feature: Add lc3_encoder_disable_ltpf

LTPF requires a lot of processing power, so disabling it might be
required for embedded low-performance devices.

include/lc3.h

@@ -355,6 +355,17 @@ LC3_EXPORT lc3_encoder_t lc3_hr_setup_encoder(
 LC3_EXPORT lc3_encoder_t lc3_setup_encoder(
     int dt_us, int sr_hz, int sr_pcm_hz, void *mem);
 
+/**
+ * Disable LTPF analysis
+ * encoder        Handle of the encoder
+ *
+ * LTPF analysis is known to take a lot of CPU time and work quite bad on
+ * synthetic signals such as sine waves, so it might be beneficial to
+ * disable it in such cases.
+ */
+LC3_EXPORT void lc3_encoder_disable_ltpf(
+    lc3_encoder_t encoder);
+
 /**
  * Encode a frame
  * encoder         Handle of the encoder

include/lc3_cpp.h

@@ -137,6 +137,11 @@ class Encoder : public Base<struct lc3_encoder> {
     }
   }
 
+  void DisableLTPF() {
+    for (auto &s : states)
+      lc3_encoder_disable_ltpf(s.get());
+  }
+
   ~Encoder() override = default;
 
   // Reset encoder state

include/lc3_private.h

@@ -113,6 +113,8 @@ typedef struct lc3_spec_analysis {
 } lc3_spec_analysis_t;
 
 struct lc3_encoder {
+    bool ltpf_bypass;
+
     enum lc3_dt dt;
     enum lc3_srate sr, sr_pcm;
 

src/lc3.c

@@ -301,7 +301,7 @@ static void analyze(struct lc3_encoder *encoder,
 
     bool att = lc3_attdet_run(dt, sr_pcm, nbytes, &encoder->attdet, xt);
 
-    side->pitch_present =
+    side->pitch_present = !encoder->ltpf_bypass &&
         lc3_ltpf_analyse(dt, sr_pcm, &encoder->ltpf, xt, &side->ltpf);
 
     memmove(xt - nt, xt + (ns-nt), nt * sizeof(*xt));
@@ -313,7 +313,7 @@ static void analyze(struct lc3_encoder *encoder,
     lc3_mdct_forward(dt, sr_pcm, sr, xs, xd, xf);
 
     bool nn_flag = lc3_energy_compute(dt, sr, xf, e);
-    if (nn_flag)
+    if (nn_flag || encoder->ltpf_bypass)
         lc3_ltpf_disable(&side->ltpf);
 
     side->bw = lc3_bwdet_run(dt, sr, e);
@@ -424,6 +424,12 @@ LC3_EXPORT struct lc3_encoder *lc3_setup_encoder(
     return lc3_hr_setup_encoder(false, dt_us, sr_hz, sr_pcm_hz, mem);
 }
 
+LC3_EXPORT void lc3_encoder_disable_ltpf(
+    struct lc3_encoder *encoder)
+{
+    encoder->ltpf_bypass = true;
+}
+
 /**
  * Encode a frame
  */