NdownNup

NdownNup is a term used in digital signal processing to describe a two-step sampling-rate conversion process in which a signal is first downsampled by a factor N and then upsampled by the same factor. The concept is often employed as an educational illustration of how sampling, filtering, and reconstruction interact when changing a signal’s sample rate.

In practice, the downsampling step reduces the number of samples by selecting every Nth sample, typically preceded by a low-pass anti-aliasing filter to limit spectral content above the new Nyquist frequency. The upsampling step inserts N−1 zero-valued samples between each original sample, followed by a reconstruction filter to interpolate the signal back to its original rate. When combined, NdownNup can either preserve the overall energy if implemented with proper filtering or introduce attenuation and phase shifts if the anti-aliasing or reconstruction filters are inadequate.

Variants of the approach include using fractional or non-integer rates, or employing polyphase filter banks to optimize performance. In educational contexts, NdownNup serves as a straightforward example to discuss aliasing, interpolation, and the importance of filtering in rate conversion.

Applications span basic signal processing demonstrations, audio and image resampling, and simulation environments where a simple rate-change mechanism is needed without specialized resampling algorithms. The term itself is largely descriptive rather than a standardized industry designation.