RELATED STUDIES

Sound engineers are always trying to improve the listening experience for the audience. That includes the reproduction system as well as the mixing and producing of the material. In general one can say that the evolution for producing and reproducing sound has gone from mono to stereo and then to surround sound, but there are some differences between theatrical and home reproduction systems. From the beginning of audio reproduction it was all mono because of the technical limits. In mono it is almost impossible to get a sense of 3D sound. To some extent it is possible to get depth by recording more of the room or by adding reverbs. The mono format was the only one for home reproduction until 1958 when the two channel stereo LP was introduced. For theatrical sound Bell Labs engineers developed and tested a multichannel reproduction system in the 1933. It was described as 3-channel system with left, centre and right loudspeakers [1]. The stereophonic reproduction is able to give more depth in the sound image, more than a mono source is able to do. Beyond that the stereo sound is also able to define sound sources on a horizontal line in front of the listener. In 1938 Walt Disney and Leopold Stokowski started to cooperate with a classical music animated film, Fantasia. Stokowski wanted the music to be in stereo, referring to the experiment in 1933. Disney on the other hand thought one step further and wanted the sound to surround the audience rather than being in front of them. Disneys engineers developed the surround array for theatres along with multitrack recording, pan potting and overdubbing. The array consisted of three frontal speakers and two surround speakers. That was essentially the beginning of the 5.1 system we see today and the beginning of all surround reproduction. Since Disneys Fantasia and that first surround system the technique has been improved and refined, although it took a long time. Disneys 5-channel array wasnt economical marketable at the time and have gone through many changes until today. One important improvement is the adding of a separate channel for bass management, the subwoofer channel. Gary Kurtz, the producer of Star Wars, and Dolby personnel considered that the low-frequency content required for the battle scenes in Star Wars couldnt be reproduced in the sound systems that were used at that time. Therefore they developed the bass management system. The idea of that is to limit the bandwidth of that channel to get the larger headroom needed for the low-frequency content being perceived as loud as the mid- and highfrequency content. [1] Because of technical limitations it was hard to implement the surround sound to home systems. First with digital techniques it became possible to deliver stereophonic sound via VHS and laser disc. Later the Dolby Pro Logic system was introduced to carry the surround sound through the two channels available on the mediums for home reproduction. The system is built on a matrix that encodes the surround information into two channels by phase and amplitude differences. The system though caused some remarkable artifacts in the reproduction. This made professionals in the film industry want a discrete multichannel system for home cinema theatres. In 1987 the SMPTE subcommittee started to work out the 5.1 system and some years later some different coding systems were introduced, among them Dolby Digital and Digital Theatre Systems (DTS)..

Before electronic music, there was a growing desire for composers to use emerging technologies for musical purposes. Several instruments were created that employed electromechanical designs and they paved the way for the later emergence of electronic instruments. An electromechanical instrument called the Telharmonium (sometimes Teleharmonium or Dynamophone) was developed by Thaddeus Cahill in the years 1898-1912. However, simple inconvenience hindered the adoption of the Telharmonium, due to its immense size. The first electronic instrument is often viewed to be the Theremin, invented by Professor Leon Theremin circa 1919-1920. Another early electronic instrument was the Ondes Martenot, which was most famously used in the Turangal la-Symphonie byOlivier Messiaen as well as other works by him. It was also used by other, primarily French, composers such as Andre Jolivet. In 1966-67 Reed Ghazala discovered and began to teach "circuit bending" the application of the creative short circuit, a process of chance short-circuiting, creating experimental electronic instruments, exploring sonic elements mainly of timbre and with less regard to pitch or rhythm, and influenced byJohn Cages aleatoric music concept. Released in 1970 by Moog Music, the Mini-Moog was among the first widely available, portable and relatively affordable synthesizers. It became the most widely used synthesizer in both popular and electronic art music. Throughout the seventies bands such as The Residents and Can spearheaded an experimental music movement that incorporated electronic sounds. Can were one of the first bands to use tape loops for rhythm sections and The Residents created their own custom built drum machine. In 1979, UK recording artist Gary Numan helped to bring electronic music into the wider marketplace of pop music with his hit "Cars" from the album The Pleasure Principle. Arguably the biggest hit single of the 80s "Blue Monday" by New Order released in 1983 from the "Power, Corruption and Lies" album solidified electronic music in the mainstream for good and paved the way for today's modern electronic music. Other influential artists in the 1970s and early 80s, who composed primarily electronic instrumental music and managed to reach beyond the academic sphere and into the popular realm, were Jean Michel Jarre, Tangerine Dream, Brian Eno, Klaus Schulze, Vangelis, Yello, Ray Buttigieg and Art of Noise. 1980, a group of musicians and music merchants met to standardize an interface by which new instruments could communicate control instructions with other instruments and the prevalent microcomputer. This standard was dubbed MIDI (Musical Instrument Digital Interface). A paper was authored by Dave Smith of Sequential Circuits and proposed to the Audio Engineering Society in 1981. Then, in August 1983, the MIDI Specification 1.0 was finalized. The advent of MIDI technology allows a single keystroke, control wheel motion, pedal movement, or command from a microcomputer to activate every device in the studio remotely and in synchrony, with each device responding according to conditions predetermined by the composer.

MIDI instruments and software made powerful control of sophisticated instruments easily affordable by many studios and individuals. Acoustic sounds became reintegrated into studios via sampling and sampled-ROM-based instruments.

Miller Puckette developed graphic signal-processing software for 4X called Max (after Max Mathews) and later ported it to Macintosh (with Dave Zicarelli extending it for Opcode) for real-time MIDI control, bringing algorithmic composition availability to most composers with modest computer programming background.

In 1983, Yamaha introduced the first stand-alone digital synthesizer, the DX-7. It used frequency modulation synthesis (FM synthesis), first experimented with by John Chowning at Stanford during the late sixties. In the last decade a number of software-based virtual studio environments have emerged, with products such as Propellerhead's Reason and Apple Logic finding popular appeal.Such tools provide viable and cost-effective alternatives to typical hardware-based production studios, and thanks to advances in microprocessor technology, it is now possible to create high quality music using little more than a single laptop computer. Such advances have, for better or for worse, democratized music creation, leading to a massive increase in the amount of home-produced electronic music available to the general public via the internet. Artists can now also individuate their production practice by creating personalized software synthesizers, effects modules, and various composition environments. Devices that once existed exclusively in the hardware domain can easily have virtual counterparts. Some of the more popular software tools for achieving such ends are commercial releases such as Max/Msp and Reaktor and freeware packages such as Pure Data, SuperCollider, and ChucK.

RELATED LITERATURE
According to CCRMA Stanford edu, the central element of most audio systems is a mixing board or console. This device allows the combination (mixing) and distribution of many audio signals and also provides the ability to increase or decrease the amplitude of the various signals. In addition to this basic function, most mixing boards allow the alteration of tonal balance (equalization) of the signals and provide access to external devices (i.e. effects processors like delay and reverberation). Some also allow the manipulation of dynamic range with compressors, gates, and mutes. Mixers vary in complexity from four input - two output (4 x 2) mixers to consoles with hundreds of inputs and outputs. While the complexity may vary, the standard functions of signal routing and amplification are the same. Mixing board input channels usually provide both microphone level and line level inputs, with a preamplifier to increase the microphone output to line level. Tape recorder inputs are provided

to allow monitoring of previously recorded tracks along with any signals currently being recorded. The tape inputs (often called tape returns) may be separate monitor channels on a different part of the mixer (split console), or they may be incorporated into the main channel input modules (in-line console). Returns may also be provided so that effects processor outputs may be recombined with the other mixed signals. In addition to inputs, consoles provide enough outputs to feed multitrack recorders and effects devices. Generally, each input channel is assignable to any of several outputs, often in stereo pairs. These outputs may be known as buses, groups or subs, which allow several inputs to be combined and controlled by a single output level control. Other outputs called sends may allow selected signals to be combined and sent to outboard processors like reverbs or delays. Additional patch points (or inserts) may be accessible on channels, allowing individual channels to be processed externally and returned to the channel strip.

RELATED THEORIES
Microphone amplification: Since microphones produce very low voltages, they must be amplified in order to record them. Most mixing boards have built-in microphone preamplifiers (mic pre-amps). These may vary in quality from atrocious to excellent. The mic preamps may account for most of the cost of mixing boards, since each input channel can cost hundreds of dollars. Fortunately, recent advances in integrated circuit design and manufacture have allowed the production of higher quality preamplifiers in the 10s of dollars range, contributing to the availability of acceptable quality affordable mixers. Since microphones often need 60 dB of amplification, their amplifiers performance is critical. Mic preamps must be able to handle large dynamic range signals without distortion, which means they need to have wide bandwidth capability without introducing noise even at very high gains. Many engineers favor using external preamps for their most critical recordings. These devices can cost as much as a low-end console for two channels of microphone amplification. There is undeniably an advantage to using these highlyoptimized preamps when working with consoles with average quality built-in preamps. Compromises must be made to fit the required number of preamps into the space and cost of any console, compromises which are not necessary in an outboard preamp. There are considerations in addition to the obvious need for noise-free undistorted gain in a microphone preamp: the input impedance of the preamp can audibly affect the microphone output, especially if it is transformercoupled. The ability to proved clean phantom power to a wide range of capacitor microphones is also a consideration. Since the output impedance and phantom power requirements of microphones varies, a preamp must be flexible to fill a range of such needs. Microphone preamplifiers are often described by whether they use solid state devices or vacuum tubes as gain elements and the circuit topology employed. These factors can make a difference in the sound of the preamp, although it is not a simple as assuming tubes always sound warmer than solid-state units. The performance of a preamp is affected by the interaction with its input device as well as its internal performance. A well-designed vacuum-tube preamp may well

sound as clean and focused as a solid-state device. Preamps using transformers often produce a characteristic sound due to the imperfections in the transformer, like low-frequency distortion and high amplitude saturation. This may well produce the warm sound we are after. Tubes also tend to produce a particular type of distortion which is sometimes desired. In addition to the type of gain element, the circuit topology may contribute to the overall sound a preamp produces. Amplifiers are described by a classification: class A, AB, B, and others. These classifications consider the way power is used by the amplifier. A class A amplifier always draws current from the power supply even with no signal present. While this results in high power consumption and heat production, it creates little distortion since the amplification devices are always on. In order to avoid the large power consumption, class B amplifiers may be constructed which use a pair of complementary output devices (either transistors or tubes), each of which conducts on half of the cycle. The problem is then that the zero-crossings may contribute distortion if not very carefully adjusted. There are variations of these types like class AB, and more exotic types with switched power applied during large voltage swings. Another amplifier design consideration is the use of negative feedback, where the output is inverted and fed back to the input. This has the effect of minimizing distortion in the amplifier and stabilizing it so that it does not tend to oscillate. Again, this must be done carefully as it also may degrade the amplifiers ability to follow rapid changes in input voltages. Since op -amps are designed to employ negative feedback, their performance may be inherently limited relative to circuits using discrete devices and less feedback. Summing amplifiers: Another type of amplifier is used to combine, or mix, signals. This amplifier must be able to add many signals together without allowing any interaction between the outputs of the devices producing the signals. This can easily be achieved with operational amplifiers in the inverting configuration. Unfortunately, this configuration inverts the signal. Usually, there are several stages of amplification involved in a mixer and hopefully the signal which emerges is a non-inverted replica of the input. This may not be the case, as some stages of amplification may not be applied to all signals. As a general rule, the more complex a mixer, the greater the likelihood of phase inversion on at least some of the signals.

Any shortcomings in a mixing (summing) amplifier will be reflected in the final stereo mix: since all signals must pass through the summing amplifier, the design of this stage is also of critical importance. The simple op-amp summing amplifier works for a small number of inputs but begins to have problems if too many inputs are connected. The more inputs to the summing amplifier, the higher gain it must deliver. So for mixing a few signals, the summing amp runs a low gain while the gain increases as more signals are added to the mix. Since op-amps may have audibly different sound characteristics based on the amount of gain they must deliver, this is undesirable. The inverting summing amplifier is also sensitive to ground loop-induced noise, since it references its output to the non-inverting input, which is connected to circuit ground. Given the size of modern large consoles, ground loops are as significant a problem as they are for the studio at large. While other circuits are possible, they are much more expensive to implement, as some of these involve a differential internal bus structure and some require heavy cables for summing buses that run the width of the console. This explains some of the cost of high-end consoles.

Signal Routing: In addition to amplitude manipulations, mixers are used to control the flow of signals from one point to another. This can be done by means of switches which send the signal to different outputs or mixing amplifiers (buses), or by potentiometers which can pan (from panorama) the signal from left to right in a stereo mix. Often, input channels of a mixer have insert points or patch points which allow each input signal to be separately routed to an external device and then brought back to the same place in the mixer so it can be mixed with other signals. Direct outputs send the signal to external devices but do not provide a signal return to the mixer. Often, a mute switch is available to temporarily turn off selected signals without altering gain or other settings. A solo circuit functions as a separate mixing bus which allows the engineer to monitor selected signals in the control room while those in the studio continue to hear the complete mix. This can be of great help in isolating noise or other problems in a complex mix. The solo signal can be taken before the channel fader (PFL=pre-fader listen) or after the channel fader (AFL). In the first case, the fader position will not affect the solo signal. This arrangement also applies to direct outs and sends. Many consoles make pre-/post- fader selection switchable or alterable with internal jumpers. For signal monitoring, it may be useful to monitor pre-fader. For effects, though, it is usually desirable to monitor post-fader so the effect send level tracks the fader level and the effect isnt fed signal when the direct (dry) signal fader is off. Faders: One of the main functions of a mixer is to allow each channels level in the mix to be manipulated, so a principal control is the fader. A fader is a linear (as opposed to rotary) potentiometer which controls each channels level sent to the summing bus. The physical construction of the fader determines how it feels when its moved and how the level increases and decreases as the fader is moved up and down. There are different materials used in the resistive element: carbon composition and conductive plastic are the most common. The exact taper of the resistance as the wiper moves along the resistive surface determines how the signal level changes as the fader is moved and several resistance curves are available, from linear to logarithmic to audio taper. Since all faders on a console must have matching behavior, their construction accuracy is important. The best faders can cost hundreds of dollars each. Some mixers are capable of automated control, which requires a motor connected to the fader to allow electronic control of fader movement. This further increases the cost. Equalization: Another function found on mixing consoles is equalization (EQ). The equalizer is a special form of amplifier that provides gain that is frequency-dependent: the engineer can set the relative gain differently for different frequency bands. The tone control found on stereos is a simple form of equalizer. Generally, each input module provides some EQ control. Many types of equalizer are available and the type employed varies from manufacturer to manufacturer. The sound of the equalizer is determined by the type and quality of the actual design employed, so functionally similar EQs on different consoles can sound different. Some engineers have distinct preferences, but it is usually possible to create a similar sound with any of the available consoles.

Specific types of equalizers commonly found on mixers include parametric, semi-parametric, and shelving types. Parametric filters allow the user to adjust the center frequency, width of the frequency band, and amount of boost/cut for each filter band. Semi-parametric EQ lacks the bandwidth adjustment of the fully parametric type. Shelving filters boost or cut all frequencies above or below the corner frequency and are commonly used as tone controls on stereos as well as on consoles. Graphic equalizers, which provide a separate boost/cut adjustment for from 5 to 30 fixed frequency bands, are rarely used on consoles but are often found on PA-type mixers and outboard equalizers. The type of electronic circuitry used in a filter can make an audible difference. Although modern designs lean heavily on capacitor/op-amp filter stages, designs using inductors are returning to popularity despite their greater cost. These circuits have a distinctive sound reminiscent of the early days of recording, possibly due to the saturation effect inductors generate when the iron cores can no longer accept the magnetic flux demanded by high-level signals. Whatever the cause, the sound of these older designs, like the Pultec equalizers, is very music-friendly. Dynamics processing: Once available only on the most expensive large-format consoles, dynamic range processing is now a standard feature of most digital mixing boards. The ability to alter the dynamic range of a signal is very helpful in constructing a complex mix of sounds so that we may still clearly perceive the individual elements. Dynamic range processors are amplifiers with variable gain, which may be controlled by a signal derived from the sound signal itself. It allows the variations of loudness of a sound to be reduced by an arbitrary amount. Think of an engineer with a hand on the fader, turning it down when it gets loud. Using the envelope of the signal as a measure of its loudness, we can use the signal to control the fader level, creating a compressor.

A compressor allows several parameters to be set which control how the gain is affected. The threshold determines the amplitude level at which gain is reduced, the slope (ratio) determines how much it is reduced when the level exceeds the threshold, the attack determines how long it takes to reduce the gain, and the release determines how long it takes for the gain to return to normal after a compression event. Make-up gain is a linear gain after the compressor which amplifies the compressed signal so that the now-reduced peaks are as loud as they were before. This also increases the level of the softer sounds which were not being compressed. The net effect is a reduction in the dynamic range. A limiter is a compressor with a very high ratio. Generally it also uses a higher threshold to absolutely limit the output level but not alter the balance of quiet to loud sounds as much as a compressor does. Another approach is the noise gate, which reduces the signal gain when the level drops below a threshold. This has the effect of reducing or eliminating background noise. Signal Level Displays: Mixing boards always provide some type of signal level indicators, either mechanical VU-type meters or LED displays. VU (volume unit) indicators are specially calibrated meters which indicate the perceived loudness of the signal being monitored. LED displays usually display the peak level, and thus are better indicators of the amplitude of short transients, which may cause overload distortion in recorders and processors. Many boards have LED overload indicators on

the preamps, allowing trim adjustment of incoming signals. The combination of metering types available should allow the engineer to monitor each signal level in the board so signal-to-noise ratio and distortion performance can be optimized. Gain Structure: Although metering is intended to convey signal levels in the mixer, there are potential problems keeping the gains of each block in the signal flow at optimal levels. Generally, the VU meters are indicating the levels at the summing bus outputs and hopefully some indicators monitor the preamp levels. But with dynamics processors and equalizers on each channel, there are potentially some circuits without adequate signal level monitoring. The level at which clipping occurs may differ for each functional block in the mixer, so we must be careful not to apply too much gain with equalizers, for example, that then lead to overloads in subsequent stages. Dont assume that because the preamp level isnt clipping that the signal cant overload at some subsequent stage. One measure of mixer quality is the amount of headroom throughout the entire system.