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Exploring voice isolation

Posted: 18 Jan 2016     Print Version  Bookmark and Share

Keywords:smartphone  neural networks  background noise  noise reduction  deep learning 

We tend to forget that a smartphone is also a telephone. With all the fantastic functions and features, somehow people have grown accustomed to the occasional dropped syllable and garbled sounds that make us repeat ourselves time and again. A recent article in Scientific American suggests that the fault is with the service providers. It's true that bandwidth is definitely a factor, but even when there is a relatively good connection, throw in a noisy environment like a coffee shop or morning traffic and communication starts to break down. A new approach to old problems—with the help of deep neural networks—may make background noise a thing of the past.

The voice band—good enough?
Since the invention of the telegraph almost two centuries ago, there has been nearly exponential improvement in bandwidth, mobility, speed, and reliability. That said, a key aspect of voice telecommunications has lagged behind: the quality and intelligibility of transmitted voice. Very early on, the standard for human voice transmission was set as the "voice band" located between 300Hz and 3.3kHz (to put this in perspective, the natural frequency span of human voice during speech ranges from about 50Hz to nearly 10kHz).

Apparently, this was satisfactory for landline usage in quiet settings, and generations of phone users came to expect poor call quality. When these standards were carried over for cell phone audio quality, and with the added woes of spotty network coverage and connection dropouts, cell phone users' expectations for call quality fell even lower.

Extending the frequency (for better or for worse)
Now that there are about about as many cell phone subscriptions as there are people on earth, one would think that there really shouldn't be any more technological excuses for poor voice quality. New standards branded as HD Voice and VoLTE promise the eventual extension of voice transmission frequency range up to 7kHz. An IEEE Spectrum article from September 2014 gave an instructive, in-depth analysis of the causes of lousy voice quality, and placed hope in the deployment of these new technologies. Their implementation requires new hardware and new networks, which will be overcome in time, but broadening the voice band does nothing to solve the other major challenge preventing great sounding calls—in fact, HD Voice and its relatives may actually make the problem worse!

Noise and the boundless crusade for its cancellation
Nearly half of all phone users today employ their mobile phones as their primary voice connection (a number sure to grow). Mobile phones, by design, are used in many different environments: in planes, trains, and automobiles; at sporting events, offices, factories, and shopping centres; on playgrounds and (yeah, that guy) in public restrooms.

Just think of the noises you might encounter walking through an urban downtown, near a construction site, or in an airport lounge. While the narrow range of the current voice band standard impairs the quality of the voice that is transmitted, it also automatically filters out any noise that may be present in higher frequency bands. By doubling the frequency span of the voice band, HD Voice and relatives increase the environmental noise power that is transmitted and, ironically, can make voice quality and intelligibility worse in everyday use cases.

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