Characterization of Audio

A project log for VMU Zero V2 [coming soon]

Reviving the Dreamcast VMU

satri360satri360 07/07/2018 at 20:400 Comments


VMU Zero is using the original piezo speaker in the VMU. Piezo actuator is essentially a capacitor and it requires higher voltage to drive.

Test Setup


  1. Set volume as 100%
    amixer cset numid=1 -- 100%
  2. [Idle] Probe the GPIO18 directly at Pi output in idle state
  3. [Frequency Response] Probe V_in at R21 and V_out at LT3469 output
  4. Play pre-generated single tone wave file
    aplay sin_1khz_100.wav
  5. Measure the sound level at 2cm proximity using SoundMetrer X on iPhone
  6. Repeat step 3-5 with varying the frequency from 10Hz to 20kHz
  7. [THD] Probe V_in at R21 and V_out at LT34469 output
  8. Play pre-generated 1kHz wave file
  9. Record the sound at 20cm proximity using WaveSpectra on PC
  10. Repeat step 7-9 with varying the peak level of the wave from 10% to 100%



Continuous 8.4kHz oscillation was observed at PWM output when no audio. Amplitude was about 1.6V and DC offset was around 1.60V.

Frequency Response - Individual

Peak volume at 5kHz.

Frequency Response - Sweep

Narrow peak frequency response from 3kHz to 6kHz. Anything below 2kHz is significantly attenuated with -20dB.


THD was almost 100% when the volume is 50% level because the PWM output was clipped above 50%. This resulted 3rd harmonic tone surpassing the fundamental of 1kHz at the speaker output.

1kHz 50%