Add support for setting the attenuator.

CONFIGURATION.md

@@ -31,11 +31,38 @@ about what went wrong.
 ## Example configuration file
 
 ```toml
+attenuator = false
 bufferSizeSamples = 512
 ```
 
 ## Options reference
 
+### Option `attenuator`
+
+*Boolean*-typed option that determines if the QA401 hardware attenuator should
+be engaged or not.
+
+If the option is set to `true`, the QA401 attenuator will stay engaged.
+
+If the option is set to `false`, the QA401 attenuator will be bypassed during
+audio streaming.
+
+Note that, contrary to the QuantAsylum Analyzer application, ASIO401 will not
+adjust sample values in any way to account for the presence or absence of the
+attenuator. In other words, defeating the attenuator has the effect of adding
+20 dB to the samples coming from ASIO401.
+
+**CAUTION:** do not apply high voltages to the QA401 inputs while the attenuator
+is bypassed. See the QA401 User Manual for details.
+
+Example:
+
+```toml
+attenuator = false
+```
+
+The default behaviour is to keep the attenuator engaged.
+
 ### Option `bufferSizeSamples`
 
 *Integer*-typed option that determines which ASIO buffer size (in samples)

src/asio401/ASIO401/asio401.cpp

@@ -398,6 +398,7 @@ namespace asio401 {
 				// If we already have two buffers queued, we can start streaming.
 				// If we have more writes queued that the QA401 can store, we *have* to start streaming, otherwise the next FinishWrite() call will block indefinitely.
 				Log() << "Starting QA401";
+				preparedState.asio401.qa401.SetAttenuator(preparedState.asio401.config.attenuator);
 				preparedState.asio401.qa401.Start();
 				started = true;
 			}

src/asio401/ASIO401/config.cpp

@@ -87,6 +87,7 @@ namespace asio401 {
 		}
 
 		void SetConfig(const toml::Table& table, Config& config) {
+			SetOption(table, "attenuator", config.attenuator);
 			SetOption(table, "bufferSizeSamples", config.bufferSizeSamples, ValidateBufferSize);
 		}
 

src/asio401/ASIO401/config.h

@@ -6,6 +6,7 @@
 namespace asio401 {
 
 	struct Config {
+		bool attenuator = true;
 		std::optional<int64_t> bufferSizeSamples;
 	};
 

src/asio401/ASIO401/qa401.cpp

@@ -84,6 +84,11 @@ namespace asio401 {
 		Log() << "QA401 is reset";
 	}
 
+	void QA401::SetAttenuator(bool enabled) {
+		// According to QuantAsylum, the attenuator is bit mask 0x02. Also preserve the bits set in Reset().
+		WriteRegister(5, 4 | (enabled ? 0 : 0x02));
+	}
+
 	void QA401::Start() {
 		Log() << "Starting QA401 streaming";
 

src/asio401/ASIO401/qa401.h

@@ -11,6 +11,7 @@ namespace asio401 {
 		QA401(std::string_view devicePath);
 
 		void Reset();
+		void SetAttenuator(bool enabled);
 		void Start();
 		void StartWrite(const void* buffer, size_t size);
 		void FinishWrite();