In this note, we are going to know about the Mechanical Phonographic Cylinder and Disc Recording of Wax Disc or Lacquer Disc. Welcome to Poly Notes Hub, a leading destination for engineering notes.
Author Name: Arun Paul.
Mechanical Phonographic Cylinder
When an audio signal passes through a coil wound on an armature and placed in a magnetic field, the armature moves to and fro in response to the audio signal. A cutting needle is fixed to the armature, and it also vibrates. The cutter moves from the edge to the centre on a rotating disc made of wax or lacquer, and hence cuts spiral grooves on the surface of the disc, which change their positions laterally by the audio signal.
In the absence of sound, the grooves shall be uniformly spaced and move spirally, ending at the centre, as shown in the figure. The audio signal causes the grooves to shift laterally, and thus, the sound is recorded on the disc as shown in the figure.
Diagram of Phonographic Cylinder and Disc Recording
A disc recording unit is shown in the figure. It consists of a powerful electromagnet giving a strong magnetic field between the north and south poles when a DC or Direct Current is passed through the field coil. An armature of soft iron is placed between the two pole pieces. A coil is wound on the armature, and a cutter stylus is attached to the lower end of the armature.
When an audio current flows in the armature coil, it produces a varying magnetic field that is superimposed on the steady field of the electromagnet. This causes the armature to vibrate in the horizontal plane. These vibrations are caused by the variation of the audio current and are transferred to the cutting needle, which is made of diamond.
Diagram of Phonographic Cylinder and Disc Reproduction
For reproduction, a device called a cartridge is made to change the vibrations of the playback needle into electrical signals, which can be amplified and then converted into sound by a loudspeaker. The working principle of a cartridge is similar to that of a microphone. Like microphones, cartridges can be of many types. A magnetic cartridge is shown in the figure. When the needle of the record player tracks the recorded grooves on the disc, it vibarates in accordance with lateral variations of grooves. These vibrations move a magnet placed in the magnetic field of a fixed permanent magnet.
Due to vibrations of the magnet, the flux density through a coil, placed in that magnetic field, changes and hence an emf is induced in the coil depending on the rate of change of flux. The electrical signal is amplified by transistor amplifiers and is fed to a loudspeaker to get the original sound.