Hybrid radio — combining broadcast radio and additional data from the internet — offers, among other features, multi-platform service following: the opportunity to switch listeners between IP and broadcast, where available.
A typical use-case for this technology might be in-car, enabling listeners to drive, for example, between Berlin and Munich — but enjoying a local Berlin station throughout the journey. Listeners might start with a signal from FM, switching to DAB+ where available, and as they leave the broadcast area, the vehicle’s in-car system would switch to internet-delivered audio.
Internet broadcasts are often delayed in comparison to FM, and DAB+ can also add a delay to the audio, making it harder to switch cleanly between different platforms. The very nature of IP broadcast also means that these delays may not always be predictable.
There are a number of different methods to lessen, or completely avoid, the timing differences between platforms when switching.
In the UK and some other territories, Radioplayer produces a hybrid radio app for the LG Stylus 2 mobile phone, which has an addressable DAB+ chip inside. To lessen data use, the app automatically prefers DAB+ broadcasts if available.
Radioplayer’s switching algorithm will instantly switch to IP if the DAB+ signal is unavailable. While connected to IP, it will monitor the available DAB+ signal. If it is available error-free for a period of around two minutes, it is judged acceptable to switch back. This minimises the amount of switches, although a user will still hear a jump in audio. As a pragmatic solution, lessening the number of switches is a simple way to reduce, if not remove, the jarring effect of a switch.
Audi has a hybrid radio receiver in the new Audi A8. For this product, Audi worked with Fraunhofer IIS to create the Sonamic TimeScaling algorithm. This has audio waveform matching, which adds a progressive delay to the broadcast audio (via FM or DAB+) to align it to the audio on the IP stream. It also adjusts the audio level to match. In this way, a switch between IP and broadcast should be imperceptible. Fraunhofer call it “seamless”.
Where a listener starts listening via broadcast radio, it might be unrealistic to buffer the broadcast audio to align with the audio delivered by IP, since a listener is used to being able to listen to broadcast radio immediately. Radio receivers may choose to slowly add a delay, as the Fraunhofer solution does, by adding near-inaudible amounts of silence in the same way as a profanity delay unit. In many use-cases, it may be simpler to only make the effort to align broadcast audio once a listener is tuned to IP — since IP will almost always be delayed when compared to broadcast.
Delays and waveform matching can also be useful when switching between FM and DAB+, or in some cases between AM and FM HD signals, since these platforms often have an inherent delay caused by encoding and different distribution paths.
Much of the metadata that enables switching is detailed in the RadioDNS specifications. Nick Piggott, Project Director for RadioDNS, said "Automatic switching between broadcast radio and IP streaming can make radio so much easier for listeners. We're here to help both broadcasters and manufacturers implement the standardized functionality so it works brilliantly every time."
Best practice for radio broadcasters include avoiding excessive delays in their IP stream. In particular, preroll audio, played before the live stream, can significantly exaggerate the delay and may make audio waveform matching unreliable. Some methods of online ad replacement may also introduce additional delays, and can defeat waveform matching algorithms.
Ideally, stations should also have similar dynamic processing on their IP and terrestrial signals, to make the switches between different sources inaudible.