The SI is using the 455, it's not something you can select. The unit automatically uses 455 for SI. There is no designated beam for the DI. H/B creates the DI image by computer programming using the SI beam. The 798 will not get as good DI images as a H/B unit that has a DI in it's model number. Unit's with a DI in the model number have a designated beam in the transducer for DI. The 798 isn't capable of using a DI transducer. My 798 transducer has a tag attached with the model number.