In the absence of any technical manual:
SviWav2Cas comes with source to decode sampled SVI3x8 tapes.
openMSX can emulate the SVI machines and creates WAV files from SAVE/BSAVE that SviWav2Cas seems to understand.
Maybe save known data from openMSX in SVI mode (say a 1024 zero bytes, or 1024 &hFF bytes) and analyze the WAV files it produces?
(the CAS ...