Early this year, after Google’s DeepMind artificial intelligence technology proved it can beat an 18-time world champion at a game as complex as Go, many wondered if that intelligence could be put to good use in the real world, and how soon. Since then, Google showed that DeepMind can be used to cut the cooling bill for one of its data centers by 40%. The company just announced another real-world breakthrough pertaining to text-to-speech (TTS) technology.
Synthetic Speech Breakthrough
According to Google, DeepMind’s technology, called WaveNet, succeeded in reducing the gap between Google’s best TTS technology and the human voice by 50%, in terms of how natural synthetic speech can sound.
Until now, Google would use concatenative TTS, where the company would first record speech fragments spoken by real humans and then combine them to form sentences. This approach is what gives text-to-speech its “robotic” tone, because the words are spoken with no context or emotion.
Google also uses the parametric approach, where all the information required to generate the data is stored in the parameters of the model, and the contents and characteristics of the speech can be controlled via the inputs to the model. However, so far the parametric technology has been successful only with non-syllabic languages such as Mandarin Chinese, but it makes syllabic languages such as English sound less natural than does the concatenative method.
How WaveNet Works
WaveNet is a fully convolutional neural network that can modify the raw waveform of the audio signal one sample at a time. That means that for one second of audio, WaveNet can modify 16,000 samples (16KHz audio), making synthesized speech sound much more natural. Sometimes, WaveNet even generates sounds such as mouth movements or breathing, which shows the flexibility of using raw waveforms.
At training time, the inputs are real waveforms recorded from human speakers. After the training, Google can sample the network to generate synthetic speech. The process for picking the samples one step at a time makes for a computationally expensive process, but Google said that it is essential to generate realistic-sounding audio.
To test how good the new text-to-speech engine was, Google did a blind test with human subjects who would give 500 ratings on 100 sentences. The results are shown below, and as you can see, WaveNet reduced the gap between Google’s best previous technology (either concatenative for English or parametric for Mandarin) and human speech by around 50%.
Google’s DeepMind team seemed surprised that directly generating each audio sample even worked at all with deep neural networks, and they were even more surprised that it could outperform the company’s previous cutting edge TTS technology.
The team will continue to improve WaveNet so that it can create synthesized speech that’s even more human-like. Presumably, the team will also want to reduce the computational costs so that Google can start using WaveNet commercially as soon as possible.
