docs: def echo(params, source, delay, strength):
      '''
      Create an echo
      :param params:
      :param source:
      :param delay:
      :param strength:
      :return:
      '''
      source = create_buffer(params, source)
      delay = create_buffer(params, delay)
      strength = create_buffer(params, strength)
      output = source[:]
      for i in range(params.length):
          d = int(i - delay[i])
          if 0 <= d < params.length:
              output[i] += source[d]*strength[i]
      return output