Chapter 5: Sound and Audio

Yamaha SD-1 (YMF825) FM Synthesizer

The Yamaha SD-1 (YMF825) serves as the primary FM synthesis engine of the X65 microcomputer, delivering rich, multi-voice sound with minimal CPU overhead. This hardware FM synthesizer is based on Yamaha’s extensive experience with frequency modulation synthesis, offering a balance between sound quality and low processing demands.

Features of the SD-1 FM Synthesizer

  • 4-operator FM synthesis, enabling complex sound generation similar to later Yamaha FM chips.

  • 16-channel polyphony, allowing multiple simultaneous sounds.

  • 29 on-chip operator-waveforms and 8 algorithms, offers a whole variety of

    sound.

  • Built-in effects processing, including envelope shaping and vibrato.

  • Integrated 3-band equalizer.

Integration with the X65

The SD-1 is connected to the system via an SPI interface, which is memory-mapped by RIA hardware, allowing efficient access by 65816 CPU. All SD-1 registers are directly accessible in a dedicated memory region, allowing direct programming. Since the SD-1 operates as a standalone digital synthesizer, the CPU only needs to send configuration commands, freeing resources for game logic and other tasks.

Mono Output and Mixing

The YMF825 produces a mono digital audio signal, which is then processed by the X65’s integrated audio mixer. This ensures that FM synthesis is seamlessly combined with other sound sources, such as the PWM audio channels.

Dual PWM Audio Channels

In addition to the FM synthesizer, the X65 features two Pulse-Width Modulation (PWM) audio channels, which serve as a flexible alternative for generating sound effects and basic waveform synthesis. These PWM channels are directly controlled by the CPU, making them ideal for custom waveform generation, simple music playback, and sampled sound effects.

Key Features of the PWM Audio System

  • Two independent channels, allowing stereo-like effects through software control.

  • 8-bit duty cycle, allowing dynamic sound shaping.

How PWM Works in the X65

Each PWM channel is controlled by a dedicated hardware timer, which determines the duty cycle and frequency of the generated waveform. By rapidly toggling the output voltage, PWM creates a square wave that can be manipulated to approximate analog signal.

While the SD-1 handles complex musical synthesis, the PWM channels provide an method for generating simple square wave, percussive sounds, and sampled audio effects. When combined, these audio subsystems create a versatile sound environment that balances hardware efficiency and creative flexibility.

Applications of PWM Audio

  • Simple programmatic beeps: Useful for system notifications, debugging signals, or simple chiptune-style sound effects.

  • Basic waveform synthesis: By carefully adjusting the duty cycle, PWM can simulate square, triangle, and other waveforms.

  • Advanced sound generation: Through precise timing and software control, complex sounds can be synthesized, including dynamically changing waveforms and amplitude modulation techniques.

  • Speech synthesis and sampled audio: With clever programming, PWM can reproduce low-resolution sampled sounds for speech effects or digital audio playback.

Integrated Mixer

The X65 features a hardware-integrated audio mixer, responsible for combining and balancing the various sound sources within the system. The mixer ensures that both FM synthesis (YMF825), PWM audio, and External Audio-In are properly blended before reaching the output.

Mixer Capabilities

  • Combines SD-1 FM audio and dual PWM channels into a single output stream.

  • Adjustable volume levels for each source, allowing software-controlled mixing.

  • Simple low-pass filtering, improving sound quality by smoothing out PWM artifacts.

  • Stereo output support, ensuring flexibility in speaker or headphone configurations.

  • Integrated gain control, preventing distortion and maintaining dynamic range.

  • Programmable audio routing, allowing different mixing configurations depending on system settings.

Summary

The X65 sound system combines hardware FM synthesis, PWM-generated sound, and an integrated mixer to provide a robust and flexible audio solution. The Yamaha SD-1 delivers high-quality FM music, the dual PWM channels offer custom waveform generation, and the hardware mixer seamlessly blends all sources for a cohesive and powerful sound experience.