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From: dsi@unccvax.UUCP (Dataspan Inc)
Newsgroups: net.analog
Subject: Some answers, some questions
Message-ID: <254@unccvax.UUCP>
Date: Tue, 30-Jul-85 10:47:36 EDT
Article-I.D.: unccvax.254
Posted: Tue Jul 30 10:47:36 1985
Date-Received: Fri, 2-Aug-85 00:41:19 EDT
Organization: UNC-Charlotte
Lines: 86
> Why were phonograph turntable speeds set at 33 1/3rd, 45, and 78 RPM?
Well, in the 33.3333 rpm case, the speed is a convenient
integer multiple of common AC hysterisis/synchronous unloaded speeds -
33.3333 is 3600/108 (60hz) and 3000/90 (50hz.) This holds up even
for 45 rpm in the 60hz case (is 3600/80) and at 78, the error is
only 3.3 %.( 3600/46.15). In the latter case, I don't think that
exact speed is 78.0 but rather 78.2600... (which is 3600/46)
This also makes a stroboscope handy for calibrating record
speeds, i.e. an integral number of lines and spaces are required
to get the beat pattern.
At 50 Hz, 45 and 78 rpm fall apart.
> Why were tape recorder speeds set at 3 3/4ths, 7 1/2, and 15 IPS?
Isn't the original standard 30 IPS ? Just curious . . .
> Why are the standard worldwide AC line-voltage frequencies 50 or 60 Hz?
Was there ever a 25 Hz standard in the U.S. ? I remember
old Sears catalogues mentioning that their power tools were not
'for use in Chicago where there was 25 hz current.'
> Why are standard US (NTSC) tv pictures composed of 525 lines?
A product of odd factors is required to generate an odd number,
and since synchronous counters and all that good stuff in LS-TTL hadn't
been invented yet, the minimum number of odd factors required to get
an odd number of lines was the basis for the number of lines (given
other tradeoffs). I suppose they used class 'C' amplifiers and
multiplied 'up' to twice the horizontal scanning frequency from the
line frequency, given extra-high-'Q' tanks. Scatter-sync must not
have been good enough for them, but with those horrible vertical
sync circuits early sets, who cared about interlace??
525 lines = 3 * 5 * 5 * 7
405 lines = 3 * 3 * 3 * 3 * 5
(old UK mono and US sequential
colour standard)
441 lines = 3 * 3 * 7 * 7
(old Philco standard ??)
625 lines = 5 * 5 * 5 * 5
(PAL-B/EBU standard)
ad nauseam. . .
Of course, an ODD number of lines is required to generate an
interlaced image of any kind. However, not only 2:1 interlace was
considered, but that's another story. . .
While the number of lines is the same, multiplication up from
the line frequency is obviously no longer the case. Everything is
now related to (63/88)(4/455) * 5 000 000 / 525 lines.
> Why were 50, 72, and 300 ohms selected as standard antenna matching impedances?
72 ohms is very nearly the impedance of an infinitely thin,
half wave dipole in free space (which is 73.0 + j42.5). If you can
get the antenna to resonate by shortening it slightly, it comes
out to 70.0 + j0 ohms exactly.
Folding the dipole (again, half wave) back over quadruples
the impedance, and assuming an element diameter of 0.0005 wavelength,
and spacing of .0063 wavelength, the impedance is 280 + j0 ohms
at resonance. (This is sufficiently close to '300 ohm' wire but
I am not sure why 300 and not 280 or 288 ohms was chosen as the
standard).
The 50.0 ohm case is left to the reader. . .
David Anthony
DataSpan, Inc
P.S. Have you burnt your fingers on a pair of 6L6-GT's lately?