M65831F

The M65831F is a digital echo processor from Renesas Electronics (formerly Mitsubishi Electric), designed specifically for adding echo and reverberation effects to audio signals. This device has been widely used in karaoke systems and audio equipment for many years, becoming a de facto standard for low-cost digital echo processing.

At the heart of the M65831F is 27KB of built-in SRAM, organized as 4096 words of 12-bit memory for each of two channels. This memory serves as the delay line for creating echo effects — the audio signal is digitized, stored in memory, and then read out after a programmable delay. Multiple delayed versions are mixed with different gains to create rich reverb effects.

The device integrates 8-bit A/D and D/A converters, eliminating the need for external converters. The sampling frequency can be set from 12kHz to 50kHz via an external clock or RC oscillator, allowing tradeoffs between audio quality and delay time. Higher sampling rates provide better audio quality but reduce the maximum delay time for a given memory size.

Echo parameters such as delay time, feedback level (regeneration), and effect level are controlled through external potentiometers or DC voltages, making the M65831F easy to integrate into analog audio systems. The SOP-24 package provides a compact surface-mount solution. With a single 3V-5.5V power supply and low 150mW power consumption, the M65831F is suitable for both mains-powered and battery-operated audio equipment.

The M65831F creates digital echo and reverb effects through the following signal chain:

1. Analog Input and A/D Conversion: The input audio signal (from a microphone or line-level source) is fed into the internal 8-bit analog-to-digital converter. The ADC samples the analog signal at the sampling frequency (set by the external clock/oscillator) and converts it to 8-bit digital samples.

2. Delay Memory: The digital samples are written into the internal SRAM delay memory. The memory is organized as a circular buffer — each new sample is written to one location while older samples are read out from a location N positions behind, where N determines the delay time. N is set by the DELAY control pin (external voltage or potentiometer).

3. Feedback Loop: A portion of the delayed signal (controlled by the FEEDBACK pin) is fed back into the delay line input, along with the original dry signal. This creates multiple decaying echo repetitions, simulating the natural reverberation of sound in a room. Higher feedback creates longer reverb decay times.

4. D/A Conversion: The delayed (wet) signal is converted back to analog by the internal 8-bit digital-to-analog converter. The output is an analog signal containing the echo/reverberation effect.

5. Mixing: In typical applications, the wet (echo) signal from the M65831F is mixed with the dry (original) signal using an external op-amp or mixer circuit. The E.LEVEL control adjusts the ratio of wet to dry signal.

6. Dual Channel Operation: The M65831F has two channels, allowing stereo echo processing. Each channel has its own delay memory (4096 × 12 bits) and D/A converter, but they share the same A/D converter and control logic. Stereo input is time-division multiplexed through the single ADC.

7. Clock and Sampling: An external clock signal (or RC oscillator circuit connected to the OSC pins) sets the sampling frequency. Higher sampling rates (up to 50kHz) provide better audio bandwidth but reduce the maximum available delay time, as the fixed-size memory fills up faster.

• Karaoke machines and home karaoke systems
• Professional audio mixers with built-in effects
• Home theater AV receivers with reverb/echo modes
• Guitar and vocal effects pedals
• Car audio sound processors
• Public address (PA) systems with reverb