Audacity Recordings with MME (or DirectSound) vs. WASAPI at Different Sample Rates

The "sound card" is the DA2USB, the Signal source is very special and precise digital audio signal generator. The OS is Windows 10 Professional, the Audacity version is 2.1.2 (all 2.1.x, even a 2.1.3 nightly build have been tested).

The problem is that Audacity doesn't want to connect in WASAPI-mode at 96 kHz to the USB interface while at 48 kHz it connects and works perfectly.

For all 5 recordings:


48 kHz, WASAPI

This recording is simply perfect. The right channel's sine wave amplitude is below the 16 bit quantization threshold but even though it is bit-precisely recorded.

48 kHz, MME

16 bit recording only with (obviously) some dithering noise added. I couldn't find how to stop that stupid dithering noise, particularly not in Preferences -> Quality.

96 kHz, WASAPI

Nothing to add.

96 kHz, Imported WAV-File from a Primitive Self-Written Recording Program

This file is imported to, but not recorded by Audacity. The recording program used is a very simple test program based on DirectSound. This import is shown here just in order to prove that the interface works correctly.

96 kHz, MME

Works, 96 kHz with 16 bit only. Also the same stupid dithering.

192 kHz, MME

Just for completeness that the interface works here, too. But the source's sample width is 16 bit only.

96 kHz, MME, Low-Pass Filtered

An interesting observation: I filtered the 96 kHz, MME recording above with a 5 kHz low-pass filter. The result shows that in the right channel the sub-16-bit information is still present. That means that dithering must have taken place before truncation to 16 bit! That must either have happenet somwehere in Windows or in Audacity. I suspect Windows.

192 kHz, MME, Test Signal 0 to 90 kHz

The following test signal was recorded in order to prove that Audacity in MME mode at 192 kHz sample and project rate is able to work correctly. The background it is that I heard (resp. read) that with MME under no circumstances sample rates higher than 44100 Hz are possible. The signal is generated by my digital audio sine wave generator which it is set up and operated as follows:

Between second 13.1 and the end only the 90 kHz tone is recorded.

You can download the 16 bit WAV file here: Rec_MME_192_0-90kHz.wav. (11.7 MByte, 16 seconds approx.)

This is the screenshot, made at the and of the recording session (note the start with a DC-signal):

A second experiment on Windows XP and Audacity 2.1.0 showed an identical result.

Uwe Beis - June 7th, 2016