When the preparation of the SRS document can begin?

SRS if I am assuming correct a Software Requirement Specification Document, can begin from day 1. One can start taking rough notes, and with more functional information gathered , the SRS can be continually refined. Though at the same time, the Business Analyst should consider the limitations of the technology used for development or depending on the reqs an appropriate platform must be chosen. After a thorough analysis of functional and technical scope an SRS can move towards finalization and can be later signed off by business users and Technical Project Managers I hope that helps