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why 12V?



 
 
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  #41  
Old March 9th 17, 08:36 PM posted to uk.tech.digital-tv
charles[_2_]
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Posts: 557
Default why 12V?

In article , Vir
Campestris wrote:
On 08/03/2017 12:44, The Other John wrote:
On Wed, 08 Mar 2017 11:43:12 +0000, NY wrote:

And why is the standard film frame rate 24?


Was it set by the Merkins? If so could it be because of their 60Hz
mains? You would get 5 positive and negative mains peaks per frame if
my sums are right, thus avoiding strobing effects.

Merkin TV is of course (almost exactly) 30Hz.


It was once, but with the arrival of color it became 59.94Hz

Charles

--
from KT24 in Surrey, England
  #42  
Old March 9th 17, 09:06 PM posted to uk.tech.digital-tv
NY
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Default why 12V?

"charles" wrote in message
...
In article , Vir
Campestris wrote:
On 08/03/2017 12:44, The Other John wrote:
On Wed, 08 Mar 2017 11:43:12 +0000, NY wrote:

And why is the standard film frame rate 24?

Was it set by the Merkins? If so could it be because of their 60Hz
mains? You would get 5 positive and negative mains peaks per frame if
my sums are right, thus avoiding strobing effects.

Merkin TV is of course (almost exactly) 30Hz.


It was once, but with the arrival of color it became 59.94Hz


I never understood why NTSC needed to have its frame and line rate tweaked
but PAL and SECAM didn't. Was it caused/exacerbated by the non-alternating
of the colour info? Was it fortuitous that European and UK PAL systems and
French SECAM didn't have to alter when colour was introduced, or were those
systems designed "better" than NTSC in some way?

Was it a baseband video restriction or a restriction in the US broadcast
system that was best solved by tweaking the baseband signal rather than the
broadcast spec?

Do *all* NTSC systems use 59.94 (maybe to remain compatible with US)?

Presumably now that analogue is very much less common, the NTSC could *in
theory* have gone back to 60.0 Hz for digital production and broadcast.

  #43  
Old March 9th 17, 09:20 PM posted to uk.tech.digital-tv
charles[_2_]
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Posts: 557
Default why 12V?

In article , NY
wrote:
"charles" wrote in message
...
In article , Vir
Campestris wrote:
On 08/03/2017 12:44, The Other John wrote:
On Wed, 08 Mar 2017 11:43:12 +0000, NY wrote:

And why is the standard film frame rate 24?

Was it set by the Merkins? If so could it be because of their 60Hz
mains? You would get 5 positive and negative mains peaks per frame
if my sums are right, thus avoiding strobing effects.

Merkin TV is of course (almost exactly) 30Hz.


It was once, but with the arrival of color it became 59.94Hz


I never understood why NTSC needed to have its frame and line rate
tweaked but PAL and SECAM didn't. Was it caused/exacerbated by the
non-alternating of the colour info? Was it fortuitous that European and
UK PAL systems and French SECAM didn't have to alter when colour was
introduced, or were those systems designed "better" than NTSC in some
way?


It's over 50 years since I learned the reason. As far as I can remember it
was to prevent patterning from the sound carrier on the broadcast system.

Was it a baseband video restriction or a restriction in the US broadcast
system that was best solved by tweaking the baseband signal rather than
the broadcast spec?


Do *all* NTSC systems use 59.94 (maybe to remain compatible with US)?


Those that rmained in monochrome stayed with 60Hz. The last one within the
USA (sort of) was the United Nations tv coverage which remained in mono for
a long time,

Presumably now that analogue is very much less common, the NTSC could *in
theory* have gone back to 60.0 Hz for digital production and broadcast.


Yess, probably, but why bother. It would involve a fair bit of expenditure
to change something that's been around for about 60 years.

--
from KT24 in Surrey, England
  #44  
Old March 9th 17, 09:42 PM posted to uk.tech.digital-tv
Paul Ratcliffe
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Posts: 2,442
Default why 12V?

On Wed, 8 Mar 2017 11:43:12 -0000, NY wrote:

Why did someone come up with 24, rather than a rounder figure like 25?


Why do you think 25 is rounder than 24?
24 is divisible by 2, 3, 4, 6, 8 and 12.
25 is only divisible by 5.
What makes a round number anyway (in your opinion) ?

Maybe I'm applying too much numerical OCD to it, in thinking that if you have
a free choice you choose either multiples of 5 or 10, or else powers of 2.


Yes, you are.
  #45  
Old March 9th 17, 10:19 PM posted to uk.tech.digital-tv
NY
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Posts: 1,122
Default why 12V?

"Paul Ratcliffe" wrote in message
...
On Wed, 8 Mar 2017 11:43:12 -0000, NY wrote:

Why did someone come up with 24, rather than a rounder figure like 25?


Why do you think 25 is rounder than 24?
24 is divisible by 2, 3, 4, 6, 8 and 12.
25 is only divisible by 5.
What makes a round number anyway (in your opinion) ?


One that is a multiple or sub-multiple of the base that I count in - 10.
That trumps (not of the Donald variety!) all other considerations.


Maybe I'm applying too much numerical OCD to it, in thinking that if you
have
a free choice you choose either multiples of 5 or 10, or else powers of
2.


Yes, you are.


Fair enough.

Given a totally free hand in setting any numerical standard, I'd always
round it to the nearest 10 or 5.

Sometimes there other considerations. For example, in packaging tin cans etc
into boxes, 12 is a better number than 10 because it has factors that are
more similar (3 and 4), and a 3x4 "almost square" box is easier to manage
than a "long thin" 5x2 box.

I suppose in an ideal world we'd have been born with five fingers and a
thumb on each hand, learned to count in base 12 with symbols for 10 and 11,
and then we'd have had the advantages of 3x4 boxes etc and yet would regard
12 as more natural than 10.

I suppose in days when division circuitry was analogue rather than digital,
it makes sense to choose standards such as number of TV lines to have lots
of small factors - hence 405, 525 and 625. Interesting that the competitor
to Baird had 240 lines - an even number which must have made interlacing
more difficult in days when one field was made up of a half-line at the
beginning and a half-line at the end to interleave the two fields (when the
vertical deflection was a continuous sawtooth rather than a staircase).


Someone raised the issue of 24 fps being 5 half-cycles of US 60 Hz mains, so
no strobing. What about in 50 Hz countries? Did they have to use DC lights
to avoid strobing? Do TV and film lights use DC (or very long persistence
with AC) to avoid strobing now? I imagine that filaments have long enough
persistence but what about HID and LED lights - in the latter case these are
normally pulsed at very high frequency, but presumably not for TV where they
need to be DC or else alternating banks of LEDs 180 degrees out of phase so
there's always half of them lit.

  #46  
Old March 10th 17, 01:13 AM posted to uk.tech.digital-tv
Johnny B Good[_2_]
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Posts: 394
Default why 12V?

On Wed, 08 Mar 2017 16:18:01 +0000, Brian Gaff wrote:

I can recall the first car radio I ever saw. it had a vibrator pack for
generating the ht as it was all valves inside. Motorola made it and the
tuning seemed to be via cores on string that was wound in and out of
coils.
One assumes this was less troublesome than capacitors?
Brian


In the case of car radio antenna setups, permeability tuning (aka,
variable inductance tuning) allowed the whip antenna to act as a pretty
effective resonant capacitive probe type for the LW and MW wavebands.

Even a 2 metre long whip antenna represents only about 1% of the
wavelength of a MW broadcast transmission at the high frequency end of
the band. Such a short, unloaded whip antenna acts as an electric field
probe antenna at these frequencies and below.

In order to get the highest signal voltage into the antenna tuned
circuit we need to minimise the tuning capacitance contributed by the
screened feeder and the evil necessity of an antenna/feeder padding
trimmer capacitor in parallel with the whip antenna's own, desirable
capacitance and maximise the inductance component that makes up the
required LC product to tune into frequencies across the range of the MW
(and optional LW) band(s).

For the American MW band, this requires a variable C or L with a max:min
ratio of 10.6:1 (in the UK and Europe, the requirement is a more modest
9.3:1). Since using a variable C results in reduced sensitivity as we
tune from the HF end to the LF end of the band[1] along with the
likelihood that the "stray capacitance" will more likely be in the
neighbourhood of 100pF rather than the more typical 30pF of a
conventional radio thus requiring a 1000pF variable tuning capacitor
rather than the more typical 300 to 500pF variety, the obvious solution
is to vary the L instead using movable iron dust cores which are able to
provide comfortably more than the 10.6:1 variation required allowing
trimming adjustments to be made in both the L and C values to calibrate
the frequency/wavelength tuning scale.

The reason why permeability tuning is rarely, if ever, used in
conventional radios is down to the additional costs involved over a dual
or triple gang tuning module based on the ubiquitous 2 or 3 gang air
spaced tuning capacitor. The issue is simply a matter of attaining the
best cost/benefit ratio which, in the case of the MW/LW car radio
restricted to the use of a very short whip antenna, is obtained by the
use of permeability tuning in preference to capacitive tuning.

[1] Such an antenna system can be considered to be the equivalent of a
capacitive volt dropper where the capacitance of the exposed whip aerial
(although effectively in parallel with the 'strays' and the trimmer
connected across the variable inductor as far as the tuned frequency
equation is concerned) can be considered as being in series with a
voltage source, in this case the electric field of the passing radio
waves.

If this circuit is tuned by a variable capacitor, the aerial voltage of
this capacitive volt dropper will reduce as we tune from the HF end to
the LF end of the band. Varying the inductance component to retune the LC
combination instead, eliminates this problem completely, hence its use in
AM car radios designed to utilise a short voltage probe whip antenna to
permit reception whilst on the move.

--
Johnny B Good
  #47  
Old March 10th 17, 03:27 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 1,746
Default why 12V?

On 09/03/2017 18:12, Graham. wrote:

I've got a Caruso 78 that is actually marked 80RPM


I've got La Paloma on an 80rpm.


Bill




  #48  
Old March 10th 17, 03:38 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 1,746
Default why 12V?

On 09/03/2017 22:20, charles wrote:

Yess, probably, but why bother. It would involve a fair bit of expenditure
to change something that's been around for about 60 years.


That's probably why Hil hasn't divorced me.

Bill
  #49  
Old March 10th 17, 04:08 AM posted to uk.tech.digital-tv
Johnny B Good[_2_]
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Posts: 394
Default why 12V?

On Tue, 07 Mar 2017 23:37:04 +0000, Graham. wrote:

On Tue, 7 Mar 2017 22:21:58 -0000, "NY" wrote:

"Bill Wright" wrote in message
news
So many things run on 12V DC. Firstly, why has that become the
standard?
Is it all because of the nominal voltage of a car battery?


I would imagine so. 12V can easily be made out of six 2V lead-acid cells
(for a car) or eight 1.5V batteries (for a portable radio etc).

I'm not sure how 5V became the standard for USB power.


TTL logic must figure strongly in the answer.


It sure does! Since the days of the very first hobbyist kit computers in
the mid to late 70s, the logic components have all been sourced from the
famous 7400 TTL series "Logic Family". The very first IBM PC had very
few, if any, dedicated VLSI (or even just LSI) chips being made up almost
exclusively of TTL logic chips to provide the 'logic glue' for the Intel
8088 cpu that IBM's cash starved fledgling PC division chose as a cheap
way to fabricate a 16 bit class desktop computer using cheap off the
shelf 8 bit data bus components.

Although the technology has moved on to cmos based chips, initially a
logic family given the base number 4000 (and still available) and able to
work on supply voltages ranging from a low of 3 to a high of 15, the cmos
versions of the 7400 TTL logic family were characterised to operate at
the standard 5v logic levels to retain compatibility as were the later
dedicated VLSI chips used to replace the original TTL logic glue chips to
reduce fabrication costs of the later motherboards where most of the
costs lay simply in the physical size of said motherboards and the
mounting of dozens of TTL chips where a single VLSI chip could now
suffice.

The 5 volt rail has been a standard supply rail in all versions of the
PC PSU from the earliest PC PSU through the AT and into the still current
ATX class PSUs. The addition of a 3.3 volt rail which was a mobile chip
voltage standard used by notebook (laptop) computers, didn't show up as a
standard rail voltage until the advent of the ATX class of PSU where it
appeared as an extra rail voltage rather than a replacement for the 5
volt rail.

Although modern CPUs have been using low voltage cores for some two
decades now (give or take a year or three - it's definitely been more
than 15 years), there's no point in getting the ATX PSU to provide a
voltage lower than 3.3 in order to directly power cpu cores which are now
typically using voltages as low as 1.2v at currents that can peak above
100 amps.

Such low voltage high current supplies are best generated local to the
cpu socket via programmable high efficiency switching converters drawing
their power from the 12 volt rail which can provide the required voltage
regulation at current demands that can exceed 100A and at voltages that
can be programmed in 5 or 10 mV increments over the range 0.8 to 1.5
volts or thereabouts. A 5% variation of a 1v supply is a mere 50mV.
There's no easy or efficient way to provide such low voltages at such
high power levels directly from the PSU.

The 5 volt standard is unlikely to vanish from the desktop (or even
notebook) PC any time soon since it's such a useful voltage to power USB
kit and internal peripherals. The time of its eventual demise in whatever
new PSU standard that will eventually replace the now venerable ATX
standard, is likely to coincide with the loss of the 3.3 and -12 volt
rails (the -5v rail disappeared from the ATX spec a few years ago now),
reducing the supply rails down to a single 12 volt rail (probably split
into separately monitored groups to allow detection of overloads that
could potentially melt a wire if such protection was solely reliant on
detecting, for example, the 30 odd amp's worth of overload that would be
needed to trip a 360W rated PSU).

Enthusiasts of low power ITX kit have been using this "Single 12 volt
rail PSU" concept for a few years now (and it's a technique that's not
just limited to ITX kit, it can be applied to low and medium power ATX
machines just as effectively).

In this case, special ATX 20 and 24 pin MoBo psu connectors complete
with switching converters to provide the -12v, the 3.3 and the 5 volt
rails powered by the single 12v supply which also directly feeds the 12v
pins allows the machine to be powered from a high efficiency 12v power
brick not unlike a high power laptop charging brick (usually rated
somewhere in the region of 90 to 120 watts output).

Additional switching converter power modules may be needed to power up
peripherals such as SSDs and HDDs, I can't recall whether the ATX adapter
header allowed for the extraction of the 5 and 3.3 volt (and 12 volt) to
power such peripherals or not. In a low power setup, this could be a
useful option otherwise excessive demands on the 3.3 and 5 volt rails
would best be served by additional converter modules (assuming the 12v
power brick can take the additional strain, of course).

An updated single rail PSU scheme would see main boards with power
conversion modules already built in to locally generate any 5 or 3.3 or
-12 (or even -5) volt rails to power their circuitry. Such main boards
could therefore still provide the standard 5 volt power to their USB
ports.

A change to single rail PSUs won't necessarily signal the demise of the
5v standard. There are no clear signs of it disappearing for another
decade or three since it's such a useful low voltage DC voltage source to
power the logic chips inside of most peripheral devices or gadgets with
modest power requirements.

Any such USB connected peripherals with power demands in excess of the
classic 2.5W limit are probably better powered from a dedicated PSU
rather than be allowed to add additional stress on the PC's PSU anyway.

The USB3 power management protocol that allows (IIRC) for up to 30 watts
to be drawn by a peripheral is a feature I find rather troubling. I
suppose if the feature can be limited or disabled in the MoBo setup when
there's an obvious overload risk to the PSU chosen to power the system,
it would be ok otherwise I'd be concerned that plugging in a 'greedy' usb3
peripheral could end up triggering a catastrophic power outage event.

--
Johnny B Good
  #50  
Old March 10th 17, 04:45 AM posted to uk.tech.digital-tv
Johnny B Good[_2_]
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Default why 12V?

On Wed, 08 Mar 2017 17:09:58 +0000, Roderick Stewart wrote:

On Wed, 08 Mar 2017 11:42:21 +0000 (GMT), Jim Lesurf
wrote:

I'm not sure how 5V became the standard for USB power.

TTL logic must figure strongly in the answer.


It's a while since I did electronics. Is 5V (or thereabouts) based on
physical properties of the silicon for the transistors in TTL? I
probably knew this at one time...


I'm trying to recall if RTL or DTL used 5Volts. I did briefly use some
RTL/DTL as an undergrad.


Yes they did.

But yes, IIUC the old TTL standard devices tended to be quite fussy
about the rail voltage.


I've always assumed 5V was chosen simply because it's easy to produce
from a 6V battery.

That wasn't the reason. I think 5 volt was chosen because it was a nice
round number and a voltage close enough to the optimum (whatever that was
at the time).

As regards this 'myth of fussiness about rail voltage', there's no basis
in fact for this. Provided the voltage rails were properly decoupled to
suppress noise artefacts and also properly regulated, the Vcc rail had a
tolerance of +/-10% (4.5 to 5.5 volt) range over which the TTL chips were
guaranteed to meet all their specifications in full.

It's no accident that the 5v rail in the PC, AT and ATX specs all
mandate a +/-5% tolerance to guarantee that the TTL would never be asked
to operate outside of its voltage tolerance limit.

As for burning TTL devices out with excess voltage, they were specced to
withstand a maximum of 7 volts for 10 seconds or less. That 7 volt limit
represents a 40% over-volting event which seems quite generous
considering that they're intended to be powered from a regulated noise
free 5 volt source maintained to a +/-5% tolerance rather than directly
from a set of four AA carbon zinc torch cells.

[1] A specification claim that was mirrored in the cmos versions of the
original TTL logic family - in this case a generously conservative claim
- it needed the 5VSB rail on the 'Silent Assassin' versions of those
infamous Bestec PSUs used in E-Machines PCs, to go to 8 or 9 volt,
usually overnight (unless you were in the habit of switching the mains
power off), before the cmos based chipsets finally started to fail (in
subtle and interesting ways to begin with).

--
Johnny B Good
 




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