MSU1 for..... the megadrive ! The Mega SD

Relikk wrote:Talking to a guy on Discord earlier, he managed to get Streets Of Rage to run with CD audio using an emulator called Regen that has a debugger.

https://www.romhacking.net/utilities/758/

Due to imperfect manufacturing technologies of the time and poor separation between the analog and digital parts of the chip, the 6581's output (before the amplifier stage) was always slightly biased from the zero level. Each time the volume register was altered, an audible click was produced. By quickly adjusting the amplifier's gain through the main 4-bit volume register, this bias could be modulated as PCM, resulting in a "virtual" fourth channel allowing 4-bit digital sample playback. The glitch was known and used from an early point on, first by Electronic Speech Systems to produce sampled speech in games such as Impossible Mission (1983, Epyx) and Ghostbusters (1984, Activision). The first instance of samples being used in actual musical compositions was by Martin Galway in Arkanoid (1987, Imagine), although he had copied the idea from an earlier drum synthesizer package called Digidrums. The length of sampled sound playback was limited first by memory and later technique. Kung Fu Fighting (1986), a popular early sample, has a playback length measured in seconds. c64mp3 (2010) and Cubase64 (2010) demonstrate playback lengths measured in minutes. Also, it was hugely CPU intensive - one had to output the samples very fast (in comparison to the speed of the 6510 CPU).

The better manufacturing technology in the 8580 used in the later revisions of Commodore 64C and the Commodore 128DCR caused the bias to almost entirely disappear, causing the digitized sound samples to become very quiet. Fortunately, the volume level could be mostly restored with either a hardware modification (biasing the audio-in pin), or more commonly a software trick involving using the Pulse waveform to intentionally recreate the required bias. The software trick generally renders one voice temporarily unusable, although clever musical compositions can make this problem less noticeable. An excellent example of this quality improvement noticeably reducing a sampled channel can be found in the introduction to Electronic Arts' game Skate or Die (1987). The guitar riff played is all but missing when played on the Commodore 64c or the Commodore 128.

Relikk wrote:A small excerpt from the Wikipedia article about the chip...

https://en.wikipedia.org/wiki/MOS_Technology_6581

Due to imperfect manufacturing technologies of the time and poor separation between the analog and digital parts of the chip, the 6581's output (before the amplifier stage) was always slightly biased from the zero level. Each time the volume register was altered, an audible click was produced. By quickly adjusting the amplifier's gain through the main 4-bit volume register, this bias could be modulated as PCM, resulting in a "virtual" fourth channel allowing 4-bit digital sample playback. The glitch was known and used from an early point on, first by Electronic Speech Systems to produce sampled speech in games such as Impossible Mission (1983, Epyx) and Ghostbusters (1984, Activision). The first instance of samples being used in actual musical compositions was by Martin Galway in Arkanoid (1987, Imagine), although he had copied the idea from an earlier drum synthesizer package called Digidrums. The length of sampled sound playback was limited first by memory and later technique. Kung Fu Fighting (1986), a popular early sample, has a playback length measured in seconds. c64mp3 (2010) and Cubase64 (2010) demonstrate playback lengths measured in minutes. Also, it was hugely CPU intensive - one had to output the samples very fast (in comparison to the speed of the 6510 CPU).

The better manufacturing technology in the 8580 used in the later revisions of Commodore 64C and the Commodore 128DCR caused the bias to almost entirely disappear, causing the digitized sound samples to become very quiet. Fortunately, the volume level could be mostly restored with either a hardware modification (biasing the audio-in pin), or more commonly a software trick involving using the Pulse waveform to intentionally recreate the required bias. The software trick generally renders one voice temporarily unusable, although clever musical compositions can make this problem less noticeable. An excellent example of this quality improvement noticeably reducing a sampled channel can be found in the introduction to Electronic Arts' game Skate or Die (1987). The guitar riff played is all but missing when played on the Commodore 64c or the Commodore 128.