Measurements

It’s not uncommon to hear or read about the “grit of the DAC in the old FM synths”, or many romantic terms applied to the noise figures and distortion values of the golden era.

While sometimes entertaining, I have learned to value those comments a lot. They are a superb guide on what to measure. And, I love measuring.

So here’s a few measurements on distortion and noise floor. The spectral view shows the C4 key, with a sine wave at -6dB amplitude.

The very first is XFM2 using the Digital Output. There’s no noise floor image because… there’s no noise floor. A silent output is digital zero.

Second one is XFM2 analog output, meaning the Adafruit UDA1334 DAC.
While picking the DAC for XFM2 I tested five different DAC/CODEC chips. The best performer was the PCM1793 using lo-noise opamps and balanced outs, and was the primary design for XFM2 for most of the development cycle. However, when I discovered the Adafruit and measured it, the difference was less than 3dB of distortion, at 1/5th of the price, and without the hassle to build it. I still have the 1793 design ready and will make it into a future product, but having the digital out available, it doesn’t really make any sense.

Third image is the legendary DX7IIFD, one of the best 80’s FM synths in regards to noise and distortion.

Fourth and fifth are XFM2 analog and the DX7IIFD noise floor graphics.

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