Symbols & Symbol Rate Video Lecture - Computer Science Engineering (CSE)

Voiceover: How is that we
should measure information
in a way that applies to
any communication system
you can think of, human,
animal or alien.
(dolphin squeaks)
Well, let's return to
the late 19th century,
where at the time we were
focused as we are today on speed.
(single handed piano music)
And one goal to improve
speed was to design a machine
which allowed operators to input letters,
which we can think of as
primary symbols and have the
machine automate
(Morse Code signals)
the lower-level signaling events,
such as pulses of electricity,
we can call secondary symbols.
And machines can be driven
by some clock source
allowing it to generate a
precise and rapid pulse stream,
which presumably would run much
faster than any human hand.
And one great example of this was
the Baudot Multiplex System.
And the design was put
into service in 1874.
And it built off the same conceptual
ideas we've seen in the shutter telegraph.
It consisted of 5 keys which could
be played in any combination.
Think of it as a chord.
(single hand piano music)
Each combination would
represent a unique message
with five notes each either
on or off, you can play
two to the power of five
or 32 different chords.
The code assigned the 32
different chords to each letter
of the alphabet with
the leftovers used for
carriage returns, new line and spaces.
So the operator would literally
play letters and their
machine would automatically output
a pulse stream representing the letters.
Voiceover: Like this, for letter "T."
Voiceover: Or like this for letter "R".
Voiceover: Or like this for letter B.
So we have an output signal
containing various combinations
of DC Impulses, a signal
that accurately represents
the message typed on the teletypewriter.
(telegram output dinging)
The iambic counter, the
mechanical nerves of the system,
change words to holes on
tape, and the holes on tape
to electrical impulses
speeding over the wires.
(soft thoughtful music)
Voiceover: Notice at the
lowest level, this system is
exchanging either the presence
or absence of electrical
current in a sequence
divided using a clock.
So, how fast can our internal clocks run?
The limiting speed was not the clock.
Then and today, the
speed of transmission was
physically limited by the minimum spaces
between these impulses or the pulse rate.
And this problem plagued
engineers who were testing
underground submarine cables using
the existing Morse Code system.
And it's similar to an
echo or a sustained note.
If one sends dots too fast
over a long undersea circuit,
they will run together
at the receiving end.
Because the symbol we receive
at the far end of the circuit
will be a slightly longer smoothed out
rise and fall, not an exact replica.
And sending pulses too fast,
results in inter-symbol interference.
This occurs, for example,
when the longer flow of a
current bleeds into the next time division
and perhaps reverses a zero to a one.
So even if we are automating
the detection of these
current levels, there is a
fundamental limit to how far
we can squeeze two pulses together.
And this is the same problem
Alice and Bob ran into with
their string communication system,
(snow rustling underfoot)
(heavy breathing)
which we called the maximum pluck speed.
If they plucked any faster
than two plucks per second,
(plucked string reverberates)
they noticed they started to bleed
together and they got confused.
(Slow string music)
So this is called the symbol rate.
Remember, a symbol can be broadly defined
as the current state of
some observable signal,
which persists for a fixed period of time.
Whether you are using fire,
(crackling)
sound,
(single string plucked)
electrical current,
anything, a signaling event
is simply a change from
one state to another.
So the symbol rate is the
number of signaling events
which can be squeezed
together in one second.
(Morse Code signals)