def hpf_zc(times_s, freqs_hz, amplitudes, cutoff_freq_hz):
    """Brickwall high-pass filter for zero-cross signals (simply discards everything < cutoff)"""
    hpf_mask = np.where(freqs_hz > cutoff_freq_hz)
    junk_count = len(freqs_hz) - np.count_nonzero(hpf_mask)
    log.debug('HPF throwing out %d dots of %d (%.1f%%)' % (junk_count, len(freqs_hz), junk_count/len(freqs_hz)*100))
    return times_s[hpf_mask], freqs_hz[hpf_mask], amplitudes[hpf_mask] if amplitudes is not None else None