Initialise TIST with consistent system time at startup

The change introduced in fd33dd4 (v2.4.0) had the consequence of
generating different timestamp offsets depending on when exactly
the mux was started. The intended goal to avoid having the NULL
symbol output at a fixed offset to avoid receiver synchronisation is
however not so important, because most commercial receivers will
cut transmission for several seconds before resuming, and the majority
of the receivers will then start resynchronising.

However, the different timestamp offsets upsets some modulator input
modules, which so not necessarily have enough buffering capacity.

This change reverts back to the previous behaviour, where the timestamp
is set to current system time, rounded to 24ms.

1 files changed, 13 insertions, 14 deletions

diff --git a/src/DabMultiplexer.cpp b/src/DabMultiplexer.cpp
index 9bd3c3f..2bd8d74 100644
--- a/src/DabMultiplexer.cpp
+++ b/src/DabMultiplexer.cpp
@@ -134,24 +134,23 @@ void DabMultiplexer::prepare(bool require_tai_clock)
         throw MuxInitException();
     }
 
-    /* TODO:
-     * In a SFN, when reconfiguring the ensemble, the multiplexer
-     * has to be restarted (odr-dabmux doesn't support reconfiguration).
-     * Ideally, we must be able to restart transmission s.t. the receiver
-     * synchronisation is preserved.
+    /* Ensure edi_time and TIST represent current time. Keep
+     * a granularity of 24ms, which corresponds to the
+     * duration of an ETI frame, to get nicer timestamps.
      */
     using Sec = chrono::seconds;
-    const auto now = chrono::time_point_cast<Sec>(chrono::system_clock::now());
-
-    edi_time = chrono::system_clock::to_time_t(now);
-
-    // We define that when the time is multiple of six seconds, the timestamp
-    // (PPS offset) is 0. This ensures consistency of TIST even across a
-    // mux restart
+    const auto now = chrono::system_clock::now();
+    edi_time = chrono::system_clock::to_time_t(chrono::time_point_cast<Sec>(now));
+    auto offset = now - chrono::time_point_cast<Sec>(now);
+    if (offset >= chrono::seconds(1)) {
+        throw std::logic_error("Invalid startup offset calculation for TIST! " +
+                to_string(chrono::duration_cast<chrono::milliseconds>(offset).count()) +
+                " ms");
+    }
     timestamp = 0;
-    edi_time -= (edi_time % 6);
-    while (edi_time < chrono::system_clock::to_time_t(now)) {
+    while (offset >= chrono::milliseconds(24)) {
         increment_timestamp();
+        offset -= chrono::milliseconds(24);
     }
 
     // Try to load offset once