USB Audio, distortion in low bits

Question

I've implemented a PIC32 as a USB sound card, using USB Audio Class 1. I'm sending a sawtooth signal from the microcontroller to the PC(windows 7, 64 bit), as 16-bit samples:

in decimal:

000
800
1600
2400
.. so on

then i try recording the received audio using Audacity, with MME -driver, as .wav or .raw. I use MATLAB to open and inspect the data, and there i see data like:

000
799
1599
2400
..

The distortion varies from -1 to +1 bit pr sample.. Anyone have any idea where the problem might be.? Windows-audio drivers.?

Answer 1

I solved my problem..

The problem was caused by the application i used to record the data, and the method i used.. I used Audacity, which supports the old windows MME audio API, and the DirectSound API. These are relatively high-level API's apparently, and are the cause of the distortion. About the Windows Core Audio APIs

Instead i used another program, called Reaper, it has an option to record using ASIO og WASAPI. This solves my problem. I've checked every sample in an 2 hour .wav file, using MATLAB, and it is completely bit-perfect.

I was probably some quantization error, but it was caused by the API. ASIO and WASAPI gave me bit-perfect sound, MME and DirectSound gave me a distorted signal.

Answer 2

Since you receive the audio signal on PC, playback it, and record it using SW, the audio signal is converted from digital to analog, and to digital again. These introduce quantization error and noise, and you see the little difference between two signals.