How is it we should measure information in a way that
applies to any communication system you can think of –
human, animal or alien?
Well, let's return to the late 19th century,
where, at the time, we were focused –
as we are today – on speed.
One goal to improve speed
was to design a machine which allowed
the operators to input letters –
which we can think of as 'primary symbols' –
and have the machine automate
the lower-level signaling events
such as pulses of electricity
[which] we can call 'secondary symbols.'
And machines can be driven by some clock source,
allowing [them] to generate a precise and
rapid pulse stream, which presumably
would run much faster than any human hand.
One great example of this was
the 'Baudot multiplex system.'
The design was put into service in 1874.
It built off [of] the same conceptual ideas
we've seen in the shutter telegraph.
It consisted of five keys, which could be
played in any combination.
Think of a chord.
Each combination would
represent a unique message.
With five notes –
each either 'on' or 'off' –
you can play 2 to the power of 5 –
or 32 – different chords.
The code assigned the 32 different chords
to each letter of the alphabet
with [those] left over used for carriage returns,
new line[s], and spaces.
So the operator would literally play letters,
and their machine would automatically output
a pulse stream representing the letters.
Like this, for letter T.
Or like this, for letter R.
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.
Behind the 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.
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?
Well, the [factor] limiting [the] 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 [and] submarine cables,
using the existing Morse-code system.
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.
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 0 to a 1.
So, even if we're automating
the detection of these current levels,
there is a fundamental limit
to how [closely] we can squeeze two pulses together.
And this is the same problem
Alice and Bob ran into
with their string-communication system –
which we called the 'maximum pluck speed.'
If they plucked any faster than two plucks
per second, they noticed [their signals]
started to bleed together,
and they got confused.
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, sound,
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 1 second.
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