Question: What is a prerequisite for seamless rate switching between all EVS primary rates in the Enhanced Voice Services (EVS) codec, as per 3GPP standards?
Option 1: Complete re-initialization of all coding tools at every bit-rate change.
Option 2: Dedicated signaling to indicate rate switch at the beginning of each frame.
Option 3: Unique bit-rate dependent setups for each tool, described in separate configurations.
Option 4: Sharing and maintaining most states and memories across all bit-rates.
Explanation: Seamless switching between EVS primary rates is facilitated by the codec's ability to share and maintain most of its states and memories across different bit-rates. This allows for reconfiguration of coding tools at the start of any frame without needing a complete restart, making Option 4 the correct answer. Options 1, 2, and 3 are incorrect as they either contradict the seamless nature (Option 1), introduce unnecessary complexity (Option 2), or only partially describe the setup process without addressing the core prerequisite for seamless switching (Option 3).