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Audio Overview

Introduction

Nintendo created a custom sound synthesis IC for the Virtual Utopia Experience Project called the VSU. Its silicon chip is housed in a 28-pin SOIC package labeled U3 and located on the top of the Virtual Boy's mainboard.

The VSU consists of six sub-units (or "channels") each capable of generating sound data in some way. The data produced by these six channels are mixed together, converted to a pair of analog signals (left and right), and sent to the Virtual Boy's speakers and headphone jack. On the way, these signals actually pass through the currently installed cartridge, which (theoretically) allows said cartridge to augment, or even entirely replace, the audio signals produced by the VSU. The output is a DC-coupled signal varying from approximately 550 to 750mV. As the amplifier itself is AC-coupled, it should accept a fairly wide range of input signals, using some kind of mixer to balance the external and internal volume levels. 1)

Channels

The six channels available for sound generation are as follows:

  • Channels 1-5 are wave-table synthesis channels.
    • Uses one of five 6-bit x 32 sample PCM2) data tables.
    • Independent left and right volume levels
    • Envelope function (ramp volume up or down over time)
    • 76.294 Hz to 156.250 KHz playback frequency range (lowest note overall is a de-tuned variant of D#2)
    • Intervals are built in.
  • Channel 5 in addition consists of added sweep and modulation functions.
    • The sweep function increases or decreases the playback frequency over time (for sweeping tone effects, etc.).
    • The modulation function modifies the playback frequency according to a table of 32 signed 8-bit values (for vibrato and similar effects).
  • Channel 6 is a noise source based on a PRNG3).
    • It uses a 15-bit Fibonacci LFSR4).
    • The frequency of the shift-register is selectable within the 244.14 Hz to 500.00 KHz range.
    • One of the two feedback taps on the shift-register can be selected from a set of eight configurations.
    • Other dynamics are the same as Channels 1-4.


2) pulse-code modulation
3) pseudo-random number generator
4) linear-feedback shift-register
 


Last modified: 23.04.2018