filtering, dither, and noiseshaping
Reply #105 – 2008-07-09 03:41:39
If I'm not mistaken, the sigma delta ADCs that are in common use today inherently dither, i.e. if you apply a DC level half way between two quantization levels then you will alternately get one and then the other. What you describe is primarily the effect of noise shaping which alone doesn't guarantee linearity. Some dithering can theoretically be achieved by randomly shifting the quantization threshold of the 1bit quantizer up and down to some extent (not too much to avoid overloading issues). I think it's really down to thermal noise and/or background noise which should be enough when recording in high resolution. Cheers, SG Good point. I suspect any consumer sigma delta ADC used in 16-bit mode is not applying sufficient dither to decorrelate quantization noise, just bandlimiting. Potentially, one could implement dither by adding a signal that's suitable to provide dither onto the analogue signal before the discriminator (thresholding device). If you can add the output of a 20 to 24-bit DAC to the input, or more simply supply its output to the threshold-level input of the discriminator, you can choose a single level for each output sample period that fits the required PDF (e.g. triangular PDF). The alternative is to record to 24-bit (where thermal noise will supply dither) and convert on-the-fly to 16-bit in the digital domain with appropriate dither, which could be implemented within the ADC chip.