|
[email protected] 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.