How Streets of Rage 2 used the Mega Drive Hardware to Create an Iconic Soundtrack

Streets of Rage 2 – called Bare Knuckle 2 in Japan – is a scrolling beat ‘em up video game that was developed by Ancient and released for the SEGA Mega Drive/Genesis console in 1992. It featured 4 playable characters – Axel and Blaze who were in the first game, as well as 2 new additions Max and Skate. The game offered players a range of combination attacks that they could use with each character, and a new versus mode where players could test their skills against each other. They could use any one of the game’s 8 stages to fight each other in, with a range of weaponry scattered around the stage. Due to its 5 difficulty levels and ability for players to choose how many lives they started the game with, Streets of Rage 2 had a low barrier to entry while still offering a challenging experience to fans of the series. Yuzo Koshiro who composed the soundtrack for the original Streets of Rage game returned for the sequel, writing all of the game’s music and sound effects except for 3 tracks which were written by Motohiro Kawashima.

When discussing Streets of Rage 2, strafefox (2018) said that it was one of the first SEGA games to be put on a 16 megabit cartridge which was 4 times the size of the cartridge for the original game. This allowed for more detail in the game overall, as well as additional space for sound effects and music. According to Ayano Koshiro – the head designer at Ancient – they only had around half a year of development time which makes the technical achievements of the game all the more impressive.

The SEGA Mega Drive was released in 1988 in Japan, 1989 in North America and 1990 for Europe. It was unique for the time as it had two on-board processors – the 6800 and Z80. The Z80 handled audio and had a 4 MHz maximum clock speed while the 6800 was in charge of all other operations and was faster with a clock speed of 7.61 MHz. The 6800 was actually a 32-bit processor that took in a 16-bit bus while the Z80 was an 8-bit processor (Mitchell, R, 2016). This meant that if lots of operations were happening at once, the music and sound effects of the game would be unaffected by any strain on the core processor (Mitchell R, 2016). By isolating the sound operations, developers did not need to worry about sounds and music stuttering which created a smoother experience for players.

A 32-bit processor has significantly more computational power than an 8-bit one. This is because it can read data from larger data buses, allowing for more accurate floating point numbers to be used. By increasing the number of bits available for a processor to use, this improves the speed at which it can complete operations. To complete an operation an 8-bit processor will need more instructions than a 32-bit processor, as the size of the commands it can understand is smaller. This meant that the 6800 was significantly more powerful than the Z80 as it took less time for the processor to complete the same action (Bond, K, 2016, p. 489).

The Mega Drive’s capacitors around the headphone jack and YM2612 chip

The Mega Drive also had two ways to output sound – through the RF port for the TV, or a headphone jack on the front of the console. The RF port could only use mono sound, while the headphone jack was capable of stereo. Additionally, it had a volume slider on the front of the console, an unusual feature that gave players finer control over the console’s volume. A reason why they might have added this slider was the headphone jack, as there would have been no other way to change the sound levels. Consequently, the circuit board is covered in capacitors to allow for the audio to be amplified through the port.

For producing sounds, the console once again had 2 chips – the YM2612 and an SN76489A clone that was integrated into the Z80. This was because the console was designed to be backwards compatible and the SN76489A was the sound chip in the Master System – the Mega Drive’s predecessor. As a result, composers for the system had access to 6 Frequency Modulation (FM) channels and 4 Programmable Sound Generator (PSG) channels which they could use to create music and sound effects. Furthermore, the YM2612 had an integrated digital to analogue converter (DAC) along with stereo sound capabilities (Collins, 2008, p. 40) which allowed for audio panning techniques to be used.

Panning is a technique in sound design which, “creates the illusion of a sound source moving from one part of the soundstage to another.” (Trandafir, 2017). It works by playing audio channels louder in one ear than the other to make it sound as though the audio is coming from a particular side. This can be used to make it feel like sound sources are moving, by changing the levels of channels gradually. Both Koshiro and Kawashima took advantage of the Mega Drive’s ability to produce stereo sound and you can notice this in tracks like Alien Power, Expander and Go Straight where different channels move from left to right throughout the tracks, giving them a more dynamic feel.

FM is a way of representing sound by changing the frequency and shape of the wave, while keeping the amplitude (height) the same. The main waveforms used in FM synthesis are sine, square, saw-tooth and triangle. These each produce a different type of sound with sine waves having a softer sound and more jagged waves like saw-tooth and triangle producing harsher tones. These basic tones could then be edited further to create unique patterns.

The different waveforms that can be produced by FM

According to Koshiro (2014), “With the introduction of FM synthesis, various instruments, like string, wind and percussion instruments, could all be played on one FM chip.” Since composers for the Mega Drive had 6 of these channels to work with, they could layer different waves on top of each other to create more complex sounds than previously possible. In the first level of Streets of Rage 2, Koshiro only uses 5 of these channels, leaving one available for the game’s sound effects. One of the channels on the YM2612 could be used to play sounds that had been recorded prior. Koshiro used this channel to play samples of the kick and snare of a Roland TR-909 (strafefox, 2018) while using FM synthesis to copy the Roland TR-808 drum beats, because they were popular drum machines at the time and he wanted to create an authentic club feel.

“Programmable sound generators (PSGs) are sound chips designed for audio applications that generate sound based on the user’s input.” (Wolf, Mark J. P, 2012). They take in an oscillating wave signal that is tuned to a set MHz value and a series of commands which are then applied to the wave signal, modifying how it sounds.

The SN7648A has 8 registers – 4 for volume control, 3 for generating tones and 1 for creating noise. As it was from a previous generation of consoles, it took its instructions in 8-bits, rather than 16. It can take a clock signal of up to 4MHz and has “a range of 10 octaves” (SMSPower, 2005). This meant that Koshiro was able to add 3 additional sounds to his composition on top of the 5 FM channels he was using from the YM2612, giving him greater depth and complexity in his compositions.

When dealing with real components, they aren’t always perfect, resulting in distortion of waveforms. This was particularly noticeable with the Mega Drive as the SN76489A had problems with the wave decaying towards zero. Furthermore, the YM2612 would truncate its 14-bit output to 9 bits which meant that the sound had to be passed through other circuitry – making the waves distort further (MD, 2015). Due to the distortion of the sound, the Mega Drive had a unique feel, known as the ladder effect as it was the only console that can produce this particular noise (Yuzo Koshiro, 2019).

His knowledge of computer programming and components made Koshiro stand out from other composers at the time since it gave him greater control over what the game sounded like compared to someone who needed a programmer to translate their compositions. When speaking on this subject he said, “I was trying to program on the computer to create sounds. It was really important for me to create music drivers or sound drivers.” (Koshiro, 2017).

To create his music, Koshiro used a variation of Music Macro Language (MML) that he created himself, called Music Love.  MML was a programming language used to notate music using text which had many different variations as it was unique to the interpreter and chips it was being used with. Therefore, it made sense for Koshiro to create his own version suited to the hardware that he was using, especially since it gave him more flexibility in how he could program his sounds as he said in an interview “Since I made my own editor and driver, I could control everything about the chip down to the fine details.” (Koshiro, 2014). The main way he modified it was by changing the language from being based on BASIC to coding “more like Assembly” (Koshiro, 2005).

Before he started to make music for the Mega Drive, Koshiro used the NEC PC-8801 for all his work and he used a variant of MML based on NEC’s particular version of BASIC. Koshiro stated during an interview with Redbull that the PC-8801 and Mega Drive conveniently had, “almost the same FM synthesis chips” (Koshiro, 2017) making music production simpler for him as the sounds he was creating on his PC would be true to how they would sound on the Mega Drive.

When they weren’t working on the game, Koshiro and Kawashima would go to clubs in Tokyo to listen to the newest club music, particularly at places like Yellow (Koshiro, 2017). They would take these new sounds and try to bring them into the game’s music. They were heavily influenced by popular dance music of the time, particularly tracks that came from abroad as both techno and the Mega Drive were more popular in North America and Europe than in Japan. Koshiro even took a trip to LA around 1988, where he “constantly had MTV on at the hotel” and he bought records to take home to Japan with him. He wanted to update the sound of the Streets of Rage franchise, in the same way that club and house music was constantly changing and evolving (Koshiro, 2014).

Streets of Rage 2 is still an influential and popular game to this day, with fans of its music across the globe. Koshiro’s music was so key to the game that he was one of the first video game composers to have a major credit for his work. The way that it uses the hardware is particularly impressive when compared to other games for the console as they had a tendency to sound the same. This was due to composer’s over-reliance on GEMS (Genesis Editor for Music and Sound effects) and the built in library of samples. It was released in 1991, and used in hundreds of games for the console, as it allowed musicians to use the MIDI format that they were comfortable with, rather than needing to learn how to program.

Both Koshiro and Kawashima continued to be successful game composers and they went on their Diggin’ in the Carts world tour in 2017 where they exclusively played music from game soundtracks that they composed together. Streets of Rage 2 was an introduction to techno music to many young people, and the game was a successful sequel although Streets of Rage 3 was less well received. Streets of Rage 4 has recently been announced for a 2019 release, leaving fans excited and questioning whether Koshiro will be returning to the franchise, 25 years later to write one more soundtrack.

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