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use accurate log10 for meter (fast_log2 is unsuitable inaccurate)

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fast_coefficient_to_dB() returns a lower bound value, unsuitable
to catch audio peaks. The difference to 20*log10 is as large as 0.4 dB!

The effective speedup of fast_log10 compared to log10f is marginal
(sweep of all 24bit values)

  i686 (1.6GHz Intel core):  2.36 [times faster]
  x86_64 (core2 2.4GHz):     1.63
  x86_64 (I3 2.80GHz):       2.03

the execution time of one log10f() averaged over a
sweep of all 24 bit values

  i686 (1.6GHz Intel core):  0.131 usec
  x86_64 (core2 2.4GHz):     0.033 usec
  x86_64 (I3 2.80GHz):       0.044 usec

PeakMeter::run() is called from dedicated non-rt, no harm done.
This commit is contained in:
Robin Gareus 2013-07-09 13:13:27 +02:00
parent 617f73f8a9
commit ee97942165
2 changed files with 10 additions and 9 deletions
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2
libs/ardour/ardour/dB.h
View File

@ -31,7 +31,7 @@ static inline float fast_coefficient_to_dB (float coeff) {
}
static inline float accurate_coefficient_to_dB (float coeff) {
return 20.0f * log10 (coeff);
return 20.0f * log10f (coeff);
}
extern double zero_db_as_fraction;

17
libs/ardour/meter.cc
View File

@ -55,6 +55,8 @@ PeakMeter::~PeakMeter ()
* Input acceptance is lenient - the first n buffers from @a bufs will
* be metered, where n was set by the last call to setup(), excess meters will
* be set to 0.
*
* (runs in jack realtime context)
*/
void
PeakMeter::run (BufferSet& bufs, framepos_t /*start_frame*/, framepos_t /*end_frame*/, pframes_t nframes, bool)
@ -95,7 +97,7 @@ PeakMeter::run (BufferSet& bufs, framepos_t /*start_frame*/, framepos_t /*end_fr
// Meter audio in to the rest of the peaks
for (uint32_t i = 0; i < n_audio; ++i, ++n) {
_peak_signal[n] = compute_peak (bufs.get_audio(i).data(), nframes, _peak_signal[n]);
if (/* TODO use separate bit-flags for mixer,meterbridge,.. */ /* 1 || */ _meter_type & MeterKrms) {
if (_meter_type & MeterKrms) {
_kmeter[i]->process(bufs.get_audio(i).data(), nframes);
}
}
@ -239,11 +241,7 @@ PeakMeter::meter ()
;
} else {
/* empirical WRT to falloff times , 0.01f ^= 100 Hz update rate */
#if 1
new_peak = _visible_peak_power[n] - _visible_peak_power[n] * Config->get_meter_falloff() * 0.01f * 0.05f;
#else
new_peak = _visible_peak_power[n] - sqrt(_visible_peak_power[n] * Config->get_meter_falloff() * 0.01f * 0.0002f);
#endif
if (new_peak < (1.0 / 512.0)) new_peak = 0;
}
_visible_peak_power[n] = new_peak;
@ -257,7 +255,7 @@ PeakMeter::meter ()
_max_peak_signal[n] = std::max(new_peak, _max_peak_signal[n]);
if (new_peak > 0.0) {
new_peak = fast_coefficient_to_dB (new_peak);
new_peak = accurate_coefficient_to_dB (new_peak);
} else {
new_peak = minus_infinity();
}
@ -283,7 +281,11 @@ PeakMeter::meter_level(uint32_t n, MeterType type) {
{
const uint32_t n_midi = current_meters.n_midi();
if ((n - n_midi) < _kmeter.size() && (n - n_midi) >= 0) {
#if 0
return fast_coefficient_to_dB (_kmeter[n-n_midi]->read());
#else
return accurate_coefficient_to_dB (_kmeter[n-n_midi]->read());
#endif
}
return minus_infinity();
}
@ -304,6 +306,7 @@ PeakMeter::meter_level(uint32_t n, MeterType type) {
}
}
}
void
PeakMeter::set_type(MeterType t)
{
@ -329,5 +332,3 @@ PeakMeter::state (bool full_state)
node.add_property("type", "meter");
return node;
}