The concept of sample rate is a bit different in MATLAB, since it isn't directly attached to a signal. In the case of System objects like DUC, it is assumed that the input and output are for the same 'time duration'.
Here is an example: Consider a signal 'x' of 100 samples to use as the input to DUC.
x = randn(100,1);
If we set the SampleRate property of DUC object to be 100 Hz, this means the input 'x' to the object is a 1 second data (100samples / 100Hz). If the InterpolationFactor of the DUC is 50, then the output of step method of DUC will have 100*50 = 5000 samples. Now, since the input and output should correspond to the same time duration, the sample rate of the output should be deduced as: 5000 samples / 1 second = 5000 Hz.
In that sense, the output sample rate of DUC is SampleRate*InterpolationFactor.
