Explain sigma-delta modulation and noise shaping.

Sigma-delta modulation uses oversampling and feedback to achieve high resolution with simple quantizer (often 1-bit). Structure: Integrator accumulates difference between input and quantized output, quantizer produces output, feedback subtracts quantized output. Noise shaping: Loop filter shapes quantization noise spectrum, pushing noise to high frequencies where it is removed by digital decimation filter. Higher-order loops provide more aggressive noise shaping but stability is challenging. Result: High SNR in band of interest despite coarse quantization. Used in audio ADCs, DACs, and instrumentation converters.