if you're watching something like well
like this on a modern display you
probably don't think too much about what
your device is doing to make all this
happen it's not really that
extraordinary in this modern world
filled with computers and micro
processors in fact I'm willing to bet
you have a rough idea of how this works
but I'll explain anyway if you get up
close to a TV or monitor you'll find
there's a grid made of millions of
little squares called pixels from far
enough away these pixels blend into each
other and our eyes and brains build a
coherent picture to actually create the
image on the screen each pixel can have
some instructions sent to it to tell it
how much light it should emit a series
of controller circuits work with
millions of transistors to methodically
give every one of these squares a
specific brightness values 60 or 120
times a second sometimes more often than
that the instructions are further
divided into three separate values for
red green and blue which when combined
together can create practically any
color you can think of but have you ever
wondered how old school TV worked we've
been sending video signals over the air
for a very long time in fact analog
television predates World War two there
weren't computer or logic circuits
decoding number of values then in fact
there weren't even pixels yet this
somehow worked how what magic is going
on to take the signal coming over this
wire and turn it into a black and white
image as me all about a single digital
circuit to find out let's make things
really simple instead of looking at
millions of pixels let's look at just
one a single pixel is really nothing
more than a point of light without using
any digital circuitry how can we tell
the light what to do easy by controlling
how much power it gets here's an LED
hooked up to a power supply by simply
varying how much voltage it gets you can
change how much light it emits using
radio technology it's really easy to
build a circuit that can control the
brightness of this LED or another light
source based on the strength of a signal
transmitted over the air of course this
is clearly not television but it's at
the core of what makes it work see we
really suck at seeing things that happen
quickly our eyes and brains are just no
good at processing fast visual
information but it's really easy to
trick our eyes and the scene something
that's not really there
if you take a light source and move it
very quickly you no longer see a single
light source instead you see a
continuous line that follows the path of
the light our brains can't process the
lights fast motion and it just blurs
together into a solid line this
phenomenon is called persistence of
vision now if you manipulate the voltage
of the light while you move it you can
make patterns in the line you can find a
lot of toys that exploit persistence of
vision this little 20 questions game
works using this principle a small
circuit board with a few LEDs on it
spins in a circle really fact too fast
for your eyes to keep track of if the
LEDs were lit up all the time it would
just appear to be a continuous circle
but the game uses sensors to track where
the LEDs are and turned each of them on
and off at very specific times by
manipulating the brightness of the leds
and timing it with their motion it can
draw simple text and graphics using just
eight points of light but a CRT
television like this has only one point
of light to work with after all this is
1920s technology and multitasking wasn't
really a thing yet
so first what actually makes the light
well CR T stands for cathode ray tube
the name comes from cathode rays which
were discovered by Johan hit to earth in
1869 William Crookes had created these
goofy tube things that were really
important to early scientific discovery
he was able to evacuate nearly all the
air from these tubes which allowed
electrons to move freely within them
though no one yet knew what electrons
were when electric current was sent
through these tubes something caused
them to glow Johan Hitler was the first
person to piece together that whatever
was causing his phenomenon travelled in
a straight line from the cathode or
negative electrode observing how
extensible between the cathode and the
surface of the tube cast a shadow
Eugene Goldstein gave the name cathode
rays just like rays of sunlight
JJ Thompson would later use these tubes
to work out what these cathode rays were
actually made of and in doing so he
happened to discover the electron so
good on him before we continue
safety warning exploring the innards of
the CRT television can be quite
dangerous a set as small as this can
generate over a thousand volts through
the flyback transformer and the CRTs
glass can store a lethal charge I know
what is and what's not okay to touch and
since you likely don't don't try this at
home if you've ever messed around with
antique grade
you'll have seen vacuum tubes which are
the precursors to transistors these
electronic components have the air
evacuated from them so electrons can
move freely
just like the crooks tubes using a
heater filament to induce thermionic
emission from a cathode they can
manipulate electric current in a bunch
of ways a television CRT is really a
specialized vacuum tube that has had its
top blown way up and out to form a
screen it's then mounted sideways in a
cabinet and you stare at the front of it
that's what brought about the phrase
watching the tube it also explains the
name of this site because it has no air
inside it has to be pretty strong to
counteract the force of the atmosphere
always try and crush it
that's why larger tube TVs are so heavy
the glass needs to be quite thick on
larger sets most of the tube is empty
space with the meat and potatoes being
at the very back here you'll find the
oftenly named electron gun this
component generates a stream of
electrons and they are shot straight out
to the front of the tube the flyback
transformer generates an extremely high
voltage in the anode to attract the
electrons to the front of the screen
coating the inside surface of the tube
with a special powder known as a
phosphor when the electron sent from the
gun hit the phosphor it gets all excited
and emits light via fluorescence but
only in the spot the electrons are
hitting it here's a working CRT with one
of the critical components to television
removes because we haven't gotten that
far don't worry we'll get there
so the CRT is doing a bang-up job
producing a stream of electrons and
they're going straight to the front of
the screen and colliding with it to make
it glow and this is the result so
fascinating but hang on there's more to
it the vast majority of the signal
coming into the TV is simply telling it
how bright to make this point of light
therefore a signal that alternates
between bright and dark will make this
happen
amazing that doesn't do that much good
ah but you see the point of light can be
moved one of the things crooks and
others noticed when mucking about with
these tube things was that a magnetic
field can bend an electron beam in other
words you can use a magnet to alter the
path of being makes through the tube
watch here's an ordinary strong magnet
used for a name tag when I move it
around the neck of the tube
the point of light moves around the
screen also mind-bending more like being
bending am i right back so then here
comes the other bit this little bundle
of wires is called the deflection yoke
this is responsible for moving the beam
really really quickly and fooling your
eyes the yoke is made of two
electromagnets that surround the neck of
the tube and they work together to move
the electron beam around in a set
pattern it does this by creating a
fairly strong magnetic field which will
deflect the beams path first I'll turn
on the horizontal deflector now rather
than a point of light we see a line this
line is being drawn on the screen
thousands of times a second way too fast
for your eyes to notice just like the
POV effect from the toy if we carefully
control how bright this line is as it
moves left and right we can create
patterns in the line like this
but the yoke contains another
electromagnet that can move the beam up
and down let's switch to that one we now
have a vertical line being drawn on the
screen and we can control its intensity
just like the horizontal one to draw
patterns this vertical movement happens
much more slowly than the horizontal
movement with the line only being drawn
60 times in a second now since we can
point the beam from left to right as
well as up and down we can point it
anywhere we want on the screen let's
turn on both electromagnets at the same
time we now have an image on the whole
screen pretty neat huh
by carefully controlling the intensity
of the beam over time we can create a
complete image if you look really
closely at a black and white television
you won't find pixels rather you'll find
lines see the image is made of lions in
fact there are roughly 525 lines that
make up the NTSC signal at about 480 are
visible on the screen the deflection
yoke is making a pattern on the screen
called a raster and in NTSC countries
it's drawn on the screen 60 times a
second there's a bit of a trick though
because the screen is really completely
redrawn 30 times a second see as the
raster is drawn only every other line is
filled in this is called a field and
it's the principle behind interlaced
video that's the eye in 1080i because
not a lot of bandwidth is available the
whole screen can only reasonably be
filled in 30 times a second but this
would be noticeable it's slicker and
could give many people headaches by
skipping every other line and then
repeating the scan to fill in the rest
the screen is drawn from top to bottom
60 times a second which was too fast for
most people to proceed flickering also
it allowed for smoother motion with the
caveat that fast moving objects would
have less detail as only every other
line is filled in with each field
however that never proved to be a huge
concern as it's hard to see detail and
fast moving objects anyway side note
it's no coincidence that the 60 Hertz
refresh rate matches the frequency of
the AC electricity sent into homes as
the 60 Hertz sine wave coming from the
socket powering the TV made for a
convenient timing reference for vertical
scanning PAL countries which have 50
Hertz electricity have a television
frame rate of 25 frames per second
interlaced with a scanning refresh rate
of 50 Hertz
so TV frame rates are what they are
because convenience so now that we have
the means to generate this raster well
how does that make a picture well it's
just like the T of the effects from the
toy or Lance a hell of a lot faster and
the light moves in two dimensions let's
slow down time and see how the TV builds
an image let's say we want to show this
on the screen at the start of the field
the deflection yoke is pointing the
electron beam at the top left of the
screen as it moves towards the right the
beam changes its intensity along with
how bright the image should be so at a
point along the line that's bright
it produces a lot of electrons and
that's that point on the screen glows
brightly dark parts sends little to no
electrons when the beam gets to the end
of the line the deflection yoke almost
instantly pulls it back to the left-hand
side and starts the next line but
remember it's skipped a line this
process or peak until it reaches the
bottom of the screen
vanyo flings the beam back to the top
and we start again filling in the
alternate lines this happens way too
fast for us to notice it so it appears
like a fully illuminated screen one
thing to note is that the vertical
deflection isn't happening in steps
rather it's a constant downward motion
this means the horizontal lines are
actually slightly slanted downwards to
the right to counteract this the
deflection yoke is mounted to the tube
ever so slightly crooked so the lines on
the screen are actually level the
constant downward travel is also how the
interlacing is accomplished the next
line will start at the same height as
the end of the first which creates a gap
you may remember an extremely
high-pitched noise coming from a TV set
whenever it was turned on this noise
actually came from the deflection yoke
and the electronics that drive it in
NTSC televisions the horizontal
deflection occurred 525 times per frame
and there are 30 frames in a second
which means the electron beam is being
deflected left and right 15,000 750
times per second in powell countries the
frame rate is only 25 frames per second
but 625 lines are drawn with each frame
which works out to 15,625 deflections
per second the yolk and the flyback
transformer along with some other
components actually vibrate at this
frequency ever so slightly which
produces audible noise this is what it
sounds like adults over the age of 25 or
so can't hear this sound as it's at the
upper limits of human hearing range
which gradually diminishes with age so
for those viewers sorry
when it comes to actually producing an
image the trickiest part is matching
that raster to an incoming television
signal to help with this the TV signal
contains triggers which assist the TV
and grabbing hold of the image and
keeping it in one place this TV is
correctly chained to channel 3 which is
currently displaying this video that
you're hearing right now but there's
absolute nonsense on the screen what
gives
well the teaming is generating its own
raster and right now it's not
synchronized with the raster coming into
the TV you're seeing all the image but
each part is in the wrong place because
it's not lined up here to show you what
the TV is looking for let's say to white
you'll notice that there are a lot of
black gaps swirling around which should
be an entirely white screen these gaps
are the horizontal blanking intervals
between individual scan lines when
horizontal hold is properly adjusted
electronics and the TV can see these
gaps and line them up hold up how can
this set tell the difference between the
blanking intervals and a black spot on
the screen well it can tell them apart
because the blanking intervals are
actually blacker than black no really
here's one line of a television signal
drawn on a graph these parts at the end
are the blanking intervals between scan
lines they are the lowest parts of the
graph because their amplitude is near
zero here's the actual start of the scan
line the higher the line goes here the
brighter that part of the scan line will
be drawn on the screen makes sense but
black is all the way up here television
sets are calibrated to not fire the
electron gun at amplitudes at or below
this amount so to the eye any amplitude
below this point won't be visible but
the electronics can clearly tell
blanking intervals from signals the
blanking interval isn't there just to
provide a reference for the beginning
and end of a scanline though it's also
there to prevent anything from being
drawn on the screen as the deflection
yoke sweeps the electron beam back to
the left hand side before the start of
the next line the TV just has to line
these low points up by catching them at
the beginning of each scan line and then
they'll fall into the TV's own Raptor
everything is hunky-dory so then when I
adjust the horizontal hold you can see
that this moves the blanking intervals
closer to each other and eventually the
image snaps
place is sort of now the image is
rolling it's continually moving
downwards ah see we've only synchronized
the televisions raster with the
horizontal components of the signal
without a reference as to what starts a
field scan the picture is just going to
roll around like this see that hunk of
black between my head and my waist
that's the vertical blanking interval
which is little more than just a bunch
of empty skin lines just like the
horizontal intervals it allows the
deflection yoke time to get back to the
top of the field again this is blacker
than black and it allows the television
to hold on to the start of each field
and keep them in one place the vertical
blanking interval also contains some
special pulses to differentiate between
the odd and even numbered fields so I'll
adjust the vertical hold and eventually
the frame snaps into place and you get a
truly stable image very intentionally
the CRT is scanning outside the borders
of the face of the tube this is called
over scan and it's done to hide the
blanking intervals as well as just
ensure the whole screen is being used on
this set you can see how the scan
extends beyond the tube itself when
looking from behind this unseen over
scan area was used later to add closed
captioning into television broadcasts on
one of the lines that make up the
vertical blanking interval alternating
white black bits created a barcode of
source that contained two digital text
information a decoder inside the
television set could read this data from
that line and when enabled place text
graphics on top of the image I think
that's pretty frigging nifty as far as
audio well that's really simple that's
nothing more than a simple FM radio
built into the TV and each channel has
an audio signal being transmitted at a
set offset frequency from the video
source since the signals are transmitted
together they are always in sync so
that's how these old things work but
there's a lot more to explore for one
how did television cameras actually
create the signal that drives this TV
and who are the people responsible for
inventing it what about color we'll
explore all that in later episodes along
with the precursor to CRT television
mechanical television so be sure to
subscribe to technology connections if
you like this video I humbly ask that
you hit that like button and maybe leave
a comment I'm doing my best to keep
videos like this headed your way thanks
for watching
darkness well we're going to give up
with that