Regular Scheduled HD Television is 75 Years Old

[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?