For better understanding, here are two videos describing about the principle of magnetic recording...
Compact Disks are optical media; they record information by burning pits in the disk and use a laser and optical sensor to detect whether a pit is present or not. A hard disk is a magnetic system - a recording head is moved close to the surface of the disk and used to read or write information.
It is these different technologies that lead to the difference in abilities; CDs are more robust and cheaper, but slower - particularly when writing or re-writing information. Hard drives can hold more information, are faster and can be rewritten indefinitely.
It is these different technologies that lead to the difference in abilities; CDs are more robust and cheaper, but slower - particularly when writing or re-writing information. Hard drives can hold more information, are faster and can be rewritten indefinitely.
1. Analog recording
Analog recording is based on the fact that remnant magnetization of a given material depends on the magnitude of the applied field. The magnetic material is normally in the form of tape, with the tape in its blank form being initially demagnetized. When recording, the tape runs at a constant speed. The writing head magnetizes the tape with current proportional to the signal. A magnetization distribution is achieved along the magnetic tape. Finally, the distribution of the magnetization can be read out, reproducing the original signal. The magnetic tape is typically made by embedding magnetic particles in a plastic binder on polyester film tape. The commonly used magnetic particles are Iron oxide particles or Chromium oxide and metal particles with size of 0.5 micrometers. Analog recording was very popular in audio and video recording. In the past 20 years, however, tape recording has been gradually replaced by digital recording.
2. Digital recording
Instead of creating a magnetization distribution in analog recording, digital recording only need two stable magnetic states, which are the +Ms and -Ms on the hysteresis loop. Examples of digital recording are floppy disks and HDDs. Since digital recording is the main process nowadays and probably in the coming future, the details of magnetic recording will be discussed in the rest of the project using the HDD as an example.
3. Magneto-optical recording
Magneto-optical recording writes/reads optically. When writing, the magnetic medium is heated locally by a laser, which induces a rapid decrease of coercive field. Then, a small magnetic field can be used to switch the magnetization. The reading process is based on magneto-optical Kerr effect. The magnetic medium are typically amorphous R-FeCo thin film (R being a rare earth element). Magneto-optical recording is not very popular. One famous example is Minidisc developed by Sony.
4. Domain propagation memory
Domain propagation memory is also called bubble memory. The basic idea is to control domain wall motion in a magnetic medium that free of microstructure. Bubble refers to stable cylindrical domain. The information is then recorded by the presence/absence of bubble domain. Domain propagation memory has high insensitivity to shock and vibration, so its application are usually in space and aeronautics.
Oberlin Smith published a description of magnetic recording in Electrical World, Sep. 8, 1888, based on his visit to Edison's lab in 1878, using an electromagnet with a string covered with iron filings. He may have built a working model but no device has survived.Above is the "original drawing of Oberlin Smith (1840-1926) published in the Electrcial World of 8.9.1888. The spoken words are transformed by the telephone A into an electrical sound signal and are recorded in the form of magnetization patterns on the sound carrier C, passing through the recording head B.
F = battery, E = take up reel, D = supply reel, J = reel brake." (from Heinz Ritter, 1988)
Valdemar Poulsen in 1894 discovered the magnetic recording principle while working as a mechanic in the Copenhagen Telegraph Company. In 1898 he patented the telegraphone, the first successful magnetic recording device.
Valdemar Poulsen in Denmark would succeed in 1898 where Smith had failed. He built and patented the first working magnetic recorder called the Telegraphone with wire wrapped around a drum and a recording/playback head that moved by a screw thread on top. Poulsen had become a telephone engineer at the Copenhagen Telephone Company in 1893 and began to experiment with magnetism to record telephone messages.
By 1899 he filed U.S. patent 661,619 for a vertical wire-covered cylinder, and in 1900 demonstrated improved drum and horizontal wire cylinder models at the 1900 Paris Exhibition. While making these improved models, Poulsen and his partner Peder O. Pedersen discovered the application of a direct current to the recording head, called dc bias, improved the sound quality on a steel tape version of the Telegraphone. At the Paris fair, Poulsen recorded the voice of Emperor Franz Joseph, today preserved in the Danish Museum of Science and Technology as the oldest magnetic sound recording in existence.
Poulsen stopped his work on magnetic recording and turned to radio after 1902, and only a small number of his machines were made in Denmark and Germany. The American Telegraphone Company acquired the patent rights in 1905 and made dictating machines, selling 50 to the Du Pont Company. However, the signal remained weak without amplification and the wire spools became twisted and were unreliable. The wax cylinder phonographs of the rival Ediphone and Dictaphone companies were cheaper and more reliable. By 1918, the company went into receivership and stopped manufacturing after 1924.
"In 1930 the Allgemeine Elektrizitatsgesellschaft [AEG] in Berlin decided to start the development of a magnetophone machine, based on the Pfleumer principle. 2 years later there was an agreement of collaboration with BASF, Ludwigshafen: AEG developed the system, BASF an appropriate sound carrier. This collaboration did not come by accident. On the contrary, BASF had the knowledge necessary for the development of magnetic tape.
Here since 1925, carbonyl iron powder in the finest particles had been produced for induction coils in telephone cables and for mass cores in the high frequency technique. In addition, experience had been gathered in the manufacture of enamel paint by milling and dispersing of dyestuffs with cellulose acetate and solvents. At the same time the development of plastics had started for the production of foils and fibres. Thanks to this rich experience in 1934 BASF was able to ship the first 50,000 meters of magnetic tape. The tape consisted of a foil of cellulose acetate as carrier material, coated with a lacquer of iron oxide as magnetic pigment and cellulose acetate as binder.