# Bandwidth confusion

Let's imagine that I took a Fourier analysis of a random voice signal that I want to sample and plotted it's frequency components in frequency domain (frequency vs amplitude). Now I want to sample it. Based on Nyquist–Shannon sampling theorem my sampling frequency should be > than 2*B (B-bandwidth). Based on a symbols that I've drawn here can you tell me what a bandwidth (B) of a signal equals to: BW or BW/2=(upper frequency-lower frequency)/2 and why? Thanks!

• That's not actually true. You can sample this sufficiently at 6.2 kHz, but it would require you to understand the concept of aliasing – Marcus Müller Sep 19 '17 at 21:22
• I understand something a bit about sampling and aliasing, but I'm a bit more confused in answering the question: "Based on my picture, is bandwidth equal 3.4kHz-300Hz (which is bandwidth of a human voice) or 3.4kHz-300Hz/2?" – Krushe Sep 19 '17 at 21:26
• why should it be (3400 Hz - 300 Hz)/2 ? I don't really see where you get that from? – Marcus Müller Sep 19 '17 at 21:34
• Please take a look at first image on this link: en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem This is why I'm confused I don't know what is the difference... – Krushe Sep 19 '17 at 21:56
• sorry, still don't understand your confusion. That image shows a two-sided spectrum from -F to +F. Your picture only shows positive frequencies. – Marcus Müller Sep 19 '17 at 22:24

Now, under some conditions, you can do bandpass sampling, where the bandwidth is considered as $3400 - 300 = 3100\text{ Hz}$, and the signal can be sampled at 6200 samples per second. You can't always do that, though; see here for the Wikipedia explanation. For the actual scholarly source, see
• @David If you sample at 3100 Hz the samples will be complex That is incorrect -- there is no way to sample a real signal and end up with a complex signal. – MBaz Sep 20 '17 at 14:02