def _write_header(self, block, fi):
        if len(block.segments) > 0:
            channel_indexes = channel_indexes_in_segment(block.segments[0])
        else:
            channel_indexes = []

        # type of file
        fi.write('NEURALSG')

        # sampling rate, in text and integer
        fi.write('30 kS/s\0')
        for _ in range(8): fi.write('\0')
        fi.write(struct.pack('<I', 1))

        # channel count: one for each analogsignal, and then also for
        # each column in each analogsignalarray
        fi.write(struct.pack('<I', len(channel_indexes)))
        for chidx in channel_indexes:
            fi.write(struct.pack('<I', chidx))
Return an (rId, description, image_size) 3-tuple identifying the
dataset/ETHPy150Open NeuralEnsemble/python-neo/neo/io/blackrockio_deprecated.py/BlackrockIO._write_header--crossover with--dataset/ETHPy150Open scanny/python-pptx/pptx/shapes/placeholder.py/PicturePlaceholder._get_or_add_image