def frame_num(pcm_lens, sample_rate, frame_length=0.025, frame_step=0.01):
    """
    compute the frame number given the length of signals
    Args:
        pcm_lens: a Tensor, of batch [batch_size]
        sample_rate: python int, sample rate
        frame_length: python float, frame length in second
        frame_step: python float, frame step in second

    Returns:
        a Tensor including the frame number
    """
    frame_length = int(sample_rate * frame_length)
    frame_step = int(sample_rate * frame_step)
    num_frames = (pcm_lens - frame_length + frame_step) // frame_step
    return num_frames