240 volts

On Tue, 04 Mar 2008 14:13:02 -0500 Michael A. Terrell
| snipped-for-privacy@ipal.net wrote: |> |> On Mon, 03 Mar 2008 23:48:04 -0500 Michael A. Terrell
|> | |> | Obviously you are. Tell me something, Phil. Do you ever watch the |> | news and see either weather satellite photos, or NASA's live video feeds |> | from space? |> |> Of course I am ... I started the thread. |> |> Why don't you ask that question in a thread you start. I don't see how |> that even connects to this thread. | | | Of course you don't, but that great video is received with the | equipment you dammed in another message. The designs you called
Maybe if there was another design, you would have received the messages even more reliably. Of course, if the design that is chosen works, there would be no incentive to improve on it. Why spend money doing a whole new better design if the poor design now in use accomplishes the goals anyway.
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snipped-for-privacy@ipal.net wrote:

You still don't get it. The design was chosen because it did work better than anything else on the market. Microdyne was THE goto 'engineer to order' company for reliable telemetry equipment. They didn't dicker on the price. All they asked were two simple questions. 'Can you do THIS'?, and 'When can we have it'. There were constant design improvements as newer components and technologies became available, and as older ones became obsolete. You have absolutely no idea what is involved in designing and building modular equipment that is intended for 24/7 use for decades, with little or no downtime. Some contracts were to take early production, 15 year old equipment and update it to the latest rev. Others were to add new capabilities to existing equipment, to save taxpayer money. In some cases it was pulling the usable modules out of an old chassis and starting over in a new one.
Some products were in production for almost 20 years, because they were so reliable. On unit was in constant use for over 30 years by NASA with ZERO maintenance. That was almost 10 years ago, and it may still be working. The cost of that item to the tax payer was down under $2 per day.
Just because you don't like the design methods does not make the design bad. NASA, NOAA and other government agencies have poured over the designs and suggested no changes. They bought millions of dollars worth of the equipment because THEY liked the conservative designs, and the long life they got for their money. Microdyne was started by two engineers and a salesman who quit Defense Electronics to market a better design. They put their former bosses out of business with that new product. They built a multi-million dollar company around that, and had hundreds of employees.
How many people work for you? What equipment have you designed that is in use in space? ZERO? That's what EXACTLY I thought.
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On Tue, 04 Mar 2008 17:07:08 -0500 Michael A. Terrell
| snipped-for-privacy@ipal.net wrote: |> |> On Tue, 04 Mar 2008 14:13:02 -0500 Michael A. Terrell
|> |> |> |> On Mon, 03 Mar 2008 23:48:04 -0500 Michael A. Terrell
|> |> | |> |> | Obviously you are. Tell me something, Phil. Do you ever watch the |> |> | news and see either weather satellite photos, or NASA's live video feeds |> |> | from space? |> |> |> |> Of course I am ... I started the thread. |> |> |> |> Why don't you ask that question in a thread you start. I don't see how |> |> that even connects to this thread. |> | |> | |> | Of course you don't, but that great video is received with the |> | equipment you dammed in another message. The designs you called |> |> Maybe if there was another design, you would have received the messages |> even more reliably. Of course, if the design that is chosen works, |> there would be no incentive to improve on it. Why spend money doing |> a whole new better design if the poor design now in use accomplishes |> the goals anyway. | | | You still don't get it. The design was chosen because it did work | better than anything else on the market. Microdyne was THE goto | 'engineer to order' company for reliable telemetry equipment. They | didn't dicker on the price. All they asked were two simple questions. | 'Can you do THIS'?, and 'When can we have it'. There were constant | design improvements as newer components and technologies became | available, and as older ones became obsolete. You have absolutely no | idea what is involved in designing and building modular equipment that | is intended for 24/7 use for decades, with little or no downtime. Some | contracts were to take early production, 15 year old equipment and | update it to the latest rev. Others were to add new capabilities to | existing equipment, to save taxpayer money. In some cases it was pulling | the usable modules out of an old chassis and starting over in a new one. | | Some products were in production for almost 20 years, because they | were so reliable. On unit was in constant use for over 30 years by NASA | with ZERO maintenance. That was almost 10 years ago, and it may still | be working. The cost of that item to the tax payer was down under $2 | per day. | | Just because you don't like the design methods does not make the | design bad. NASA, NOAA and other government agencies have poured over | the designs and suggested no changes. They bought millions of dollars | worth of the equipment because THEY liked the conservative designs, and | the long life they got for their money. Microdyne was started by two | engineers and a salesman who quit Defense Electronics to market a better | design. They put their former bosses out of business with that new | product. They built a multi-million dollar company around that, and had | hundreds of employees. | | | How many people work for you? What equipment have you designed that | is in use in space? ZERO? That's what EXACTLY I thought.
YOU designed this equipment? I think not. Well, maybe one day you might have been able to. But you clearly do not understand enough about math to accomplish that today. You couldn't even come close on figuring out the phase shift over time of 2 very close frequencies (2 Hz apart during the time of 1 TV scan line in the NTSC system).
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snipped-for-privacy@ipal.net wrote:

A project that size is designed by a team, or it would never make it to market in time. Of course, no one expects you to know this simple fact of engineering. No one cares about you, or your anal retentive crap.
As far as the chroma phase shift, I suppose you know EVERYTHING possible about Quartz crystals? If you did, you would know that they never lock, exactly. The further apart thery are, the more phase noise and chirp occurs. I'll bet you're going to go into great detail about them, but I've seen and used some you'll likely never see, like the 125 MHz FUNDAMENTAL CUT crystals we used in some of our tuner modules. They were in gold based TO-5 cans, as well.
The burst is only availible for seven cycles per horizontal line. The early color burst circuts had to be set up by disabling the burst input, and trimming the oscillator as close as possible to 3.579545 MHz. When the burst was turned back on, you had to fine tune it, to center the tint range to the center of the control's range. If this wasn't followed, the color could gothrough a full 360 degree change in one line, as the set drifted.
Why don't you do the universe a favor by calculating exactly how long it will take your tiny brain to exit the back of your vacuum pack head, when you finally pull the trigger on that .357 magnum if it placed exactly four inches from your unibrow?
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On Tue, 04 Mar 2008 23:37:34 -0500 Michael A. Terrell
| snipped-for-privacy@ipal.net wrote: |> |> YOU designed this equipment? I think not. Well, maybe one day you might |> have been able to. But you clearly do not understand enough about math to |> accomplish that today. You couldn't even come close on figuring out the |> phase shift over time of 2 very close frequencies (2 Hz apart during the |> time of 1 TV scan line in the NTSC system). | | | A project that size is designed by a team, or it would never make it | to market in time. Of course, no one expects you to know this simple | fact of engineering. No one cares about you, or your anal retentive | crap.
I know _you_ don't care. But why is it _you_ need to keep making that point?
| As far as the chroma phase shift, I suppose you know EVERYTHING | possible about Quartz crystals? If you did, you would know that they | never lock, exactly. The further apart thery are, the more phase noise | and chirp occurs. I'll bet you're going to go into great detail about | them, but I've seen and used some you'll likely never see, like the 125 | MHz FUNDAMENTAL CUT crystals we used in some of our tuner modules. They | were in gold based TO-5 cans, as well.
You seem to have some problem understanding locked vs. not locked.
If it is locked, it will have the same number of cycles as the source in the long term. In that case, there is no accumulated phase shift. The phase shift may jitter or wobble around. But it will go back the other way just as much over that long term.
If it is NOT locked, then over the long term, there will be fewer or more cycles, and the phase shift will accumulate.
If the local oscillator is NOT locked, your color will be distorted or constantly changing. For example, if it gains or loses exactly one cycle every field, you'll see the rainbow effect go completely full circle from top to bottom of the screen. If it gains or loses that one cycle every line (or more radical difference), that color will vary full cycle from left to right. If it gains or loses exactly one cycle every 10 seconds (rather close, but still not in sync), you'll see the color cycle around slowly over those 10 seconds back to the same as it was 10 seconds ago.
You better have it locked, or your color will suck.
If your crystal oscillator can't lock, then your color will suck.
If you have an oscillator that shifts in phase by N degrees leading then shifts back to N degrees lagging, and back again, over some period of time, but never accumulates or loses any cycles over the long term, then it *IS* locked. It may be locked poorly, but it is locked. The quality can be measured by how far the phase shift goes and the modulation of the phase shift.
| The burst is only availible for seven cycles per horizontal line. | The early color burst circuts had to be set up by disabling the burst | input, and trimming the oscillator as close as possible to 3.579545 | MHz. When the burst was turned back on, you had to fine tune it, to | center the tint range to the center of the control's range. If this | wasn't followed, the color could gothrough a full 360 degree change in | one line, as the set drifted.
That is a case where it loses or gains a full cycle. That is NOT locked, at least not to the proper subcarrier. If it happens like this over the course of just one line, as you say, then it is locked TO A SIDEBAND of the subcarrier, plus or minus the horizontal frequency from it. That would be about 3563811 Hz or 3595280 Hz for NTSC. The crystal would have to be quite a ways off frequency (at least half the horizontal frequency from the subcarrier) to get locked out there. A good RC circuit could do better than that. The reason to use a crystal is so that there is very little phase noise across the line. Ideally, it will be very close in phase when it gets to the next burst, preferrably within a degree or two (any more than that and you'd see the phase drift as color shift). Then the next burst can pull it back to being in phase, again.
How many degrees of phase drift do you think is OK?
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snipped-for-privacy@ipal.net wrote:

You still haven't told us WHICH burst circuit you are using. There are a lot of variables, and no single answer to your vague question.
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On Wed, 05 Mar 2008 02:18:39 -0500 Michael A. Terrell
| snipped-for-privacy@ipal.net wrote: |> |> How many degrees of phase drift do you think is OK? | | You still haven't told us WHICH burst circuit you are using. There | are a lot of variables, and no single answer to your vague question.
I'm not talking about a specific circuit.
You can talk about any circuit you wish. Just be specific about whether it will produce exactly the same number of cycles out to match the source it is supposed to be locked to, or if it will produce some finite number of additional or fewer cycles over some specified period of time (such as a video line time, or video field time), or if it will produce and output that cannot be characterized as having some known relationship in number of cycles relative to the locking source.
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snipped-for-privacy@ipal.net wrote:

In other words, you have no clue how rthe different circuits work. That's no surprise.

You started this crap, so it is YOUR problem to find a circuit and analyze it. Do all the math you want, but you won't get an answer without a prototype.
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On Wed, 05 Mar 2008 12:59:13 -0500 Michael A. Terrell
| snipped-for-privacy@ipal.net wrote: |> |> On Wed, 05 Mar 2008 02:18:39 -0500 Michael A. Terrell
|>
|> |> |> |> How many degrees of phase drift do you think is OK? |> | |> | You still haven't told us WHICH burst circuit you are using. There |> | are a lot of variables, and no single answer to your vague question. |> |> I'm not talking about a specific circuit. | | | In other words, you have no clue how rthe different circuits work. | That's no surprise.
If that is your conclusion, you have no clue of logic.
|> You can talk about any circuit you wish. | | | You started this crap, so it is YOUR problem to find a circuit and | analyze it. Do all the math you want, but you won't get an answer | without a prototype.
I clearly didn't need any "answer" you offered.
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snipped-for-privacy@ipal.net wrote:

Right. that's why you keep spouting your inanae crap. You have no clue as to what works in the real world.
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snipped-for-privacy@ipal.net writes:

Has it ever occurred to you that people may avoid pointing out errors in what you say because of your combative debating style, not because they agree with what you said? I expect to be attacked just for saying anything that disagrees with you, no matter how correct or well-supported what I write is.
For the record, here is something I wrote (a long time ago) to explain where the magic numbers in NTSC come from. The ratio 63/88 does not appear anywhere in the original standard that I could see. There are a number of other ratios that do appear, and a particular product of them can be reduced to 63/88. So that value is theoretically exact - but knowing it doesn't tell you anything about where it came from. The note below does.
Dave
-----------------------------------------------------------------------------
This is based mostly on the NTSC committee's own report, with a little bit of guessing on my part (in the section about prime factors of divisors).
Original B&W standard:
* 60 Hz vertical frequency, so "hum bars" from poor power supply rejection are stationary on screen
* Horizontal frequency is 525*60 Hz. Odd number gives interlaced image, to give off better vertical spatial resolution in a fixed bandwidth
* Channel spacing is 6 MHz, with 4.5 MHz offset between sound and video carrier
* Video is transmitted vestigial sideband, with 4.2 MHz video bandwidth.
The new color standard needed to be compatible with existing B&W receivers:
* Colour information would be encoded on subcarrier; subcarrier would be visible on B&W receivers in areas of saturated color.
* To minimize visibility of subcarrier, lock subcarrier to H sync so any resulting pattern is stationary, not moving.
* Use odd multiple of half line frequency, so subcarrier forms a fine "checkerboard" instead of lines - less visible.
* The higher the subcarrier frequency, the less visible on B&W sets, but the less bandwidth available for carrying color information. Tests showed the best compromise frequency to be around 3.6 MHz.
* The two constraints above mean that subcarrier should be approximately 457/2 times horizontal frequency. But 457 is a prime number, and dividing by 457 is hard - there are no cheap digital dividers available in 1950.
* Looking at nearby odd numbers, 453 = 3*151, 455 = 5*7*13, 459 = 3*3*3*17, and 461 is prime. 455 is the easiest divisor to generate - all its prime factors are 13 or less. So subcarrier is set to Fh * 455/2.
* So, at this point, the magic numbers are: Fv = 60 Fh = 60 * 525/2 = 15750 Fsc = Fh * 455/2 = 3583125
But there's a problem: to minimize visibility of any beat frequency between color subcarrier and sound carrier, it is desirable to have the difference between the two be an odd multiple of half the line frequency.
* With numbers above, offset is 916875 Hz. 916875/Fh = 58.21 = 116.4/2. So nearest odd multiple of Fh/2 is 117/2.
* Thus new sound carrier offset should be Fh*(455 + 117)/2 = 4504500 Hz. This is (exactly) 1001/1000 times the old sound offset.
* But (in those days) TV sound used a separate FM transmitter and possibly a separate antenna; changing sound offset means retuning the sound transmitter.
* To avoid this, the NTSC moved all the *video* frequencies down by a factor of 1000/1001 instead, giving the desired relationship between subcarrier and sound carrier.
* So subcarrier becomes 3583125 * 1000/1001 = 3579545.4545 (rounded to 3579545 in the original standard).
* New frequencies (without intermediate rounding) Fsc = 3579545.4545 Fh = Fsc * 2/455 = 15734.266 Fv = Fh * 2/525 = 59.94
* The tolerance on these is 3 PPM, so the range of permitted values is entirely within the looser tolerances of the old B&W frequencies, so B&W TVs should continue to work at the new frequencies (though hum bars will now roll slowly).
* If you happen to have a precise 5 MHz frequency standard, to derive Fsc from it your need a multiplier of
(60 * 525/2 * 455/2 * 1000/1001) / 5000000 = 63/88 (exactly)
So the numbers 63 and 88 never appear in the NTSC standard; they are just the rational number defined by all those *other* numbers above, reduced to simplest form.
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Dave Martindale wrote:

<snipped an excellent review>
I was a junior engineer at Bell Labs at about the time all this was happening, but not directly involved with TV. Thanks for the insight!
Aren't we glad that the CBS method of using a whirling "color wheel", didn't become the standard. I actually built a working model in 1949. The DLP TV sets have gone back to this way of getting color!
It would be interesting to attempt to do your analysis on PAL or SECAM, (or even the new digital standards).
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Thanks.
I'd be interested as well, but it would probably have to be done by someone with better access to PAL or SECAM documentation. I just happen to have a copy of the NTSC committee's original report, but nothing similar for the other standards.
What I do know:
PAL started from a monochrome system with different frame and field rates, wider channel spacing, and thus more video bandwidth. This allowed them to use a higher subcarrier frequency, reducing its visibility. Despite that, there was room in the channel to transmit both colour components double sideband, so they could encode (R-Y) and (B-Y) directly instead of having to generate I and Q.
But they did a couple of weird things. They specified different RGB colour primaries (which are probably more realistic than the NTSC ones). But instead of recalculating the RGB to YUV transform matrix for these new primaries, they re-used exactly the same matrix as NTSC. And the relationship between subcarrier and H sync is more complex, with a 90 degree phase shift per frame that I don't understand at all.
    Dave
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snipped-for-privacy@cs.ubc.ca (Dave Martindale) writes:

[snip]
Fascinating. That also explains the mystery why the vertical synch was defined to be 59.94 Hz when it "should have been" 60 Hz to avoid power line interference effects.
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On Wed, 5 Mar 2008 22:01:09 +0000 (UTC) Michael Moroney
| snipped-for-privacy@cs.ubc.ca (Dave Martindale) writes: | |>For the record, here is something I wrote (a long time ago) to explain where |>the magic numbers in NTSC come from. The ratio 63/88 does not appear |>anywhere in the original standard that I could see. There are a number of |>other ratios that do appear, and a particular product of them can be reduced |>to 63/88. So that value is theoretically exact - but knowing it doesn't tell |>you anything about where it came from. The note below does. | | [snip] | | Fascinating. That also explains the mystery why the vertical synch was | defined to be 59.94 Hz when it "should have been" 60 Hz to avoid power | line interference effects.
And it's a good thing they did design it the way they did. Otherwise the backronym for NTSC would have become (N)o (T)elevision (S)ince (C)olor.
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snipped-for-privacy@world.std.spaamtrap.com (Michael Moroney) writes:

It actually was 60 Hz in B&W days.
In retrospect, it seems like changing all of the video frequencies to avoid touching sound was a bad decision. If they'd just left sound alone, it wouldn't be quite an odd multiple of half the line frequency, but it's FM anyway - it's not a fixed frequency. Or they could have move the sound carrier up a bit - a 4500 Hz shift in sound frequency in a system with 25 kHz deviation should not have screwed up sound receiption in the old B&W TV sets.
Either way, we could have had exactly 60 Hz frame rate, and drop-frame time code would never have been invented, and the slow time error accumulation of even drop-frame code would not worry anyone. And 24 FPS films could be run at exactly 24 FPS instead of 23.98. Many potential headaches would disappear.
On the other hand, ironically, the multiplier to generate subcarrier from a 5 MHz reference source would become 5733/8000 instead of the value of 63/88 we have now. By pure blind luck, the 1000/1001 multiplier used to shift the video frequencies just happens to cancel most of the prime factors of 455:
    455/1001 = (5*7*13)/(7*11*13) = 5/11
and that's mostly why you get the simpler 63/88.
    Dave
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| snipped-for-privacy@world.std.spaamtrap.com (Michael Moroney) writes: | |>Fascinating. That also explains the mystery why the vertical synch was |>defined to be 59.94 Hz when it "should have been" 60 Hz to avoid power |>line interference effects. | | It actually was 60 Hz in B&W days. | | In retrospect, it seems like changing all of the video frequencies to | avoid touching sound was a bad decision. If they'd just left sound | alone, it wouldn't be quite an odd multiple of half the line frequency, | but it's FM anyway - it's not a fixed frequency. Or they could have | move the sound carrier up a bit - a 4500 Hz shift in sound frequency | in a system with 25 kHz deviation should not have screwed up sound | receiption in the old B&W TV sets. | | Either way, we could have had exactly 60 Hz frame rate, and drop-frame | time code would never have been invented, and the slow time error | accumulation of even drop-frame code would not worry anyone. And 24 FPS | films could be run at exactly 24 FPS instead of 23.98. Many potential | headaches would disappear.
ATSC has real 60 Hz (and 30 Hz and 24 Hz) as optional modes, in addition to those that are 1000/1001 lower.
| On the other hand, ironically, the multiplier to generate subcarrier | from a 5 MHz reference source would become 5733/8000 instead of the | value of 63/88 we have now. By pure blind luck, the 1000/1001 | multiplier used to shift the video frequencies just happens to cancel | most of the prime factors of 455: | | 455/1001 = (5*7*13)/(7*11*13) = 5/11 | | and that's mostly why you get the simpler 63/88.
A fixed number of audio samples per field would be nice, too. That is not the case with 59.94 Hz and either 44100 Hz or 48000 Hz. With 44100 Hz, we get exactly 735.735 audio samples per field (see how 1001 affects that). With 48000 Hz we get exactly 800.8 audio samples per field (see how 1001 also affects that, too. If we had exactly 60 Hz, we would have exactly 735 or 800 samples per field. OTOH, exactly 120000 Hz as an audio sample rate will give a whole number of audio samples per field for all field rates (5005 at 23.976 fps, 5000 at 24 fps, 4800 at 25 fps, 4004 at 29.97 fps, 4000 at 30 fps, 2400 at 50 fps, 2002 at 59.94 fps, 2000 at 60 fps, as well as 1001 at 119.88 fps and 1000 at 120 fps for those shooting video in those special high frame rate cameras).
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Dave Martindale wrote:

Headaches? No. Minor problems, at best. You have to realize how primitive electronics was when NTSC was developed. Or they could have just kept TV monochrome, until digital technology was perfected. The development of NTSC video mandated compatibility with existing monochrome TV sets, and introducing a sync buzz wasn't acceptable. There was also a problem with chroma noise crawling up the screen if they kept the original timing.

Considering that frequency division was done with vacuum tube multi-vibrators, they had to keep the ratios as simple as possible. We are talking tubes like the 6SN7, octal based tubes. There were no fancy phase locked loop synthesizers back then.

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On Thu, 06 Mar 2008 21:42:34 -0500 Michael A. Terrell
| Considering that frequency division was done with vacuum tube | multi-vibrators, they had to keep the ratios as simple as possible. We | are talking tubes like the 6SN7, octal based tubes. There were no fancy | phase locked loop synthesizers back then.
OTOH, 63/88 would not have been hard to do, given the range of frequencies.
I don't know if they knew of this method, or not. But they most certainly did know the pre-requisites, which were to generate harmonics, filter a narrow band, do phase comparison, and work at UHF frequencies. What could be done is take the 63rd harmonic of 5 MHz (or 21st harmonic of 15 Mhz if that would be easier), and take the 88th harmonic of 3.579545 MHz (or the 22nd harmonic of 14.318182 MHz if they wanted that), bandpass filter them at 315 MHz, and do the phase comparison there. Then the phase error would be used to tweak the derived frequency oscillator.
An actual circuit design using tubes of the era is left as an exercise for the readers that really care.
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| snipped-for-privacy@ipal.net writes: | |>There's no ignorance in what I say. If there was, anyone who wanted to |>point it out would have been specific and said exactly what was wrong. |>No one did. | | Has it ever occurred to you that people may avoid pointing out errors in what | you say because of your combative debating style, not because they agree with | what you said? I expect to be attacked just for saying anything that | disagrees with you, no matter how correct or well-supported what I write is.
Are you making that determination about debating style based on how I answer posts that fail to point out specific errors? You cannot conclude what the style would be for specific topical challenges when what you see is how I respond to personal attacks from people who have a history of doing so.
| For the record, here is something I wrote (a long time ago) to explain where | the magic numbers in NTSC come from. The ratio 63/88 does not appear | anywhere in the original standard that I could see. There are a number of | other ratios that do appear, and a particular product of them can be reduced | to 63/88. So that value is theoretically exact - but knowing it doesn't tell | you anything about where it came from. The note below does.
I got that expression from the FCC rules. I came up with something else when I worked backwards from the original number. That something else is equivalent to 5000000*63/88.
| ----------------------------------------------------------------------------- | | This is based mostly on the NTSC committee's own report, with a little bit of | guessing on my part (in the section about prime factors of divisors). | | Original B&W standard: | | * 60 Hz vertical frequency, so "hum bars" from poor power supply rejection | are stationary on screen | | * Horizontal frequency is 525*60 Hz. Odd number gives interlaced image, to | give off better vertical spatial resolution in a fixed bandwidth | | * Channel spacing is 6 MHz, with 4.5 MHz offset between sound and video | carrier | | * Video is transmitted vestigial sideband, with 4.2 MHz video bandwidth. | | The new color standard needed to be compatible with existing B&W receivers: | | * Colour information would be encoded on subcarrier; subcarrier would be | visible on B&W receivers in areas of saturated color. | | * To minimize visibility of subcarrier, lock subcarrier to H sync so any | resulting pattern is stationary, not moving. | | * Use odd multiple of half line frequency, so subcarrier forms a fine | "checkerboard" instead of lines - less visible. | | * The higher the subcarrier frequency, the less visible on B&W sets, but the | less bandwidth available for carrying color information. Tests showed the | best compromise frequency to be around 3.6 MHz. | | * The two constraints above mean that subcarrier should be approximately | 457/2 times horizontal frequency. But 457 is a prime number, and dividing | by 457 is hard - there are no cheap digital dividers available in 1950. | | * Looking at nearby odd numbers, 453 = 3*151, 455 = 5*7*13, 459 = 3*3*3*17, | and 461 is prime. 455 is the easiest divisor to generate - all its | prime factors are 13 or less. So subcarrier is set to Fh * 455/2. | | * So, at this point, the magic numbers are: | Fv = 60 | Fh = 60 * 525/2 = 15750 | Fsc = Fh * 455/2 = 3583125 | | | But there's a problem: to minimize visibility of any beat frequency between | color subcarrier and sound carrier, it is desirable to have the difference | between the two be an odd multiple of half the line frequency. | | * With numbers above, offset is 916875 Hz. 916875/Fh = 58.21 = 116.4/2. | So nearest odd multiple of Fh/2 is 117/2. | | * Thus new sound carrier offset should be Fh*(455 + 117)/2 = 4504500 Hz. | This is (exactly) 1001/1000 times the old sound offset. | | * But (in those days) TV sound used a separate FM transmitter and possibly | a separate antenna; changing sound offset means retuning the sound | transmitter. | | * To avoid this, the NTSC moved all the *video* frequencies down by a factor | of 1000/1001 instead, giving the desired relationship between subcarrier | and sound carrier. | | * So subcarrier becomes 3583125 * 1000/1001 = 3579545.4545 (rounded to | 3579545 in the original standard). | | * New frequencies (without intermediate rounding) | Fsc = 3579545.4545 | Fh = Fsc * 2/455 = 15734.266 | Fv = Fh * 2/525 = 59.94 | | * The tolerance on these is 3 PPM, so the range of permitted values is | entirely within the looser tolerances of the old B&W frequencies, so | B&W TVs should continue to work at the new frequencies (though hum bars | will now roll slowly). | | * If you happen to have a precise 5 MHz frequency standard, to derive Fsc | from it your need a multiplier of | | (60 * 525/2 * 455/2 * 1000/1001) / 5000000 = 63/88 (exactly) | | So the numbers 63 and 88 never appear in the NTSC standard; they are just the | rational number defined by all those *other* numbers above, reduced to | simplest form.
Excellent summary. I had read most of that in various places before. But this sums it up succinctly. Thanks.
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|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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