ISM metadata discontinuity in FEC

The ISM metadata (MD) are coded in active frames while there is no MD transmission in inactive frames (VAD=0 and low-level noise) in no-DTX mode of discrete ISM. In the case of FEC, the last frame MD is simply replicated.

When there is a transition from inactive to active frame, the first active frame contains true MD values. In case the MD changes in the inactive segment and the first active frame is lost, the concealed MD values might be far from the true values while the incorrect values can be propagated up to 10 frames which cause a significant subjective artifact.

A fix is proposed such that

• MD is transmitted in an inactive frame when their values significantly change, i.e. a difference between the true MD value and the last transmitted value is more than 10 degrees
• MD is transmitted at least every 25 frames