VOIP dialler tone for outgoing call iOS webrtc

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Implementing Dial Tone for Outgoing WebRTC Calls on iOS

An iPhone screen showing a dial pad with a WebRTC call in progress, symbolizing dial tone functionality.

Learn how to integrate a realistic dial tone experience into your iOS WebRTC application for outgoing calls, enhancing user feedback and call initiation.

Providing a dial tone for outgoing calls is a crucial user experience element in traditional telephony. When building a VoIP application using WebRTC on iOS, replicating this familiar audio cue can significantly improve user perception and indicate that the call initiation process has begun successfully. This article will guide you through the process of generating and playing a dial tone, specifically focusing on the DTMF (Dual-Tone Multi-Frequency) standard, which is commonly used for dial tones.

Understanding Dial Tone Generation

A standard dial tone is typically a continuous sound composed of two distinct frequencies played simultaneously. In many regions, this is a combination of 400 Hz and 450 Hz. To generate this programmatically, you'll need to create an audio buffer containing these combined sine waves. The WebRTC framework itself doesn't directly provide a dial tone generator, so we'll leverage iOS's audio capabilities, specifically AVAudioEngine or AudioToolbox for low-level audio synthesis.

flowchart TD
    A[User Initiates Call] --> B{Generate Dial Tone Frequencies}
    B --> C[Mix Frequencies (e.g., 400Hz + 450Hz)]
    C --> D[Create Audio Buffer]
    D --> E[Play Audio Buffer via `AVAudioEngine`]
    E --> F{WebRTC Call Connects?}
    F -- Yes --> G[Stop Dial Tone]
    F -- No --> H[Stop Dial Tone & Handle Error]

Flowchart of dial tone generation and playback in an iOS WebRTC application.

Implementing Dial Tone Playback with AVAudioEngine

The AVAudioEngine framework in iOS provides a powerful and flexible way to handle audio playback and processing. We can use it to create a custom audio source that generates the dial tone frequencies and then play it through the device's speakers. This approach allows for precise control over the audio characteristics and easy integration with other audio components if needed.

import AVFoundation

class DialToneGenerator {
    private var audioEngine: AVAudioEngine!
    private var audioPlayerNode: AVAudioPlayerNode!
    private let sampleRate: Double = 44100.0
    private let frequency1: Double = 400.0 // First dial tone frequency
    private let frequency2: Double = 450.0 // Second dial tone frequency
    private let amplitude: Float = 0.1

    init() {
        setupAudioEngine()
    }

    private func setupAudioEngine() {
        audioEngine = AVAudioEngine()
        audioPlayerNode = AVAudioPlayerNode()
        audioEngine.attach(audioPlayerNode)

        let format = AVAudioFormat(standardFormatWithSampleRate: sampleRate, channels: 1)!
        audioEngine.connect(audioPlayerNode, to: audioEngine.mainMixerNode, format: format)

        do {
            try audioEngine.start()
        } catch {
            print("Error starting audio engine: \(error.localizedDescription)")
        }
    }

    func startDialTone() {
        let bufferSize: AVAudioFrameCount = 4096
        let audioBuffer = AVAudioPCMBuffer(pcmFormat: audioPlayerNode.outputFormat(forBus: 0), frameCapacity: bufferSize)!
        audioBuffer.frameLength = bufferSize

        let bufferPointer = audioBuffer.floatChannelData![0]

        var phase1: Double = 0.0
        var phase2: Double = 0.0

        // Generate continuous dial tone
        audioPlayerNode.scheduleBuffer(audioBuffer, at: nil, options: .loops) { [weak self] in
            guard let self = self else { return }
            for i in 0..<Int(bufferSize) {
                let sample1 = sin(2 * .pi * self.frequency1 * phase1 / self.sampleRate)
                let sample2 = sin(2 * .pi * self.frequency2 * phase2 / self.sampleRate)
                bufferPointer[i] = Float((sample1 + sample2) / 2.0) * self.amplitude

                phase1 += 1.0
                phase2 += 1.0
            }
            // Reschedule the buffer for continuous playback
            self.audioPlayerNode.scheduleBuffer(audioBuffer, at: nil, options: .loops, completionHandler: nil)
        }

        audioPlayerNode.play()
        print("Dial tone started.")
    }

    func stopDialTone() {
        audioPlayerNode.stop()
        audioPlayerNode.reset()
        print("Dial tone stopped.")
    }

    deinit {
        audioEngine.stop()
        audioEngine = nil
    }
}

// Usage Example:
// let dialTone = DialToneGenerator()
// dialTone.startDialTone()
// ... when call connects or fails ...
// dialTone.stopDialTone()

Swift code for generating and playing a dial tone using AVAudioEngine.

Integrating with WebRTC Call Lifecycle

The key to a seamless user experience is to integrate the dial tone playback with your WebRTC call's lifecycle. The dial tone should start immediately after the user initiates an outgoing call and stop as soon as the call connects (e.g., when you receive an onAddStream or onIceConnectionStateChange event indicating connection) or if the call fails to connect.

// Assuming you have a WebRTC client class, e.g., `WebRTCClient`

class WebRTCClient: NSObject, RTCPeerConnectionDelegate {
    var dialToneGenerator: DialToneGenerator?

    func initiateCall(to recipient: String) {
        // ... WebRTC signaling to initiate call ...

        dialToneGenerator = DialToneGenerator()
        dialToneGenerator?.startDialTone()
    }

    // MARK: - RTCPeerConnectionDelegate

    func peerConnection(_ peerConnection: RTCPeerConnection, didChange state: RTCIceConnectionState) {
        switch state {
        case .connected, .completed:
            print("ICE Connection State: Connected. Stopping dial tone.")
            dialToneGenerator?.stopDialTone()
            dialToneGenerator = nil
        case .failed, .closed, .disconnected:
            print("ICE Connection State: \(state). Stopping dial tone.")
            dialToneGenerator?.stopDialTone()
            dialToneGenerator = nil
        default:
            break
        }
    }

    func peerConnection(_ peerConnection: RTCPeerConnection, didAdd stream: RTCMediaStream) {
        // This delegate method indicates that a remote stream has been added,
        // often signifying that the call has connected and media is flowing.
        print("Remote stream added. Stopping dial tone.")
        dialToneGenerator?.stopDialTone()
        dialToneGenerator = nil
    }

    // ... other RTCPeerConnectionDelegate methods ...
}

Example integration of DialToneGenerator with a WebRTC client's lifecycle events.