Speech Recognizer API – What Do You Need to Know?

Adding appropriate keys along with their descriptions to the file info.plist

• NSSpeechRecognitionUsageDescription – a key telling you what we will use SFSpeechRecognizer for in the application.
• NSMicrophoneUsageDescription – in case you use the microphone to analyze live speech.

Calling request for permission SFSpeechRecognizer

SFSpeechRecognizer.requestAuthorization { authStatus in
            /*
                The callback may not be called on the main thread. Add an
                operation to the main queue to update the record button's state.
            */
            OperationQueue.main.addOperation {
                switch authStatus {
                    case .authorized:
                         //..
                    case .denied:
                         //..
                    case .restricted:
                         //..
                    case .notDetermined:
                         //..
                }
            }
        }

Creating speech recognition request

There are two types of such a request: SFSpeechURLRecognitionReqest – used to analyze the speech from a previously recorded audio file SFSpeechAudioBufferRecognitionRequest – used to analyze live speech (using a microphone) For both requests SFSpeechRecognizer class will be needed to perform an analysis of the speech.

//..
let speechRecognizer = SFSpeechRecognizer(locale: Locale(identifier: "pl-PL"))
//..

SFSpeechURLRecognitionReqest

Handling such a request comes down to a few lines of code:

let fileURL = URL(fileURLWithPath: Bundle.main.path(forResource: "audio", ofType: ".mp3")!)
let request = SFSpeechURLRecognitionRequest(url: fileURL)

SFSpeechAudioBufferRecognitionRequest

Handling analysis of live speech requires a little more work. You should additionally use AVAudioEngine to capture speech from the microphone of the device. The next and final element which is necessary for speech recognition is: SFSpeechRecognitionTask.

//..
recognitionRequest = SFSpeechAudioBufferRecognitionRequest()
        
        guard let inputNode = audioEngine.inputNode else { fatalError("Audio engine has no input node") }
        guard let recognitionRequest = recognitionRequest else { fatalError("Unable to created a SFSpeechAudioBufferRecognitionRequest object") }
        
        // Configure request so that results are returned before audio recording is finished
        recognitionRequest.shouldReportPartialResults = true
        
        // A recognition task represents a speech recognition session.
        // We keep a reference to the task so that it can be cancelled.
        recognitionTask = speechRecognizer.recognitionTask(with: recognitionRequest) { result, error in
            //..
        }
        
        let recordingFormat = inputNode.outputFormat(forBus: 0)
        inputNode.installTap(onBus: 0, bufferSize: 1024, format: recordingFormat) { (buffer: AVAudioPCMBuffer, when: AVAudioTime) in
            self.recognitionRequest?.append(buffer)
        }
        
        audioEngine.prepare()
        try audioEngine.start()

SFSpeechRecognitionResult includes bestTranscription and alternative transcripts.