Data over songs: Hiding bits in pop hits

400 bits per second achieved in ideal conditions with no impact on listening pleasure

A song can convey so much; a story, a mood, an emotion and up to 400 bits per second, thanks to new technique developed by researchers at ETH Zurich.

Typically, data is sent over cables, radiowaves, microwaves and infrared, but a growing body of work is seeking to transmit data over soundwaves at both audible and inaudible frequencies. The data is translated into a sequence of sounds, which can be decoded by another device – kind of like an audio QR code.

In a paper presented at a recent IEEE conference in London, the ETH researchers showed how data could be sent within any song, without an average listener noticing the difference.

“Our goal was to ensure that there was no impact on listening pleasure,” said author Manuel Eichelberger.

The technique takes the dominant notes in a piece of music, and overlays each with two marginally deeper and two marginally higher notes that are quieter than the dominant note. It also makes use of harmonics of the strongest notes, inserting slightly deeper and higher notes there, too.

These additional notes carry the data, and can be received and processed by a smartphone microphone but are imperceptible to the human ear.

“When we hear a loud note, we don’t notice quieter notes with a slightly higher or lower frequency. That means we can use the dominant, loud notes in a piece of music to hide the acoustic data transfer,” Eichelberger said.

Extremely high notes, which humans can barely register, are used as an anchor to tell the decoder algorithm in the smartphone where to look for the data.

Songs with lots of dominant notes – particularly loud, pop music – worked best at the data transfer, the researchers said. Up to 400 bits per second were achieved in ideal conditions, but in a more realistic scenario the transfer rate was closer to 200 bits per second. The technique achieves higher bitrates, transmittable over longer distances, than previous attempts.

Up to 400 bits was about the limit for the technique – called differential acoustic orthogonal frequency-division multiplexing – before the sound suffers and is noticed by listeners.

The researchers demonstrated their technique by repeating a URL to a news article every 0.7 seconds within a performance by their university’s big band. The algorithm for receiving the data is not yet publicly available as a smartphone app.

Un-wired for sound

The transfer of data over sound waves offers a number of benefits.

“By default, smartphones, laptops and tablets are equipped with microphones. At the same time, at many public places such as stores, stadiums, train stations and restaurants, speakers play background music,” said author Simon Tanner.

“Our technique opens the potential for an easy communication path from speakers to microphones without the requirement of additional hardware or any setup. That would be handy in a hotel room, since guests would get access to the hotel Wi-Fi without having to enter a password on their device,” he added.

A number of start-ups are already established in the ‘data over sound’ space, including Chirp, Lisnr and Trillbit.

Last year, Chirp partnered with the UK’s EDF Energy for a trial at one of its nuclear power plants. The successful trial involved retrofitting equipment to transmit status data, in an environment where radio transmissions are prohibited. The technology was also used to communicate with mobile workers.

Chirp has also been used in promotional activities for video game publisher Activision and children’s toy maker Hijinx.

Lisnr is focused primarily on enabling payments over sound and has also partnered with Ticketmaster to deliver digital gig tickets, while Trillbit is exploring use cases in retail, indoor wayfinding and checking in to events.

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