In my last video, we explored how analog television
works.
You can check out the whole video either through
this card or through the link in the description,
but here’s a brief overview.
At its core, analog television is just an
amplitude modulated radio transmission where
the strength of the signal dictates brightness
of a light source, with a strong signal producing
a bright light, and a weak signal producing
little to no light.
The television set uses an electromagnet to
deflect the source of the light, an electron
beam, in a pattern called a raster, which
is really just a ton of horizontal lines.
This has the effect of producing a glowing
series of lines on the face of a picture tube.
The electronics of the TV set line up the
incoming signal with the movement of the beam
to create an image, with each part of the
imaging being drawn brightly or darkly along
with the signal’s instantaneous strength.
With everything in alignment, you get an image.
By far the most complicated part of making
an image appear on the screen is making that
raster pattern.
The electronic components and other crucial
parts such as the picture tube and deflection
yoke are primitive by today’s standards,
but still pretty complicated.
We really just need a way make a repeating
pattern of lines from a light source, there’s
got to be a simpler way to do it!
Enter: Mechanical Television.
The earliest televisions actually got some
of their inspiration from fax machines, really?,
and relied on a couple of important developments.
So first a bit about the fax machines.
Facsimile transmission actually predates the
telephone?
(what?), with images such as signatures being
commercially reproduced over telegraph wires
as far back as 1865, and the earliest fax-like
device being invented by Scottish inventor
Alexander Bain in 1846.
Now, I’m simplifying a great deal here,
but the theory was that if you could synchronize
the movement of a scanning device with a drawing
device, you could replicate an image.
If you scanned a piece of paper line by line
and sent a signal over a wire to match the
darkness of the ink, you could reproduce the
image by syncing up a drawing mechanism with
the scanning one.
These early fax machines worked, but they
were very slow.
Nevertheless, they showed us that you could,
via electro-mechanical means, reproduce an
image.
Fast forward to 1884, and 23-year old Paul
Julius Gottlieb Nipkow created the Nipkow
disc.
This is the core of most mechanical television
systems.
Nipkow realized that a spinning disc could
methodically scan an image line-by-line simply
by placing evenly-spaced holes in a spiral
pattern.
This is a home-made Nipkow disc.
I took a really awful vinyl record that I’d
be happy to destroy, and marked 32 divisions
around the circumference, like 32 very skinny
pie slices.
Then I methodically drilled a hole along these
lines, with each hole being drilled a 32nd
of an inch (roughly .8 milimeters) closer
to the center than the last.
The result is a spiral pattern of holes.
This might not seem like much, but it’s
actually extremely clever.
If you put a square-ish shaped mask in front
of the holes, its height being slightly less
than the distance between the holes, you’ve
made a device which mechanically creates a
raster scan using these physical holes.
John Logie Baird realized that with this Nipkow
disc, you could in theory focus an image with
a lens onto the disc, and you could use a
light sensor to give an instantaneous reading
of how bright each part of the image was,
with the holes in the nipkow disc serving
as a way to divide the image into transmittable
pieces.
Back in 1873, Willoughby Smith discovered
the photoconductivity of selenium, and with
this knowledge Baird used some selenium to
create the light sensor for his televisor.
I’ve mounted this Nipkow disc to an AC motor
which will spin it at 1,800 RPM, giving a
complete revolution 30 times per second.
Before I turn it on, look through the mask.
I’ve put an extremely bright LED behind
the disc so you can see the holes.
As I slowly turn the disc, you’ll see that
only one hole is visible at a time, and each
hole gets closer to the left than the next
one.
When I switch the motor on, the holes blend
into a moving line, and as it gets faster,
the line seems to widen into a square.
This square is very uneven because my homemade
Nipkow disc was made hastily and with poor
precision.
But here’s the key.
Only one of the holes is actually visible
through the mask at any given moment.
It’s just moving too fast to see.
Baird used the selenium light sensor to create
a signal from an image being scanned by the
disc, and on the receiving end, another identical
disc would spin at precisely the same speed,
and a light source such as a neon lamp would
vary its brightness along with the signal
strength presented by the light sensor, and
thus, you’d get an image.
I shall now attempt to show you how this worked.
Now before you get too excited, I’ll admit
that my mechanical television doesn’t work
as well as I had hoped.
And that’s all on my insistence in using
crap I had laying around, rather than going
through the process to make a proper LED driver.
However, I hope you’ll get an understanding
of what’s going on.
This 10W LED chip is what we’ll use as a
light source.
It’s really bright and, importantly, it
can react very quickly to changes in the voltage
it receives.
First, I’ll simply power the LED continuously.
As the disc spins up, the lines start to blend
into each other, and eventually the whole
“screen” is illuminated.
Now, I’m going to switch the LED on and
off at a higher and higher frequency.
First, 5 hz.
The screen appears to just be flashing, nothing
too extraordinary, but you might be able to
see some odd stuff happening as the light
switches states.
Now I’ll switch it on and off at 60 hz.
Something odd starts to be visible here.
See, the disc makes a complete revolution
30 times per second, and with the light flashing
at twice that frequency, only some of the
holes are lit up as the disc passes over the
LED.
Now let’s move to 1,800 hz.
Frequencies that are a multiple of 30 will
appear stable as an even number of pulses
fit within each revolution.
If you mess with that, though, things get
weird.
Bumping the frequency up just a tad makes
the pattern move in relation to the disc.
The holes in the disc are directly responsible
for creating the patterns you see.
Without the disc, the LED appears to just
be continuously illuminated.
But, it’s not.
It’s flashing really quickly.
The disc allows for that flashing to be visible
because it physically obscures different parts
of the light source over time.
This is just like the electron beam in the
CRT television, except instead of electromagnets
moving a beam across the surface of a picture
tube, the light source is physically moved
via the location of these holes.
It’s a pretty crafty way of producing a
raster scan, and it actually works.
This is the best imagery I could get my televisor
to produce.
This pattern was generated through manipulating
audio samples in Audacity.
To give you an idea of how poorly this mechanical
TV works, well the image I intended to make
was not a map of the world as this vaguely
suggests, but that of a circle.
Here’s a look at true video.
What you’re seeing here is a very low contrast,
very low resolution image of Seth Meyers.
I mean obviously, how could you not recognize
him?
Yeah OK, it’s garbage, but you can see that
there is certainly something there and it’s
moving slightly.
Like a talk show host’s head might when
said talk show host is talking.
On his show.
To make this image, I simply placed my phone
behind the televisor with the screen brightness
all the way up, and I placed this solar panel
with an audio cord patched into it into one
of my trusty Tascam DR-05 audio recorders,
which I use all the time.
In fact there’s one in my pocket right now.
And yes, that’s directly from a solar garden
light.
The solar panel would produce a high current
whenever it saw bright light, and it would
produce low current with less light.
Duh.
As the disc spun, it would only allow the
tiniest bit of the image through to the solar
panel at any given time.
This would produce a quickly varying signal
with amplitude corresponding to image brightness.
The TASCAM would just encode these relative
brightnesses as sound samples, at a sample
rate of 48 kilohertz, and then because I’m
really lazy, I just hooked my LED into an
audio amplifier and played that sound back.
The LED would become brighter with a stronger
signal from the amplifier, though as it’s
a diode it would filter out any AC components
of the signal.
Quite honestly I’m amazed it produces anything
at all.
I opened the file in Audacity just to see
what it looked like, and it’s pretty intriguing.
Here’s what it sounds like, for those interested.
Now, in case it’s not obvious, let’s go
over the reasons mechanical television didn’t
catch on.
First, up until now, I’ve not let you hear
what this sounds like.
Here’s what a 12 inch vinyl disc sounds
like at 1,800 RPM.
I’m sure that would never get old.
But aside from that, there are just so many
practical concerns with mechanical TV.
First of all, the image is tiny.
And it’s a horribly low resolution--only
32 lines.
That’s the only reason a signal can be recorded
as an audio file.
Not a lot of bandwidth is needed.
Because the disc obscures almost all of the
light source, hardly any light gets through.
This LED is fricken bright, it’s painful
to look at directly, and yet through the Nipkow
disc, nearly all of the light is blocked,
and it produces a dull image.
When these devices were first in development,
the light source would often be a neon lamp,
like the orange light in a powerstrip’s
switch.
Imagine how dark the image would be with only
that for a light source.
One of the biggest troubles with mechanical
television is image synchronization.
Because we’re using a big spinning thing
to divide the light into chunks, the disc
has to be in precisely the right place if
you want the image to land where it should.
If we take the mask away, you can see that
the image just repeats itself over and over.
But each adjacent image is actually shifted
one line up or down.
The most critical part of synchronization
was ensuring the disc is spinning at the exact
same speed as the scanning disc of a camera,
but it would also be necessary to slow down
or speed up the disc in slight increments
to get the image aligned with the viewing
mask, and with the top and bottom in the right
place.
But the most damning problem is that of geometry.
Imagine we wanted to make a display with the
resolution and size of this small CRT television.
Well, the face of the tube is about 15 cm
wide.
With 480 lines of resolution, there would
need to be 480 holes in the nipkow disc.
Remember, only one hole can be seen through
the mask at once for this to work, so the
holes have to be at a minimum 15 centimeters
apart.
So the disc’s circumference would have to
be 72 meters, with a diameter of roughly 23
meters, or about 75 feet.
I live in a building that’s 6 stories tall.
A mechanical television to rival this TV would
be taller than my building!
And, it would have to spin at 1,800 RPM just
like this one to make 30 frames per second
possible.
This things scares me spinning this fast.
I’m pretty sure a 75 foot disc would just
explode.
In fact, let’s do the math.
A 72 meter circumference means that the edge
of the disc would travel 2.160 kilometers
per second, or well above mach 6.
Yeah.
If the disc were rolling, it would make it
from New York to Los Angeles in about 35 minutes--not
in a straight line, mind you, but by traveling
along actual roads.
So, the Baird television system didn’t get
too far.
It was certainly genious and is an important
part of the history of television.
But is was far too limited, clunky, and, to
be honest, it had crappy image quality.
I’ve added some links in the description
to videos of mechanical televisions that actually
work, and I think you’ll agree that’s
they’re pretty cool, but it’s a damn good
thing they didn’t become mainstream.
As always, thank you so much for watching.
If you liked this video, a thumbs up would
be most appreciated.
I’m absolutely thrilled that this channel
has over 21,000 subscribers now!
I never thought that would be possibl.
If you’re not one of the people in that
number, and you liked this video, I humbly
ask that you become one of them by pressing
that subscribe button.
I’m doing my best to keep videos like this
headed your way, and I’ll see you next time!
