The 421 could get comms over HDMI from a Meridian player such as the 800 but it seems to me that the HD621 doesn't have this option and you probably don't have a Meridian player anyway.

It looks like you need a DIN to BNC adaptor or lead to take comms from the HD621 to the C15.

regards

Chris


Lounge/Cinema, Trinnov, DSP8000XE, 4x 320.2 in-ceiling, M6 rear, 2x DSW.2
Roon Nucleus+ and 320.2 in-ceiling in many rooms (KEF in-ceiling in bathrooms)
Spare: DSP5000, 598, 562v2, 500Mk2, 518, 519, 504, 101B, M3, 861v4, 800 DVD, MS600, MC200