Reproducing downsampling effect on guitar tone

Question

I've noticed if I downsample my classical guitar recordings from 44100Hz to 8000Hz, the result is a bassy vintage electric guitar sound, which I really like in some songs of mine.

I know because of the sampling theorem, the downsampling involves cutting all frequencies above 8000/2=4000Hz. I did the same on the original 44100Hz signal, applying a low pass filter at 4000Hz with 48db steepness (maximum allowed by the Audacity software I am using), but could not recreate the same downsampled tone with just this.

I used Audacity Spectral Selection and then the spectral delete plugin to delete all frequencies above 4kHz, just like downsampling at 8kHz does but without loosing quality. This is a great way to simulate a guitar pickup if you record your classical guitar with a good external condenser mic like I do.

Do you know what else is going on downsampling? Or how can I find out out? I would like to have the same altered tone I get with downsampling, but without loosing sound quality, keeping the signal sampled at cd quality.

Answer 1

1. Audacity (and probably other resampling algorithms as well) use filters that are steeper than 48 dB/octave. The difference might be audible.

2. As the goal of filtering in resampling is to cut frequencies above the new fs/2, the filter frequency needs to be placed a bit below fs/2. Again, the difference might be audible.

3. Concerning the sentence in your answer "spectral delete plugin to delete all frequencies above 4kHz" note that no filter is perfect, and every filter comes with some limitations and compromises. There is no way to delete all components in a given frequency range, though good plugins these days certainly can do a very decent job. I'm glad you found a solution that works for you.