NTSC Question / 6845 CRTC Registers
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Posted by FyberOptic on July 21, 2007, 9:49 pm
Please log in for more thread options only for winter to roll around and put a damper on my soldering (couldn't open the windows), so I pretty much fell out of the electronics mood for quite a while. But that urge to build has returned, and I'm back with idears on things I want to try and get back to making my own little computer! I'm still really interested in being able to generate my own NTSC signal, and having poured over more documents on the subject again this summer, I think I'm pretty close to another attempt. Particularly since I've ordered a new microcontroller to try with, since my PIC I was attempting it on died last summer. But this time it's a 8052 variant, which I think I've fallen in love with due to the fact it can have both an externally accessible address/data bus as well as still have the all-in-one charm of a PIC. This one is also capable of one machine cycle per clock, as long as you stick to using the internal flash/sram. Anyhoo, I'd just like to try this signal generation manually first, in black and white, pumping out the sync and levels myself. Only downside is the amount of misinformation on the internet. It would seem that some say the sync pulse is about -0.4v, blanking around 0v, black around 0.3v, and white at 1v. Other people say sync can be 0v, blanking/black 0.3v, and white 1v. I've also seen sync at 0, blanking 0.3, black 0.4, white 1, etc etc. I mean, does it really matter that much? Could I get by with just using 0, 0.3, and 1? I really don't want to have to try and start getting a negative voltage if avoidable, since to be honest, I'm not entirely sure how I'd do it at the moment. Televisions seem like they're fairly lenient, and with all the methods out there I've seen, I would assume most/all of them work, albeit possibly with less quality in some respect. I also found some clashing information on sending vertical sync, with some saying it's okay to just send repeating inverted sync pulses the whole time (and for the whole line, instead of doubles) to get back to the top instead of surrounding it with equalizing pulses. Aside from doing it the raw way, I also want to try using a 6845 I ordered last year to generate video. I think I've wrapped my head around how it works fairly well, but after looking at CGA register values, I found myself confused. It seems that if you're going to run in either 320x200 or 640x200 graphics modes, you don't set the number of vertically displayed rows to 200 as one might think, but 100 instead. I don't understand this, or how the screen would then be displayed properly, since it seemingly is only doing half the screen. As far as I know, CGA doesn't ever interlace the actual screen, despite the graphics memory being laid out interlaced (which I'm sure is related to this somehow). If anyone can shed any light on this for me, I'd sure appreciate it! | |||||||||||||
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Posted by John McGaw on July 22, 2007, 10:05 am
Please log in for more thread options Well, as someone who worked in television broadcasting/CCTV of various sorts for decades I can confirm that "NTSC levels" are sometimes ambiguous, especially when one gets away from real studio equipment and into home/amateur types. Computers bring a new level of confusion to levels. Basically, if it isn't 1V p-p and the video/sync ratio isn't 100/40 then in doesn't stand a chance of being "real" NTSC. This page shows pretty much everything you might want to know about NTSC levels. It is from Tektronix and you can take it as gospel: http://www.tek.com/Measurement/cgi-bin/framed.pl?Document=/Measurement/App_Notes/Component_Puzzle/standards.html&FrameSet=television That URL will have broken badly but I'll leave it for you to put back together. Or just go to google and search for "NTSC levels" and choose the tek link. I haven't designed with an MC6845 for a _very_ long time (before 1980 for sure)and can't contribute anything useful after all this time. -- John McGaw [Knoxville, TN, USA] http://johnmcgaw.com | |||||||||||||
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Posted by Paul on July 22, 2007, 11:25 am
Please log in for more thread options John McGaw wrote:
> FyberOptic wrote:
>> Hiya folks. I got all interested in project building last summer,
>> only for winter to roll around and put a damper on my soldering >> (couldn't open the windows), so I pretty much fell out of the >> electronics mood for quite a while. But that urge to build has >> returned, and I'm back with idears on things I want to try and get >> back to making my own little computer! >> >> I'm still really interested in being able to generate my own NTSC >> signal, and having poured over more documents on the subject again >> this summer, I think I'm pretty close to another attempt. >> Particularly since I've ordered a new microcontroller to try with, >> since my PIC I was attempting it on died last summer. But this time >> it's a 8052 variant, which I think I've fallen in love with due to the >> fact it can have both an externally accessible address/data bus as >> well as still have the all-in-one charm of a PIC. This one is also >> capable of one machine cycle per clock, as long as you stick to using >> the internal flash/sram. >> >> Anyhoo, I'd just like to try this signal generation manually first, in >> black and white, pumping out the sync and levels myself. Only >> downside is the amount of misinformation on the internet. It would >> seem that some say the sync pulse is about -0.4v, blanking around 0v, >> black around 0.3v, and white at 1v. Other people say sync can be 0v, >> blanking/black 0.3v, and white 1v. I've also seen sync at 0, blanking >> 0.3, black 0.4, white 1, etc etc. I mean, does it really matter that >> much? Could I get by with just using 0, 0.3, and 1? I really don't >> want to have to try and start getting a negative voltage if avoidable, >> since to be honest, I'm not entirely sure how I'd do it at the >> moment. Televisions seem like they're fairly lenient, and with all >> the methods out there I've seen, I would assume most/all of them work, >> albeit possibly with less quality in some respect. I also found some >> clashing information on sending vertical sync, with some saying it's >> okay to just send repeating inverted sync pulses the whole time (and >> for the whole line, instead of doubles) to get back to the top instead >> of surrounding it with equalizing pulses. >> >> Aside from doing it the raw way, I also want to try using a 6845 I >> ordered last year to generate video. I think I've wrapped my head >> around how it works fairly well, but after looking at CGA register >> values, I found myself confused. It seems that if you're going to run >> in either 320x200 or 640x200 graphics modes, you don't set the number >> of vertically displayed rows to 200 as one might think, but 100 >> instead. I don't understand this, or how the screen would then be >> displayed properly, since it seemingly is only doing half the screen. >> As far as I know, CGA doesn't ever interlace the actual screen, >> despite the graphics memory being laid out interlaced (which I'm sure >> is related to this somehow). If anyone can shed any light on this for >> me, I'd sure appreciate it! >> >
App_Notes/Component_Puzzle/standards.html&FrameSet=television
> Well, as someone who worked in television broadcasting/CCTV of various > sorts for decades I can confirm that "NTSC levels" are sometimes > ambiguous, especially when one gets away from real studio equipment and > into home/amateur types. Computers bring a new level of confusion to > levels. Basically, if it isn't 1V p-p and the video/sync ratio isn't > 100/40 then in doesn't stand a chance of being "real" NTSC. > > This page shows pretty much everything you might want to know about NTSC > levels. It is from Tektronix and you can take it as gospel: > > http://www.tek.com/Measurement/cgi-bin/framed.pl?Document=/Measurement/ >
> > That URL will have broken badly but I'll leave it for you to put back > together. Or just go to google and search for "NTSC levels" and choose > the tek link. > > I haven't designed with an MC6845 for a _very_ long time (before 1980 > for sure)and can't contribute anything useful after all this time. > This article tells you how to use an emitter follower configured transistor, to convert logic level signals, into something suitable for driving a 75 ohm coax at video levels. As near as I can tell, this circuit cannot swing below ground, and probably relies on "DC restoration" in the TV, to get the levels where they should be. Note the simple resistor summing scheme, to combine RGBS to make a single signal. That summing scheme is how you could combine a composite sync or separate sync signals, with your video signal. Note that the signals need to have the correct polarity, for things to sum the correct way. (Sometimes, you need a CMOS inverter, to invert the signal, and also give it a rail to rail logic swing.) http://www.circuitcellar.com/library/print/1006/Geraets195/3.htm I built something like you are trying to do, back in the early 80's, with one of these. SMC CRT5037 family. This thing is so old, it is a dual rail chip. They might have used dual rails, to give 12V to run the high speed circuitry, while using 5V to generate standard CMOS logic levels on their output pins. There was a time, when CMOS could be run from high voltages, like 15 volts. CRT5037 datasheet - looking just as bad as the paper xerox copy I got :-) http://www.tranzistoare.ro/datasheets2/59/590424_1.pdf Paul | |||||||||||||
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Posted by Paul on July 26, 2007, 2:21 am
Please log in for more thread options Paul wrote:
> John McGaw wrote:
>> FyberOptic wrote:
>>> Hiya folks. I got all interested in project building last summer,
>>> only for winter to roll around and put a damper on my soldering >>> (couldn't open the windows), so I pretty much fell out of the >>> electronics mood for quite a while. But that urge to build has >>> returned, and I'm back with idears on things I want to try and get >>> back to making my own little computer! >>> >>> I'm still really interested in being able to generate my own NTSC >>> signal, and having poured over more documents on the subject again >>> this summer, I think I'm pretty close to another attempt. >>> Particularly since I've ordered a new microcontroller to try with, >>> since my PIC I was attempting it on died last summer. But this time >>> it's a 8052 variant, which I think I've fallen in love with due to the >>> fact it can have both an externally accessible address/data bus as >>> well as still have the all-in-one charm of a PIC. This one is also >>> capable of one machine cycle per clock, as long as you stick to using >>> the internal flash/sram. >>> >>> Anyhoo, I'd just like to try this signal generation manually first, in >>> black and white, pumping out the sync and levels myself. Only >>> downside is the amount of misinformation on the internet. It would >>> seem that some say the sync pulse is about -0.4v, blanking around 0v, >>> black around 0.3v, and white at 1v. Other people say sync can be 0v, >>> blanking/black 0.3v, and white 1v. I've also seen sync at 0, blanking >>> 0.3, black 0.4, white 1, etc etc. I mean, does it really matter that >>> much? Could I get by with just using 0, 0.3, and 1? I really don't >>> want to have to try and start getting a negative voltage if avoidable, >>> since to be honest, I'm not entirely sure how I'd do it at the >>> moment. Televisions seem like they're fairly lenient, and with all >>> the methods out there I've seen, I would assume most/all of them work, >>> albeit possibly with less quality in some respect. I also found some >>> clashing information on sending vertical sync, with some saying it's >>> okay to just send repeating inverted sync pulses the whole time (and >>> for the whole line, instead of doubles) to get back to the top instead >>> of surrounding it with equalizing pulses. >>> >>> Aside from doing it the raw way, I also want to try using a 6845 I >>> ordered last year to generate video. I think I've wrapped my head >>> around how it works fairly well, but after looking at CGA register >>> values, I found myself confused. It seems that if you're going to run >>> in either 320x200 or 640x200 graphics modes, you don't set the number >>> of vertically displayed rows to 200 as one might think, but 100 >>> instead. I don't understand this, or how the screen would then be >>> displayed properly, since it seemingly is only doing half the screen. >>> As far as I know, CGA doesn't ever interlace the actual screen, >>> despite the graphics memory being laid out interlaced (which I'm sure >>> is related to this somehow). If anyone can shed any light on this for >>> me, I'd sure appreciate it! >>> >>
>> Well, as someone who worked in television broadcasting/CCTV of various >> sorts for decades I can confirm that "NTSC levels" are sometimes >> ambiguous, especially when one gets away from real studio equipment >> and into home/amateur types. Computers bring a new level of confusion >> to levels. Basically, if it isn't 1V p-p and the video/sync ratio >> isn't 100/40 then in doesn't stand a chance of being "real" NTSC. >> >> This page shows pretty much everything you might want to know about >> NTSC levels. It is from Tektronix and you can take it as gospel: >> >> http://www.tek.com/Measurement/cgi-bin/framed.pl?Document=/Measurement/ > App_Notes/Component_Puzzle/standards.html&FrameSet=television
>>
>> >> That URL will have broken badly but I'll leave it for you to put back >> together. Or just go to google and search for "NTSC levels" and choose >> the tek link. >> >> I haven't designed with an MC6845 for a _very_ long time (before 1980 >> for sure)and can't contribute anything useful after all this time. >> >
> This article tells you how to use an emitter follower configured > transistor, to convert logic level signals, into something suitable > for driving a 75 ohm coax at video levels. As near as I can tell, this > circuit cannot swing below ground, and probably relies on "DC restoration" > in the TV, to get the levels where they should be. Note the simple resistor > summing scheme, to combine RGBS to make a single signal. That summing > scheme is how you could combine a composite sync or separate sync > signals, with your video signal. Note that the signals need to have the > correct polarity, for things to sum the correct way. (Sometimes, you > need a CMOS inverter, to invert the signal, and also give it a rail to > rail logic swing.) > > http://www.circuitcellar.com/library/print/1006/Geraets195/3.htm > > I built something like you are trying to do, back in the early 80's, > with one of these. SMC CRT5037 family. This thing is so old, it is a > dual rail chip. They might have used dual rails, to give 12V to run > the high speed circuitry, while using 5V to generate standard CMOS > logic levels on their output pins. There was a time, when CMOS could > be run from high voltages, like 15 volts. > > CRT5037 datasheet - looking just as bad as the paper xerox copy I got :-) > http://www.tranzistoare.ro/datasheets2/59/590424_1.pdf > > Paul In terms of the signal swing issue, this Conexant TV encoder chip is powered by 3.3V and GND, and has no negative power rail. To me that implies that the sync tips will be a 0 volts, black will be at +0.330V and white would be more positive than that. They don't show how to build a driver (since the chip has a driver capable of driving a doubly terminated transmission line to begin with), but at least this is an example of driving a compliant output signal without a negative going output. Pg 136 and others... Doc also makes passing reference to some standards. http://www.conexant.com/servlets/DownloadServlet/DSH-200671-001.pdf?docid=672&revid=1 Paul | |||||||||||||
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Posted by Franc Zabkar on July 29, 2007, 3:09 am
Please log in for more thread options finger to keyboard and composed:
>In terms of the signal swing issue, this Conexant TV encoder chip is
>powered by 3.3V and GND, and has no negative power rail. To me that >implies that the sync tips will be a 0 volts, black will be at +0.330V >and white would be more positive than that. They don't show how to build a >driver (since the chip has a driver capable of driving a doubly >terminated transmission line to begin with), but at least this is an >example of driving a compliant output signal without a negative going >output. AFAICS, your output doesn't need to swing below zero because the inputs of AV devices are capacitively coupled. - Franc Zabkar -- Please remove one 'i' from my address when replying by email. | |||||||||||||
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> only for winter to roll around and put a damper on my soldering
> (couldn't open the windows), so I pretty much fell out of the
> electronics mood for quite a while. But that urge to build has
> returned, and I'm back with idears on things I want to try and get
> back to making my own little computer!
>
> I'm still really interested in being able to generate my own NTSC
> signal, and having poured over more documents on the subject again
> this summer, I think I'm pretty close to another attempt.
> Particularly since I've ordered a new microcontroller to try with,
> since my PIC I was attempting it on died last summer. But this time
> it's a 8052 variant, which I think I've fallen in love with due to the
> fact it can have both an externally accessible address/data bus as
> well as still have the all-in-one charm of a PIC. This one is also
> capable of one machine cycle per clock, as long as you stick to using
> the internal flash/sram.
>
> Anyhoo, I'd just like to try this signal generation manually first, in
> black and white, pumping out the sync and levels myself. Only
> downside is the amount of misinformation on the internet. It would
> seem that some say the sync pulse is about -0.4v, blanking around 0v,
> black around 0.3v, and white at 1v. Other people say sync can be 0v,
> blanking/black 0.3v, and white 1v. I've also seen sync at 0, blanking
> 0.3, black 0.4, white 1, etc etc. I mean, does it really matter that
> much? Could I get by with just using 0, 0.3, and 1? I really don't
> want to have to try and start getting a negative voltage if avoidable,
> since to be honest, I'm not entirely sure how I'd do it at the
> moment. Televisions seem like they're fairly lenient, and with all
> the methods out there I've seen, I would assume most/all of them work,
> albeit possibly with less quality in some respect. I also found some
> clashing information on sending vertical sync, with some saying it's
> okay to just send repeating inverted sync pulses the whole time (and
> for the whole line, instead of doubles) to get back to the top instead
> of surrounding it with equalizing pulses.
>
> Aside from doing it the raw way, I also want to try using a 6845 I
> ordered last year to generate video. I think I've wrapped my head
> around how it works fairly well, but after looking at CGA register
> values, I found myself confused. It seems that if you're going to run
> in either 320x200 or 640x200 graphics modes, you don't set the number
> of vertically displayed rows to 200 as one might think, but 100
> instead. I don't understand this, or how the screen would then be
> displayed properly, since it seemingly is only doing half the screen.
> As far as I know, CGA doesn't ever interlace the actual screen,
> despite the graphics memory being laid out interlaced (which I'm sure
> is related to this somehow). If anyone can shed any light on this for
> me, I'd sure appreciate it!
>