diff --git a/lib/src/st2110/st_header.h b/lib/src/st2110/st_header.h
index ba63ec846..a19ee7a65 100644
--- a/lib/src/st2110/st_header.h
+++ b/lib/src/st2110/st_header.h
@@ -1054,8 +1054,6 @@ struct st_tx_ancillary_session_pacing {
   uint64_t cur_epochs;        /* epoch of current frame */
   /* timestamp for rtp header */
   uint32_t rtp_time_stamp;
-  /* timestamp for pacing */
-  uint32_t pacing_time_stamp;
   uint64_t cur_epoch_time;
   double tsc_time_cursor; /* in ns, tsc time cursor for packet pacing */
   /* ptp time may onward */
@@ -1224,8 +1222,6 @@ struct st_tx_fastmetadata_session_pacing {
   uint64_t cur_epochs;        /* epoch of current frame */
   /* timestamp for rtp header */
   uint32_t rtp_time_stamp;
-  /* timestamp for pacing */
-  uint32_t pacing_time_stamp;
   uint64_t cur_epoch_time;
   double tsc_time_cursor; /* in ns, tsc time cursor for packet pacing */
   /* ptp time may onward */
diff --git a/lib/src/st2110/st_tx_ancillary_session.c b/lib/src/st2110/st_tx_ancillary_session.c
index bc17b1af8..5cf60281f 100644
--- a/lib/src/st2110/st_tx_ancillary_session.c
+++ b/lib/src/st2110/st_tx_ancillary_session.c
@@ -260,7 +260,7 @@ static int tx_ancillary_session_sync_pacing(struct mtl_main_impl* impl,
                                             bool sync, uint64_t required_tai,
                                             bool second_field) {
   struct st_tx_ancillary_session_pacing* pacing = &s->pacing;
-  double frame_time = pacing->frame_time;
+  long double frame_time = pacing->frame_time;
   /* always use MTL_PORT_P for ptp now */
   uint64_t ptp_time = mt_get_ptp_time(impl, MTL_PORT_P);
   uint64_t next_epochs = pacing->cur_epochs + 1;
@@ -309,12 +309,20 @@ static int tx_ancillary_session_sync_pacing(struct mtl_main_impl* impl,
   if (epochs < next_epochs) s->stat_epoch_onward += (next_epochs - epochs);
 
   pacing->cur_epochs = epochs;
-  pacing->cur_epoch_time = tx_ancillary_pacing_time(pacing, epochs);
-  pacing->pacing_time_stamp = tx_ancillary_pacing_time_stamp(pacing, epochs);
-  pacing->rtp_time_stamp = pacing->pacing_time_stamp;
+
+  if (required_tai) {
+    pacing->cur_epoch_time = required_tai + frame_time;
+    pacing->rtp_time_stamp =
+        ((uint64_t)((required_tai / frame_time) * pacing->frame_time_sampling) &
+         0xffffffff);
+  } else {
+    pacing->cur_epoch_time = tx_ancillary_pacing_time(pacing, epochs);
+    pacing->rtp_time_stamp = tx_ancillary_pacing_time_stamp(pacing, epochs);
+  }
+
   pacing->tsc_time_cursor = (double)mt_get_tsc(impl) + to_epoch;
   dbg("%s(%d), epochs %" PRIu64 " time_stamp %u time_cursor %f to_epoch %f\n", __func__,
-      s->idx, pacing->cur_epochs, pacing->pacing_time_stamp, pacing->tsc_time_cursor,
+      s->idx, pacing->cur_epochs, pacing->rtp_time_stamp, pacing->tsc_time_cursor,
       to_epoch);
 
   if (sync) {
diff --git a/lib/src/st2110/st_tx_fastmetadata_session.c b/lib/src/st2110/st_tx_fastmetadata_session.c
index 5a04ceb13..84e40a939 100644
--- a/lib/src/st2110/st_tx_fastmetadata_session.c
+++ b/lib/src/st2110/st_tx_fastmetadata_session.c
@@ -260,7 +260,7 @@ static int tx_fastmetadata_session_sync_pacing(struct mtl_main_impl* impl,
                                                bool sync, uint64_t required_tai,
                                                bool second_field) {
   struct st_tx_fastmetadata_session_pacing* pacing = &s->pacing;
-  double frame_time = pacing->frame_time;
+  long double frame_time = pacing->frame_time;
   /* always use MTL_PORT_P for ptp now */
   uint64_t ptp_time = mt_get_ptp_time(impl, MTL_PORT_P);
   uint64_t next_epochs = pacing->cur_epochs + 1;
@@ -309,12 +309,20 @@ static int tx_fastmetadata_session_sync_pacing(struct mtl_main_impl* impl,
   if (epochs < next_epochs) s->stat_epoch_onward += (next_epochs - epochs);
 
   pacing->cur_epochs = epochs;
-  pacing->cur_epoch_time = tx_fastmetadata_pacing_time(pacing, epochs);
-  pacing->pacing_time_stamp = tx_fastmetadata_pacing_time_stamp(pacing, epochs);
-  pacing->rtp_time_stamp = pacing->pacing_time_stamp;
+
+  if (required_tai) {
+    pacing->cur_epoch_time = required_tai + frame_time;
+    pacing->rtp_time_stamp =
+        ((uint64_t)((required_tai / frame_time) * pacing->frame_time_sampling) &
+         0xffffffff);
+  } else {
+    pacing->cur_epoch_time = tx_fastmetadata_pacing_time(pacing, epochs);
+    pacing->rtp_time_stamp = tx_fastmetadata_pacing_time_stamp(pacing, epochs);
+  }
+
   pacing->tsc_time_cursor = (double)mt_get_tsc(impl) + to_epoch;
   dbg("%s(%d), epochs %" PRIu64 " time_stamp %u time_cursor %f to_epoch %f\n", __func__,
-      s->idx, pacing->cur_epochs, pacing->pacing_time_stamp, pacing->tsc_time_cursor,
+      s->idx, pacing->cur_epochs, pacing->rtp_time_stamp, pacing->tsc_time_cursor,
       to_epoch);
 
   if (sync) {
diff --git a/lib/src/st2110/st_tx_video_session.c b/lib/src/st2110/st_tx_video_session.c
index 2c93f4b08..00f6e707d 100644
--- a/lib/src/st2110/st_tx_video_session.c
+++ b/lib/src/st2110/st_tx_video_session.c
@@ -18,9 +18,8 @@ static inline double pacing_time(struct st_tx_video_pacing* pacing, uint64_t epo
   return epochs * pacing->frame_time;
 }
 
-static inline double pacing_first_pkt_time(struct st_tx_video_pacing* pacing,
-                                           uint64_t epochs) {
-  return pacing_time(pacing, epochs) + pacing->tr_offset - (pacing->vrx * pacing->trs);
+static inline double pacing_first_pkt_time(struct st_tx_video_pacing* pacing) {
+  return pacing->tr_offset - (pacing->vrx * pacing->trs);
 }
 
 /* pacing start time(warmup pkt if has warmup stage) of the frame */
@@ -33,25 +32,26 @@ static inline double pacing_start_time(struct st_tx_video_pacing* pacing,
 /* time stamp on the first pkt video pkt(not the warmup) */
 static inline uint32_t pacing_time_stamp(struct st_tx_video_session_impl* s,
                                          struct st_tx_video_pacing* pacing,
-                                         uint64_t epochs) {
+                                         uint64_t epochs, uint64_t tai) {
   uint64_t tmstamp64;
-  double time;
-  if (s->ops.flags & ST20_TX_FLAG_RTP_TIMESTAMP_EPOCH) {
-    /* the start of epoch */
-    time = pacing_first_pkt_time(pacing, epochs);
-  } else if (s->ops.flags & ST20_TX_FLAG_RTP_TIMESTAMP_FIRST_PKT) {
+  long double frame_time = pacing->frame_time;
+  double time = pacing_time(pacing, epochs);
+
+  if (s->ops.flags & ST20_TX_FLAG_USER_TIMESTAMP) {
+    time = tai;
+  } else if (s->ops.flags & ST20_TX_FLAG_RTP_TIMESTAMP_EPOCH) {
+    time += pacing_first_pkt_time(pacing);
+  } else {  // if (s->ops.flags & ST20_TX_FLAG_RTP_TIMESTAMP_FIRST_PKT) {
     /* the start of first pkt */
-    time = pacing_first_pkt_time(pacing, epochs);
-    if (pacing->warm_pkts) time -= 3 * pacing->trs; /* deviation for VRX */
-  } else if (s->ops.rtp_timestamp_delta_us) {
-    int32_t rtp_timestamp_delta_us = s->ops.rtp_timestamp_delta_us;
-    time = pacing_time(pacing, epochs) + (rtp_timestamp_delta_us * NS_PER_US);
-  } else {
-    /* default to the start of first pkt */
-    time = pacing_first_pkt_time(pacing, epochs);
+    time += pacing_first_pkt_time(pacing);
     if (pacing->warm_pkts) time -= 3 * pacing->trs; /* deviation for VRX */
   }
-  tmstamp64 = (time / pacing->frame_time) * pacing->frame_time_sampling;
+
+  if (s->ops.rtp_timestamp_delta_us) {
+    time += (s->ops.rtp_timestamp_delta_us * NS_PER_US);
+  }
+
+  tmstamp64 = (time / frame_time) * pacing->frame_time_sampling;
   uint32_t tmstamp32 = tmstamp64;
 
   return tmstamp32;
@@ -600,14 +600,16 @@ static int tv_sync_pacing(struct mtl_main_impl* impl, struct st_tx_video_session
   int idx = s->idx;
   struct st_tx_video_pacing* pacing = &s->pacing;
   double frame_time = pacing->frame_time;
+  double to_epoch;
   /* always use MTL_PORT_P for ptp now */
   uint64_t ptp_time = mt_get_ptp_time(impl, MTL_PORT_P);
   uint64_t next_epochs = pacing->cur_epochs + 1;
   uint64_t epochs;
+  uint64_t ptp_epochs;
   bool interlaced = s->ops.interlaced;
 
   if (required_tai) {
-    uint64_t ptp_epochs = ptp_time / frame_time;
+    ptp_epochs = ptp_time / frame_time;
     epochs = required_tai / frame_time;
     dbg("%s(%d), required tai %" PRIu64 " ptp_epochs %" PRIu64 " epochs %" PRIu64 "\n",
         __func__, idx, required_tai, ptp_epochs, epochs);
@@ -638,7 +640,22 @@ static int tv_sync_pacing(struct mtl_main_impl* impl, struct st_tx_video_session
 
   /* epoch resolved */
   double start_time_ptp = pacing_start_time(pacing, epochs);
-  double to_epoch = start_time_ptp - ptp_time;
+  if (required_tai) {
+    to_epoch = (double)required_tai - ptp_time;
+    if (to_epoch > NS_PER_S) {
+      dbg("%s(%d), required tai %" PRIu64 " ptp_epochs %" PRIu64 " epochs %" PRIu64 "\n",
+          __func__, s->idx, required_tai, ptp_epochs, epochs);
+      s->stat_error_user_timestamp++;
+      to_epoch = NS_PER_S;  // do our best to slow down
+    }
+    if (to_epoch < 0) {
+      /* time bigger than the assigned epoch time */
+      to_epoch = 0; /* send asap */
+    }
+  } else {
+    to_epoch = start_time_ptp - ptp_time;
+  }
+
   if (to_epoch < 0) {
     /* time larger than the next assigned epoch time */
     dbg("%s(%d), to_epoch %f, ptp epochs %" PRIu64 " cur_epochs %" PRIu64
@@ -665,8 +682,13 @@ static int tv_sync_pacing(struct mtl_main_impl* impl, struct st_tx_video_session
   }
   if (epochs < next_epochs) s->stat_epoch_onward += (next_epochs - epochs);
   pacing->cur_epochs = epochs;
-  pacing->cur_epoch_time = pacing_time(pacing, epochs);
-  pacing->rtp_time_stamp = pacing_time_stamp(s, pacing, epochs);
+  if (s->ops.flags & ST20_TX_FLAG_USER_TIMESTAMP) {
+    pacing->cur_epoch_time = required_tai;
+  } else {
+    pacing->cur_epoch_time = pacing_time(pacing, epochs);
+  }
+
+  pacing->rtp_time_stamp = pacing_time_stamp(s, pacing, epochs, required_tai);
   dbg("%s(%d), old time_cursor %fms\n", __func__, idx,
       pacing->tsc_time_cursor / 1000 / 1000);
   pacing->tsc_time_cursor = (double)mt_get_tsc(impl) + to_epoch;