No you don't.
You just change the transducer type in the Sonar menu Transducer Select from Compact Sidescan to Hi-def Sidescan. The unit determines the sonar function, not the transducer.
You can use the Hi-def Sidescan setting with the XNT 9 SI 180 T if you want to use both SI frequencies (800 and 455).
The ONLY difference in the two transducers is the SI crystal length. The HDSI is longer and has longer SI crystals. Those longer SI crystals produce a thinner SI pulse and sharper images. The HDSI can improve the images of any unit that comes standard with the Compact SI transducer.
The less expensive models don't have the 800 kHz capability so the HDSI will not add that frequency to those units, just produce sharper images with the 455 kHz Imaging frequency.
Edited by Wayne P. (08/18/15 08:05 PM)
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, SOLIX 12 SI, 360, RC-1