Why did old analog television sets have separate VHF and UHF channel knobs?

[oldnewb]

Was it merely convention (e.g., users expected it), or was there a technical consideration?  Surprisingly, Google was not forthcoming with an answer.  I could hazard a few guesses but figured I'd ask first.

[Zero999]

VHF TV? I don't think the UK ever used it along side UHF. When it was monochrome, TV was all VHF, then it was dropped and replaced with UHF, when we moved to colour.

[Ian.M]

Errrr no. The UK switchover took 15 years, so there was a long period in which dual standard 405 line (VHF) and 625 line (UHF) sets were popular.  See https://en.wikipedia.org/wiki/405-line_television_system

The use of two selector knobs for VHF and UHF was because of turret tuners that mechanically rotated a pre-tuned frequency selector card into position to connect to the active parts of the tuner circuit to select each channel.   It was technically possible to build a combo turret tuner which used different contact positions for VHF and UHF so either a VHF or UHF channel selector card could be fitted in any turret drum position, but the result was notably bulky.  Separate turret tuners, with the UHF one selected by a dummy position on the VHF one (as there was never going to be a need for more VHF channels), that also switched the line timebase from 405 to 625 lines/frame, was cheaper, typically lower noise, and allowed manufacturers an easy factory fitted upgrade path for their existing set designs.

[Gyro]

The youth of today! 

I remember our family's Philips TV set of the time. It was pretty featured for the time:

- LDR automatic brightness compensation.

- Automatic channel change... A push button which caused a little shaded pole induction motor to engage (starter motor style) to rotate the VHF turret tuner to the next populated slot (kerchunk, kerchunk, kerchunk).

- A big chunky slide switch at the bottom of the panel which switched over to the UHF tuner (simple knob and tuning scale). That was only used to watch BBC2 - the first channel to be launched on UHF, everything else was initially still on VHF initially.

These sets required quite a bit more complexity in the scanning circuits, VHF being 405 line and UHF being 625 line.

There was also a socket for a (plug in) remote control. I eventually found one in Lisle street - it had analogue thumbwheel knobs for volume, brightness and contrast, together with a channel change (kerchunk...). It had no facility for VHF / UHF switching (the slide switch on the TV was switching IF and scan ciircuitry). I also remember that the remote control cable was about the thickness of a 13A mains lead, just a bit stiffer! 


P.S. Not forgetting that you also had to get a UHF aerial installed to plug into the second aerial socket!

[Fungus]

Was it merely convention (e.g., users expected it), or was there a technical consideration?  Surprisingly, Google was not forthcoming with an answer.  I could hazard a few guesses but figured I'd ask first.

Simple: There was a time when there were two different types of TV signals being broadcast.

One tuner circuit in your TV wasn't enough.

[Benta]

These sets required quite a bit more complexity in the scanning circuits, VHF being 405 line and UHF being 625 line.

"Fog in Channel - Continent cut off"   

The 405 line VHF system was a pure British thing (probably also included the colonies).

Everywhere else it was 525/625 line systems (Russia excepted).

[BrianHG]

Here in North America, having 12 channels on the 2 VHF sets 2-6, then 7-13, it was easy to make a knob which rotated and stopped in 12 slots after 1 revolution, on 2 separate bands.  VLF was channels 2 through 6 (approx 54Mhz-88Mhz), VHF was 7 through 13(approx 174Mhz-216Mhz).  These channel knobs had to change tuner calibrations, or had fixed tuned switched capacitors for each channel depending on design.

But, for UHF, the same 525 line video system, going from channel 14 through 83, that's 70 channels (approx 470Mhz through 890Mhz).  It was much simpler to just have a 1 turn varicap, or volume control driving a tuning diode, to tune through 75 channels.  At times, the tuning knob had dentin steps, but this was nothing more than a a fine tooth gear with a spring bearing ball faking the approximate channel position.  Also considering the frequency difference between the VHF and UHF, at the time, sometimes a separate tuner was used.

https://en.wikipedia.org/wiki/North_American_television_frequencies

[CJay]

It was technically possible to build a combo turret tuner which used different contact positions for VHF and UHF so either a VHF or UHF channel selector card could be fitted in any turret drum position, but the result was notably bulky.  Separate turret tuners, with the UHF one selected by a dummy position on the VHF one (as there was never going to be a need for more VHF channels), that also switched the line timebase from 405 to 625 lines/frame, was cheaper, typically lower noise, and allowed manufacturers an easy factory fitted upgrade path for their existing set designs.

Turret tuners... 
*weeps quietly in corner*

[coppice]

VHF TV? I don't think the UK ever used it along side UHF. When it was monochrome, TV was all VHF, then it was dropped and replaced with UHF, when we moved to colour.

UK TV used VHF for 405 line TV, and UHF for 625 line TV.

When BBC2 launched (about 1963), BBC1 and ITV were still 405 line stations on VHF frequencies. BBC2 launched as 625 line monochrome, on UHF channels. After a couple of years BBC2 went colour. A couple of years after that (1967) BBC1 and ITV launched 625 line colour services on UHF channels, in parallel with their 405 line monochrome transmissions. In 1969 Thorn and Philips launched the first all semiconductor colour TVs that got the cost down, and the reliability and picture quality up, to the point where sales (or more often rentals) started to take off. Those sets no longer had any need to support 405 line modes, or VHF channels, and were pure 625 line UHF designs.

[Harb]

generally speaking there was only ever limited bandwidth in the VHF spectrum, so the UHF system was a no brainer to allow more licences to be granted for extra TV channels in many countries.

The Zenith TV company was probably the leaders in single tuning knob tuners, with a very smart replaceable tuning strip design tuning unit....that allowed the set to only have a single knob.....very very smart design having an upgrade path for existing sets.

Unlike today with digital channels, back then there were spectrum limits to the amount of analogue channels that could be fit into a local regions bandwidth limits, so the tuners capacity was never really a problem, but only one company seemed to see the sense in the approach Zenith took.....the rest just seemed to cheap out by just adding an extra tuner....rather that working on a solution, or paying patent rights on the Zenith system.

Here is a great video on Zenith thats worth a watch on what happened back in the day....

[T3sl4co1l]

A lot of (all?) UHF dials were continuous tuned (i.e., the knob turned a variable capacitor and/or inductor), and had notches just for convenience -- it was pure coincidence that the notches lined up with real channels, but most of the time it was pretty close anyway, and a little fine adjustment (the fine adjust covered about a channel's span, so you could always hit a channel even if you got "unlucky") would get you there.

I also remember having a B&W TV that had a UHF dial out to ~80.  UHF was redefined in the 80s to have 70 channels, and the rest was reallocated for cellphone use -- the earliest analog cell phones.  You could see dot and line patterns, and hear sizzling or buzzing, or sometimes garbled voice, up there...

The same tuning mechanism wasn't suitable for the wide frequency span of VHF channels -- in particular, VHF isn't a single band, but is several, separated by gaps.  It made more sense to use presets -- the turret tuners mentioned above.

You would also have separate antenna inputs, for similar reasons.  Also, I don't think wideband antennas (anything that would be flat, as in, within 20dB kind of flat) ever caught on, probably because of material and manufacture expense, and bulk.  So they were always adjustable in some way.  Or people just used coat hangers, which honestly will do just as well after some lucky adjustments to a given channel.

When digital tuners took over, a PLL handled all the frequency control with exact precision, and all that was needed was a tuner with a wide range, a high 1st IF to match, and a bit of brain to set the control voltage.  TV tuners make an excellent starting point for a quite reasonable spectrum analyzer.

Tim

[TimFox]

The VHF tuner and UHF tuner were separate circuits in the vacuum-tube TV era.  The UHF tuner output went to the VHF tuner, and had a separate local oscillator (UHF triode).

[Gyro]

These sets required quite a bit more complexity in the scanning circuits, VHF being 405 line and UHF being 625 line.

"Fog in Channel - Continent cut off"   

The 405 line VHF system was a pure British thing (probably also included the colonies).

Everywhere else it was 525/625 line systems (Russia excepted).

Ha, I've got one word for you - SECAM, go and laugh at the French, they're on your continent! 

Anyway, who invented television? 

[Benta]

Sorry, Chris, but SECAM is also 625 lines.   

[Gyro]

Yes, but it's a 'pure French thing'. 

[coppice]

Sorry, Chris, but SECAM is also 625 lines.   

Sometimes it was 625 lines. Sometimes it was 819 lines.

[coppice]

Yes, but it's a 'pure French thing'. 

Actually, SECAM was used in other countries, such as the USSR.

[Gyro]

Actually, SECAM was used in other countries, such as the USSR

Ok, You're right. I surrender. 

[Ian.M]

With or without cheese & monkeys?

[Richard Crowley]

• Original broadcast TV was VHF only
• The UHF (470-894 MHz) band is widely separated from the VHF bands (54-216 MHz) combining tuning function was impractical
  
• When UHF was rolled out, it was a simply "added" to VHF receivers as an additional, separate module.
• For decades, VHF tuning was accomplished with a "turret" contraption with a separate strip of L & C components for each channel
• Digital synthesis technology was too expensive/fiddly to use for consumer TV receivers in that era
• But varactor-tuned LO (local oscillator) technology had become ready for mass-market TV receivers

[karoru]

Yes, but it's a 'pure French thing'. 

Actually, SECAM was used in other countries, such as the USSR.

And wonderful SECAM decoders were made by Soviet engineers to adapt American RCA NTSC color TV designs to SECAM. I had "Rubin" ("Ruby" in English) color TV (mod of some RCA chassis bought/reverse-engineered by USSR) that had two (sic) additional knobs for regulating colour balance and even with them we got Never Twice Same Color reproduction quality in Europe:) On that particular TV with tongue at a right angle I managed to do only two settings - nearly pure green or LSD trip video style.

[james_s]

I don't think UHF ever really caught on in the US, I don't recall ever having any UHF channels in my area. Back before we had cable and relied on an antenna outside there were just a handful of VHF channels. I remember we got 4, 5, 7, 9, 11 and 13 at our house growing up. That was it for TV, if you wanted to watch something else you could rent a VHS tape.

[TimFox]

In my youth, UHF channels were found in major cities (such as Chicago) where they ran out of space in the limited VHF band.  Note that adjacent channels (e.g., 3 and 4) were not assigned in a given market due to interference.  Tuning UHF on the old tuners was a pain, but with the shorter distances required in cities the system worked.  One Chicago TV personality always wanted to move to a "single-digit station", but only moved from 32 to 20 as station managements changed.

[Richard Crowley]

I don't think UHF ever really caught on in the US, I don't recall ever having any UHF channels in my area. Back before we had cable and relied on an antenna outside there were just a handful of VHF channels. I remember we got 4, 5, 7, 9, 11 and 13 at our house growing up. That was it for TV, if you wanted to watch something else you could rent a VHS tape.

That is probably the perception of people who lived in the top 50 markets in the USA.  But it was very different out smaller cities and rural areas.

Of course, now ALL broadcast TV stations (even the ones called "2" thru "13") are on UHF.  The VHF band was removed from TV broadcast service starting in 2009.  And then broadcasters on channels > UHF 36 were forced to move again by the "reverse-auction" last year.  All of the former VHF TV bands and most of the UHF TV bands have been taken away from television and sold off to mobile operators.

[james_s]

Perhaps it was an urban thing, that could explain why I never encountered it. With the exception of downtown Seattle this whole area was mostly rural communities back then. As late as the early 90s there was still a field with cows in it in downdown Woodinville. Liked it better that way, but that's beside the point.