I have an application that displays several signals from different sensors at various sample rates. In order to display the data, I "stretch" the signal by repeating samples to match the highest sample rate. For example: if I display a 500 Hz signal and a 100 Hz signal at the same time, every sample in the 100 Hz signal will be repeated 5 times so it appears to have a 500 Hz sample rate.
When I apply filters to the signals (all second-order IIRs: LPF, HPF, notch) I use filters designed for the highest sample rate. In my example here, I'm using 500 Hz filters on an upsampled 100 Hz signal.
How "bad" is this? Is my only option to design an interpolation filter during upsampling, and if so, can anyone lead me to good references on how to design such a filter?