In article <telecom24.124.8@telecom-digest.org>, Neal McLain
<nmclain@annsgarden.com> wrote:
> Michael D. Sullivan <userid@camsul.example.invalid> wrote:
>> 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....
> Whereupon Robert Bonomi (bonomi@host122.r-bonomi.com) wrote:
>> Plausable, just 'false to fact'. <wry grin>
>> In the early days of TV receivers, they were equipped with
>> continuous-tuning knobs/dials, just like an AM radio receiver.
>> For the TV band, however the indicator assembly was marked by
>> "channel", *not* by frequency.
>> I used to have a 1930's Crosley TV that had that kind of
>> continuous tuner. *BIG* gap on the dial, between channel 6 and
>> 7, It actually tuned across that entire 'midband' space -- with
>> all kinds of interesting results. You could "see" aircraft
>> band transmissions, and hear stuff on broadcast FM, 2m Ham, and
>> business-band.
> Sullivan is correct.
His recitation of history is factual.
He is *NOT* accurate with regard to 'cause and effect' of 'detent'
tuners leading to "channel number" common-usage. Detent tuners were
at least 'second generation'; the prior generation (analog
continuous-tune) sets all used _numbered_ channels,
Crosley, in those days, was a "high end" manufacturer. If a
high-tech, 'detent' tuner design had been available, they *would* have
been using it.
Thus we've got numbered channels (_without_ frequency numbers) in
"common use" well before any 'detent' (turret switching, or other)
tuners were in vogue.
In the -very- early days stations _were_ identified by the frequency/
frequencies they transmitted on, There wasn't any option on the
matter, since it was being done under experimental provisions of
_amateur_radio_ licenses, and where they were broadcasting could
change from day to day. :) But this is WW-I era.
> As Sullivan acknowledged, some old TV sets did work like Bonomi's
> 1930s Crosley: they required "tuning in on a single band by twiddling
> an analog variable tuning capacitor to the right frequency [which]
> would have been difficult."
Actually it was surprisingly easy. Not even as hard as tuning an AM
receiver. Or use the markings on the dial to get 'close', then you
ignore the markings and tweak for maximum clarity. All it takes is
"reasonable" gearing. :)
Add in two-stage gearing -- the first few revolutions in either
direction were moved the frequency in small increments, once the
'limit' in either direction was reached, things went much faster.
Thus, when you overshot a "little bit" on the first attempt, you had
"automatic" fine-tuning as you started going in the other direction.
Somewhat complex mechanically, but amazingly easy to _use_.
> But by the 1950s, TV set manufacturers were installing "turret tuners"
> to simplify VHF tuning. A single knob rotated a cylindrical mechanism
> fitted with twelve little hand-wired circuit boards, one for each
> channel. Each circuit board had a bunch of capacitors, some
> hand-wound coils, and a row of metal contacts that mated with metal
> springs. As each circuit board was brought into position by the
> rotating mechanism, the springs mated with the contacts on the board,
> placing that board in the circuit.
> After the introduction of UHF, turret tuners were manufactured with 13
> circuit boards, one for each VHF channel + one that switched to a
> separate UHF tuner. The UHF tuner was tuned in one continuous-tuning
> dial.
Yuppers. 'Practicality' strikes again. 70 UHF channels would have
meant a detent roughly every 5 degrees of rotation. A 70-sided
turret, with circuit strips that were only 1/2" wide, would have had
to be nearly a _foot_ in diameter. Gotta wait for a better
technology. :)
> Sullivan continued:
>> Later on, [turret] tuners had separate fine- tuners for each channel
>> so one wouldn't need to retune when switching from station to
>> station.
> On each channel, the fine-tuning control engaged a tuning slug inside
> one of the little hand-wound coils.
Sometimes a tuning slug in a coil, often a trimmer capacitor.
Either way, it provides a frequency adjustment over a limited range,
with relatively high precision and stability.
Sets with this kind of fine tuning could _almost_always_ be identified
by the fact you had to 'push in' the fine-tuning knob/ring to engage
the adjustment mechanism on the 'active" turret segment.
> Here's a link to a picture showing a turret VHF tuner (left) and
> what appears to be a continuous-tuning UHF tuner (right). This
> particular photo happens to be on a British website, but the basic
> structure of the turret mechanism is the same in the USA.
> http://www.thevalvepage.com/valvetek/guidgrid/GGRID2.JPG
> Back in my cable TV days during the 70s, turret tuners used to drive
> us nuts. There was only one VHF TV station in the market (Channel 3),
> so if a viewer wasn't hooked to cable, the only exercise the tuner got
> was from getting flipped back and forth between UHF and 3. This kept
> the contacts on UHF, 2, and 3 clean, but the rest of the contacts got
> pretty dusty and/or corroded. If this viewer then connected to cable
> (just 12 channels in those days), suddenly, all 12 VHF circuit boards
> were needed. We spent a lot of time explaining, "I'm sorry, sir, your
> TV set's tuner needs to be cleaned ... please take it to the TV repair
> shop of your choice ... no we do not repair television sets ... our
> franchise agreement specifically prohibits it."
> Things got even worse when we introduced our first pay service (HBO)
> in 1978. We "hid" it in the midband on cable channel 17, "in the
> clear" (not scrambled, not trapped). We provided each HBO sub with
> a primitive converter: a little box with a single two-position
> switch:
> - One position converted channel 17 to channel 2 for HBO.
> - One position passed the incoming cable signal through
> unaltered for channels 2-13.
> It wasn't very good security, but the powers-that-were considered it
> to be good enough, since turret tuners couldn't tune it.
> Well, it wasn't long before local TV shops discovered a new line of
> business: retuning one of the lesser-used turret circuit boards (like
> public access) to channel 17.
> Of course, Bonomi's old Crosley would have tuned to it.
B&W only, a roughly 11" _round_ tube, medium-lousy contrast range,
moderately long-persistence phosphor, and a few other drawbacks --
hardly worth the trouble. :)
And, of course, no 75-ohm coax input. Would have had to use a balun
on the twin-lead screw terminals. ;)
*nice* audio, though.
> 1970s, virtually all TV sets used turret VHF tuners. Varactor tuners
> with digital displays were just coming on the market, and the old
> continuous-tuning models had just about disappeared.
I don't think anybody selling into the U.S. market at the end of WW-II
was making a continuous-tuning model. Turret tuners were
simpler/easier/ cheaper to manufacture, once you worked out the right
combinations of resonant frequencies and band-pass filters.
And *much* more "user-friendly".
> A few years later, we moved HBO to channel 2 (so we could sell HBO to
> hotels and motels), installed negative traps to secure it, .....
What science can take away, science can put back. Those traps did _not_
*completely* eliminate the signal getting into the customer premises, they
just made it so weak that a conventional TV set couldn't amplify it enough
to make a decent picture. A decent high-gain single-channel pre-amp, on
the other hand, installed 'in front of' the TV receiver, could do a
surprisingly good job of resurrecting the 'killed' signal. <grin>
Robert Bonomi (bonomi@host122.r-bonomi.com)