def am_tx(m,a_mod,fc=<NUM_LIT>):
m24 = interp24(m)<EOL>t192 = np.arange(len(m24))/<NUM_LIT><EOL>m_max = np.max(np.abs(m24))<EOL>x192 = (<NUM_LIT:1> + a_mod*m24/m_max)*np.cos(<NUM_LIT:2>*np.pi*fc*t192) <EOL>return x192, t192, m24<EOL>
AM transmitter for Case Study of Chapter 17.

Assume input is sampled at 8 Ksps and upsampling
by 24 is performed to arrive at fs_out = 192 Ksps.

Parameters
----------
m : ndarray of the input message signal 
a_mod : AM modulation index, between 0 and 1
fc : the carrier frequency in Hz

Returns
-------
x192 : ndarray of the upsampled by 24 and modulated carrier
t192 : ndarray of the upsampled by 24 time axis
m24 : ndarray of the upsampled by 24 message signal

Notes
-----
The sampling rate of the input signal is assumed to be 8 kHz.

Examples
--------
>>> n = arange(0,1000)
>>> # 1 kHz message signal
>>> m = cos(2*pi*1000/8000.*n)
>>> x192, t192 = am_tx(m,0.8,fc=75e3)