Moving average before downsampling effect on Nyquist frequency?

1. What happens when a signal is sampled at less than the Nyquist rate?
2. Does downsampling increase frequency?
3. How does downsampling cause aliasing?
4. What is the effect of downsampling?

What happens when a signal is sampled at less than the Nyquist rate?

When the sampling frequency drops below the Nyquist rate, the frequencies will crossover and cause aliasing.

Does downsampling increase frequency?

To answer the OP's question, downsampling alone does NOT change the frequency resolution. The simple answer is that the frequency resolution is determined from the total time duration of the data (this applies universally whether we are in the analog or frequency domain).

How does downsampling cause aliasing?

If a discrete-time signal's baseband spectral support is not limited to an interval of width 2 π / M radians, downsampling by M results in aliasing. Aliasing is the distortion that occurs when overlapping copies of the signal's spectrum are added together.

What is the effect of downsampling?

Performers might sometimes downsample a signal as an effect, to achieve a low-fi sound. The anti-aliasing filtering will reduce the bandwidth of the signal and attenuate the high frequencies, and the interpolation process, depending on how it is done, can add noise or "grittienss" to the sound.