When sending video (st20p) and enabling ST20P_TX_FLAG_USER_PACING, the RTP timestamp of the frame/field should correspond accurately to the TAI (send) time provided.

However, we are observing an RTP offset of ~55 (media clock ticks at 90kHz) using analysis tools (ie. EBU List).

SOLUTION 1

We feel, that in this mode, the RTP timestamp should be calculated as follows:

TAI time (provided by user) gets translated to the RTP timestamp (straight math)

If ST20_TX_FLAG_RTP_TIMESTAMP_EPOCH is set, then Round RTP timestamp to Epoch

If user provided rtp_timestamp_delta_us, add it to the RTP timestamp.

SOLUTION 2

Perhaps rework the use of the frame data-structure to allow a user to pass in BOTH the TAI (pacing) timestamp, and the RTP timestamp individually.
The fields are there, but they are not used this way on Tx.

(MTL main branch)