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In digital signal processing, downsampling, compression, and decimation are terms associated with the process of resampling in a multi-rate digital signal processing system. Both downsampling and decimation can be synonymous with compression , or they can describe an entire process of bandwidth reduction ( filtering ) and sample-rate reduction.
The frequency, f, in Hz is divided by the sample-rate. The periodicity and Nyquist frequency are then represented by constants, 1 and 1/2 respectively. The bottom pair depict a different normalized frequency scale, used by the Z-transform... the periodicity and Nyquist frequency are respectively represented by 2π and π.
Sample-rate conversion prevents changes in speed and pitch that would otherwise occur when transferring recorded material between such systems. More specific types of resampling include: upsampling or upscaling; downsampling, downscaling, or decimation; and interpolation.
The bandwidth, B, in this example is just small enough that the slower sampling does not cause overlap (aliasing). Sometimes, a sampled function is resampled at a lower rate by keeping only every M th sample and discarding the others, commonly called "decimation". Potential aliasing is prevented by lowpass-filtering the samples before decimation.
a)1-D Decimation b)1-D Interpolation. Theoretically, explanations of decimation and interpolation are: [1] • Decimation (Down-sampling): The M times decimated version of x(n) is defined as y(n)= x(Mn), where M is a nonsingular integer matrix called decimation matrix. In the frequency domain, relation becomes
For example, if compact disc audio at 44,100 samples/second is upsampled by a factor of 5/4, the resulting sample-rate is 55,125. Fig 1: Depiction of one dot product, resulting in one output sample (in green), for the case L=4, n=9, j=3. Three conceptual "inserted zeros" are depicted between each pair of input samples.
In the case of decreasing the pixel number (scaling down), this usually results in a visible quality loss. From the standpoint of digital signal processing, the scaling of raster graphics is a two-dimensional example of sample-rate conversion, the conversion of a discrete signal from a sampling rate (in this case, the local sampling rate) to ...
Additionally, a semi-mechanistic downscaling approach can be applied as for example used for the CHELSA data of downscaled model output. In this example, the temperature algorithm is based on statistical downscaling and the precipitation algorithm incorporates orographic predictors with a subsequent bias correction. [7]