"John Howells" wrote in message
...

"Martin Underwood" wrote

But then the obscure numbers that have become enshrined in standards
have
always intrigued me. Why are audio tape speeds not integer numbers of
inches
per second (eg 2, 4, 8, 16 etc) or else round numbers of cm/sec?

Early wire recorders ran at 60ips (and were VERY dangerous for the
operators
if the wire broke, which happened all too frequently). Speeds of tape
followed, and halved as technology allowed, only going "wrong" below
15ips.

Fair enough. If modern tape speeds are derived from successively halving 60
ips, that makes sense. I wonder how the original figure of 60 was arrived
at. And how do vinyl record speeds of 16 2/3, 33 1/3, 45 and 78 rpm
originate. 33 1/3 is exactly 1/3 of 100 rpm, but how about the others?

The weirdest one is the spec for high-definition TV which uses 1080
lines -
neither 2xPAL (1250) nor 2xNTSC (1050) lines, so *both* standards will
have
to be interpolated using weird conversion factors (leading to loss of
sharpness) when showing old low-definition material on high-definition.

FWIW, the comparison with 1080 should be 1152 for PAL (2x576 active lines)
and 960 for NTSC (2x480), so from NTSC 1080 = 9/4 * 480, and from PAL 1080
=
15/8 * 576. Not such weird conversion factors, so maybe the standards guys
did know something.

Ah, the 1080 "lines" is the number of active lines, is it, not the total
number of line-periods as for PAL/625 and NTSC/575? Fine. I'm surprised
that, given the US/Japanese dominance in the field, the standard isn't 960
active lines, with Europe having to make whatever it can of the resulting
mess ;-)

"Dave Fawthrop" wrote in message
...
On Sat, 28 Feb 2004 18:50:00 GMT, "Martin Underwood"
wrote:

| Why is the
| UK railway gauge 4' 8½" rather than 4' 6", 4' 9" or 5' (or else round
| numbers of centimetres)?

That one at least is easy, it depends on the width of the bums of two
horses. The original ?railway? trucks were ordinary carts, with wheels
4" 8 1/2" pulled by horses, along wooden rails. Wooden rails were much
smoother than cart tracks. As improvements like iron rails, and steam
locomotives the gauge stayed the same. Roman roads have ruts of much the
same gauge.

http://www.spartacus.schoolnet.co.uk/RAgauge.htm

Isombard Kingdom Brunell had a great idea when he designed the Great
Western Railway. He built it in the technically much better Broad Gauge
2.2m but the standard won out against the better.

You can see a replica of the broad gauge Iron Duke locomotive at National
Railway Museum in York.

All standards happened in a similar sort of way :-(((((((((

Yes, I've heard this one. The width of two average horses' bums gives the
approximate gauge, but why not then round the gauge to the nearest whole
number of inches, centimetres, or other measurement system? Surely not all
horses' bums were exactly the same size to the nearest half inch! I
understand that different collieries used slightly different gauges - it
just so happened that the one that George Stephenson modelled his railways
on had a gauge of 4' 8½" - which says to me "why did that colliery use this
gauge rather than a round number of inches?".

And why did Brunel choose 7' 0¼" rather than 7' exactly? In one of the sheds
at Didcot Railway Centre I've seen a "derivation" which involves pi, though
I'm not sure whether this is actually Brunel's.

"Martin Underwood" wrote

Fair enough. If modern tape speeds are derived from successively halving
60
ips, that makes sense. I wonder how the original figure of 60 was arrived
at.

No idea. But it was probably a compromise, as are most standards, with an
arbitrary value chosen that gives acceptable quality and still give useful
performance.

And how do vinyl record speeds of 16 2/3, 33 1/3, 45 and 78 rpm
originate. 33 1/3 is exactly 1/3 of 100 rpm, but how about the others?

No idea.

Ah, the 1080 "lines" is the number of active lines, is it, not the total
number of line-periods as for PAL/625 and NTSC/575?

Yes. With a digital system there is no continuous line frequency clock, so
for digital systems the resolution numbers are more like computer displays -
WYSIWYG.

Fine. I'm surprised
that, given the US/Japanese dominance in the field, the standard isn't 960
active lines, with Europe having to make whatever it can of the resulting
mess ;-)

Quite! But I doubt it's an accident that 576 = 480 * 6 / 5.

John Howells

And why did Brunel choose 7' 0¼" rather than 7' exactly?
4' 8½" rather than 4' 6", 4' 9" or 5

Originall 4ft8in and 7ft

the extra was added on when it was found the gauge was a bit too tight so
rather than shorten axles widen gauge

Martin Underwood wrote:

Fair enough. If modern tape speeds are derived from successively halving 60
ips, that makes sense. I wonder how the original figure of 60 was arrived
at. And how do vinyl record speeds of 16 2/3, 33 1/3, 45 and 78 rpm
originate. 33 1/3 is exactly 1/3 of 100 rpm, but how about the others?

I seem to remember that around 100 years ago, records companies used different
standards varying from 72 rpm to 84 rpm. 78 works out to be the average.
Some turntables come with a speed adjustment which will allow for the
exact speed to be played.

The difference between speeds came down manufacturer and the ideal speed
for the old handcranked mechanical gramophones.

--
John

Why is cinema film shot at 24
frames/sec rather than 20, 25 or 30, given that 50Hz (halved to derive 25
Hz) and 60Hz (halved to derive 30 Hz) are the standard mains frequencies?


The rates for film have absolutely nowt to do with mains power. And like
most of the other answers given here have more to do with practicality.
Silent film was run at anything from 8 up to around 20 fps, eventually 18fps
was standardised upon as the ideal rate to give smooth motion without
jerkiness (When each frame was show twice, giving an effective frame rate of
36fps). With film costs of the time, anymore would have been too expensive.
Most film was also 70mm, but was cut in half by film makers (to give twice
the length) who couldn't afford the cost. When sound came along 18fps wasn't
enough length to squeeze in good enough optical or magnetic sound quality,
and the best quality / cost compromise was 24fps (Again each frame is still
shown twice). Film being mechanical could easily be any speed regardless of
the mains voltage or cycle. (Indeed some modern projectors require 3-phase).
25fps only came later when interlaced electrical television was invented as
the easiest way to work with the 50 / 60Hz cycle of modern power.

You might say why not standardise film at 25fps or 30??? Well Hollywood
would never bow down to television, and most projectors can't be easily
adjusted and are fixed at the correct speed. The beauty of their mechanical
nature is that I work today with machines which were built in the 1940s, are
still going strong with spares still readily available. I'd like to see the
correct incarnation of digital / DLP projectors still running in a theatre
in 2060......

Martin Underwood wrote:

Ah, the 1080 "lines" is the number of active lines, is it, not the total
number of line-periods as for PAL/625 and NTSC/575? Fine. I'm surprised
that, given the US/Japanese dominance in the field, the standard isn't 960
active lines, with Europe having to make whatever it can of the resulting
mess ;-)

You're closer than you realise.

1080 is the number of active lines in Japanese 1125-line HDTV, which has
been on air since the early 1980s. Yup, they were there 20 years ago,
and UK broadcasters still regard it as too new-fangled to contemplate.

Still, at least everyone is agreed on 1920x1080 square pixels for HDTV,
which might not have happened if earlier European efforts had borne fruit.

--
Richard Lamont

OpenPGP Key ID: 5ABEC9C3 http://www.stonix.demon.co.uk/key.txt
Fingerprint: 9DEE 7113 DF02 A516 404C 22AC 1FF6 185D 5ABE C9C3

Still, at least everyone is agreed on 1920x1080 square pixels for HDTV,
which might not have happened if earlier European efforts had borne fruit.

Square pixels WOOOHOOOO!!!!!! You don't know how happy that makes me.... No
more pratting about with graphic aspect ratios in Photoshop ;)

In article m, Martin
Underwood wrote:
Fair enough. If modern tape speeds are derived from successively halving 60
ips, that makes sense. I wonder how the original figure of 60 was arrived
at. And how do vinyl record speeds of 16 2/3, 33 1/3, 45 and 78 rpm
originate. 33 1/3 is exactly 1/3 of 100 rpm, but how about the others?

I've been told (though I can't quote a reference) that the speed of 33+1/3
came about because it simplified the gearing for turntables that were linked
to cameras and projectors for the early sound films, which had their sound
tracks on disk.

Rod.

In article , Mat Overton
wrote:
Film being mechanical could easily be any speed regardless of
the mains voltage or cycle. (Indeed some modern projectors require 3-phase).
25fps only came later when interlaced electrical television was invented as
the easiest way to work with the 50 / 60Hz cycle of modern power.

It's nothing to do with interlace. Television frame rates were chosen to be
the same as local mains frequency to minimise the effect of inadequate
smoothing of power supplies, which would cause hum bars on the screen. It was
thought that if the hum bars were stationary or only moving very slowly, they
would be less annoying. Modern power supplies can use many more active
components (because they don't have to be thermionic valves), and so power
supplies can be much more effectively regulated and smoothed, making hum bars
practically non-existent, so monitors can display at virtually any scan
frequency.

You might say why not standardise film at 25fps or 30??? Well Hollywood
would never bow down to television, and most projectors can't be easily
adjusted and are fixed at the correct speed. The beauty of their mechanical
nature is that I work today with machines which were built in the 1940s, are
still going strong with spares still readily available. I'd like to see the
correct incarnation of digital / DLP projectors still running in a theatre
in 2060......

I'm told that many cinema projectors are actually run at 25fps, though whether
this is just in Europe or America as well, I don't know.

Rod.