MSM6258

OKI MSM6258

Overview

The OKI MSM6258 is an ADPCM audio chip developed by OKI. Unlike FM synthesizers such as the Yamaha YM2151, the MSM6258 is designed primarily for the playback of digital sampled audio.

The chip is best known for its use in the Sharp X68000, where it is paired with the YM2151. This combination provides both synthesized FM music and digital sample playback.

The MSM6258 uses 4-bit ADPCM to reduce the amount of memory required for sampled audio while retaining useful speech, music and sound-effect quality for the period.

Technical characteristics

• Manufacturer: OKI
• Model: MSM6258
• Type: ADPCM audio codec
• Channels: 1
• Encoding: 4-bit ADPCM
• Typical maximum sample rate: 15.625 kHz
• Typical clock: 4 MHz / 8 MHz
• Typical computer: Sharp X68000

ADPCM sample playback

ADPCM, or Adaptive Differential Pulse-Code Modulation, stores audio by encoding changes between successive samples rather than storing each sample as a complete PCM value.

This significantly reduces the amount of data required for a recording. For a computer with limited memory and storage, ADPCM therefore provided a practical way of adding sampled voices, percussion and sound effects.

The MSM6258 provides a single hardware playback channel. Its playback frequency is determined by the chip clock and divider rather than by a freely programmable pitch system. Supported configurations can produce rates including approximately 7.8 kHz, 10.4 kHz and 15.6 kHz. :contentReference[oaicite:4]{index=4}

MSM6258 and the Sharp X68000

The Sharp X68000 combines the MSM6258 with the Yamaha YM2151. The YM2151 provides eight channels of FM synthesis, while the MSM6258 provides a dedicated ADPCM playback channel. :contentReference[oaicite:5]{index=5}

This division between synthesis and sampling was particularly effective. Music drivers could use the YM2151 for melodic instruments while reserving the MSM6258 for drums, voices and other sampled effects.

Tracker and music-driver relevance

The MSM6258 is an important component of the X68000 music ecosystem. Rather than functioning as a traditional tracker synthesizer, it is normally used as the sample playback part of a larger sound system.

X68000 software could also use software techniques to multiplex samples, effectively allowing more apparent sample voices than the hardware's single ADPCM channel would suggest.

Historical importance

The MSM6258 illustrates the transition from purely synthesized computer music towards hybrid systems combining FM synthesis and digital samples.

Its pairing with the YM2151 gave the X68000 a particularly powerful sound architecture for its era and contributed significantly to the distinctive sound of Japanese computer games.

Related soundchips

• Yamaha YM2151 — Eight-channel FM synthesizer used alongside the MSM6258.
• Yamaha YM2608 — Yamaha chip combining FM, PSG, rhythm and ADPCM.
• Commodore Amiga Paula — Multi-channel sample playback hardware used in the Amiga.

Further reading

• Furnace — MSM6258 documentation
• VGMRips — OKI MSM6258
• X68000 Sound Hardware reference