signal.py 文件源码

def pb2bb(x, fs, fc, fd=None, flen=127, cutoff=None):
    """Convert passband signal to baseband.

    The baseband conversion uses a low-pass filter after downconversion, with a
    default cutoff frequency of `0.6*fd`, if `fd` is specified, or `1.1*fc` if `fd`
    is not specified. Alternatively, the user may specify the cutoff frequency
    explicitly.

    For communication applications, one may wish to use :func:`arlpy.comms.downconvert` instead,
    as that function supports matched filtering with a pulse shape rather than a generic
    low-pass filter.

    :param x: passband signal
    :param fs: sampling rate of passband signal in Hz
    :param fc: carrier frequency in passband in Hz
    :param fd: sampling rate of baseband signal in Hz (``None`` => same as `fs`)
    :param flen: number of taps in the low-pass FIR filter
    :param cutoff: cutoff frequency in Hz (``None`` means auto-select)
    :returns: complex baseband signal, sampled at `fd`
    """
    if cutoff is None:
        cutoff = 0.6*fd if fd is not None else 1.1*fc
    y = x * _np.sqrt(2)*_np.exp(2j*_np.pi*fc*time(x,fs))
    hb = _sig.firwin(flen, cutoff=cutoff, nyq=fs/2.0)
    y = _sig.filtfilt(hb, 1, y)
    if fd is not None and fd != fs:
        y = _sig.resample_poly(y, 2*fd, fs)[::2]
    return y