Digital Signal Analysis & Processing

Design a digital low-pass filter with the following specifications:

1. Passband magnitude constant to 0.7 dB below the frequency of $0.15\pi$
2. Stopband attenuation at least 14 dB for the frequencies between $0.6\pi$ to $\pi$

Use Butterworth approximation as a prototype and use impulse invariance method to obtain the digital filter.