Click here to purchase the entire book in PDF format. Linear phase If you plot the phase response of a filter for all frequencies, chances are you'll get a smooth, fancy-looking curve like the ones in Figure 6.18. Some filters, on the other hand, have a phase response plot that's a straight line if you graph the response on a linear frequency scale (instead of a log scale like we normally do...). This line usually slopes upwards so the higher the frequency, the bigger the phase change. In fact, this would be exactly the phase response of a straight delay line - the higher the frequency, the more of a phase shift that's incurred by a fixed delay time. If the delay time is 0, then the straight line is a horizontal one at 0$^{\circ }$ for all frequencies. Any filter whose phase response is a straight line is called a linear phase filter. Be careful not to jump to the conclusion that, because it's a linear phase filter, it's better than anything else. While there are situations where such a filter is useful, they work well in all situations to correct all problems. Different intentions require different filter characteristics.