SDR Connect (Working Title)

SDR Connect is a high-performance Software Defined Radio (SDR) client for iOS, built with Swift and SwiftUI. It connects over the network to an rtl_tcp server, allowing users to explore the radio spectrum with a real-time spectrogram and waterfall display, tune frequencies, and demodulate signals directly on their iPhone.

This project is currently in the alpha stage. The core visualization and NFM demodulation pipelines are functional, but there are known issues and many features yet to be implemented.

Current State

The app can successfully:

• Connect to an rtl_tcp server over the network.
• Receive and process I/Q data in real-time.
• Display a performant, interactive spectrogram and waterfall.
• Control key hardware parameters of the RTL-SDR dongle (frequency, sample rate, gain, AGC, etc.) via a tabbed UI.
• Tune a specific signal using an interactive VFO (Virtual Frequency Oscillator).
• Adjust VFO bandwidth with a pinch gesture.
• Demodulate Narrowband FM (NFM) signals and play the audio through the speaker.
• Adjust an audio squelch level.

Project File Structure

The project is organized using the MVVM (Model-View-ViewModel) architecture to ensure a clean separation of concerns between the UI, state management, and core logic.

📁 rtl_tcp/
│
├── 📁 App/
│   └── 📄 rtl_tcpApp.swift
│
├── 📁 Features/
│   └── 📁 Radio/
│       ├── 📂 View/
│       │   ├── 📄 RadioView.swift
│       │   ├── 📂 Controls/
│       │   │   ├── 📄 ConnectionSettingsView.swift
│       │   │   ├── 📄 ConnectionStatusView.swift
│       │   │   ├── 📄 DSPControlsView.swift
│       │   │   ├── 📄 HardwareControlsView.swift
│       │   │   └── 📄 TuningControlsView.swift
│       │
│       └── 📂 ViewModel/
│           └── 📄 RadioViewModel.swift
├── 📁 Bookmarks //TODO
        |   ├── 📂 View //TODO
        |   |   ├── 📄 BookmarksListView.swift     // Displays the list of saved frequencies. //TODO
        |   |   ├── 📄 BookmarkDetailView.swift    // View for adding/editing a bookmark. //TODO
        |   |
        |   ├── 📂 ViewModel //TODO
        |       ├── 📄 BookmarksViewModel.swift    // Manages the logic for fetching and saving bookmarks. //TODO
│
├── 📁 Core/
│   ├── 📁 Networking/
│   │   └── 📄 RTLTCPClient.swift
│   │
│   ├── 📁 DSP/
│   │   ├── 📄 DSPEngine.swift
│   │   ├── 📄 SpectrogramProcessor.swift //TODO
│   │   ├── 📄 VFOProcessor.swift  //TODO
│   │   ├── 📄 CircularBuffer.swift //TODO
│   │   └── 📂 Demodulators/
│   │       ├── 📄 Demodulator.swift
│   │       ├── 📄 WFMDemodulator.swift //TODO
│   │       ├── 📄 NFM_AM_Demodulator.swift //TODO
│   │       ├── 📄 SSBDemodulator.swift //TODO
│   │       ├── 📄 RAWDemodulator.swift //TODO
│   │       └── 📄 NFMDemodulator.swift
│   │
│   └── 📁 Audio/
│       └── 📄 AudioManager.swift
│
├── 📁 Data/
│   ├── 📄 RadioSettings.swift
│   └── 📄 SampleRate.swift
│
└── 📁 UI Components/
    ├── 📄 FrequencyDialView.swift
    ├── 📄 SpectrogramView.swift
    ├── 📄 TuningIndicatorView.swift
    └── 📄 WaterfallView.swift

Known Issues & TODO List

This list tracks the next steps for development and bug fixes.

🟥 Critical Bugs & Performance Issues

• [TODO] DSP Performance: The performVFO and performFFT functions still rely on some manual Swift for loops for data manipulation (re-interleaving). Profiling shows these IndexingIterator calls are a major bottleneck. These loops must be replaced with highly-optimized C-based functions from the Accelerate framework (like cblas_scopy or vDSP_ztoc) to achieve maximum performance and reduce battery drain.

🟨 High-Priority Features

• [TODO] Implement a Real Low-Pass Filter: The lowPassFilter function in NFMDemodulator.swift is currently a placeholder. A proper FIR (Finite Impulse Response) filter needs to be designed and implemented. This is the most important step for improving audio quality, as it will reject adjacent channel noise before demodulation.
• [TODO] Add More Demodulation Modes:
• [TODO] Audio Resampling: The NFM demodulator currently outputs audio at a sample rate close to 48 KHz but not exactly. A proper audio resampler (e.g., a polyphase FIR filter) should be implemented to convert the demodulated audio stream to the exact 48000.0 Hz required by the AudioManager for the highest quality audio playback.
• [TODO] Dynamic Sample Rate in DSP: The DSPEngine and Demodulator classes have the device's sample rate hardcoded. This needs to be dynamically passed down from the RadioView so that the decimation and filter calculations are always correct for the user's selected sample rate.

🟦 Medium-Priority Features & UI Polish

• [TODO] Frequency Bookmarking:
  • Implement SwiftData or CoreData to store frequency bookmarks.
  • Build the UI screens (list, add/edit) for managing bookmarks.
• [TODO] Scanning Functionality: Add logic to scan through a list of bookmarked frequencies, stopping when the squelch opens.
• [TODO] Custom Frequency Step: The "Custom..." button on the frequency dial is a placeholder. This should present a dialog or text field to allow the user to input a custom tuning step value.
• [TODO] More Robust Connection Logic: The DispatchQueue.main.asyncAfter call in RadioViewModel's setupAndConnect is a "hack". This should be replaced with a more robust Combine-based approach that waits for the client.isConnected property to become true before sending the initial batch of commands.

Low-Priority & Future Ideas

• [TODO] Windowing Function: Apply a windowing function (e.g., Hann or Blackman-Harris) to the I/Q data before the FFT in performFFT. This will reduce spectral leakage and result in a cleaner, more accurate spectrogram with less "smearing" of strong signals.
• [TODO] Multiple VFOs: Refactor the DSPEngine to support multiple independent VFO processing chains to demodulate more than one signal within the captured bandwidth simultaneously.