Align the silence scale range between Validate() and ScaleSilence

After #3745, ScaleSilence accepts [0.01, 2] but OfflineTtsConfig::Validate()
still only rejects silence_scale < 0.001. Values in [0.001, 0.01) and above 2
therefore pass validation and are then silently skipped at run time, so the
user asks for scaling and gets unscaled audio.

Use one pair of constants for both, document the range in the --tts-silence-scale
help text, and raise the upper bound from 2 to 10.

Rejecting in Validate() also makes bad input fail before synthesis rather than
after it. The check in ScaleSilence stays, because callers of the Generate API
pass GeneratedAudioConfig::silence_scale directly and bypass Validate().

Author: ekenberg

sherpa-onnx/csrc/offline-tts.cc

@@ -31,19 +31,26 @@ struct SilenceInterval {
   int32_t end;
 };
 
+// Supported range for --tts-silence-scale. The lower bound keeps a pause from
+// collapsing to nothing; the upper bound is a sanity limit, far below the
+// value at which interval_length * scale would overflow an int32_t.
+static constexpr float kMinSilenceScale = 0.01f;
+static constexpr float kMaxSilenceScale = 10.0f;
+
 GeneratedAudio GeneratedAudio::ScaleSilence(float scale) const {
   if (scale == 1) {
     return *this;
   }
 
-  // scale is used to shorten long pauses, so it is normally within (0, 1).
-  // Values outside this range are rejected: n below is computed as
-  // interval_length * scale and converted to an int32_t, and NaN, infinity or
-  // a very large scale make that conversion undefined. Note that any
-  // comparison with NaN is false, so NaN is rejected here as well.
-  if (!(scale >= 0.01f && scale <= 2.0f)) {
-    SHERPA_ONNX_LOGE("Silence scale %f is not in [0.01, 2]. Skip scaling.",
-                     scale);
+  // scale is normally within (0, 1), since it is used to shorten long pauses,
+  // but scaling a pause up is also useful. Values outside the supported range
+  // are rejected: n below is computed as interval_length * scale and converted
+  // to an int32_t, and NaN, infinity or a very large scale make that
+  // conversion undefined. Note that any comparison with NaN is false, so NaN
+  // is rejected here as well.
+  if (!(scale >= kMinSilenceScale && scale <= kMaxSilenceScale)) {
+    SHERPA_ONNX_LOGE("Silence scale %f is not in [%.2f, %.2f]. Skip scaling.",
+                     scale, kMinSilenceScale, kMaxSilenceScale);
     return *this;
   }
   // if the interval is larger than 0.2 second, then we assume it is a pause
@@ -195,7 +202,7 @@ void OfflineTtsConfig::Register(ParseOptions *po) {
 
   po->Register("tts-silence-scale", &silence_scale,
                "Duration of the pause is scaled by this number. So a smaller "
-               "value leads to a shorter pause.");
+               "value leads to a shorter pause. Must be in [0.01, 10].");
 }
 
 bool OfflineTtsConfig::Validate() const {
@@ -221,8 +228,10 @@ bool OfflineTtsConfig::Validate() const {
     }
   }
 
-  if (silence_scale < 0.001) {
-    SHERPA_ONNX_LOGE("--tts-silence-scale '%.3f' is too small", silence_scale);
+  if (!(silence_scale >= kMinSilenceScale &&
+        silence_scale <= kMaxSilenceScale)) {
+    SHERPA_ONNX_LOGE("--tts-silence-scale '%.3f' is not in [%.2f, %.2f]",
+                     silence_scale, kMinSilenceScale, kMaxSilenceScale);
     return false;
   }