Introduction

Back to: Sampling & Reconstruction

The Distortion Introduced by Sampling lesson lists three sources of distortion related to sampling:

1. Distortion due to signal bandwidth  (section 2)
2. Distortion due to DAC interpolation (section 3)
3. Distortion due to quantization (section 4)

For each source, increasing the sample rate Fs can improve several sampling characteristics at the possible expense of other factors such as system cost and power consumption. To be more specific:

• Increasing the sample rate allows signals to have larger bandwidths.
• Increasing the sample rate results in less error at the output of a sample-and-hold DAC.
• Increasing the sample rate while maintaining an acceptable SNQR decreases the number of required bits per sample.

These statements are true for standard PCM consulting.

Delta Modulation

The Distortion Introduced by Sampling lesson also introduced a tool for reducing SQNR by encoding the difference between consecutive samples rather than encoding each sample as-is. This encoding method can be called delta encoding or delta modulation.

The next section builds upon the methods of over-sampling and delta modulation. It also introduces a third tool: noise shaping.

Noise shaping pushes a large portion of the quantization noise energy into the higher frequency bands while keeping the desired signal energy in the lower frequency bands. The system can then low-pass filter the output. This yields a high energy signal and low quantization noise⁠—that is, a high SNQR.

The delta-sigma encoder is a simple method to accomplish a high SNQR.