What is ‘Sound for Games’?
When we talk about computer (or video) games we are absolutely convinced that the sounds in the game are as essential as the animations itself. Try playing any game with the sound muted and you’ll see what I mean. Even the earliest games from 1970s (when the games emerged as a form of entertainment) had sounds in them. Back then the only way to embed sound into a game was by directly programming it into the computer chips. Therefore early video games musicians were required to program those sound chips and transcribe their music into the hardware. The music was monophonic and synthetic. Why synthetic? Because you could not embed audio files into the program. Synthesizers were the only choice. In 1960s and 70s subtractive synthesis was discovered.
Subtractive synthesis starts by generating a spectrally rich source signal that is then filtered with a time-varying filter. As the source signal one of the classical geometric waveforms, the sawtooth, the rectangular pulse, or the triangle wave-form, is typically used. Sometimes a mix of the above-mentioned waveforms or a noise signal is used. The filtering is typically performed with a lowpass filter that has a controllable resonance close to its cutoff frequency.
Classic games such as Super Mario and Pac Man are good examples.
In the 1980s and 1990s, subtractive synthesis was overtaken as the most popular sound generation principle mainly by frequency modulation synthesis and sampling synthesis.
Frequency Modulation Synthesis (or FM synthesis) is a form of audio synthesis where the timbre of a simple waveform (such as a square, triangle, or sawtooth) is changed by modulating its frequency with a modulator frequency that is also in the audio range, resulting in a more complex waveform.
“Sampling” an instrument means simply to record it under a wide variety of useful playing conditions. Not every key needs to be recorded, and it is quite typical to use one recorded sample to cover several adjacent keys by modifying its playback rate to scale the pitch. Additionally, memory can be saved by looping the steady-state portion of the sound.
The technology has advanced very far since 1970s. Now it is possible to use a large number of samples, loops and even the whole complete musical tracks in any of the modern games. However, there’s one thing that has not changed in four decades – the reason why the sounds are used in computer games. The sole purpose of sounds in computer games is to enhance the gameplay experience, make the world of the game seem as real as possible so the player forgets his/her real life with its worries, disappointments and imperfections and instead, fights against various forms of evil eventually emerging a victor. Good dose of self-confidence guaranteed.
The evolution of sound technology in games
The Computer Space arcade game (1971) was the first game with sounds. It had no microprocessor, no RAM, no ROM and no sound chip. Instead it used electrical components & a printed circuit board (logic board). There was only one channel of sound, output in mono.
Pong arcade game (1972) was the first popular video game with sounds. It also used electrical components & a printed circuit board. There was only one channel of mono sound generated by wiring one wire of a speaker to ground of the printed circuit board and the other a the point giving the most desirable sound (a simple bleep).
Gunfight (Midway games 1975, arcade). The 1st game with a microprocessor (Intel 8080) to replace hardwired electronic circuits. It had one channel of sound, output in mono.
Atari VCS (video computer system) /2600 (1977). This Second generation console was one of the 1st successful home gaming consoles ever made. It had an 8 bit microprocessor, programmed with cartridge based game code.
128 bytes RAM and 4 kB ROM. It used a custom sound & graphics chip called the television interface adapter (TIA). The chip’s 2 oscillators provided two channels of sound, which were then output in mono. For each oscillator there was a choice between two possible square waveforms, one treble or one bass, a sine wave, a saw wave or noise.
Each oscillator could produce 32 possible note values. A four bit volume control meant there were 16 possible output levels. The note tuning for each channel was different, which made it difficult to compose music for the game. Sounds were programmed using assembly language
The Pokey chip was a custom programmable sound generator (PSG sound chip), used in Atari’s 8 bit home consoles & arcade games during the 80s. The chip provided 4 audio channels of sound, which were then output in mono.
Each channel could produce a variable pulse wave at a certain pitch & amplitude or white noise. The design of the POKEY made it possible for games to have polyphonic music (though the music suffered from detuning issues).
Atari’s Tempest 1981 used two Pokey chips for 8 voices of polyphony. Sounds were programmed using assembly language
The SID (Sound Interface Device) chip was a high quality programmable sound generator (PSG sound chip), used in the Commodore 64, 8 bit home consoles during the 80s. The SID chip was patented in 1983 with the hope of eventually using it in synthesizers.
The SID chip provided 4 audio channels of sound, which were then output in mono. 3 of its voices were generated by 3 oscillators, each having four selectable waveforms of either sawtooth, triangle, pulse wave, or noise and an 8 octave range.
It had 3 envelope generators and one multi mode filter featuring a low-pass, a high-pass and band-pass outputs and also had 3 ring modulators.
The SID chip was also 4 bit sample playback used for realistic voice synthesis.
Nintendo NES (1985) Third generation 8 bit console
The NES (Nintendo Entertainment System) was an extremely popular game console famous for its music composed for titles such as the Legend of Zelda & Super Mario bros (Koji Kondo).
It had a 5 channel custom made PSG sound chip (Audio Processing Unit). This PSG had two pulse wave oscillators, each with an 8 octave range. Pulse width could be adjusted for additional timbres.
Phasing & echo effects could be generated by manipulating the timing and the level of the pulse wave channels. Pitch bend was also possible. Another oscillator produced a triangle wave one octave lower than the pulse waveforms but with limited note capacity.
A noise channel was used for sound effects & percussion, A PCM (digital audio) channel was capable of low quality sample playback.
The NES could generate five individual sounds at once, output in mono.
Game composers still had to be programmers, writing programming codes and working with little cartridge memory. Limited memory meant looping was a technique commonly used in game music of this era.
Sega Genesis/Mega Drive (1989)
16 bit processor (Fourth Generation) had 8 channels of stereo audio and much more memory but still had 8 bit sound. Six channels of stereo audio were generated by a FM synthesis chip.
A PSG (3 + 1) chip was used for sound effects & music. An additional PCM voice synthesis chip provided one channel of 8 bit sample playback for vocals.
8 kB RAM was dedicated to sound. Difficult to program, as assembly language was needed to edit the sounds for both chips. For music sequences, loops & transpositions were used to save time & memory.
SNES 1991 (16 bit processing) Also known as the Super Famicom
Had superior sound compared with the Sega Genesis,with a 32kHz, 16 bit DAC for audio playback & 64kB of RAM reserved for audio.
The sound module had a dedicated processor and supported 8 channels of stereo audio. Wavetable synthesis was used to generate realistic sounds.
Sounds were shaped with filters & envelope generators. Channels could be panned and effects such as reverb added.
The sound module had a standard MIDI instrument set making it easier for musicians to compose game music with MIDI controller keyboards. Using MIDI to program sounds also saved on memory.
Many games, such as Judge Dredd (1995) were movie tie–ins, so more of the music was licensed. In addition, electronic music composers were commissioned to write game song.
Doom (shareware PC game MSDOS) 1993
A First person shooter 3D game, typical of this era.
Surround sound allowed the sound of demons to be positioned anywhere within 3 dimensional space. The volume of sounds could also be automatically adjusted to simulate distance from sound sources.
Sega Saturn 1994 (32 Bit processor, CD ROM)
The Sega Saturn had two Saturn custom sound processors developed by Yamaha. Each had a 16 bit DAC providing CD quality sample rates. Each custom sound processor had a 32 channel sound generator, for either FM synthesis or PCM digital audio playback.
Built in Digital signal processing then allowed 16 possible sound effects to be added to the sound.
Supported MIDI via an additional MIDI interface unit. This meant a MIDI controller such as a keyboard could access the sound processor.
A limited amount of RAM was reserved for sound.
Sony Playstation 1994 (32 Bit processor, CD ROM)
The popular Sony Playstation could play back CD quality audio files. Its sound chip provided 24 channels of stereo sound with real time envelope shaping, pitch modulation and reverb.
MIDI was supported for ease of programming. Its CD ROM drive could also play audio CDs. For certain games, game audio could be substituted for the players own choice of music.
Redbook CD audio meant higher quality sounds but this was at the expense of dynamic, adaptable audio and meant hard transitions between audio tracks. Looping was heavily relied upon.
Microsoft Xbox (2001) 6th generation Console.
Provided 64 3D channels of audio and up to 256 stereo voices
2MB dedicated sound memory. Allowed 44.1 or 48KHz sample rates. Was the first game to feature real time, adaptive Dolby digital 5.1 surround sound.
Games used 5.7 GB DVD discs, and the console had 8GB of hard drive memory.
The Xbox’s media player could be used to play CD’s and even rip them to the console’s hard drive.
Nintendo Wii (2006) 7th generation console.
The game is stored on Wii optical discs and so it has limited functionality as a media player. Comes with 512 MB of flash memory and additional removable SD card memory. The SD memory can be used for loading mp3 audio, which can be used for custom in-game music
Its most innovative feature is a wireless controller called the Wiimote, which has a built in speaker in the controller for dynamic sound reproduction. Capable of surround sound.
Microsoft Xbox One (2013) 8th generation console.
Announced on May 21, 2013, the Xbox One will place a large emphasis on internet-based features; including the ability to record and stream gameplay, and the ability to integrate with a set-top-box to watch cable or satellite TV through the console with an enhanced guide interface and Kinect-based voice control.
It has 1.75 GHz AMD x86-64 eight core CPU codenamed “Jaguar”.
Audio I/O section has HDMi (one in, one out), Optical Toslink, DTS-HD Master Audio, Dolby True HD, DTS HD, High Resolution Audio, Dolby Digital. HDMi means High-Definition Multimedia Interface.
Theory of sound in computer games
Usually game’s action (or gameplay) is taking place in some sort of environment, such as street, room, forest, stadium, interstellar space, narrow cave or under the water. Each of those environments kind of makes the main player expect certain sounds to be present. This is not a conscious expectation but rather something that we all usually take for granted without being aware of it. However, we might become aware if the environmental sounds do not match the environment. So the task of a good game’s sound designer is to choose the most suitable sounds and, ironically, the better the sounds the more chances that the player will not be aware of them. Nevertheless it will significantly enhance the game experience (or immersion). That is the main goal.
And the main tool of a sound designer for achieving this goal is a reverb. The hearing of the reverb is giving our brains a lot of information about our surroundings. Is this a street or a tunnel? How wide is the street? What material the tunnel walls are made of? Are there other people or objects near by? And so on. Our brains do all these calculations automatically and build an accurate 3D (three dimensional) image of our surroundings. There are quite a few blind people in the world who can get around by clicking their tongues and listening to the reverb, a method similar to echolocation used by bats and some other nocturnal creatures. This creates a 3D representation of the surrounding sound.
When talking about 3D sounds in the game, we mean those sounds that change depending on where the game character (or listener) is at any moment. For example as you approach a sound source it gets louder and vice versa. Or if the noisy object is on the right side of the screen, the sound should come from the right-side speaker or headphone. And if the character turns around the sound moves to the left speaker.
2D sounds are also used in the game but most of the time they’re not part of the game environment. Background music and game menu sounds are two examples of 2D sounds.
In other words 3D sounds in game are diegetic and 2D sounds are non-diegetic. In the next chapter I will talk more about the term diegetic.
Which Sounds to use in the Game?
To analyse game sound in more detail we can use IEZA model. IEZA model is an academic tool (or framework) used to analyze sounds in computer games. There are a number of different tools but IEZA seems to be the most helpfull and easy to understand. All sounds in the game can be divided into four categories: Interface, Effect, Zone and Affect, hence the name – IEZA. (introduce IEZA model more properly), For those who would like a really in-depth description of it click this link: The IEZA Model.
But for now I’m going to try to explain the IEZA model in a simplest way that I am able to come up with. Firstly all the sounds can be divided into two categories:
a) sounds that are meant to be heard by the game character only (diegetic sounds)
b) sounds that are meant to be heard by the player only (non-diegetic).
Secondly all the sounds can also be divided into other two categories:
c) sounds that are influenced directly by player’s actions (activity)
d) sounds that are not directly influenced by player’s actions (setting)
So now those two dividing lines slice the whole bunch of sounds into four smaller categories:
Interface. Those are mostly sounds from the menu window, such as Load, Save, Delete. Not part of the gameplay, non-diegetic. They are there for the player only, the main character in the game can’t hear them.
Effect. Those can be shooting, walking, running jumping and fighting sounds. They are triggered by the keyboard (controller) presses or mouse clicks. They are main part of the gameplay, diegetic.
Zone. Ambience of the game world environment, the hum of factory machinery, crowd cheering, wind, rain and other nature sounds. Player has no direct control over them, but the character is meant to hear them, so they are diegetic. (physical setting)
Affect. This can be a musical track in the game. The music might change from one room to another. Alternatively, sounds from this category can work as a feedback to the player about the status of his character and/or gameplay. Game character is not meant to hear them. They are non-diegetic. Unlike Zone, which represents the physical setting, the Affect is more of a psychological setting. It forewarns the player of what might be happening soon and affects his mood and emotions.
Sound Design and Sound Production Methods
Sound design is the creative construction and manipulation of sound. Sound design methods are divided into following categories:
Foley Recordings (sounds recorded in the recording studio). Word “foley” came from the man who first started recording different sounds for movies in order to enhance their sound. His name was Jack Donovan Foley (1891 – 1967). He had begun this form of art when working with Universal Studios in 1914 during silent movie era.
Location Recordings (recordings done outside the studio)
Sound Libraries (when you don’t have time to do the recordings yourself)
Sound Creation (Editing) (manipulating and mixing prerecorded sounds). Sampling, audio editing.
Sound Creation (Synthesis). Subtractive synthesis, additive synthesis, FM synthesis.
Outsourced sound creation (contracting someone else to do the work)
Any combination of the above.
LIMBO is a puzzle-platform video game, the first title by independent Danish game developer Playdead.
The player guides an unnamed boy through dangerous environments and traps as he searches for his sister. The developer built the game’s puzzles expecting the player to fail before finding the correct solution. Playdead called the style of play “trial and death”, and used gruesome imagery for the boy’s deaths to steer the player from unworkable solutions.
The game is presented in black-and-white tones, using lighting, film grain effects and minimal ambient sounds to create an eerie atmosphere often associated with the horror genre.
The game’s audio was created by Martin Stig Andersen, a graduate from the Royal Academy of Music in Aarhus. Andersen’s specialisation was in acousmatic music, non-traditional music created from generated sounds that have no apparent visual source.
Andersen avoided the use of easily-recognizable sounds, distorting them when needed as to allow players to interpret the sounds’ meanings for themselves. Many reviews for the game stated that there was no music in Limbo, but Andersen countered that his sound arrangements helped to evoke emotions; the acousmatic music was intended to leave room for interpretation by the player in the same manner as the game’s art and story. Andersen noted that this helps with immersion within the game by making no attempt to control the emotional tone; “if the players are scared it will probably make them more scared when there’s no music to take them by the hand and tell them how to feel”. Due to fans’ requests, Playdead released the game’s soundtrack on iTunes Store on July 11, 2011 (source: Wikipedia. Limbo game).
Analysis using IEZA model:
At the start of the game we are taken to the game menu. We can hear an ambient music. It sounds more like a drone consisting of few different sounds: a synth string pad, white noise and a mosquito-like buzz. This sound belongs to Affect category. As we select the different options of the game we can hear a percussive mid-frequency sine wave of various pitches. These sounds belong to Interface category. Both music and menu sounds are non-diegetic.
When the actual gameplay starts the ambient music is replaced with ambient sound. At first it is just a brown noise. As the character moves from one environment to the next one this ambience changes slightly. The change is rather subtle and if you’re playing the game yourself you won’t perceive it. Because of this slight change this sound most probably diegetic and belongs to the Zone category. But it could also be classed as an Affect as well. The dividing line is blurry.
Character’s footsteps, sounds of an object being pushed by the character or sounds of falling objects can be categorised as Effect, because those are directly influenced by player’s actions.
There’s another interesting use of sound in this game. In one instant, when the character is climbing up the tree, the high frequencies of the noise become louder. And when he’s climbing down the high frequencies get cut off.
The near-constant presence of brown or white noise and not many other sounds puts the player into a state of always waiting for something bad (or exciting) to happen. The Player expects something but has no idea when or where the disaster will strike. He gets absorbed into this continuous unnerving state, which is, in fact, the biggest appeal of this game for me personally. This artificially induced anxiety is what keeps the Player on the edge of his/her seat, ready to jump. The brain is, at that moment, preparing the adrenaline to be released into the bloodstream, just in case. The subtle use of reverberation is giving our subconscious mind plenty of cues about the size of the immediate environment: sometimes vast and spacious (suddenly you feel small), and sometimes small and claustrophobic (“Oh! I want to get out of here!”). But there is no escape, you are going to die, ha ha ha (evil laugh)!!!
The diegetic sounds are very realistic, be it a mosquito buzz, water splash or a noisy blade of the circular saw on an excruciatingly slow approach. The realism of the sounds allows you to forget that it’s just a two-dimensional, monochromatic puzzle game. The sounds of dying, coupled with the visuals, are really gruesome. The cracking of the bones, wet splashes of guts being thrown about and so on. There are many ways to die in this game and there is a sound for each of them.
2. Hitman: Contracts
Hitman: Contracts is an action-adventure stealth video game developed by IO Interactive and published by Eidos Interactive. It is the third entry in the Hitman series.
Gameplay centers around the exploits of a hitman, Agent 47 as he is sent to various locations to assassinate targets. An array of weapons can be used, from kitchen knives to belt-fed machine guns. While stealth and subterfuge is encouraged, the game allows the player to take a more violent approach and gunfight their way to their mission goals. As players progress through the game, they can collect the various armaments found in the levels, allowing them to be used in future missions. Aside from the more straightforward ways of killing targets such as gunplay and strangulation, several missions allow the player more subtle ways to eliminate hits, such as judicious use of poison, or arranging “accidents” like a heat-induced heart attack inside a sauna.
Audio-wise, Contracts follows the series tradition of employing the considerable talents of Jespar Kyd, who has conjured an exceptional soundtrack that for the most part is an unobtrusive backdrop to the already dark and moody ambience on show elsewhere. The score was awarded the title of “Best Original Music” at the 2005 BAFTA Games Awards.
The following video of the Hitman: Contracts starts with an intro sequence which is not part of the actual gameplay but it sets the mood or “tone” of the game. You can hear the voice of the protagonist (main character), some dark synth pad, sounds of the surgeon operating on the agent 47 body and removing the bullet. During the gameplay you can hear the sound of rain and the traffic. They belong to Zone category. The music that is playing inside the building is of the same category too, because the character can also hear it and it pans left or right as the character moves around.
There’s only a couple of scrolling sounds that belong to the Interface category. I guess game developers didn’t expect anyone spending too much time going through the menu.
Effect category consist of the usual mix of character’s footsteps, shooting, picking a door lock, opening the door, taking and dropping an object. Some of the characters actions trigger a response from other characters (like screams or sounds emmited when one is being strangled or injected with some sleeping medicine from a syringe.
When the 3D music is not playing the 2D music replaces it in a very subtle transition. And it can be categorized as an Affect. It is there just to affect the players state of mind. The music changes slightly from one room to another (to make it less boring to wander around) or when one of the main targets is eliminated (to heighten the excitement and pump some adrenalin into the blood.
At first, while there is no non-diegetic music, the player feels that it is safe to just play the game casually, without too much excitement. There is only sound of rain outside and the diegetic rock music coming from the bar inside the building. As the main character is approaching the room of his first target, the quiet sound of ride cymbal being touched by a soft timpani mallets starts to be heard as a constant, dark drone. It is joined by a low male choir pad and a deep bass. These sounds are starting to make the player anxious about the task ahead, namely killing a person without raising the alarm.
After the first target is killed, the sound-scape changes dramatically. The synthetic, distorted and rhythmical audio loop begins to play at a faster pace, in effect mirroring the increased heart rate of the player. There is no going back now, he must finish what he started. Now it’s time to hurry up and get out of here before someone finds the body. When the Hitman gets to the basement to find his second target, the distorted rhythmical loop is replaced by another slow and dark, drone-like sound, evoking the feeling of danger in the players subconscious mind. Danger of being seen in these clothes. However, when the Hitman gets changed into the bikers clothes, the dark drone fades away and is replaced by a sound of the Turkish Oud (or lute) string being plucked and a shaker. The music is not as menacing now and the player feels slightly more relieved. As the Hitman is approaching his second target, the sound of oud is joined by the sound of Turkish Nay (or flute) playing a slow Arabic melody in harmonic minor key. The sounds might be suggesting the nationality of the second target. After he’s killed, Arabic music stops and the non-diegetic interface beep is heard providing the Player with the sense of achievement. The sinister synth pad starts playing, suggests that this is a good time to get out of here. When the Hitman reaches the ground floor, sounds of drums from the bar seem very cheerful in comparison and that relaxes the Player a bit. Once the Hitman is outside the building, there is only sound of rain. And when he gets out the perimeter fence, the square wave synth sound urges the Player to hurry up to the Exit point of the game.
3. Bus Driver
Created by Czech developer SCS Software, Bus Driver is a mundane procedural exercise in urban public transportation. The game features 12 different bus styles, from traditional school buses to double-deckers, and a fictionalized multi-district city backdrop in which to manoeuvre them. The core gameplay premise compels you to complete 30 different routes over five unlockable tiers, with “missions” ranging from inner-city passenger pick-ups to a helicopter-chaperoned prisoner transport (source: http://www.gamesradar.com/bus-driver-review/).
Very simple game. Simple sounds. The developers had a plan to make it super realistic. Most of the sounds are diegetic. What you’d hear on the real bus is what you get in this game.
The Effect category contains bus engine sounds, doors opening/closing, indicators’ (blinkers) sounds and sounds of crashing into other cars or objects.
The Interface category has music playing on top of the usual scrolling and clicking sounds. The developers must have thought that a little bit of music after all that tiresome driving would be a welcome change. Plus there’s a lot of info in the menu to get through so I guess having music really helps. Also after each “mission” you get the crowd cheering and clapping sounds which do become annoying after a while. At times passengers get upset and scream when you break too suddenly.
There are no sounds that you can ascribe to the Affect category and the Zone category has only sounds of brakes’ air pressure release and sounds of passing cars.
The game developers didn’t bother stuffing the game diegesis with any sounds that were not related to the game’s main activity. The driving itself is stressful enough so you don’t need any music or special sound effects to tell you that. The Player is not on a mission to murder anyone or do other naughty and antisocial things, but rather to practice his/her bus driving skills. No mystery, no dramas, no nonsense.
It feels good after you have completed one of the “missions” and there is a sound of a crowd clapping and cheering. However, the crowd still claps even if you get zero points. So you need to employ your willpower to stay motivated.
No attempted psychological manipulation using sounds or music have been observed while playing this game.
The story of the game revolves around infiltrating a high-tech base filled with robots that you need to destroy. The great thing about this game is the fact that even though it is only a demo, it is very well designed, since it offers a great variety of opponents (robots).
You will have to face various enemies (machines) that, as you advance, become harder to defeat, requiring you to do a lot of dodging and running, but ultimately having an amazing time. The game doesn’t actually end. Instead, the last stage keeps on repeating itself. (source: http://macdownload.informer.com/angry-bots/).
Because it is a demo game, it is incomplete. The Interface category has no sound at all. There’s one non-diegetic sound that appears to be of the Affect category, and it is heard when the character enters the Lab Room. But there’s plenty of different ambient sounds that belong to Zone category (rain outside, mechanical whirring and hum in various rooms, computer CPU crackling, Patrol Mechs patroling) and Effect category (character’s footsteps, shooting sounds, explosions, doors opening/closing, computers being hacked by the main character).
Out of these four games that I’m analysing this one is more active than the others. Even though it’s just a demo. When the Player starts shooting at the enemies, the sound of the machine gun is paramount to what the Player is feeling. If it’s a massive, heavy gun sound, the Player will feel massively satisfied. The next ones on the list of important sounds are the explosions and destruction of the enemies. The bigger and louder the sounds, the better the Player will feel. Loud sounds of a gun fight makes the heart rate to rise as it’s pumping more adrenalin through the system. On my version of the sounds for this game I added distorted electrical guitar sounds in the areas of the heaviest fighting to compliment the feel of the scene. I hope this increases the production of adrenalin within the Player.
In the areas with less fighting I put the slower drone-like sounds to remind the player to breathe but not to relax completely. There is always a possibility of danger round each bend of a corridor.
Wikipedia (2013) Limbo (video game). Available at:
Academic Dictionaries and Encyclopedias (2013) Video Game Music. Available at: http://en.academic.ru/dic.nsf/enwiki/205362
Devmag (2013) Video Game Audio: Diegesis Theory. Available at: http://devmag.org.za/2012/04/19/video-game-audio-diegesis-theory-2/
Stephan Schutze. Sound Design Methods. Available at: http://www.stephanschutze.com/sound-design-methods.html (accessed on 21.11.2013)
Bit Depth (2011) Sound Design: How is Sound Design Different to Sound Producing. Available at:
Wikipedia (2013) Hitman: Contracts. Available at: http://en.wikipedia.org/wiki/Hitman:_Contracts
iTunes (2004) Hitman: Contracts – Music. Available at: https://itunes.apple.com/us/album/hitman-contracts/id120313910
Soundcloud (2004) Hitman theme. Available at:
Gamesradar (2013) Bus Driver. Available at: http://www.gamesradar.com/bus-driver-review/
MacDownload (2013) AngryBots for Mac. Available at: http://macdownload.informer.com/angry-bots/