Cheap Audio Measurements!

The Problem…

Most engineers are familiar with the Audio Precision line of instruments—the gold standard for high-quality audio measurements. However, even second-hand units on eBay cost thousands of dollars.

Quant Asylum devices offer a more cost-effective solution for hobbyists, but they’re still far from cheap.

You could use an oscilloscope and waveform generator for basic sweeps and FFT measurements, but their resolution and speed are limited for detailed analysis.

Another option is connecting a USB audio interface to a PC and using REW (Room EQ Wizard) or similar open-source tools. This can work well, but setup takes time, and incorrect input or output gain settings can ruin the measurement accuracy.

This Project

This project aims to provide a low-cost alternative for practical, near–real-time audio measurements.

Performance is limited by the PCM3060 codec and NE5532 input/output amplifiers. So far it has been able to achieve ~86 dB SNR, -95 dB THD, and ~95dB dynamic range

This is a capable device for studio-grade audio measurements for analog and digital systems. Powered by the RP2040, it provides fast data processing and analysis. In it’s basic configuration it can update the display with real-time measurement results at up-to ~3.8 fps.

Measurement Capabilities

  • Frequency and Levels (Vrms | Vp-p | dBu)

  • Total Harmonic Distortion (THD | THD+N)

  • Signal-to-Noise Ratio (SNR) + Distortion (SINAD)

  • Fast Fourier Transform (FFT)

  • Frequency sweeps (Bode plots | THD vs. Frequency)

  • Amplitude sweeps (Transfer Curves)

  • Oscilloscope (Voltage vs. Time)

Measurement Update Speed

  • 260 ms — Levels metrics

  • +200 ms — Additional THD metrics

  • +50 ms — Additional noise metrics

  • +290 ms — Band limited frequency detection

  • +750 ms — Full band frequency detection

Software Features

  • 4096 sample capture buffer

  • 4096 point FFT

  • Up-to 240 sweep points

  • Touch-screen calibration sequence

  • ADC calibration sequence (using an external signal source)

  • DAC self-calibration routine

  • Adjustable frequency and level (dBu)

  • Optional frequency detection for measuring external sources

Hardware Features

  • I2S ADC and DAC @ 24 bit - 48kHz

  • Auto-range between -10dBV | +4dB

  • Selectable input impedance: 50 Ω / 1 MΩ

  • Loopback (DAC -> ADC) with no signal cables connected

  • 320×240 touchscreen

FFT with 20x averaging (4096 points)

Bode Plot

THD vs. Frequency

Wave View

Transfer Curve

PC Analysis Included:

Sweep data analysis on PC via USB output and supporting tool. Includes averaging for FFT, storing sweeps, and exporting (all) recorded data

Raw data analysis via USB output

Next

DSP Module