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Earlier today, I was listening to my digital radio, and was intrigued as to how they are able to encode the information displayed on the LCD screen (which was basically a short string of text giving a brief description of the currently playing record/discussion topic), alongside the audio transmission.

My question is:

How is extra information (such as "currently playing song"), transmitted alongside the audio in digital radio encoding?

I am somewhat familiar with the workings of a radio, along with some basic understanding of RF-transmission, but nothing particularly advanced. And I was intrigued as to how they can transmit information within the same bandwidth of the audio transmission without causing interference. My first thought was that they could polarize both text and audio transmissions so their planes of oscillation are orthogonal to each other, and thus no interference would occur, but I didn't know how plausible that would be.

Thanks in advance, apologies if this a stupid question.

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up vote 3 down vote accepted

It's not actually encoded within the audio signal itself. The digital radio transmission consists of frames, and while most of the frames contain audio data, some of them contain metadata. The receiver is responsible for sorting out the frames and handling the contents of each appropriately.

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Is the time-base for each of these frames determined by a standard, if not, how does the receiver sort the frames? – Shaktal Jun 11 '12 at 15:56
Each frame contains enough information to identify the purpose of the frame. – Ignacio Vazquez-Abrams Jun 11 '12 at 15:57
Do you have any references regarding this, which I could have a look at for further information, if you could add one or two to your answer I'd be happy to accept it. Thanks! – Shaktal Jun 11 '12 at 16:03
Here's the spec for Vorbis. The specs for MP3, HE-AAC, etc. are all grossly similar, but not gratis. – Ignacio Vazquez-Abrams Jun 11 '12 at 16:21
Ahh, thanks very much! – Shaktal Jun 11 '12 at 16:26

A reasonable clear description is given in Wikipedia's Digital Audio Broadcasting. DAB is a European Digital Radio system, used in e.g. the UK.

The extra information in this case would be called Dynamic Label Segments. They're inserted as X-PAD groups in the PAD (Programme Associated Data), at the end of DAB audio frames.

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With a non-digital broadcast radio you can embed the same digital data in an FM broadcast

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