Advanced Theory of Operation
How does the Pyrodigital system work?
The Pyrodigital (PD) system is essentially a computer. It contains a cpu, memory, input and output ports, most everything
your standard PC would contain with a few small differences. Instead of a monitor, it has an LCD (Liquid Crystal Display)
screen. Instead of a hard drive, it contains flash memory. Instead of a keyboard, it contains a keypad. These are some
of the basic differences between your PC and the PD system. The main difference between a computerized firing system and
your typical "manual" firing system is that the computer proccesses discrete information, and the "manual" system uses analog.
The real world is all in analog, so in the end (at the slave),
an analog signal must be sent through the electric match. Hugh? What?! Keep reading.
What do you mean by discrete and analog?
Analog = continuous. Discrete = Either on or off. Everything in the real world is continuous. There are an infinite number
of different values between 0 and 1 in the real world. For example, 0.01, 0.02, or even smaller 0.001, 0.002, so on and so forth.
You can keep choping it down into smaller and smaller numbers, meaning there is a continous flow from 0 to 1, and not just a
sudden jump. However, in a computer system, the computer only understands "on" or "off".
Which is represented in the binary number system as "0" = "off" and "1" = "on". This would constitute the sudden "jump"
from 0 to 1 and is what we mean by "discrete".
So how does all this apply to the Pyrodigital system?
So now that we understand that a computer only deals with "1's" and "0's", we can say that the computer sends out streams
of these values to the slave modules. Each of these values represents 1 bit of data. Eight of these "bits" are known as a byte.
The computer itself is not sending an analog
voltage to the electric match since that is not possible due to it being a discrete system. So how does the computer fire
the electric match? The computer sends out the data, which in our case is the address of a particular shot to be fired. This
data is then interpreted by the slave module. If the cue to be firied is on that slave, a discrete in / analog out type
device such as a transistor is triggered by the discrete components within the slave and will flow analog current out
through the electric match. In other words, the computer is turning the cue "on" and "off". Each slave will activate a firing
pulse of a duration long enough to ensure the electric match fires. This pulse duration is also dependent on the system design.
What exactly is Time Code?
Now that you understand what "data" is, time code is simply a stream of data (usually a byte worth), which represents a time value. In our case,
such as for Pyrodigital FSK, it represents a sequential number, which the Field Controller (FC) interprets as a time. Each number
in the PD FSK time code is unique, and therefore, can only represent one value of time. That is the whole reason behind being able to
plug the time code into the FC at any point during the show, and the FC will be able to sync up to the current point in the soundtrack,
without having had to be sync'd up at the beginning. Makes sense? This is what seperates the PD FSK from the other forms of time code.
So for a simple example (these numbers are made up and do not represent the actual PD values), the byte 00000001 (binary) would equal
time 00001 (decimal), and byte 00000010 would equal 00002, so on and so forth. Each of these different decimal values would represent a unique
time to the FC.
What exactly does FSK mean?
FSK is short for Frequency-Shift-Keying. It is a type of Frequency Modulation (FM). Remember, the soundtrack (we shall assume is
playing from a CD) is sending the time code data (the 1's and 0's) to the FC. How do we send these bits of data to the FC through an
audio system? Simple (figuratively speaking), we modulate an audio carrier with our information. A CD player and all associated
audio equipment, filters all frequencies that are not within the audio spectrum (roughly 40Hz to 20kHz), hence the reason we need to
modulate our information signal (the time code) onto a carrier frequency which falls in the audio spectrum.
This also allows us to broadcast the time code as a "voice" over a
simple hand held radio (yet another great feature of the PD system).
Ok, so then how exactly does FSK work?
Let's say we have an audio carrier (pure sinusiod) of 10KHz. And we have data (our 1's and 0's) coming at 1200bps (bits per second).
If we Frequency Modulate our 10KHz carrier with our data, we get the time code signal. To "Frequency Modulate" simply means to vary the
frequency of the carrier signal according to our data signal. So, our 1's and 0's (on's and off's) represent only two different values
or "changes" that we need to make to the carrier signal. So, we can say that we need our carrier signal to change back and forth bettween
two different frequencies depending on which data value (a 1 or a 0) is currently present. See the three different graphs below for
clarification on FSK modulation.
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