In the post a few days ago about the newly released "SDR Receiver" app for iOS, we briefly mentioned that another iOS app called "RTL_TCP SDR" has just been released out of beta and put onto the Apple store as well.
"RTL_TCP SDR" is a little different to "SDR Receiver" because it contains a full spectrum analyzer and waterfall display, whereas "SDR Receiver" only allows you to listen via presets or manual tuning. Both apps can not access the RTL-SDR directly on the iOS device due to Apple limitations. An external server on a Raspberry Pi or PC running rtl_tcp is required. Programmer HotPaw writes about his App:
An RTL-SDR Software Defined Radio receiver for iOS devices (requires an external rtl_tcp server). Listen to VHF AM and FM radio signals. View a waterfall of the RF spectrum. Connect, via the rtl_tcp network protocol, to a networked RTL-SDR USB peripheral.
iOS devices do not currently support the direct connection of USB devices such as an RTL-SDR. Thus, the use of this app requires network access to a server, such as a Raspberry Pi (or Mac), with an RTL-SDR unit plugged into its USB port, and running the rtl_tcp protocol at an TCP/IP network address accessible from your iOS device. The Raspberry Pi acts, essentially, as a USB port adapter for your iOS device.
No support is provided for installing any of the software needed to use this app with a Raspberry Pi. Please do not download this app unless you are already familiar with Software Defined Radio, have an RTL-SDR USB device, and have already installed and tested rtl_tcp on your Raspberry Pi, Mac, or other server.
Over on his Reddit discussion thread he also mentions:
Since Apple's iOS doesn't allow an RTL-SDR to be plugged directly into a Lightning port (even with a USB adapter), an rtl_tcp adapter, such as a Raspberry Pi (or Pi Zero) server is required.
This app is an experiment in real-time DSP and SDR coding using Apple's Swift and Metal GPU-shader programming languages. It includes a spectrum waterfall, and supports demodulating FM, AM, and SSB. Also, includes beta test support for the AirSpy HF+.