It has to do with how to hit a consumer price point with a receiver for a complex modulation scheme using only a couple dozen active devices and cheap-to-manufacture filters.
Original NTSC monochrome television is a 6 MHz wide channel, the video carrier is 1.25 MHz up from the bottom, and at the transmitter a filter lops off most of the lower sideband.
For receivers, the problem is the IF filter. Filters with steep skirts are hard to build and align, and are expensive. So... channel allocation was done with the assumption that most consumers would have receiving sets with poor adjacent-channel rejection because of using cheap filters. This was solved by channel allocation -- do not let two stations with overlapping grade-A signal contours be on adjacent channels. Problem solved.
In the very early days in the US, channel 1 was 48-54 MHz. They found out that the percentage bandwidth was too wide to be able to build a practical transmitting antenna for channel 1. The SWR was too high at the band edges, and high SWR on analog TV means the transmission line generates ghost pictures. So channel 1 was dropped. Now hams have the 6 meter 50-54 MHz band, and cheap license-free R/C toys and walkie-talkies have 48-50 MHz.
Channel 2 is 54-60, 3: 60-66, 4: 66-72 and then a gap: 72-74 is radio control, then channel 5 is 74-80, and so forth, and if I recall correctly there is a big gap between channels 7 and 8 so even though they are sequential numbers they are not frequency-adjacent channels. (7 and below being the so-called "low VHF" and 8-13 the "high VHF" TV channels.)
The 2 MHz gap between channels 4 and 5 was also used to advantage, as it was enough to eliminate the adjacent-channel interference problem on reception. I recall when I lived in Minneapolis, that channels 2, 4, and 5 were all allocated in the metro area. Channel 3 was Mankato, far enough away that if you lived between Minneapolis and Mankato you could receive all of those stations without interference problems, being in the grade-B signal contour of both.
And..... this entire post contains obsolete information of historical interest only, and even at that, of interest to very few. Now... excuse me because I have to readjust the spark gap on my 200 meter transmitter...
The US Department of Commerce publishes a wall chart showing how the difference parts of the spectrum are allocated to different services. Notice how bands for "Broadcasting (Television)" are broken up and interspersed with those for other services such as aeronautical navigation and FM radio, despite the TV channels having consecutive numbers:
Original NTSC monochrome television is a 6 MHz wide channel, the video carrier is 1.25 MHz up from the bottom, and at the transmitter a filter lops off most of the lower sideband.
For receivers, the problem is the IF filter. Filters with steep skirts are hard to build and align, and are expensive. So... channel allocation was done with the assumption that most consumers would have receiving sets with poor adjacent-channel rejection because of using cheap filters. This was solved by channel allocation -- do not let two stations with overlapping grade-A signal contours be on adjacent channels. Problem solved.
In the very early days in the US, channel 1 was 48-54 MHz. They found out that the percentage bandwidth was too wide to be able to build a practical transmitting antenna for channel 1. The SWR was too high at the band edges, and high SWR on analog TV means the transmission line generates ghost pictures. So channel 1 was dropped. Now hams have the 6 meter 50-54 MHz band, and cheap license-free R/C toys and walkie-talkies have 48-50 MHz.
Channel 2 is 54-60, 3: 60-66, 4: 66-72 and then a gap: 72-74 is radio control, then channel 5 is 74-80, and so forth, and if I recall correctly there is a big gap between channels 7 and 8 so even though they are sequential numbers they are not frequency-adjacent channels. (7 and below being the so-called "low VHF" and 8-13 the "high VHF" TV channels.)
The 2 MHz gap between channels 4 and 5 was also used to advantage, as it was enough to eliminate the adjacent-channel interference problem on reception. I recall when I lived in Minneapolis, that channels 2, 4, and 5 were all allocated in the metro area. Channel 3 was Mankato, far enough away that if you lived between Minneapolis and Mankato you could receive all of those stations without interference problems, being in the grade-B signal contour of both.
And..... this entire post contains obsolete information of historical interest only, and even at that, of interest to very few. Now... excuse me because I have to readjust the spark gap on my 200 meter transmitter...