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  • This video is remarkable.

  • You're hearing these words months, maybe years, after I've spoken them, yet everything is as clear as if we were sitting in the same room.

  • The ability to record and transmit different kinds of information is a core part of modern engineeringand the world as we know it.

  • That's why some say we're living in the information age.

  • Whether you're using your phone, turning on the radio, or strumming your electric guitar, you're sending and receiving signals all the time.

  • And to get all that information where it needs to go, you'll need signal processing.

  • [Theme Music]

  • As an engineer, communicating means more than having a chat in the break room.

  • Whether you're watching YouTube videos, using satellite navigation, or just making a phone call, there's communication happening.

  • Signals are representations of the information we're sending when we do this.

  • Text, sounds, images, and even computer files will all be converted into a signal when you send them.

  • And that's really what communication is, sending stuff from one place to another to convey information.

  • The basic task is to take content, turn it into a signal, transmit it, and then turn it all back into content on the other end.

  • These steps are known as signal processing.

  • The signal itself will be a current running through a wire or an electromagnetic wave, like radio or light.

  • However you choose to relay it, the overall process is basically the same.

  • The problem of communicating remotely is one engineers faced long before digital computers came onto the scene.

  • We saw an example of this in the history of electrical engineering, with Samuel Morse's 1837 telegraph.

  • In his design, the operator pushed down a lever, called a key, to complete a circuit and transmit an electric current down a wire.

  • At the other end, a machine called a register would receive that current and mark a piece of paper.

  • By pressing down the key for different lengths of time, the operator could make the register draw little dots and dashes that spelled out a message.

  • The key and register in Morse' telegraph are both examples of what are called transducers.

  • Transducers take physical information, like the operator's press of the lever, and turn it into a signal or vice versa.

  • To record this video, for example, the input transducers were the microphone and the camera I'm speaking to,

  • which measures the sound and light in this environment and converted them to electrical signals.

  • Watching the video involves output transducers, things like your headphones and monitor.

  • Unlike Morse' system, however, the signal won't stay in one form between transducers.

  • It might start out as an electric current in the camera that gets converted into a file on a memory card.

  • That's transmitted again as a signal when we send the file to a computer or upload it to the internet, where it's stored on YouTube's servers.

  • At least, until you request that the signal be sent to you in its final form, to be converted back into light and sound.

  • Morse's system was popular because it was simple and remarkably easy to use, ushering in the era of instant communication we enjoy today.

  • The ingenious part was finding a way to take information as people understand it, in terms of ordinary letters and words,

  • and encode it in a form that could be transmitted as electricity.

  • Encoding is a key part of signal processing.

  • Signals need a transmission-friendly way of representing the information you're trying to relay.

  • A hundred years after Morse unveiled his telegraph, it was replaced by more sophisticated and convenient forms of communication, like telephones and radios.

  • But these methodsand everything up to the internet todayare still based on encoding.

  • It's the way the information is encoded and how it's transmitted that's changed.

  • Consider radio waves, like the kind used to transmit signals between your phone and a cell tower.

  • It's the wave nature of radio that lets your phone encode the information you need to make a call.

  • Engineers design hardware that changes, or modulates, the behavior of that wave to encode information about the pressure of the air near the microphone

  • in other words, the physical effects of sound.

  • Two of the most common ways of doing this are Amplitude Modulation and Frequency Modulation,

  • or AM and FMthat's where the names on your radio dial come from!

  • One adjusts the amplitude, or strength of the wave, while the other changes the frequency, or distance between one peak and the next.

  • Much like telegraph signals, the transmitted wave carries the information you want, which is then decoded on the other side.

  • Similar methods can even represent sounds and images, which is how television broadcasts work.

  • But these methods have two pretty big limitations!

  • The first is capacity.

  • The signal of a radio wave can be thought of as a combination of other, simpler waves put together.

  • Specifically, you can represent a signal as the sum of radio waves with different frequencies.

  • The range of different frequencies you can represent is called the bandwidth,

  • and it limits how much information can be encoded by your signal, as well as how many of them can be sent at the same time.

  • Think of signals as fluids and radio channels as pipes; the bandwidth is like the size of the pipe, which controls how much fluid can flow at once.

  • The other problem is noise.

  • As they travel through the atmosphere, radio waves interfere with each other and are warped by objects in their path, which both cause distortions.

  • So the signal the other person receives usually ends up pretty different from the one that you sent!

  • Noise is anything that changes your signal from its original form, usually in a random way.

  • The greater the noise, the more distorted and unrecognizable the received message will be.

  • That's why old TV sets sometimes ended up with 'static' in the image!

  • To go back to the pipe analogy, noise would be any contamination the pipe puts into the fluid, changing its concentration.

  • A tiny, contaminated pipe does a pretty terrible job of delivering lots of clean water.

  • So as you can imagine, noisy channels with low bandwidth aren't great for sending signals that can be reliably decoded on the receiving end.

  • Worse still, both of these problems happen for wired communications as well.

  • The signal traveling down a wire is also a wave, where the amplitude is represented by the the power of the electric current at any given point in time.

  • That's how we modulate electric currents to carry signals, but it also means that those signals suffer from noise and capacity issues, too.

  • Radio and wired communications faced these sorts of problems during World War II,

  • which brought them to the attention of engineer and mathematician Claude Shannon.

  • In 1948, he published A Mathematical Theory of Communication,

  • which revolutionized how engineers consider information itself, and what it takes to send information reliably.

  • Among Shannon's contributions was a mathematical formula for determining the conditions needed for sending a signal at a particular rate.

  • Imagine sending a Morse Code message down a noisy wire.

  • Each segment of the code represents a dot or a dash, what you might call a “bitof the message.

  • Bitstands forbinary digit”, because each part of our message only occupies one of two states.

  • In his paper, Shannon developed a formula that determines the number of bits you can transmit per second, orbit-rate” –

  • given the power of your signal, the amount of noise, and the bandwidth of the channel.

  • When your internet provider advertises a speed of 50 megabits per second, that's Shannon's bit rate!

  • He figured out that it's the ratio of the power of the signal to the power of the noise that determines the bit rate.

  • So either the signal needs to be strong enough, or the bandwidth needs to be large enough

  • for there to be so many frequencies representing the signal that noise can't affect them all at once.

  • As well as this handy formula, Shannon laid out lots of groundwork for calculating the exact conditions needed for reliable communication.

  • Just as importantly, he worked out what kinds of signals you might need to represent the information you're trying to communicate.

  • That work would be vital once signal processing entered the digital age.

  • Digital signals represent information using a small set of distinct states rather than the continuous variation of a wave.

  • Instead of FM radio, where changes in frequency translate exactly to changes in sound,

  • digital radio sends the data piece by piece and everything is reassembled on the receiving end.

  • Because the different states of the signal can be more distinct, they're much less susceptible to noise.

  • A large difference is easier to distinguish than a small one, even when it gets distorted.

  • Morse code, with its dots, dashes, and spaces, was an early digital communication system.

  • But it would take the advent of computers for digital signaling to really take off.

  • And it was Shannon's work that allowed computer scientists and electrical engineers to find ways of encoding different kinds of information in terms of 1s and 0s – what we now call binary code.

  • Digital signals have come to form the basis of computing, and every form of data associated with it.

  • All of which are still used today!

  • Of course, we've only just skimmed the surface.

  • Signal processing overlaps with some serious technical challenges.

  • There's the task of actually encoding different sorts of information as signals, and creating channels like phone lines and WiFi routers to transmit them.

  • And there's the challenge of building hardware that transmits the final output, like computer monitors and headphones.

  • But the end result is that you can stream videos like this one at the click of a button, virtually anywhere in the world.

  • I might be a little biased, but I think that it's pretty darn cool.

  • In this episode, we looked at the fundamentals of signal processing.

  • We saw the need to represent information as a signal so it can be transmitted, and an example of that in Morse Code.

  • We explain how wired and wireless communications can suffer from the problems of bandwidth capacity and noise,

  • and how Claude Shannon helped quantify the problem so that engineers could build around those limitations and bring about the digital age.

  • Next time, we're headed out to sea to talk about moving physical objects with ships and marine engineering.

  • Crash Course Engineering is produced in association with PBS Digital Studios,

  • which also produces It's Okay To Be Smart, a show about our curious universe and the science that makes it possible, hosted by Dr. Joe Hanson.

  • Check it out at the link in the description.

  • Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C. Kinney Studio with the help of these wonderful people.

  • And our amazing graphics team is Thought Cafe.

This video is remarkable.

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