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Audio Device Distortion (THD+N) Measurement

In this recipe, we will explain how to measure "distortion" in devices such as audio interfaces and amplifiers using the Distortion Analyzer widget.

The low "distortion" in audio equipment is an indicator of performance in how faithfully the original sound can be reproduced and recorded.

What is Distortion Rate (THD+N)?

THD+N (Total Harmonic Distortion + Noise) is an indicator of the "purity" of a signal. Even though a pure sine wave (single frequency) is supposed to be input, the output may contain other components (harmonics and noise).

  • THD (Total Harmonic Distortion): The sum of frequency (harmonic) components that are integer multiples of the input signal.
  • THD+N: The sum of "all components other than the objective signal," which includes background noise in addition to THD.

The smaller this value (the lower the %, or the larger the absolute value of dB), the higher the performance of the equipment.

Measurement Procedure (Loopback Test)

First, perform a "loopback measurement" to measure the distortion of the audio interface itself. This becomes the limit value (baseline) that can be measured in that environment.

Physical Connection and Preparation

  1. Directly connect Output L (or Ch 1) of the audio interface to Input L (or Ch 1) with a cable.
  2. Start the Distortion Analyzer widget.

Measurement Settings and Execution

  1. Set the following in the Signal Generator section:
    • Signal: Sine Wave
    • Frequency: 1000 Hz (standard)
    • Amplitude: around -6 dBFS
      • Distortion changes depending on amplitude. Distortion tends to increase near the maximum level.
  2. Press the Start Measurement button.

Confirmation of Results

Measurement results are displayed in real-time.

  • THD+N: An indicator of overall distortion.
  • Harmonics Tab: The level (ratio to the fundamental wave) for each component, such as the 2nd harmonic (2nd), 3rd harmonic (3rd), etc., can be confirmed in the graph and table.

This numerical value is "the lower measurement limit of your measurement environment (sound device)." It is not possible to accurately measure equipment with distortion smaller than this.

Measurement of DUT (Device Under Test)

Next, connect the device you want to measure (DUT: Device Under Test) in between.

  1. Change the connection: [Output] -> [DUT] -> [Input]
  2. Perform the measurement again.

If the distortion rate is larger than during the loopback measurement, that difference can be judged as the distortion of the DUT itself. Conversely, if the value is the same as during loopback, the performance of the DUT is very high, and it is possible that it exceeds the measurement limit of the audio interface.

Limits and Points to Note for This Instrument

Regarding the Measurement Method

This Distortion Analyzer removes the fundamental wave (such as 1kHz) with a method called a digital notch filter, analyzes the remaining components with FFT, and calculates the distortion rate.

This method is versatile and fast, but there is a measurement limit of approximately -120dB (0.0001%).

Visualization of the Measurement Limit

Try measuring by switching the signal output setting at the bottom right of the main window from "Physical Output" to "Loopback (Virtual)". The value displayed at this time (probably around -120dB to -130dB) is the measurement lower limit of this algorithm itself. Noise and distortion lower than this are buried in calculation errors and the limits of the filter.

When Even Higher Precision Measurement is Required

If you want to measure ultra-high performance DACs and amplifiers that fall below -120dB, consider using the Lock-in THD Analyzer. By using the lock-in method, it is possible to observe even deeper noise floors.

Other Distortion Measurement Tools

The following widgets are also available for those who want to perform more advanced analysis.

  • Advanced Distortion Meter:
    • MIM (Multitone Intermodulation): Measures distortion in a state closer to a music signal by sounding multiple sounds simultaneously.
    • PIM (Phase Intermodulation): Measures phase modulation distortion.
    • SPDR (Spurious Free Dynamic Range): Measures spurious-free dynamic range.