In some DSP applications, it can be very helpful to know how many times your signal has crossed the zero-line (amplitude origin).
How is this helpful? Well, zero-crossings can tell you very quickly if your signal is composed of high frequency content or not. Let's say your sample rate is 50kHz and over a small window of 1,000 samples there are 500 zero-crossings. That would mean that every two samples crosses the zero-line (i.e. 12.5kHz)
In speech processing, the zero-crossing counts can help distinguish between voiced and un-voiced speech. Un-voiced sounds are very noise-like ('Shh' and 'Sss' for example). In addition, zero-crossings could also be used to determine if your signal has a DC offset. If you signal is 'muted' and you are not seeing alot of zero-crossings might mean that your signal is offset from the zero-line
One nice thing about the matlab code below is that it is implemented in a very DSP-friendly way. It ports very easily into C-Code and does minimizes the amount of conditional statements for faster processing time.
function count = zero_crossings(x) % Count the number of zero-crossings from the supplied time-domain % input vector. A simple method is applied here that can be easily % ported to a real-time system that would minimize the number of % if-else conditionals. % % Usage: COUNT = zero_crossings(X); % % X is the input time-domain signal (one dimensional) % COUNT is the amount of zero-crossings in the input signal % % Author: sparafucile17 06/27/04 % % initial value count = 0; % error checks if(length(x) == 1) error('ERROR: input signal must have more than one element'); end if((size(x, 2) ~= 1) && (size(x, 1) ~= 1)) error('ERROR: Input must be one-dimensional'); end % force signal to be a vector oriented in the same direction x = x(:); num_samples = length(x); for i=2:num_samples % Any time you multiply to adjacent values that have a sign difference % the result will always be negative. When the signs are identical, % the product will always be positive. if((x(i) * x(i-1)) < 0) count = count + 1; end end