One other consideration since you said this was a CATV headend switch... For mcast to work with IGMP snooping enabled whatever device you have on those ports you want to receive the udp video traffic via multi-cast must know the mcast ip and porrt and make a valid join request. If the switch does not see a valid join then the mcast will not be sent to that port. Unless you have changed the settings igmp snooping is enabled on all vlans by default.
You can do command show igmp snooping detail to confirm this. If you just want to flood all the traffic to those ports then disable igmp snooping and all mcast will be treated as a broadcast frame and the udp stream will be sent to every port you have the vlan on. The device will still need to subscribe to the stream via the ip address and port you are using in the encoders..
Hope this makes sense good luck.
