor. If you are using a VCR type video signal, you can get a quick start by adjusting the bias trimmer (5Kohm pot) to obtain 1.0volt at location Vz while insuring that the data output lines on the printer port are set to 0FFH. Once you get going you can fine tune the reference voltage by adjusting the trimmer to obtain the best picture. The video input comes in on the RCA phono jack. The 75ohm resistor provides the proper load for the connection. The .1mf capacitor isolates the signal from the rest of the circuit and is used as a part of the level shifter to clamp the video input to ground reference on the computer. The 1N914 diode and 4.7K ohm resistor provides a voltage reference 0.7V above ground for the 2N2222 clamp transistor. The transistor and diode get their supply voltage from the DO3 output pin. Typically this is around 3.3volts and should have enough power for our needs assuming that the output driver is a 74LS374 driver which is the standard part for a PC printer port. The transistor provides a low impedance charge path for the capacitor to clamp the sync tip to ground so as to insure the video signal sync tip is referenced to ground. The 100K ohm resistor provides a high resistance discharge path for the capacitor. Enough to provide a discharge path for the capacitor, but not enough to seriously affect the video signal level. The clamped video signal is then applied to all four comparators in the LM339. The LM339 gets its power from the DO2 output pin. Like the clamp transistor, normally there should be enough power from the data output pin to supply the IC. A .1mf capacitor is used to stabilize the power source. You can probably getaway without using the capacitor, but I'm paranoid. The four comparators receive their reference voltage from a resistor ladder consisting of 360ohm, 220ohm, and 360ohm resistors in series with the 120ohm resistor tied to the common ground. The voltage level on the 120ohm resistor provides the sync detector reference. The other three resistor positions provide three increasing levels of amplitude detection. The resistor ladder voltages can be shifted up or down by the computer using the DO6 and DO7 data output lines on the printer port. This allows 12 gray scale levels to be detected instead of the basic 4 levels that would otherwise be available. The resistor ladder requires a regulated source voltage with an adjustment to allow compensation for various input signal levels depending on the signal source. The 5Kohm trimmer pot provides for the bias adjustment which allows the circuit to be adapted to the absolute signal source level. The regulated source voltage is derived from the red LED in series with a 1N914 diode. A red LED will have a typical voltage drop of about 1.7 volts and the diode has a typical voltage drop of 0.7 volts giving a total of 2.4 volts for the reference. The absolute level is not important since the trimmer can compensate for minor differences. The LED gets its power through the two 330ohm resistors attached to the D0 and D1 outputs. Two outputs are used here to insure that there is enough power to drive the LED and the resistor ladder while maintaining a regulated voltage source across the LED. The LED has a 1N914 diode in series with it to get the reference voltage in the range that we need it. This is the one area where I go beyond spec for the system. The 74LS374 driver is specified to have an output voltage of greater than 2.5volts at 2.5ma. I'm actually relying on the true output voltage for the part which is typically 3.3volts. Should your printer port use a different part than the 74LS374, the circuit may or may not work properly. A 74HTC374 cmos type part will pull up to 5volts. This is not a problem, since all we need is 3 volts or better source voltage to supply the circuit. If the output device supplies less voltage, the circuit will not work because the LED and diode will not be able to maintain the required 2.4volt reference source. The voltage reference is supplied through a 220ohm resistor to the bias adjustment trimmer pot. The 220ohm resistor is a load source for the 1Kohm and 510ohm gray level shift control resistors attached to the DO6 and DO7 data output pins. The gray level shift control lines change the reference voltage on the resistor ladder to allow the computer to adjust the reference voltage to four different levels. This allows us to multiply the gray level detection by four by reading in four sequential video frames at the four possible reference control levels and selecting the highest determined video level from the four frames that are collected. This gives us a total of 12 gray levels instead of the four levels that would otherwise be available. The down side to this is that we have to collect four separate video frames to do this which means that motion will be blurred. The comparators receive their pull-ups from the control lines DC0, DC1, DC2, and DC3. In a standard printer port these lines are open collector outputs with 4.7Kohm pull up resistors. It is presumed that this is the case for this circuit. It saves us the four pull up resistors. If you feel nervous about this, you can attach 4.7Kohm pull up resistors to the comparator outputs and attach two each to the remaining DO4 and DO5 output pins (pins 6 and 7). Note that you should not draw more than 2.5ma from any data output pin or the output voltage may sag. When wiring the circuit, remember that you are working with video signals. That means you are working with signals in the RF range (up to 4MHz). Keep all wiring as short as possible and the components close together. All grounds should be tied to one single point at the 25pin connector. The circuit must be built as close as possible to the DB25P connector. Do not attach this device to the computer with a long cable. The DB25P connector must plug directly into the computer to prevent signal loss. Any attaching cable should be no more than six inches long (alright, I've gotten away with cables as long as three feet, but it isn't recommended). If possible, I suggest building the circuit into the back shell of the connector so that there is no cable involved. Remember that we are dealing with high speed signals here. Long cables don't work. Failure to keep the connection short will cause the sync signal not to be detected and the picture to blur or smear. The video cable attached to the RCA jack can be any length (within reason) since it is not as sensitive to cable length problems. Video cable lengths longer than 20 feet may cause problems, I haven't tried them that long. If you are good at this stuff, it is possible to wire the whole circuit inside the connector shell (don't use a shielded shell, it will short out the circuitry). It's a very tight fit, but if you pick a large shell, and are very careful, it can be all stuffed in there. The trimmer pot should be made available for easy adjustment (I drilled a hole in the shell for access). Also, the led should be visible (another hole to drill) to be able to quickly determine if the circuit is working. To make it all work, simply plug the connector into a printer port on the PC, and run the frame grabber program. When running the program, the LED should turn on. That indicates that the circuit has enough power to operate. If the LED does not come on, or it is very dim, there may not be enough power, and you can't use that printer port with the circuit. Try another port or a different computer. Note that you may see some minor flickering on the led when changing the DO6 and DO7 data output lines. This is normal and indicates that the circuit is working properly. Changing the DO6 and DO7 data output lines causes the current flow through the voltage reference ladder to change. This means that more or less current will be available to the LED, causing it to change in brightness. The LED should not go out completely nor become very dim. That would indicate that there is not enough power available to drive the voltage reference or a problem in your program. You can operate in the simple mode by outputting a 0FFH to the data output port which sets all the output lines high. This provides power to the comparators, voltage reference, and clamp transistor. It also disables the gray scale control lines. To shift the gray scale reference voltage level, send either a 03FH (gray ref 0), 07FH (gray ref 1), 0BFH (gray ref 2), or 0FFH (gray ref 3) to the data output port. The resulting detected output from the port is as follows: 76543210 1000xxxx sync 0000xxxx level 0 (black) 0001xxxx level 1 (dark gray) 0011xxxx level 2 (gray) 0111xxxx level 3 (white) Note that bits 0-3 are unknown and should not be relied on as being valid. ----------------------------------------------------------------- Program Description This program requires a 286 or better computer. A printer port with the attached device described above with a video signal driving it. And a MCGA or VGA type display system. You start the program up by entering its name at the DOS prompt followed by the printer port number you wish to use. Examples: DCFG2 1 <-- loads program for use with LPT1. DCFG2 2 <-- loads program for use with LPT2. If you don't give it a number, the program will default to LPT1. Grabbing a Frame: The program works by grabbing samples from the printer port in frames. Once each time through the loop, it sucks in a video frame from the video input. Actually, more than a frame is grabbed. How much is grabbed depends on the speed of access to the printer port on your computer. On a typical computer this is 1.5us per byte. At the 1.5us access rate, 63.5us per scan line, and 265.5 scan lines per video frame, that results in 11240 bytes per frame required to be collected. The collection size is currently set to 20,000 bytes. That provides enough space to handle buss speeds up to 12MHz which should be adequate for most computers. The down side is that we collect 210 extra scan lines on a normal computer. In most situations this isn't a problem since three to four frames will typically be lost while the collected video data is being processed anyway. The extra partial frame has only a slight effect on the over all collection time and results. The effect is minimized on a fast machine since nearly the entire array is filled anyway. The maximum sample size allowed is 65520. More than that and we run out of segment space in memory. Also during this period I keep the system interrupts off so as to not have distortions of the collected data caused by the system deciding to go off and do something else. In addition to the collection time, there is a pre-configure time while the loop is waiting for the vertical sync pulse to come in. I don't collect any data prior to the vertical sync pulse since it would be ignored anyway. The vertical sync is determined by watching the sync line. When it goes on for longer than 10us, you are in the vertical sync time. Less then 10us, and it is a horizontal sync pulse. Once the video data is collected, I release the interrupts to allow the computer to return to normal operation. Should no sync come in during the vertical sync detection period, I terminate the loop after a while to allow the computer to continue to operate, and return an error condition so that the program will know that no data was collected. If you have an O'scope, you can verify the video collection time by observing data output line DO7 (pin 9). This line should never go high for longer than 50ms. Typically it should be around 20 to 40ms in length. This should be the same whether you have a signal going into the video line or not. Though there may be a difference between the timing when a signal is present verses when it is not present. Video Data Translation: Once I get the data, I convert it to an intermediate form to be stored in the interpretation array. This array keeps the gray level values collected on the previous frames. The display routine will later interpret the values stored in the interpretation array into an absolute gray scale level for each display pixel. The video data is converted by searching for the horizontal sync pulse. Once found, we search for the end of the sync pulse. Following the end of the sync pulse is the actual video data. Each collected byte in the scan line is read and converted to the intermediate gray scale level. There are only four levels involved; 00, 01, 10, and 11. Note that since sync is not video data, it is not included in the gray scale level. Even if it were included, it would be a 00 value. Once converted, the byte is then shifted into it's proper frame position in the interpretation array byte and stored in the array. This is repeated until either the full width of the interpretation array is filled, or a new sync pulse is encountered. Should a sync pulse be encountered, the rest of the interpretation array values are filled with 00. Should the full width of the interpretation array be encountered, then the rest of the scan line is discarded until an new sync pulse is encountered. This sequence is continued until the entire frame has been converted. Displaying the Data: Once the frame has been converted, the interpretation array is then displayed. Each byte is read from the interpretation array and translated to an absolute gray level using the translation array located in the first 256 bytes of the interpretation array. There are 12 levels of gray available, plus black. The translation array was copied into the interpretation array at the time the interpretation array was created. The translation array is formed from the following gray level conversion chart: gray level interpretation chart frame data gray F3 F2 F1 F0 F3 = frame 3, F2 = frame 2 level: 76 54 32 10 F1 = frame 1, F0 = frame 0 12: 11 xx xx xx each group of two bits 11: <11 11 xx xx represent the video level 10: <11 <11 11 xx for the frame indicated 9: <11 <11 <11 11 8: 10 <11 <11 <11 xx = any bit pattern 7: <10 10 <11 <11 <11 = less than 11; (10, 01, 00) 6: <10 <10 10 <11 <10 = less than 10; (01 or 00) 5: <10 <10 <10 10 11, 10, 01, or 00 = the indicated 4: 01 <10 <10 <10 absolute bit pattern 3: 00 01 <10 <10 2: 00 00 01 <10 the gray level for the specified 1: 00 00 00 01 bit pattern is shown at the left 0: 00 00 00 00 There are no sync bytes in the interpretation array since they were stripped out during the process of converting the video data to the interpretation data. Note that it takes four video frames to completely update the interpretation array. However, the actual displayed data will reflect the highest gray level determined from the four previous reads. This means that if the video level shifts towards the white level, the change on the display will be immediate. If the video level moves towards the black level, the change on the display will take up to four frame grabs to reflect the lower video level for that pixel. One big problem is that since the number of scan lines collected is much greater than the resolution of pixels collected in each scan line, the aspect ratio of the picture would be out of whack if we were to display the data pixel for pixel as we received it. In order to get the aspect ratio back to a reasonable value, we must throw data away. We do this by discarding three scan lines for every one that we show. If you have a very fast buss, you may want to change this to discard two scan lines instead of three in order to get a more reasonable aspect ratio. The other thing I do is to pull a little trick to improve the horizontal resolution. Normally there is a typical horizontal resolution of about 40 pixels. This is not very much, and it would be useful if there were a better resolution. There is a halfway solution. What I do is display the even pixels on a scan line from the scan line that I'm currently processing. I then display the odd pixels from the next scan line on the succeeding video frame. What this does is to provide a random selection on the odd pixels between the normal two sequential scanned pixels that effectively improves the display resolution by about 50%. The downside is that it worsens the jitter/twinkle problem since the odd pixel is developed from a pseudo-random sampling mechanism which is not consistent. One thing to keep in mind is that the frame grab routine normally takes between one and three video frames to collect data, and during this time the interrupts are disabled. What that means is that if you are operating on a computer which has a clock interrupt faster than the normal 55ms, you will lose time, and may experience problems. Also note that since the interrupts are disabled, anything that uses high speed interrupts during the time the video data is being collected (such as RS232), will not work. The interrupts will be lost. The main factor that affects speed on slower machines are the conversion and display routines. The INS instruction used to collect the video data is relatively cpu performance independent. The main thing that affects its operation is the buss performance of the computer. The faster the buss, the more video pixels that can be collected per scan line. One possible consideration on slower machines would be to discard the extra scan lines in the Video to interpretation array conversion rather than in the interpretation to display routine. That way the extra scan lines would not even be processed, thus increasing the speed of the overall process loop. --- Future considerations for someone to do if they wish to improve on the design would be to improve the synchronization to reduce the jitter problem. At this point I don't have any ideas on how to improve the problem. Another improvement that is possible to do would be to analyze the captured video frame to determine the proper values to use for capturing the data such as scan line length in pixels, vertical sync length, total scan lines, etc. A mechanism to increase the size of the displayed picture, and maintain the best resolution possible while doing so. Save an image to disk or a series of images to make a movie. (see update information) Be able to print a captured image. Integrate the image with sound. Add color (that's a tough one). And of course integrating this stuff into Windows would be nice. (I'm working on it.) That's all folks! <eof>