OPTICAL RECORDING OF SOUND ON FILMS
• This is done by converting the audio signals into variations of light intensities falling on the photographic films. The intensity of the light is varied in accordance with sound pressure variations.
• This varying light falls on the one edge of the main movie film, photograph of varying light intensity is recorded in the same way as variations of light from a picture are recorded. It is a movie recording. It has high intensity.
• There are some methods for recording sound on a film. They are:
(i) Variable density method.
(ii) Variable area method.
i) Variable Density Method:
• The intensity of light coming from the lamp is varied in accordance with sound pressure variations in the audio signal. This varying light falls on a moving photographic film and is recorded in the form of bars of varying density and distance on the film. That's why, it is called the "variable density method".
• In this method, the sound is picked up by the microphone and then converted into an electrical signal. This electrical signal is then amplified by using an amplifier.
• The amplified audio signal is fed to the anode of the AEO (Alkaline Earth Oxide) lamp through the transformer. The Anode of the AEO lamp is connected to a high d.c. voltage [High tension] and filament (acts as a cathode) are connected to a low d.c. voltage [low tension (LT)]
• The intensity of light coming from the lamp is varied in accordance with the audio signal. This varying light is passed through a slit and a focusing lens.
• Slit: A slit is an instrument that is used for light gathering and without wastage.
The purpose of the lens is to concentrate the light on to the one point of the moving photographic film.
• The focused light falls on a moving photographic film, where the audio signal is recorded in the form of varying density and distance in the film. These variations are according to variations in audio signals.
Variable Area Method:
• The intensity of light coming from the lamp is varied in accordance with sound pressure variations in the audio signal. This varying light falls on a moving photographic film and is recorded in the form of dark and bright areas on the film.
• In this method, the lamp gives constant intensity of light. Mirror Galvanometer consists of a magnet and coil assembly, and a mirror is attached to the coil assembly is deflects when light falls on it in accordance with the sound pressure variations in the audio signal.
• The microphone converts the sound signals into electrical signals and these electrical signals are amplified by using an amplifier. The amplified audio signal is the coil assembly of a mirror galvanometer attached to the coil assembly is deflected because the light falling on the mirror is varied in accordance with the sound pressure variations in the audio signal.
• The intensity of light coming from the lamp is varied in accordance with sound pressure variations in the audio signal. This varying light falls on a moving photographic film and is recorded in the form of dark and bright areas on the film.
• In this method, the lamp gives constant intensity of light. Mirror Galvanometer consists of a magnet and coil assembly, and a mirror is attached to the coil assembly is deflects when light falls on it in accordance with the sound pressure variations in the audio signal.
• The microphone converts the sound signals into electrical signals and these electrical signals are amplified by using an amplifier. The amplified audio signal is the coil assembly of a mirror galvanometer attached to the coil assembly is deflected because the light falling on the mirror is varied in accordance with the sound pressure variations in the audio signal.
The reflected light from the mirror is passed through the slit and falls on the photographic films and is recorded in the form of variable dark and bright areas.
MP3 AND MP4 FORMATS
• MPEG-1 or MPEG-2 audio layer-III more commonly referred to as MP3, is a digital encoding format for digital audio that uses a "lossy data compression".
• MP3 is an audio-specific format that was designed by the Moving Picture Experts Groups (MPEG) as part of its MPEG-1 standard and later extended to MPEG-2.
• MP3 is a lossy data compression designed to greatly reduce the amount of data required to represent audio recording and still sound like a faithful reproduction of the original uncompressed audio for most listeners.
• The sounds that can't be detected by the human ear are dropped because the other sounds override it so that the human ear cannot perceive it.
• MP3 is an audio-specific format that was designed by the Moving Picture Experts Groups (MPEG) as part of its MPEG-1 standard and later extended to MPEG-2.
MP3 files can be constructed with bit rates of 128 K bits/second and 320 kbit/sec.
Limitations of MP3 Format:
1. Low resolution.
2. The quality of the audio signal is less.
3. Bit rate is limited to a maximum of 320 Kbits/sec.
4. Encoder/Decoder overall delay is not defined, which means there is no official provision for gapless playback.
5. MP3 cannot switch joint stereo mode for specific scale factor bands. If a joint stereo is used, it has to be used for all the bands.
MP4 Format:
• MP4, the acronym for MPEG-4 Part 14 designed by the Motion Pictures Experts Group. It is a digital multimedia format that is widely used for both audio and video storage. MP4 videos are also can converted into MP3 format to extract music from videos.
The following data are recommended (for compatibility reasons) to be embedded in MPEG-4:
• Video: MPEG-4, MPEG-2 and MPEG-1.
• Audio: MPEG-4 AAC, MP3, MP2, MPEG-1 Part-3, MPEG-2 Part-3, CELP (speech), Twin VQ (very low bitrates), SAOL (midi).
• Pictures: JPEG, PNG.
• Systems: Allows animation, interactivity, and DVD-like menus.
