Over on our forums poster hotpaw2 has released news about his new RTL-SDR app for iOS (iPhones/iPads). If we're not mistaken, this will be the first app that enables RTL-SDR usage on iOS. However, as iOS devices don't allow RTL-SDRs (or any arbitrary USB device) to connect directly to devices, you still need to use a Raspberry Pi or other network connected computing device as an rtl_tcp server. So the RTL-SDR does not plug directly into the iOS device. Currently he is looking for beta testers to help test a pre-release of the software. Hotpaw2 writes:

Hi. A first version of my iOS SDR app is nearing completion. So I'm interested finding a few users who would like to beta test a pre-release of the app, and provide some feedback. The beta test requirements are having a 64-bit iOS device (iPhone or iPad) running iOS 11.2.x or newer, having Apple's TestFlight app installed, having a Mac, PC, Raspberry Pi (or other Linux box) that already has rtl_tcp installed and ready to run. (And an RTL-SDR obviously.) The rtl_tcp server must be on a fast WiFi network reachable by your iOS device. Note that iOS TestFlight app distributions do have an expiration date.

iOS does not recognize arbitrary USB devices such as an RTL-SDR. This is even true when using Apple's Lightning Camera Connection kit to provide an iPhone with a wired USB port. So an adapter must be used. I use a headless Raspberry Pi 3 running rtl_tcp as the USB adapter to provide raw IQ samples from the RTL-SDR to the iOS app. A Raspberry Pi Zero W would also work. I then connect to the server either over WiFi, or via wired ethernet. 

This iOS SDR app is fairly simple. I've been experimenting with developing low-level DSP code in Swift. So this SDR app was written from scratch in the Swift programming language. Because the app is targeted for the iOS App store, it uses none of the existing SDR C++ code base. 

The app currently demodulates AM, N-FM, and mono W-FM. It also displays a spectrum and rudimentary waterfall, and allows one to swipe-to-tune. There are not a lot of controls, as screen real-estate on an iPhone is quite limited. But I can walk around the house and, from my iPhone, monitor if my RTL-SDR or AirSpy HF+ are picking up any interesting signals.

Contact info for beta testing can be found here: http://www.hotpaw.com/rhn/hotpaw/ 

Source code to librtlsdr and rtl_tcp can be found in many repositories on github, but zero support for finding or installing such, and/or setting up your Raspberry Pi, will be provided by me.