I am using MIT-BIH arrhythmical database where I have a digital signal of 1200 Hz so 1200 samples per second. This means that analog filters have already applied to remove the frequencies over Nyquist frequency, so no aliasing.
However, I want to take equispaced sampling, every two sample, simply by the following in Matlab
I am reading Andre Quinquils' book Digital Signal Processing Using Matlab 2008:
It is always necessary to use an anti-aliasing filter before the sampling stage in order to avoid any spectral aliasing risk and to set an appropriate sampling frequency. In practice, a causal approximation of this ideal filter is used. Thus, depending on the chosen filter synthesis method, some imperfections are introduced, such as a passband amplitude ripple, a transition band and a stopband finite attenuation.
Does this mean that I need to apply a new anti-aliasing filter before the sampling stage in oder to avoid aliasing? I think the sampling stage here is the equalspaced sampling. I have not applied any new special anti-aliasing filter.
Which antialiasing filter can you use before equalspaced sampling stage?