The prewar TV standard was pretty much the same as modern B&W NTSC, except it was 441i instead of 525i. Also the sound was AM, not FM. For a rough simulation of the picture, you could get a "widescreen" DVD movie, and feed the DVD player's luma output to a B&W TV set (via a TV modulator). Mask off with a paper cutout about 20% of the CRT (10% top, another 10% bottom, and also left and right). Or set the vertical and horizontal width 20% too big (I doubt you'd want to mess with these, thus the paper mask for this demo). Instead of 480 active video lines it would have been 404 lines. But the standard still called for 4.2MHz of horizontal resolution, so you would have seen more resolution left-right than up-down. Not a lot more, but some (about 20%). Like what you see with a direct baseband video connection from the DVD player.

To complete the similation, we have to make the sound sound like that from an AM signal 4.5Mhz above the picture carrier. A significant problem in TV set design is that, to make the sound come in despite tuner mistuning, intercarrier is used (pick off the 4.5MHz sound carrier off the video detector). Problem is that the sound carrier gets contaminated with buzzes and such from the video carrier and low frequency video content. FM is much more robust in rejecting this source of interference. To create this interference on a baseband audio signal (from the DVD player) that feeds an audio amp, mix in a small amount of the luma video signal. 30dB down would be about as much one could reasonably expect to achieve in a prewar TV set. You'd also hear (as well as see) car ignition and noisy brush motors.

If the TV set did not use the intercarrier system, you'd lose the sound very quickly if the tuner is mistuned. Ever play with an analog tuned SW radio on one of its highest SW bands, the one going from 12 to 30MHz. Breath on the tuning dial and you lose that SW station. It would be worse with a TV tuning from 45 to 100MHz (where the original TV channels were to reside). The sound would probably sound better with less interference from the video carrier.

As much as Sarnoff freaked, the FCC made a wise move to make the sound carrier FM. Maybe had Armstrong suggested to Sarnoff that FM sound for TV would have made TV even a better killer product, well....

Ummm . . . have you ever seen a prewar TV operating? Ever fiddled with the fine tuning on one to bring in the sound? I have...and your "simulation" is greatly exaggerated. And let us remember that my experience has been, of course, tuning to receive FM sound with a prewar TV's AM detector. While the comparison in that case to tuning shortwave is reasonable, your description is still exaggerated, and tuning the original AM modulated sound carrier would've been quite a bit less critical still. And if you don't have intercarrier sound (no prewar TV did), you don't get intercarrier buzz!

The BBC used the television sound channel without video for years to broadcast classical music. They got along without FM sound until color came in.

Never tuned a prewar with AM sound, but do remember tuning postwar TV with split-sound audio. As I recall, the basic definition of split-sound is the use of a separate detector for the sound signal. Some used a take off near the tuner to feed the audio IF; others connect it to a video IF. As I further recall, the much narrower bandwidth of the audio IF compared to the video IF would make it much touchier than an intercarrier set, all other parameters being equal. Am scratching my head, but I think I remember confirming the effect many years ago with a 630 clone. I just cheated and looked up the problem in a vintage Fink book. Local oscillator drift, especially on VHF-hi channels, caused the undetected sound carrier to slide out of the sound IF stage bandpass, which results in distorted audio, so it is/was much more touchy to keep in tune – at least until the LO settled down and stabilized.

Tuning sound on prewar sets is usually very easy. As TDRyan pointed out, tuning has to be more precise when using FM sound with a prewar set, since the set has to be tuned exactly on the slope of the bandpass to work right. With an AM sound source, the local oscillator can be plus or minus 200 kHz or so and still produce good sound, since the IF stages are relatively broad (most are in the 4-8 mHz range). Some sets had their local oscillator adjustments hidden, since they didn't need to be changed very often.

As for the advantage of FM over AM in television audio, I think this is overstated. The video becomes badly degraded at relatively high signal to interference ratios, well before AM sound does. All FM sound accomplishes is to allow undegraded audio while the video is trashed. As for the higher fidelity of FM, just look at the speakers and audio sections of TV sets - it was rare for manufacturers to take advantage of it.

I'd love to own an RCA TT-5. AM sound on a set would be really cool. My 630TS is the closest thing I come to in terms of a prewar set...

Jonathan

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We sell a modulator kit, made primarily for the British 405 line standard that was used before the war and into the 60s. It has AM sound, and can be used for American prewar sets. We've sold about 60 of them to collectors here and in Europe:

http://www.earlytelevision.org/405_modulator.html