1. Overview of Recording
Fall 2007
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2. ELECTROSTATIC RUBBINGS Think about this stuff
dead cat
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3. Electrical Charge The negative charge is identified with electrons.
Electrons can be “pushed around” wired (circuits) with the use of a battery.
“Resistors” impede this “current”
Ohm’s Law: V=iR
Electrons are components of atoms. An Atom contains a nucleus of protons and other junk. The protons are the fundamental positive charges.
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4. Various Kinds of Electrical Materials
Conductors
Electrons move easily
Some are poor conductors – Resistors!
Insulators
Electrons are held tightly in place by their chemical bonds.
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5. Charge stuff Potential Difference or Voltage Current
The work per unit charge required to move a charge from one point to the other
Current
The amount of charge that passes a single point in a circuit per unit time (1 sec).
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6. Plus …
Resistance
The ratio of the potential difference across a resistor (R=Ohms) to the current flowing THROUGH the resistor.
This is Ohm’s Law
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7. A Magnet +Q
S N
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OP Survey

8. Magnets come in all shapes and sizes
Magnets come in all shapes and sizes. Almost every refrigerator door has two to provide that last, snug pull when it closes.
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9. Like poles repel and unlike poles attract
Like poles repel and unlike poles attract. The magnetic field lines point away from the north pole and point toward the south pole. But they don’t start or stop on those poles, they form continuous loops (inside the magnets).
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10. In 1820 Hans Christian Oersted was showing his students in a lecture that the electric current in the wire did not deflect the compass needle (Figure a). When he held the compass above the wire as in Figure (b) he saw to his amazement that the compass needle deflected. Figure (c) shows the magnetic field created by the current. It is vertically downward where the compass in figure (a) was located and so couldn’t rotate the compass needle.
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11. Magnet Induces a Current in a Closed Circuit
A Changing Magnetic Field Induces a Current
Exercise 35 Chapter 20. Electrical energy has to come from somewhere! Here the falling magnet generates current in the coil. The current has a magnetic field that interacts with the magnetic field of the falling magnet and resists its downward motion. It is conservation of energy again, the energy generating the electric current comes from the gravitational potential energy being lost by the falling magnet.
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12. Magnets Magnets Do NOT attract chages.
Magnetism is a very different phenomenon.
Magnets have N and S poles
Like poles repel
Unlike poles attract
Where have we seem this before??
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13. Other Observations
A magnet moving into a coil produces an electric current (and voltage!).
A wire moving near a magnet will have a current generated in it.
There is a “magnetic field” around a wire.
A loop of wire acts like a small magnet.
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14. electricity and magnetism!
Now you know
everything there
is to know about
electricity and magnetism!
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15. What Reached the Ear?
This is an ANALOG signal. The ear doesn’t respond to digital signals.
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16. The Process
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17. Storage Methods Analog Storage Digital Storage
Mechanical Electrical (Record, cylinder)
Magnetic (Tape, Wire)
Digital Storage
Magnetic (Tape)
Optical (CD)
Electrical (MP3 file on your “Flash Memory”)
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18. Issues We want the process to be fast.
We want to be able to widely distribute the recorded product.
We want the product to reproduce, as well as possible, the original sound.
We want to ENJOY the final reproduction.
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19. OLDEN DAYS – (Screech of Chalk)
Bell's ear Phonautograph was a very unusual variation on the basic technology. The recording mechanism was the human ear. By removing a chunk of skull including the inner ear from a human cadaver, and attaching a stylus to the moving parts of the ear, he was able to use this bio-mechanical device to make a recording of the sounds that entered a recording horn. It recorded on a moving glass strip, coated with a film of carbon, so there are probably no original recordings from it.
Sound
Human
Ear
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20. Gramophone
The graphophone in its original form was an improved form of the phonograph. One main difference, which Edison would soon adopt, was the use of a cardboard-coated wax cylinder instead of a sheet of tinfoil. The exact construction of the cylinders and the materials used changed considerably in later years, though the basic concept of recording into a soft, plastic material was retained. (image from NMAH)
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21. Development - Platter
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22. “HIS MASTERS VOICE”
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23. Western Electric
Western Electric's recorder used electronic amplifiers to drive an electromagnetic cutting head, rather than relying on the acoustic horn. The result was a louder, clearer record.
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24. The Need for the Microphone
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25. An Old Carbon Microphone
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26. The Microphone
The microphone is a device that received the sound vibrations
converts it to an electrical “signal”
Which is then sent to the next stage in the process (later).
The signal tends to be small and gets weaker as it travels down a long wire.
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27. The Microphone Process
MECHANCAL >  ELECTRICAL
Microphone
Signal on
a wire
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28. Consider a powder of metal
Particles of Metal are pressed
closer together.
Resistance is reduced
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29. How does it work?
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30. The “Crystal” Microphone
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31. The Record
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33. Dynamic Microphone
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34. Movies??
Stretched
Horizontally
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35. 1920 Wire Recorder
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36. 1930s Magnetic Tape
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38. Playback
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39. Today
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40. CD
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41. CD
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42. CD OPERATION
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43. Latest and Greatest Many GB FLASH
MP3 !
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44. How do you squeeze all of that stuff on one iPOD?
Compression!
Standard CD records about 74 minutes of music.
This fills 780 MB … almost a GIG!
Compression makes use of what we know about our heads.
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45. What do we know?
If there are two tones sounding at the same time and one is much louder than the other, we would only hear the louder one.
So omit the data for the softer sound.
Omit other sounds that we probably won’t hear.
Give up a little in fidelity.
This will allow a 5 fold reduction in storage space.
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46. Back to your head
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49. Exploded View
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50. FULL CIRCLE!
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