How is video recording even possible? How can changing the magnetic polarities of microscopic metal particles, that are bonded to a flat ribbon of tape, create energy when being passed over by an equally tiny revolving pick up coil? Wow, that one made you think. In this first section we are going to explore just the basics of how it's done. It really is a fascinating story. One involving something that can't be seen, smelled, tasted or heard and isn't even fully understood, electricity. The discovery of electricity only happened very recently, but it has always been around. Only within the last 150 years or so have we begun to understand some of it's properties. It's staggering to try and comprehend all we already know on this subject so we are going to concentrate on just the part that deals with the electromagnetic recording of video and audio. It sounds complicated but it's really easy to understand and fun to know, so lets take the plunge.
Video recording is possible because of one of the smallest things in the universe, the atom. On the right side of the above graphic are three illustrations of what an atom might look like. No one knows exactly how they look because they are too small to see, but by using indirect observation and mathematics we humans have decided how they should look. We also know how and why they behave the way they do and most of what they are made of. Although we are still smashing them to find smaller and smaller basic particles that make up the atoms. Of the many special particles and forces that go into making an atom we are only going to concern ourselves mostly with the electrons. They are shown in the graphic above as yellow balls with the minus (-) signs on them. The positively charged protons in the nucleus, shown with the plus (+) signs on them, hold them in a spinning circular orbit similar to the earth orbiting around the sun. But they are not permanently locked in these orbits and can move about when enough force is applied. This ability to move gives electrons some very interesting capabilities, like carrying electricity, holding a charge and creating magnetism. Most of their time is spent just flying around in those orbits (also called shells or clouds) and being kept fairly stable by that nucleus located at the atoms center. It is the number of protons in an atom nucleus that dictates the type of element and its characteristics. The number of electrons usually matches the number of protons. Electrons clouds or orbits seem to want to hold a specific number of electrons to reach a state of electrical neutrality or charge. The first orbit out from the nucleus (called the K orbit) wants to hold only two electrons, then it's happy and not looking to borrow any more. The second orbit (or L) wants to hold eight. There are seven orbits known so far to exist and no orbit can hold more than 32 electrons. Most elements have electrons with orbits that incomplete so they get together with other atoms and share electrons with each other. These can be atoms of the same element but most of the time the atoms sharing electrons are different. This joining of dissimilar elements is called a chemical reaction and it results a new material called a compound* (a common example of this is two hydrogen atoms and one oxygen atom bonded together to form water, or . Relax, this is as far as we go with the chemistry lesson, but If you want to see a great interactive chart of all of the atomic elements (called the "Periodic Table"), click here.
Even though elements and compounds are pretty stable their electrons can easily move around each other. And they sometimes can get knocked off. All it takes is a little energy. When two air masses rub against each other they create friction, which is a form of energy. When enough electrons get loose and out of orbit they can pile up in one place. This creates a very large negative charge. Nature likes everything to be neutral and "in balance" so at some point the negative is going to want to find a positive in order to bring everything back to even. When this happens a massive discharge can occur. Lightening is an example of electrons moving violently to the ground or between clouds in an attempt to bring opposing charges back to a neutral state. (The earth is huge compared to the atmosphere and can absorb a lot electrons.) Another way to store up a charge of electrons is to rub your feet along the floor (and then touch a door knob to release it). Stray electrons can be controlled and when they are a lot of good can come from it. In fact, life would probably not be possible if it weren't for the harnessing of electrons. (Of course all the subatomic particles are important but the electron is a major player.) Back to the lightning. Before it strikes, the electrons build up in the clouds (on the water droplets as they slide against each other) and gather together until they reach a critical point. Once they pass critical they discharge within the cloud, or if the surrounding air becomes charged (a condition called plasma) the electrons bolt down to earth. But friction is not the only way to get electrons moving. Chemical batteries can release electrons through a chemical breakdown reaction. The sun can force electrons to move in certain types of atoms when light strikes them. But by far the most useful and controlled way to get electrons moving is by using a generator. Power plants are giant electron generators. They make electrons move by passing a magnet over a wire. Click on the photo.
Now we're going to zero in on electricity and magnetism. Everything conducts electricity to some degree (that is to say moving electrons from one atom to the other) but some materials are better at it than others. Atoms or compounds that freely let electrons hitch a ride over their outer shells (with little resistance) are called conductors. Atoms or compounds that resist electron movement are called insulators. (Please refer to the above illustration.) Insulation is important because when used to surround a conductor the electrons can be encouraged to follow a specific path without wandering off. (Electrons like taking the path of least resistance, but don't we all?) If the conductor is copper (an element that is a good at passing electrons) and the insulator is plastic or rubber (both good at resisting the flow of electrons) electrons can be sent over great distances without the majority of them getting lost along the way. An insulated conductor is generally called a wire. In the lower right of the picture above you can see just such a wire with an insulator (insulation) formed around it. But let's look now at the conductor shown on the atomic level at the top. The reddish colored balls represent copper atoms, their orange centers are the nuclei and the yellow dots are the electrons (with the negative signs on them). Air would be the insulator in this case but the illustration is mainly here to show how electrons hop from one atom to the other (see the curved black arrows). They don't have to travel the entire length of the wire all at once. But the same number that goes in on one end needs to come off at the other (only theoretically because some would get lost due to resistance and heat, no conductor is perfect). All you have to do to get a flowing of electrons (to create what is called a circuit) is to put a positive potential (called a well or hole) on one end and then provide electrons to the other end (the negative). Electrons will then happily travel along from one end to the other. As long as there are electrons being added at the negative end and are being removed at the positive end they will continue to flow creating what is called a current. While they are doing this they are creating magnetism (a magnetic field). Loop enough wires together with electrons traveling though them and some really good things can happen. Wrap it around a bar of a suitable metal and it can make motors, transformers, electromagnets, relays and a whole list of other electrical gizmos. Now would be a good time to take a break, get some refreshments and when your ready, we'll discuss how magnetism rocked our world.
Up to now we've talked about electrons only going in only one direction, but when it comes to passing them through a wire there are two ways they can travel. They can just as easily move back the other way, in reverse. But there's more. They can more back and forth (alternate) very rapidly. This oscillation is called alternating current, or simply AC for short. Same electrons as in the direct kind we've already discussed (called DC) but with AC current the polarity or direction is switched in the wire. A hole or well now alternates back and forth between the two ends of the wire. This switching action makes the electrons jump back and forth so they only travel the length to the atom on either side. The obvious advantage here is shorter traveling time, less overall resistance in the wire, less heat and fewer electrons lost, since they don't have to move very far (just side to side). AC and DC are both important and have special characteristics that make each of them indispensable in our daily lives. (Magnetism can be created when using either AC or DC current.) But for our discussion here we are going pretty much ignore their differences (for now) and treat them the same. So let's go back and explore the magnetism created from making the electrons move and let's use it for something specific, like magnetic recording. Click on the picture.
As mentioned earlier if a wire is looped in a coil around a suitable metal, such as iron, an electromagnet can be created. As the electrons move through the wire they will cause the atoms in the iron to line up in a special way that causes them to temporarily transfer magnetism from the wire. Iron isn't the only element that has this ability, but it's just very good at it. On the atomic level the atoms of many metals are (magnetically) polar and have what is known as opposite instances from one side of the atom to the other. Some metals can retain their magnetism almost permanently, but others revert back to neutral if the electric current is turned off. Magnetic attraction is different from the atomic attraction between the electrons and protons. The two poles of magnetism don't cancel each other out and one can't build up stronger on one side than the other. Because of this the two opposite forces of a magnet are labeled differently. One end is called the north pole and the other is the south, and you never find a north without a south. The attractive forces in a magnet are called the field (or lines of force). Magnets can pick up a number of metals like iron, steel, chromium, nickel and others. Everything in nature is influenced to some degree by magnetism but the ferrous family of metals (see the periodic table of elements) are especially attracted. Looking at the above picture you can see that the shape of the electromagnet doesn't have to be straight. It can be formed in a "U" so that the opposite poles are brought very close together. When this is done the resulting air gap between them creates a strong field that can be specially shaped and easily controlled. This "U" shape (seen on the right in the picture) is used a lot in recording and the gap, shown as the radiating rings of white, is where the recording takes place. During video tape recording a strip of film with metal particles is passed close to the gap. Using this process allows a unique magnetic image or recording to be transferred to the metal material on the tape. But before we discuss recording in more detail let's look at the interesting way iron (or other magnetic alloys) are bonded to a ribbon of film so it can be used for recording. Click on the picture.
The process you see above is used, in one form or another, to make all kinds magnetic recordable materials. That includes hard drives, floppy disks and recording tape. Looking at the illustration we see that finely ground, then shaped metallic particles are mixed with solvents and binders, lastly an encapsulated lubricants is mixed in and this is done in a big hopper. Once in the hopper it is precisely placed upon a long thin sheet of film called the substrate. The applied mixture is magnetically aligned then heated and bonded to the surface. It is engineered to be uniform and to remain permanently attached. (Lubricants are not used in memory disks, hard drives, etc.) The film is cooled to make it stable and then it is cleaned and polished then slit and wound upon large rolls (called pancakes) for loading into video cassettes. Now that we know how magnetic tape is made let's see how magnetism makes it store information. Let's go to the next picture.
From the picture above you can see that what is to be recorded is being picked up by an electrical device, in this case a microphone. (In this example we are only recording sound.) The sound waves traveling in the air hit upon the internal components of the microphone and generates a small amount of electricity. creates a movement of electrons or current to create a magnetic field in an electromagnet. In this case the field is operating a recording head (left). The process goes like this: the physical movement of sound waves makes electrical pulses in a special pickup device (a microphone) that in turn through amplification causes magnetic waves of force in the tiny air gap near the tape. Pass a tape across this gap at a uniform rate and the fluctuations (pulses) will produce a pattern of magnetism along the tape (shown as north and south or NS in the picture). The magnetic force will have aligned the iron particles in such a way that they will remain locked in a stable position until they acted upon by another field during some later recording event (this can be done over and over again). Once the particles are magnetized the tape can be rewound and passed across the same head for playback. During playback the head becomes a pick up device (it is passive). The magnetized particles in the tape will now generate a small field in the head as they pass by (induction). This in turn creates electricity in the coils which can be amplified, converted and made to move a wire coil attached to a moving cone in a speaker. This will reproduce a sound that duplicates the original (mechanical waves of force). Simply put we have sound going in using a microphone, converted, passed by an electromagnet and saved to magnetic tape. The tape is then rewound, passed over the head again for playback, converted and amplified so the sound that was recorded earlier comes out of a speaker. There you have it. Now, there are many factors that can affect the quality and strength of the recording and we're going to look at some of these now. Next picture.
One of the major factors that affects tape recording is the speed of the tape travel. The faster it goes by the head the more information stored in a given time frame. But there is a physical and practical limit. Too fast and friction causes things to heat up, and mechanical components start to move too fast (inertia). To help offset this problem tape manufacturers make the magnetic particles on the tape very small so they can hold more information. Making the gap smaller on the heads also helps. But there is a limit. This is leading somewhere so stay with me. Look at the picture now and you will see three charts. The speed of tape travel is shown here as IPS, inches per second (CMS or centimeters per second is also used). The squiggly line going across these graphs is the frequency response (how much force the atom particles will hold before they can't hold any more). What you're looking at is graph showing the physical limits of tape verses speed as it applies to the recording capability of magnetic tape. These charts are for sound recording and as you can see when the tape that is going by at 7 1/2 IPS shows that the higher frequencies above 30,000 cycles and above are being lost (cycles is also called Hertz in honor of a famous scientist so the expression 30 kHz or 30 kiloHertz is usually used). They are being lost because not enough particles are passing by (and taking their maximum charge of magnetism) in a short enough time to register these higher pulses on the tape. So to overcome this limitation we crank the speed up. Now at 15 IPS, twice as fast, we start to see that much of the higher frequency info is getting there, twice as many atoms are going by per second. 30 IPS and it gets even better, but as stated before, there is a physical limit. Why are high frequencies harder to record than low? The highs are vibrating very fast (cycles) and require more particles to be magnetized for recording. Lows are large, vibrate slow and are easy to record.
Now so far we've concerned ourselves with audio but things get really dicey when you try to record a signal as high as video requires. Click on the picture.
Now, that wasn't very hard was it? And we covered a lot of ground. So next we look at how this long ways scanning system was first used in broadcasting and then eventually made it's way into your home. To go to the panel on reel-to-reel recording click here.
*A special condition exists when the nucleus holds the exact number of protons to hold onto a perfect number of electrons completing an outer orbit. That atom becomes super stable and resists combining with any other atom. These are called the noble gasses and they are chemically neutral. They are helium, neon, argon, krypton, xenon and radon. Since there are seven known electron orbits there could be seven noble gasses, but only six exist. The seventh has yet to be artificially created.
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To got to the next panel "Reel-To-Reel" click here.