experimental LTC sub-frame drift compensation for scope testing

git-svn-id: svn://localhost/ardour2/branches/3.0@13384 d708f5d6-7413-0410-9779-e7cbd77b26cf

diff --git a/libs/ardour/session_ltc.cc b/libs/ardour/session_ltc.cc
index e24a233b7b632976fb3768eb33520a987205acc2..8d976ea4da1b35814c9fd1a76174b82021571d6a 100644 (file)
--- a/libs/ardour/session_ltc.cc
+++ b/libs/ardour/session_ltc.cc
@@ -50,9 +50,13 @@ using namespace Timecode;
  * yet with most sound-cards a square-wave of 1-2 sample
  * introduces rather some ringing and small oscillations.
  * so we low-pass filter the signal a bit, depending
- * on the sample-rate
+ * on the sample-rate.
+ *
+ * TODO: this should become an adaptive value, when
+ * the playback speed is increases so that 1 bit < 3-4
+ * audio samples, we should fall back to 25 us.
  */
-#define LTC_RISE_TIME MIN (150, MAX(25, (4000000 / engine().frame_rate())))
+#define LTC_RISE_TIME MIN (100, MAX(25, (2000000 / engine().frame_rate())))
 
 void
 Session::ltc_tx_initialize()
@@ -465,6 +469,24 @@ Session::ltc_tx_send_time_code_for_cycle (framepos_t start_frame, framepos_t end
                DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX5 restart @ %1 + %2 - %3 |  byte %4\n",
                                        ltc_enc_pos, ltc_enc_cnt, cyc_off, ltc_enc_byte));
        }
+#if 1 /* experimental sample bit alignment */
+       else if (ltc_speed != 0 && (fptcf / ltc_speed / 80) > 3 ) {
+               /* We may get away without a DLL if speed-changes are uniform enough and
+                * no oscillation takes place, the linear approx here should reduce the
+                * jitter sufficiently when generating LTC from another LTC source or
+                * JACK-transport or ardour internal clock.
+                *
+                * Note that the granularity of the LTC encoder speed is 1 byte =
+                * (frames-per-timecode-frame / 10) audio-samples.
+                *
+                * Thus, tiny speed changes won't have any effect and larger ones
+                * may lead to oscillations.
+                * To be better than that, resampling (or a rewrite of the encoder) is
+                * required.
+                */
+               ltc_speed -= ((ltc_enc_pos + ltc_enc_cnt - poff) - cycle_start_frame) / engine().frame_rate();
+       }
+#endif
 
 
        // (6) encode and output
@@ -529,6 +551,12 @@ Session::ltc_tx_send_time_code_for_cycle (framepos_t start_frame, framepos_t end
                        ltc_enc_byte = (ltc_enc_byte + 1)%10;
                        if (ltc_enc_byte == 0 && ltc_speed != 0) {
                                ltc_encoder_inc_timecode(ltc_encoder);
+#if 0 /* force fixed parity -- scope debug */
+                               LTCFrame f;
+                               ltc_encoder_get_frame(ltc_encoder, &f);
+                               f.biphase_mark_phase_correction=0;
+                               ltc_encoder_set_frame(ltc_encoder, &f);
+#endif
                                ltc_tx_recalculate_position();
                                ltc_enc_cnt = 0;
                        } else if (ltc_enc_byte == 0) {