alsa: implement timing callback

diff --git a/libs/backends/alsa/alsa_audiobackend.cc b/libs/backends/alsa/alsa_audiobackend.cc
index 8090ac8c9db8d22f51175904736293db511c7a66..18de4dc0b800cf72a1d9139466e86d5f0962aad7 100644 (file)
--- a/libs/backends/alsa/alsa_audiobackend.cc
+++ b/libs/backends/alsa/alsa_audiobackend.cc
@@ -50,6 +50,8 @@ AlsaAudioBackend::AlsaAudioBackend (AudioEngine& e, AudioBackendInfo& info)
        , _freewheel (false)
        , _freewheeling (false)
        , _measure_latency (false)
+       , _last_process_start (0)
+       , _process_speed_samples_per_ms (0)
        , _audio_device("")
        , _midi_driver_option(_("None"))
        , _device_reservation(0)
@@ -500,6 +502,13 @@ AlsaAudioBackend::_start (bool for_latency_measurement)
                _ports.clear();
        }
 
+       /* reset internal state */
+       _dsp_load = 0;
+       _freewheeling = false;
+       _freewheel = false;
+       _last_process_start = 0;
+       _process_speed_samples_per_ms = 0;
+
        release_device();
 
        assert(_rmidi_in.size() == 0);
@@ -696,7 +705,25 @@ AlsaAudioBackend::sample_time_at_cycle_start ()
 pframes_t
 AlsaAudioBackend::samples_since_cycle_start ()
 {
-       return 0;
+       if (!_active || !_run || _freewheeling || _freewheel) {
+               return 0;
+       }
+       if (_last_process_start == 0 || _process_speed_samples_per_ms) {
+               return 0;
+       }
+
+       const int64_t elapsed_time_us = _last_process_start - g_get_monotonic_time();
+       const int64_t nomial_time = 1e6 * _samples_per_period / _samplerate;
+
+       assert(elapsed_time_us >=0);
+
+       /* linear extrapolation, using [low pass] filtered process-speed. */
+       const pframes_t processed_samples = elapsed_time_us * _process_speed_samples_per_ms * 1e-3;
+
+       if (processed_samples >= _samples_per_period) {
+               return _samples_per_period;
+       }
+       return processed_samples;
 }
 
 
@@ -1465,6 +1492,8 @@ AlsaAudioBackend::main_process_thread ()
                                        memset ((*it)->get_buffer (_samples_per_period), 0, _samples_per_period * sizeof (Sample));
                                }
 
+                               /* call engine process callback */
+                               _last_process_start = g_get_monotonic_time();
                                if (engine.process_callback (_samples_per_period)) {
                                        _pcmi->pcm_stop ();
                                        _active = false;
@@ -1504,6 +1533,12 @@ AlsaAudioBackend::main_process_thread ()
                                clock2 = g_get_monotonic_time();
                                const int64_t elapsed_time = clock2 - clock1;
                                _dsp_load = elapsed_time / (float) nomial_time;
+
+                               const double ps = 1e3 * _samples_per_period / elapsed_time;
+                               // low pass filter, The time-constant should really be
+                               // 1.0 - e^(-2.0 * π * v / SR);
+                               // with v = _samples_per_period / N, and SR = _samples_per_period;
+                               _process_speed_samples_per_ms  = _process_speed_samples_per_ms + .05 * (ps - _process_speed_samples_per_ms) + 1e-12;
                        }
 
                        if (xrun && (_pcmi->capt_xrun() > 0 || _pcmi->play_xrun() > 0)) {
@@ -1539,6 +1574,7 @@ AlsaAudioBackend::main_process_thread ()
                                rm->sync_time (clock1);
                        }
 
+                       _last_process_start = -1;
                        if (engine.process_callback (_samples_per_period)) {
                                _pcmi->pcm_stop ();
                                _active = false;

diff --git a/libs/backends/alsa/alsa_audiobackend.h b/libs/backends/alsa/alsa_audiobackend.h
index 3823db22eddaa50618403fa193121e7cfac2fd83..fe29caf1662f83548516eb853ffab4e048cdcc08 100644 (file)
--- a/libs/backends/alsa/alsa_audiobackend.h
+++ b/libs/backends/alsa/alsa_audiobackend.h
@@ -305,6 +305,9 @@ class AlsaAudioBackend : public AudioBackend {
                bool  _freewheeling;
                bool  _measure_latency;
 
+               uint64_t _last_process_start;
+               double   _process_speed_samples_per_ms;
+
                static std::vector<std::string> _midi_options;
                static std::vector<AudioBackend::DeviceStatus> _audio_device_status;
                static std::vector<AudioBackend::DeviceStatus> _midi_device_status;