BHB3@PSUVM.BITNET (08/04/89)
Does anybody know hwat the loewst cost device that would generate a 60Hz video sync signal would be. Preferably it would be best crystal locked. I have already tried the sync out from a commercial RCA video camera. It didn't work well enough. I was able to borrow a digital time base corrector and the sync sigal for it worked. The specific application is an Ardent Workstatio computer. The RS-170(NTSC) output from it requires that you put in a external sync signal.
cook@stout.ucar.edu (Forrest Cook) (08/05/89)
In article <89216.120136BHB3@PSUVM> BHB3@PSUVM.BITNET writes: >Does anybody know hwat the loewst cost device that would generate a 60Hz >video sync signal would be. ... >The specific application is an Ardent Workstation computer. >The RS-170(NTSC) output from it requires that you put in a external ???????????? >sync signal. RS-170 and NTSC are very different color signals, RS-170 uses separate R G and B lines and NTSC puts it all on the same signal. In 3 wire RS-170 setups, the sync signals (negative pulses) are often sent along the same line as the green video signal (positive waveform) during retrace. A fairly simple comparator circuit set to trigger on the Vsync level should work as a Vertical Sync Separator. It is best to look up some kind of RS-170 spec (preferably from Ardent in your case) to get the exact specs on the signal. ^ ^ Forrest Cook - Beware of programmers who carry screwdrivers - LB /|\ /|\ cook@stout.ucar.edu (The preceeding was all my OPINION) /|\ /|\ {husc6|rutgers|ames|gatech}!ncar!stout!cook /|\ /|\ {uunet|ucbvax|allegra|cbosgd}!nbires!ncar!stout!cook
leach@neptune.uucp (Tom Leach) (08/05/89)
In article <3881@ncar.ucar.edu> cook@stout.UCAR.EDU (Forrest Cook) writes: >In article <89216.120136BHB3@PSUVM> BHB3@PSUVM.BITNET writes: >... >>The specific application is an Ardent Workstation computer. >>The RS-170(NTSC) output from it requires that you put in a external > ???????????? > >RS-170 and NTSC are very different color signals, RS-170 uses separate >R G and B lines and NTSC puts it all on the same signal. Nope, I think that you got that switched around. RS-170A refers to an NTSC-encoded composite signal. You can use and encoder to take component RGB signals and convert them into a RS-170A signal, but the RS-170 is definately an NTSC signal. (Feb 89 iss of AV Video has a short piece on RS-170A and RGB in the Q&A column) Tom Leach Internet:leach@OCE.ORST.EDU UUCP:{tektronix, hp-pcd}!orstcs!leach ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Middle-of-the-road, man, it stanks. Let's run over Lionel Richie with a tank. >>>Disclaim: It's me, not OCE.<<< B. Catt, Deathtongue. (c 1986)
poynton@vector.Sun.COM (Charles A. Poynton) (08/05/89)
> Does anybody know what the lowest cost device that would generate a 60Hz > video sync signal would be. To cut a long story short, I suggest a VIDI/O BOX (TM) from Truevision, Inc. 800-858-8783, list $995. Certainly crystal stability will be required for your application (therefore you need 59.94 Hz field rate, not 60 Hz). Just horizontal and vertical 4 V 'sync' might work, it depends on the device being fed, but I suggest (and the VIDI/O BOX provides) 1 V "black burst" which includes colour burst and pedestal, and is in fact exactly a legal black video signal. All respectible (broadcast or industrial) video equipment will free-run and generate legal timing in the absence of reference video in, this may or may not be true of your workstation. I suspect that your workstation requires reference sync only so that an external sync generator can generate subcarrier locked to the sync. This is to allow you to externally encode a coherent NTSC colour signal, that is, a signal in which the colour subcarrier is phase-locked to the horizontal sync. The VIDI/O BOX includes an encoder. I suspect that your workstation outputs is colloquially called "RGB with RS-170-A timing" and not what you refer to as "RS-170(NTSC)"; the distinction is the subject of a following article. C. ----- Charles A. Poynton Sun Microsystems Inc. <poynton@sun.com> 2550 Garcia Avenue, MS 8-04 415-336-7846 Mountain View, CA 94043 "Japan has no laws against damage to its flag, but it has strict laws forbidding the burning of foreign flags lest this give offense to the country in question." -- The Economist, July 1, 1989, p. 19. -----
mmm@cup.portal.com (Mark Robert Thorson) (08/06/89)
I'm not really sure I understand your question, but if you want a source of a composite sync signal, you can use the National Semiconductor MM5320 TV camera sync generator chip (if they still make it). It outputs a composite sync signal. It can be sync'ed up to an external signal. Ferranti also makes a composite sync generator, but I don't recall if it can be sync'ed up to an external signal. I believe it can. The National chip is much cheaper, unless you count the cost of a -12V power supply. (Ferranti is 5V only.) I haven't heard of National re-casting the 5320 in n-MOS. It would be a sensible thing to do. I've worked with the p-MOS version. Here are some tips: 1) The outputs are TTL compatible, but they're not TTL. They pretty much swing between near +5 and near -12. This is important to know if you are designing a resistor summer to mix the sync with the video. 2) This chip really surges on the power consumption at certain points in its cycle. You need a good-size decoupling capacitor on the -12 supply right near the chip. I used 22 uF. 3) This chip runs hot. Take appropriate precautions. THe last time I bought chips, it was from Advanced Computer Products somewhere near LA. Half the chips didn't work. They ignored my requests for a refund. I do not recommend doing business with them.
jimc@iscuva.ISCS.COM (Jim Cathey) (08/07/89)
In article <3881@ncar.ucar.edu> cook@stout.UCAR.EDU (Forrest Cook) writes: >RS-170 and NTSC are very different color signals, RS-170 uses separate >R G and B lines and NTSC puts it all on the same signal. RS-170 is monochrome. Isn't the 3-wire RGB RS-170-like video spec RS-343? I can't remember anymore. +----------------+ ! II CCCCCC ! Jim Cathey ! II SSSSCC ! ISC-Bunker Ramo ! II CC ! TAF-C8; Spokane, WA 99220 ! IISSSS CC ! UUCP: uunet!iscuva!jimc (jimc@iscuva.iscs.com) ! II CCCCCC ! (509) 927-5757 +----------------+ "With excitement like this, who is needing enemas?"
rpw3@amdcad.AMD.COM (Rob Warnock) (08/08/89)
In article <21056@cup.portal.com> mmm@cup.portal.com writes: +--------------- | I'm not really sure I understand your question, but if you want a source | of a composite sync signal, you can use the National Semiconductor MM5320 TV | camera sync generator chip (if they still make it). It outputs a | composite sync signal. It can be sync'ed up to an external signal. +--------------- Ah, yezzz... used it once in a project I was doing... Jameco (a local/mail_order distributor) still carries it; cost is about $12 each. But it's listed as a "closeout item, will not be reordered". (But it's been listed that way for over a year now...) +--------------- | ... The National chip is much cheaper, | unless you count the cost of a -12V power supply. | ...some tips: | 1) The outputs are TTL compatible, but they're not TTL. They pretty much | swing between near +5 and near -12. This is important to know if you are | designing a resistor summer to mix the sync with the video. +--------------- Or if you want to feed something other than TTL, like CMOS. The current that's sourced from the -12 supply can blow CMOS inputs (or even blow the whole chip by causing the CMOS to go into "latch-up"), so use a "real" TTL or LS-TTL chip as a buffer first. The National chip's outputs are actually *designed* to use the input undershoot diodes of the following TTL input as a clamp on the output voltage. (Ugh!) +--------------- | 2) This chip really surges on the power consumption at certain points in | its cycle. You need a good-size decoupling capacitor on the -12 supply | right near the chip. I used 22 uF. | 3) This chip runs hot. Take appropriate precautions. +--------------- Ottherwise, it works o.k., I guess... But for just those reasons, plus the uncertainty about future availability, I gave up using it and built a PROM/counter-based circuit instead. A naive version would use 455 x 525 locations (clocked at Color-Burst x 2), but you can do what the National chip does and clock at (CB * 4) / 7 = 2.045454286 Mhz which gives 130 clocks per H line, or 130 x 525 = 68250 locations in the PROM. [Note that "CB*4" is supplied on the bus of most IBM PC clones.] Since most EPROMs are 8 bits wide, you can use some of the extra bits that you wouldn't need for outputs to do some run-length compression, which lets you pack the pattern into a 4Kx8 EPROM (2732 or equiv). [A slightly simpler counter circuit uses more ROM but still gets it into a single 2764 EPROM, at about the same price. You also get an extra usable output, which I use for a "test pattern".] Using a 29C64 EPROM and 74HCTxxx counters, the power drain is pretty low. The total parts cost is *less* than the National chip, though the board area is greater. Besides, you get to completely choose the outputs. The outputs I needed were NTSC composite sync, NTSC composite blanking, start_of_line (similar to H_sync, but keyed at the edge of blanking), start_of_field (similar to V_sync, but keys on at line 22), and "test_pattern". (The other three bits go back to the first counter section and control how many clocks to repeat each output pattern.) If you feed the composite sync, blanking, and test pattern outputs to a simple resistive adder, you get an analog output monochrome composite-video test pattern. Using a bigger PROM, you could use an address bit to select between several patterns stored in the ROM, for example, between NTSC and PAL. More details if anybody really needs them... Rob Warnock Systems Architecture Consultant UUCP: {amdcad,fortune,sun}!redwood!rpw3 DDD: (415)572-2607 USPS: 627 26th Ave, San Mateo, CA 94403
f-liste@skakke.uio.no (Matthew P.H. Liste) (08/08/89)
You can also use a Philips SAA1043, Universal sync generator. It will generate sync signals for eight differeant standards: * SECAM 1 * SECAM 2 * PAL/CCIR * NTSC 1 * NTSC 2 * PAL-M * video game 624 and 624-line standards. It can be synchronized with an external sync signal, and the inputs and outputs are CMOS compatible. It runs on a 5.7 to 7.5 positive voltage. It will generate the vertical frequency of 54.94 Hz you are looking for. For use with colour video equipement it can be used with the Philips SAA1044 for generating the subcarrier horizontal scan frequencies. Here in Norway the SAA1043 costs about $ 20. -Matthew Liste (f-liste@ifi.uio.no)
myers@hpfcdj.HP.COM (Bob Myers) (08/09/89)
>RS-170 is monochrome. Isn't the 3-wire RGB RS-170-like video spec RS-343? >I can't remember anymore. Nope. RS-170 IS monochrome, but the version of RS-170 which includes the NTSC color encoding scheme (with accompanying slight changes in timing) is RS-170A. RS-343 (now superseded by at least RS-343A) was intended as a high-resolution closed-circuit standard; RS-343A never really caught on as far as its recommended timing (it is an interlaced standard), but the signal level definitions are used by most high-resolution computer graphics displays today. The signal levels defined for these standards are (all into 75 ohms, and assuming the BLANK (not the BLACK) level as the reference: RS-170/170A: White is 1V POSITIVE from reference Sync tips 0.4V NEGATIVE from reference. (Overall 1.4V p-p signal) RS-343/343A: White is 0.714V POSITIVE from reference. Sync tips 0.286V NEGATIVE from reference. (1.0V p-p overall.) In RGB systems conforming to these standards, the sync is on the GREEN signal. The red and blue signals conform to the signal level standards w/o the sync, i.e., the red and blue in an RS-343-level system would be expected to have a max. swing of 0.714Vp-p. Sync may also be provided to the monitor via a separate input or inputs, which are usually TTL-compatible. Back to the original question - I don't quite understand the needs of your system. If the unit simply requires an external Vsync pulse to "sync up to" before it will produce video, then simply feed it the output of a pulse generator set to make 60 Hz with about a 2-3% duty cycle (sync pulse about 0.3 - 0.4 msec long, out of the 16.67 period). This will result in video which should be usable by any RS-170 monochrome display; no need to get the 59.94 Hz NTSC timing exactly, unless you're planning on syncing up to some external video. But in the latter case, you'd want to derive the Vsync signal from that external source anyway - and that's a whole 'nother can of worms! Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers%hpfcla@hplabs.hp.com | sentient life-form on this planet.
brown@astroatc.UUCP (Vidiot) (08/09/89)
In article <26659@amdcad.AMD.COM> rpw3@amdcad.UUCP (Rob Warnock) writes:
<
<Otherwise, it [MM5320] works o.k., I guess...
Yep, the part works just fine. I use it to drive the sync inputs on the
3/4" Umatic decks that I use for editing (through a TTL buffer). I also
have a sync separator part that I use to set off a one-shot to resync the
sync generator every frame. The one shot is used because the timing of the
signals available do not exactly match what the generator needs, but since
one of them is early enough, I put it through a one-shot.
The problem with the 3/4" Sony decks (older ones) is that when the input
video is used for sync, a very minor glitch will cause the recorder to
lose gen-lock. It is worse when editing and the source tape is used as the
sync source. So, by feeding external sync into both decks, there isn't a
sync separator problem in the 3/4" deck. I looked at the internals of the
3/4" and noticed that the composite video signal used as sync was about
1/5th of the size of external TTL sync. This extra level internally provided
enough to keep glitches from screwing things up.
I use the sync separator to take in video signals from the TV and provide
re-sync for the sync generator. This keeps minor glitches from screwing up
the 3/4" deck when recording off-the-air stuff. Also, the one-shot is such
that it finishes a short while before the next frame starts, keeping random
glitches between frames was causing random re-syncs. Of course, massive
changes in original sync timing will still cause glitches. If only I had
a frame-store unit :-)
--
harvard\ att!nicmad\
Vidiot ucbvax!uwvax..........!astroatc!brown
rutgers/ decvax!nicmad/
ARPA/INTERNET: brown%astroatc.UUCP@spool.cs.wisc.educook@stout.ucar.edu (Forrest Cook) (08/09/89)
In article <2590@iscuva.ISCS.COM> jimc@iscuva.ISCS.COM (Jim Cathey) writes: >In article <3881@ncar.ucar.edu> cook@stout.UCAR.EDU (Forrest Cook) writes: >>RS-170 and NTSC are very different color signals, RS-170 uses separate >>R G and B lines and NTSC puts it all on the same signal. >RS-170 is monochrome. Isn't the 3-wire RGB RS-170-like video spec RS-343? >I can't remember anymore. Oops, I looked it up in our Seiko CH5301 hardcopier manual and it says that it is compatible with both RS-170 and RS-343 RGB composite video as well as R+G+B+Sync composite sync. Our Ramtek GM-850 monitors say they use "EIA Std RS-343A (separate composite sync availible)" which is 1280 X 1024 X 256 color resolution in our case. We just rolled in a couple of Ardent workstations that provide 1280 X 1024 X 16,777,216 color resolution. The manual only says RGB with sync on green 60Hz noninterlaced, no RS numbers are shown but It appears to be RS-343. Does anybody spec the actual color resolution of their monitors? From my experience, it varies considerably with the age of the CRT. Perhaps some well informed individual could post a table of common video formats and their associated scan frequencies, resolutions, interlace characteristics, etc. ^ ^ Forrest Cook - Beware of programmers who carry screwdrivers - LB /|\ /|\ cook@stout.ucar.edu (The preceeding was all my OPINION) /|\ /|\ {husc6|rutgers|ames|gatech}!ncar!stout!cook /|\ /|\ {uunet|ucbvax|allegra|cbosgd}!nbires!ncar!stout!cook
stevel@tybalt.caltech.edu (Steve Ludtke) (08/10/89)
If you just need a simple sync signal, and you have some sort of video source, the LM1881 is an excellent choice. It's pretty new; it provides composite sync, vertical sync, burst/back porch, and odd/even frame output, all in a 8 pin package. It takes any NTSC (might deal with PAL too) video signal as an input, and requires only 3 external components and a +5 thru +12 volt power supply. I've tried it with a number of video sources for genlocks, video decipering, etc ... and it's worked quite well. Even better, they only cost $4-6 each. I've seen a number of different scemes for accomplishing the same thing as this single chip. It usually involved several IC's and numerous resistors and cap's, and was usually less reliable than this. In any case, it's made by national, and might be worth looking into. --------------------------------------------------------------------------- Steve Ludtke stevel@tybalt.caltech.edu ..!cit-vax!tybalt.caltech.edu!stevel stevel@citiago (Bitnet) OBS949 (Amer PPl lnk) 72335,1537 (Compuserve) XJM16487 (Genie)
myers@hpfcdj.HP.COM (Bob Myers) (08/11/89)
>We just rolled in a couple of Ardent workstations that provide 1280 X 1024 >X 16,777,216 color resolution. The manual only says RGB with sync on green >60Hz noninterlaced, no RS numbers are shown but It appears to be RS-343. >Does anybody spec the actual color resolution of their monitors? From my >experience, it varies considerably with the age of the CRT. The "RS-343" designation on most monitors used with computer systems refers only to the fact that the video signal *levels* conform to those set by the RS-343 standard (white 0.714 up from blank, sync tips 0.286V down from blank), and not the timing proposed in that standard. (RS-343 was developed as a high-resolution closed-circuit standard, and include a number of recommended timings - I think four different timing standards, but I'm too lazy to dig the standard out right now. All were *interlaced*.) I'm a little uncertain as to what you mean by "color resolution". The number you give for the Ardent workstation mentioned could be misleading if expressed in this manner, as a display with 1280x1024 pixels can't possibly display 16 million colors *simultaneously*. Now, in RGB monitors, the color resolution is *not* artificially limited as it is in, say, NTSC color encoding (where there were some very good reasons for imposing a limit on the bandwidth of the color signals). Assuming that the video amplifiers in your monitor are up to snuff, and the dot pitch of the CRT was properly chosen, the monitor is in fact capable of displaying 1280x1024 pixels, each a different color. Age should not affect this unless the amplifiers degrade or there is damage to the CRT, especially in the shadow mask. This latter would affect only a certain area of the CRT, though. Whether or not you would actualy be able to SEE, say, 1280 distinct colors in a given line is a completely different question - one reason that the NTSC TV standard was able to get away with limiting the color resolution is that the human eye isn't very good at resolving fine color detail - in other words, our "color spatial resolution" isn't as good as our "monochrome" vision. (Reason? More rods, which respond to lumninance only, than cones, which respond to certain colors.) >Perhaps some well informed individual could post a table of common video >formats and their associated scan frequencies, resolutions, interlace >characteristics, etc. This information is readily available for the various "PC" formats (CGA, EGA, VGA, etc.), all of which (except for the new 8514A 1024x768 displays) are non-interlaced. I won't bother posting 'em here - partly because I can't put my hands on those files right now (gotta get this drawer organized... mumble, mumble...). There are no real "standards" for the higher-end displays, such as those used for the workstation market (as opposed to the PC industry, where everyone simply heeds The Word: "And IBM said, 'Let there be CGA!' And lo, it was so!"). The problem is that there can be a number of different specific timings for a given resolution, depending on the retrace times required by the monitors, vertical refresh rates, etc.. However, a good ballpark set of numbers can easily be obtained for a non-interlaced display as follows (examples are for a 1024x768 image): 1. Find the total vertical period. For a 60 Hz display, that's 16.67 msec. 2. Typically, about 5% of the total vertical time is used for the vertical blanking. So, 95% of the above number gives us 15.84 msec "active" vertical time. 3. This active time must be evenly divided between the displayed lines. So, divide this number by 768, and we get 20.62 usec per line, for a horizontal sweep frequency of about 48.5 kHz. 4. Typically, something like 20-25% of the total horizontal time is used in horizontal blanking (the percentage tends toward the high end of this range for the higher resolutions). If we guess right in the middle, then 77.5% of the total H time gives an "active" horzontal time of 15.98 usec. 5. This active time must be divided between the pixels in a single line. Dividing 15.98 usec by 1024 give 15.6 nsec per pixel, or a "dot clock" of about 64 MHz. Just as a point of comparison, the display I'm using right now happens to be a 1024x768 display - an HP 98545A display card in a Series 300 workstation. It uses a 64.11 MHz dot clock, and a 47.7 kHz sweep frequency; so you can see we got pretty close with our estimate. Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers%hpfcla@hplabs.hp.com | sentient life-form on this planet.
king@dciem.dciem.dnd.ca (Stephen King) (08/12/89)
In article <11525@cit-vax.Caltech.Edu> stevel@tybalt.caltech.edu.UUCP (Steve Ludtke) writes: >If you just need a simple sync signal, and you have some sort of video source, >the LM1881 is an excellent choice. [...] >I've tried it with a number of video sources for genlocks, >video decipering, etc ... and it's worked quite well. Even better, they >only cost $4-6 each. [...] Another interesting component is (was) the Motorola MC1378 video overlay synchronizer. Put NTSC in and get everything out, including phase locked 10xSc clock (35.8 Mhz). External circuitry has to feed H sync back to the chip - we accomplished that with one PAL. The thing also had RGB and overlay enable inputs. Anyway, what we worked with was a pre-production sample (labelled XC1378), and I haven't heard whether or not these things ever made it to full production. Anyone got more info? -- Se non e` vero, e` ben trovato ...{utzoo|mnetor}!dciem!dretor!king king@dretor.dciem.dnd.ca
mmm@cup.portal.com (Mark Robert Thorson) (08/12/89)
If you do as Rob Warnock suggest (use a counter to drive a PROM to generate your sync signals) you may need a reference for the TV sync signal itself. I once got an excellent booklet on the timing of standard TV signals for free from the local Tektronix representative. I believe the book was called TELEVISION BROADCAST MEASUREMENTS. It is a book intended for the engineer at a TV station, and tells what measurements you need to take, and how to take them, in order to comply with FCC standards.
cook@stout.ucar.edu (Forrest Cook) (08/15/89)
In article <17660014@hpfcdj.HP.COM> myers@hpfcdj.HP.COM (Bob Myers) writes: >>We just rolled in a couple of Ardent workstations that provide 1280 X 1024 >>X 16,777,216 color resolution. >I'm a little uncertain as to what you mean by "color resolution". The number >you give for the Ardent workstation mentioned could be misleading if expressed >in this manner, as a display with 1280x1024 pixels can't possibly display >16 million colors *simultaneously*. Ardent calls their 24 bit color table "Truecolor" and I was just wondering if it was possible to see the finest steps in a smooth transition on the monitor. While you can only display 1310720 unique colors at a time on the screen, it would be possible to make a 16 frame "movie" loop that displayed all of the possible colors. A possible test would be to have R increase with X, G increase with Y, and B increase with time/frame #. >Assuming that the video amplifiers in your >monitor are up to snuff, and the dot pitch of the CRT was properly chosen, >the monitor is in fact capable of displaying 1280x1024 pixels, each a >different color. Age should not affect this unless the amplifiers degrade >or there is damage to the CRT, especially in the shadow mask. I was referring to the aging of the Cathode on the CRT. Some of our Ramtek monitors are approaching 10 years of on time and their colors have changed intensities over the years. An old color table that we designed to have 17 discrete colors looks ok on a new crt but some of the color differences are hard to notice on an old CRT. This is after both monitors have been set up with a Black->Red, Black->Green, Black->Blue and Black->white test pattern. If you draw one of these primary color bars with 256 levels and adjust the monitor so that it is black on one side and saturated on the other, the color change will not appear to be linear. It will be different from an old monitor to a new monitor. It will also differ between the red, green, and blue phosphors. It will be very different on a video photograph. The individual's eyes and the ambient lighting will also play a role. All of these parts have non-linear response curves. Ardent was clever enough to put what they call "Gamma Correction Tables" in between their color table and each video DAC to allow for correcting the curves. I have not discovered how they determine the curves, but it sounds like they can be changed fairly easily. These promise to be very useful in correcting for video photography. Now, if I can only figure out how to generate the curves :-) ^ ^ Forrest Cook - Beware of programmers who carry screwdrivers - LB /|\ /|\ cook@stout.ucar.edu (The preceeding was all my OPINION) /|\ /|\ {husc6|rutgers|ames|gatech}!ncar!stout!cook /|\ /|\ {uunet|ucbvax|allegra|cbosgd}!nbires!ncar!stout!cook
myers@hpfcdj.HP.COM (Bob Myers) (08/17/89)
>Ardent calls their 24 bit color table "Truecolor" and I was just wondering if >it was possible to see the finest steps in a smooth transition on the monitor. >While you can only display 1310720 unique colors at a time on the screen, >it would be possible to make a 16 frame "movie" loop that displayed all of >the possible colors. A possible test would be to have R increase with X, >G increase with Y, and B increase with time/frame #. OK, I understand what you mean now. What you're really asking here is if the video amplifies provide a proper response such that the brightness of each color (of red, green, and blue) increases linearly and monotonically over a 256-step range. The answer to this, for most current color monitors, is both yes and no (don't you just love these precise answers?). Making an amplifier with a linear, monotonic response over this range is not difficult. However, the output of the amplifier (the signal sent to the CRT cathodes) does not linearly correspond to brightness, and therein lies the first problem. An additional problem is that, even if the system DID correctly resolve these color, it is very doubtful that the human eye will be able to tell the difference in all cases. Now, more on the CRT non-linearity, etc.: >pattern. If you draw one of these primary color bars with 256 levels and >adjust the monitor so that it is black on one side and saturated on the >other, the color change will not appear to be linear. It will be different >from an old monitor to a new monitor. It will also differ between the red, >green, and blue phosphors. It will be very different on a video photograph. >The individual's eyes and the ambient lighting will also play a role. >All of these parts have non-linear response curves. > >Ardent was clever enough to put what they call "Gamma Correction Tables" >in between their color table and each video DAC to allow for correcting >the curves. I have not discovered how they determine the curves, but it >sounds like they can be changed fairly easily. These promise to be very >useful in correcting for video photography. Now, if I can only figure out >how to generate the curves :-) The cathode of a CRT (actually, three cathodes in the color CRT's case) does age with time. However, you should be able to correct gross errors by performing a white balance adjustment as recommended by the manufacturer. Gamma correction is the next step beyond this, in a way, and can provide pretty accurate results if you understand how it works. As mentioned above, the absolute brightness level (as measured by a photometer, not as perceived by the eye) does not vary linearly with the applied signal level in a typical CRT. Instead, the increase follows a curve described by the formula I = k(Vg)^(gamma) where I is the intensity of the light resulting from control grid voltage Vg ("k" is a constant that hides a multitude of sins in getting from voltage to brightness). The value of gamma is usually determined empirically for each CRT/monitor design; typical values are between 2 and 3. To provide a linear response, some systems (or monitors, as this could also be done in the video amp design) provide "gamma correction." If done in the display logic, as in your case, this usually consists of an additional set of look-up tables. You can generate the correction easily enough yourself, by playing with the equation above and re-writing these tables. Starting with a gamma of about 2.5 is probably as good as any, then move up or down by 0.1 or so and see how you like the results. It's reasonably safe to assume that the video amplfiers themselves are linear, so the Vg is the above equation corresponds directly with the signal coming out of your DACs. Bob Myers | "One man's "magic" is another man's engineering. myers%hpfcla@hplabs. | "Supernatural" is a null word." hp.com | - Lazarus Long/Robert A. Heinlein