Mallory tech book on TV in 1942

[wa2ise]

Found a book, copyright 1942, with a couple of chapters on this new thing called television. Then WWII happened. As most of us know, TV was going to be 441 lines interlaced with a frame rate of 30Hz and field rate 60Hz. Channel width 6MHz, and sound carrier 4.5MHz above the picture carrier. Vestigial lower sideband, picture carrier 1 1/4 MHz above the bottom of the channel. Just like NTSC except the number of scan lines, and the sound carrier was going to be AM, not FM. If you thought picture buzz on NTSC's FM sound was bad, AM should have sounded worse.

The FCC allocated channels TV channels from 44 to 108 MHz, with a few gaps for other services. And a tentative allocation of 12 channels up from 156 to 294MHz. Another chapter of this book, on FM radio, mentions the FM band then was 42-50MHz...

And a few pictures of TV sets from then. 2 RCA's, a Philco, and a Fermseh.

[Steve McVoy]

Actually, the picture buzz in FM sound is due to the use of intercarrier IF, which didn't become standard in TV sets until the early 50s. AM sound had its own problems (compare AM to FM radio), so FM sound was a superior choice.

[David Roper]

For a book with a copyright of 1942, it sure wasn't very up to date. The FCC set the 525-line standard with FM sound in 1941.

[wa2ise]

Originally Posted by David Roper

For a book with a copyright of 1942, it sure wasn't very up to date. The FCC set the 525-line standard with FM sound in 1941.

I wonder what sort of lead time a book has from its last edit to when it hits the bookstores? And when the copyright is asserted? It seems likely the book reflected the state of the art a year or two before its copyright date.

As for FM sound for TV, I understand that RCA's Sarnoff was really unhappy about that. As he didn't want RCA to license Armstrong's FM patents, as Armstrong became an ex-friend... But the FCC did move the FM radio band from 45MHz to today's 100MHz band, this move Sarnoff hoped would kill FM radio, to cut competition to RCA's new baby, television. And Sarnoff didn't want to build all new FM broadcast stations for NBC for FM radio. Before this, RCA asked Armstrong to develop a noise free radio system, and came up with FM. I suspect Sarnoff wanted a noise free AM radio receiver, that would receive existing AM broadcast stations. If Armstrong mentioned that FM would make an excellent sound channel for television, helping make it the killer app it did become, maybe Sarnoff would have thought it a great idea...

Intercarrier AM would really sound bad, but maybe an independent sound IF and detector would have sounded not as bad. You could build as wide an AM audio bandwidth as you'd want, the video is really just very wideband AM modulation. But you'd also hear residual high freq video with AM, which FM's capture effect would ignore.

[Steve McVoy]

I'd like to hear the full story of the decision to go with FM sound. The selection of FM meant that good audio could be received at signal levels way below what was needed for good video. AM sound would have resulted in sound and picture requiring about the same signal levels for acceptable performance. So, in a way, FM sound didn't really have any advantages over AM in television.

The reason wasn't intercarrier IF, since it didn't exist in 1941. And FM detectors were more complex than AM ones at the time.

Anyone know the full story why FM was chosen?

[wa2ise]

Originally Posted by Steve McVoy

... The selection of FM meant that good audio could be received at signal levels way below what was needed for good video. AM sound would have resulted in sound and picture requiring about the same signal levels for acceptable performance.
... And FM detectors were more complex than AM ones at the time.

That would mean that less transmit power would be needed for the FM sound, to get the same area coverage for it as the video signal would have. Thus less interference to the next higher channel in the next market area. Thus the FCC deciding it was a better choice. The fact that FM would require a little bit more circuitry wasn't that big a deal, as the video part required a large amount of circuitry anyway.

[Steve McVoy]

Thanks. That makes sense. I know that the aural carrier was 6db below the visual carrier right after the war. Do you know what the ratio was when AM sound was used?

[tubesrule]

The AM sound carrier was 6dB down on the pre-war American 441 and British 405 services. As you mentioned Steve, the FM level wasn't dropped to -12dB and -16dB until the 1950's, so they were still broadcasting the FM carrier at the same level as the AM carrier for quite some time.

Darryl