setup/destination: frequency

lib/setup/setupDestination

@@ -354,11 +354,37 @@ setupDestination() {
 	comeagain
 	destinationchannels["$destination"]=$value
 	cat <<-EODesc
+
+        Sampling rate (Hertz, integer):
+            Files will be resampled as needed to the specified sampling-rate.
+            Shoutcast/Icecast streams require a constant sampling-rate.
+            Telephony systems often require a sampling-rate of 8000Hz.
 	EODesc
+	if [[ ${destinationformat["$destination"]} == opus ]]
+	then
+		cat <<-EODesc
+
+                Please note that Opus supports only the following sample-rates:
+                8000, 12000, 16000, 24000, and 48000 Hz. So don't set
+                resampling on an Opus destination to any other value or files
+                will be resampled twice.
+		EODesc
+	fi
 	comeagain() {
-		:
+		read							\
+			-e						\
+			${destinationfrequency["$destination"]+-i${destinationfrequency["$destination"]}}\
+			-p'Sampling-rate: '				\
+			value
+		if ! [[ $value =~ $expr ]]
+		then
+			echo "Invalid frequency value: $value" >&2
+			comeagain
+		fi
+		unset expr
 	}
 	comeagain
+	destinationfrequency["$destination"]=$value
 	cat <<-EODesc
 	EODesc
 	comeagain() {