 Hello friends, myself Mr. Alia Arsanjane, Assistant Professor from Department of Electronics, Walchen Institute of Technology, Solapur. Today we are going to see the topic optical recording and reproduction of sound on two different surfaces. So, what are the learning outcomes from this particular topic? At the end of this topic, student is able to describe the working principle of optical recording and reproduction systems on film. Also, he will be able to describe the working principle of optical recording and reproduction system on compact disc. So, optical recording and reproduction of sound can take place in two different methods. So, first one is variable density method and second one is variable area method. So, we are going to see these two methods one by one. So, what is the basic principle of recording on photographic film? First we are going to see this. So, this is the recording of sound on photographic film is done by converting audio signals into variations of light intensities falling on the film. So, such a type of sound which is recorded it is called as track which is having a width of 2.5 mm and it is on the edge of the movie film. So, intensity of the sound basically the sound intensity from a slit it is made varied in accordance with the sound pressure variations and hence it is also called as sound modulation. So, this varying light basically it falls on the edge of main film hence a photograph of varying light intensity gets recorded in the same way and the intensity basically it is related to the different types of pictures which are getting imprinted on the film by the light. So, first we are going to see variable density method. So, this is the diagram which shows the variable density method mechanism as here you can see basically this diagram has three different three to four different parts. So, this is the input side this is the this is the AEO bulb through which the light variable light gets incident onto the slit and then there is a lens this particular lens is a convex lens. So, by using this the light variable light intensity is concentrated on the edge of the film. So, how or what how this this mechanism basically works this mechanism works as given. So, here the sound is first picked up by mic and then it is converted into amplified signal. These amplified audio signals are then fed to a special type of vacuum tube which is called as AEO lamp which contains helium gas. The anode of the the anode of the vacuum tube gets high DC voltage in series with the volt with the audio voltage. Hence, the intensity of light coming out from the lamp varies in accordance with the audio signals which are at given at the input side. This light is then passes through the slit and it gets incident on the focusing lens. Hence, the light falls on the moving photographic film where image is recorded in the form of bars of varying densities which are basically called as track and these are having width these are having a width of 2.5 mm. So, the second method is variable area method. So, you can look at this particular diagram this diagram is the mechanism which is used for recording of sound by using variable area method. As you can see here there is a magnet there is a spring this spring assembly is attached to the mirror and the input is given at the mirror side and there is a lamp which is giving constant intensity. The light rays are then concentrated onto the mirror by using a convex lens the reflected light then passes through the slit and then it gets concentrated on the photographic film. So, here how this particular method or mechanism works? So, in this method constant light intensity is falling on a slit the area of slit varies in accordance with the variations in sound pressures which are given at input side. Hence, the light falls on variable area on soundtrack at the edge of the film. Hence, the area which is bright to the light varies the area of the slit is made variable with the help of galvanometer or the mirror. Here the sound first gets converted into electrical signals by using mic these signals are amplified and then given to the coil of galvanometer where the deflection takes place in accordance with the sound variations. Hence, the light from the lamp duly focused by a lens system falls on the mirror and hence the reflected light from the mirror goes on to the to a narrow slit where the mirror deflects the slit area exposed to the light and hence slit gets illuminated partially. The extent of slit illumination basically depends on the mirror deflections and the galvanometer deflection and hence on the audio current. The light from variable area of slit falls on to the soundtrack edge of the film and hence the sound gets recorded on to the photographic film by using a variable area method. So, how does the sound is reproduced from the film? It is reproduced by using this particular mechanism as you can see here this is the film and this film is then made it is then made incident to the light which is coming from the lamp. This light is then focused by using a focusing lens it then passes through the slit. The dispersed light coming out of the slit is again focused on to the by using a convex lens on to the film where the sound the sound is converted into different light variations. These light variations are then converted into electrical signals by using a photo cell and then given to the audio amplifier and then to the loudspeaker for the hearing purpose. There is also the second method in this you can you will you will get a clear cut idea that this is the take up assembly upon which the film is bounded as soon as it gets unwound it passes through these two pulleys and then the film is gets incident to the light it gets incident to the light and the width of the film or the picture gate or the movie film and the sound gate these are the three major components which on the film surface. So, movie film is nothing but the image and sound is nothing but it is which which has been recorded at the edge of the film and then the field rail is nothing but where the film photographic film gets bounded again. So, how does the sound recording takes place on the disc? So, this is the mechanism here you can see there is a magnet onto this magnet a field coil is bounded and there is a armature assembly which consists of a coil a cutter and then this is the armature. So, as soon as and and this is the disc surface which is coated with wax and lacquer coating. So, as soon as the DC is nothing but the audio signals. So, when the DC current or the audio signals are given to the field coil what exactly happens a EMF is generated this EMF is then passes through the magnet and it gets induced into the coil. So, the EMF is nothing but it is a varying one and these varying EMF is analogous to the input sound sound variations sound variations. So, according to that the EMF gets induced into the coil due to this the armature starts to move the armature assembly starts to move and hence the cutter also starts to move by which the cutter then cuts out the the wax or lacquer coating as soon as it moves onto the disc surface and in this way the sound gets recorded. So, how the sound is reproduced it is reproduced by using this particular mechanism here as you can see this is a stationary magnet this is a movable magnet onto which a needle arm is placed and then there is a stylus. So, when the disc is the recorded when the recorded disc starts to move the stylus is kept onto the surface of the disc what the stylus does is it trace out the recorded portion from the disc and as that as the as the recorded portion gets detected the assembly starts to move and hence the magnet will will start to move into and flow directions. Due to this movement and EMF is generated this EMF is analogous to the recorded sound to the recorded sound and this EMF is further then given to the amplifiers where the signal is amplified and then given to the loudspeaker. So, this is the sound reproduction system for from the disc. So, as you can see here this is the disc surface basically the above structure is formed on the surface during the disc recording this is formed as the cutter moves on the surface of the disc cutting the wax coating which is coated on the surface a structure this structure basically denotes the surface disc surface when the disc is kept in ideal condition or when it is not used for recording purpose. Hence, it is called as unmodulated grooves. Secondly, the B structure is due to the cutter which moves onto the surface of the disc when an audio signal is given as input. The input in turn vibrates the armature assembly to which cutter is attached hence creating the modulated grooves. The vibration in the armature assembly is due to the variations in audio input signals which are nothing but high and low pitches. During the reproduction of sound the grooves which are created by the cutter are traced by using a stylus the stylus then in turn moves the moves onto the grooves as it moves due to the variations in depth of the grooves the stylus vibrates in turn creating vibration vibrating the armature assembly hence these vibrations are then converted into electrical signals due to the movement of magnet. So, these are the references for you people for the further study. Thank you for watching the video.