Regular Scheduled HD Television is 75 Years Old

[wa2ise]

Originally Posted by Penthode

In 1937, RCA and other experimental broadcasters moved up to 441-lines, AM sound. This was the RMA (Radio Manufacturer’s Association) standard at the time.

I can imagine how AM sound would have sounded with the intercarrier system developed later on.

You could do a reasonable simulation of 441i using a DTV converter box set to letterbox mode, and a TV set, and assuming that there's enough oomph in the vertical jack up the height to make the black bars at top and bottom disappear to vertically fill the screen with picture, and increase the horizontal width to make the picture look reasonably correct. Or mask off the CRT to shrink the screen size. You could (assuming you have the woodworking skills for furnature quality work) build a prewar style console cabinet (or convert a common radio console cabinet that's missing its radio chassis, herisey!), and insert a decent B&W solid state TV or video monitor set inside, masked off to have it show approz 441i lines. And build a tube audio amp to drive a large speaker mounted below.

[Penthode]

It is interesting you should mention intercarrier. All the 405 line receivers were split sound as intercarrier would not work for AM sound.

In an American set with FM split sound design, one would have to carefully tune to optimize the sound. Intercarrier sets would maintain the sound quality over a wider range of fine tuning.

Later, fully superhet UK 405 sets with channel selectors had wide bandwidth sound IF channels. The VHF reduced noise and combined with the wide bandwidth ensured that the sound quality remained high over a wider range of fine tuning than American split sound sets.

Further, the wide bandwidth provided high fidelity sound. In other words, AM negated the need for intercarrier sound.

Note that the immediate prewar UK TV sets were almost all TRF video receivers. There was only one channel and so from the antenna through to the detector, the amplifier would be generally stagger tuned to the incoming 45 MHz video carrier with double sideband AM. The audio was generally tuned from the built-in separate AM superhet radio with a band covering the 41.5MHz AM TV audio.

Perhaps the later adoption of FM sound in 1941 for American TV was a compromise to the E H Armstrong interests?

[earlyfilm]

Originally Posted by Penthode

Perhaps the later adoption of FM sound in 1941 for American TV was a compromise to the E H Armstrong interests?

I'd guess that RCA and Armstrong were too busy suing each other to do that.

FM can live within the NTSC video sideband, but AM cannot as it would pick up the video highs. The AM sound carrier would have to be located outside of the video sideband and that would require a wider band width.

This is like phase modulation used in NTSC color can live within the video sideband and (with a little luck) not be messed up by the AM video.

James.

[Penthode]

I'd guess that RCA and Armstrong were too busy suing each other to do that.

Actually the acrimony was just beginning. The law suits came later.

I am only surmising but it does seem credible that RCA traded acceptance of FM for TV in trade for frequency band allocations (42MHz to 50MHz) for FM Radio broadcast in 1941. Remember, early 40's technology dictated lower frequencies and RCA and the NTSC gave away a chunk of bandwidth which could have been used for an extra TV channel.

FM can live within the NTSC video sideband, but AM cannot as it would pick up the video highs. The AM sound carrier would have to be located outside of the video sideband and that would require a wider band width.This is like phase modulation used in NTSC color can live within the video sideband and (with a little luck) not be messed up by the AM video.

I do not believe this to be so. The BBC experimented with NTSC color in the fifties and sixties for the 405 line system with no change in channel or audio parameters. The audio carrier is located entirely separate from the video in both 525 line with FM sound and 405 with AM sound. Adding color is a matter of using suppressed carrier quadrature modulation and frequency interleaving in the video band only.

[Penthode]

You could do a reasonable simulation of 441i using a DTV converter box set to letterbox mode, and a TV set, and assuming that there's enough oomph in the vertical jack up the height to make the black bars at top and bottom disappear to vertically fill the screen with picture, and increase the horizontal width to make the picture look reasonably correct.

I thought about this and you have to be careful here. Picture resolution is dependent on a number of factors.

An important factor which is being overlooked is the Kell Factor. Kell attempted to define the required channel bandwidth (affecting horizontal resolution) against the number of lines. He asserted that because of the gaps between the lines, vertical resolution is lost which means that the horizontal resolution hence video channel bandwidth can be reduced. The Kell factor is the ratio or vertical resolution against horizontal resolution and its maximum value is unity or "1".

When vestigial sideband transmission was introduced for 441 television in the late '30's, the same 6 MHz channel containing the same 4.2 MHz video channel was used for the later 525 line system. The only thing that changed was the reduction of the Kell Factor to about 0.6. This means that the horizontal resolution of 525 video was less than 441 line video to compensate for the increase in the number of lines. Even in those days, it was felt that artificial picture "sharpness" could help compensate for the loss of resolution with the switch from 441 to 525. And the the horizontal resolution of 525 was further reduced with the introduction of color. The Kell factor with color broadcasts on US sets made after 1953, color and monochrome was now less tthan 0.5. However 405 line had a Kell factor of unity hence had considerably higher than expected horizontal resolution.

This may explain why when CBS engineers went to England in the late 50's and examined 405 line pictures first hand, they exclaimed that in many instances the 405 line pictures looked sharper than 525 line pictures.

[ppppenguin]

Originally Posted by Penthode

An important factor which is being overlooked is the Kell Factor.

The Kell factor is largely an empirical fudge. Nothing wrong with that because it reflects how we actually perceive H and V resolution. I don't have the evidence to prove it but I strongly suspect that the work that Kell did applies only to displays with an approximately gaussian spot shape. This is more or less true for CRTs and totally wrong for LCDs. Not sure what happens to Kell if you use spot wobble on a CRT.

It was widely observed in the UK that 625 pictures seemed less sharp than 405. This may have been due to relatively poor HF response in some sets of the time. It may also be a psycho-optical effect of a visible line structure being perceived as sharper.