dsi@unccvax.UUCP (Dataspan Inc) (07/30/85)
> 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?
larry@kitty.UUCP (Larry Lippman) (08/03/85)
> > 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.' Not only was 25 Hz the first U.S. standard power frequency, but it is still used in many parts of the country, especially the eastern states, and in particular New York State! All of the first major industrial plants had 25 Hz. As 60 Hz (there was also some 50 Hz here) became common in the 1920's, the 25 Hz frequency remained for heavy industrial loads. A *great* deal of industrial construction occured between 1910 and 1930, all of which used 25 Hz transformers and switchgear. While almost no new 25 Hz installations occured after 1930, the major industrial plants already in place expanded and perpetuated the use of 25 Hz. Believe it or not, there are *many* industrial 25 Hz motors still in service; some of these motors are over 60 years old and still running. I am talking about LARGE motors, like 100 hp to 5,000 hp whose replacement cost with 60 Hz counterparts would be enormous. As a result of pressure from large industrial customers, 25 Hz is still distributed in many areas. I personally know of several chemical plants in Niagara Falls, NY [ home of the Love Canal :-) ] which get 128 kV 25 Hz primary service. I have seen installations where live front (i.e., knife switches) power switchboards handling 550 volts 25 Hz still exist today. Since they are 'grandfathered', the only concession to OSHA is a guard rail. At one plant where we do consulting work, they have a 4160 volt 2 MW 60 Hz primary service. In addition, they have a 200 amp 220 volt 25 Hz service (which is absurdly puny by comparison!) just to run some freight elevator motors. The plant was built in the 1920's, and while they have converted to 60 Hz for everything else, they have kept the freight elevators 25 Hz since a conversion cost would require total rebuilding of the elevators at a cost of well over $ 100K. As long as the power company continues to offer the 25 Hz service, they have no incentive to convert. At this time, the power companies are obligated by tariff to supply all existing 25 Hz services. Believe it or not, the power company offered them a $ 10K credit if they would discontinue the 25 Hz service, but the company refuses. I guess I can't blame 'em. Larry Lippman Recognition Research Corp. Clarence, New York UUCP {decvax,dual,rocksanne,rocksvax,watmath}!sunybcs!kitty!larry {rice,shell}!baylor!kitty!larry syr!buf!kitty!larry VOICE 716/741-9185 TELEX {via WUI} 69-71461 answerback: ELGECOMCLR "Have you hugged your cat today?"