Michael D. Sullivan wrote about Re: What Happened To Channel 1? on Sat, 19 Mar 2005 08:02:17 GMT
Television, on tho other hand, started out in two discontiguous VHF
> bands, with somewhat variable spacing between channels and a need for
> precise tuning, and tuning in on a single band by twiddling an analog
> variable tuning capacitor to the right frequency would have been
> difficult. This tuning method was used on some early TVs; I don't know
> whether they were tuned by numeric frequency or by channel number, but
> it would not have been very convenient. The TV industry instead
> standardized on TV tuners that had 12 discrete fixed settings, pre-tuned
> to channels 2-13, with a fine tuning control that allowed one to tune
> the frequency higher or lower to account for offsets. Later on, tuners
> had separate fine-tuners for each channel so one wouldn't need to retune
> when switching from station to station. Given the move to fixed-
> position tuning, the use of "digital" numbering of channels instead of
> analog-like frequency designations was an obvious simplification.
I had such a set in the early 50s. Tuning was analog, with detents and, IIRC, a fine-tuning wheel on the back of the main wheel. Band switching was done by turning the whole assembly of the main wheel. It worked.
When UHF was added, it used a single contiguous band, and most sets
> initially required a separate converter box, which had an analog-style
> variable tuning capacitor that required careful attention to get the
> station one wanted (the pointer is between 30 and 40, is that channel 33
> or 36?), but the tradition of using channel numbers instead of
> frequencies prevailed due to the established TV tradition. Eventually,
> TVs incorporated the analog-style continuously variable UHF tuner and
> later adopted a fixed-position tuner for UHF.
This was required by the FCC: they required parity in tuning (on all but the cheapest sets -- under 12 inches IIRC) between UHF and VHF to promote UHF.
[TELECOM Digest Editor's Note: This is true, however if you look at
> some FCC documents on FM radio frequency allocations (for example,
> documents on which frequencies are available on which places for
> 'low power' FM). All those documents show both 'frequency' and
> 'channel number' for the spaces between 87.6 FM and 108.1 FM. I
> think they have the 'channels' beginning at 201 and numbering
> upward. PAT]
If you look at a digital "world band" radio, you will find somewhere a "europe-america" switch. It's often well hidden. I have one where it's in the battery compartment. In the Americas the channels on the AM band are spaced 10 kHz apart. In Europe they are spaced 9 kHz apart, allowing them to squeeze in a few extra stations. This is significant only in digital tuning, especially in digital search.
There is something similar in FM. I've forgotten the exact details, but in US we use only the "odd" frequencies: 88.1, 88.3, 88.5, 88.7,
88.9 MHz. I think in some parts of the world they use the even frequencies: 88.2, 88.4 etc. This is to get the necessary spacing between the broadcasts.
One other peculiarity: in most countries, FM is about 88 to 108 MHz. In Japan it's about 78 to 98 MHz. There are a few radios that will receive the entire band, 78 to 108, but most, including Japanese brands sold outside Japan, miss the low end of the Japanese band.