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Lyra (codec)
Filename extension	.lyra
Developed by	Google
Initial release	2021
Latest release	1.3.2; December 20, 2022; 23 months ago
Type of format	speech codec
Free format?	Yes (Apache-2.0)

Lyra is a lossy audio codec developed by Google that is designed for compressing speech at very low bitrates. Unlike most other audio formats, it compresses data using a machine learning-based algorithm.

Features

The Lyra codec is designed to transmit speech in real-time when bandwidth is severely restricted, such as over slow or unreliable network connections.^[1] It runs at fixed bitrates of 3.2, 6, and 9 kbit/s and it is intended to provide better quality than codecs that use traditional waveform-based algorithms at similar bitrates.^[2]^[3] Instead, compression is achieved via a machine learning algorithm that encodes the input with feature extraction, and then reconstructs an approximation of the original using a generative model.^[1] This model was trained on thousands of hours of speech recorded in over 70 languages to function with various speakers.^[2] Because generative models are more computationally complex than traditional codecs, a simple model that processes different frequency ranges in parallel is used to obtain acceptable performance.^[4] Lyra imposes 20 ms of latency due to its frame size.^[3] Google's reference implementation is available for Android and Linux.^[4]

Lyra version 1 quality comparison

Original

Resampled to 16 kHz

Lyra at 3 kbps

Opus at 6 kbps

Speex at 3 kbps

Problems playing these files? See media help.

Quality

Lyra's initial version performed significantly better than traditional codecs at similar bitrates.^[1]^[4]^[5] Ian Buckley at MakeUseOf said, "It succeeds in creating almost eerie levels of audio reproduction with bitrates as low as 3 kbps." Google claims that it reproduces natural-sounding speech, and that Lyra at 3 kbit/s beats Opus at 8 kbit/s.^[2] Tsahi Levent-Levi writes that Satin, Microsoft's AI-based codec, outperforms it at higher bitrates.^[5]

History

In December 2017, Google researchers published a preprint paper on replacing the Codec 2 decoder with a WaveNet neural network. They found that a neural network is able to extrapolate features of the voice not described in the Codec 2 bitstream and give better audio quality, and that the use of conventional features makes the neural network calculation simpler compared to a purely waveform-based network. Lyra version 1 would reuse this overall framework of feature extraction, quantization, and neural synthesis.^[6]

Lyra was first announced in February 2021,^[2] and in April, Google released the source code of their reference implementation.^[1] The initial version had a fixed bitrate of 3 kbit/s and around 90 ms latency.^[1]^[2] The encoder calculates a log mel spectrogram and performs vector quantization to store the spectrogram in a data stream. The decoder is a WaveNet neural network that takes the spectrogram and reconstructs the input audio.^[2]

A second version (v2/1.2.0), released in September 2022, improved sound quality, latency, and performance, and permitted multiple bitrates. V2 uses a "SoundStream" structure where both the encoder and decoder are neural networks, a kind of autoencoder. A residual vector quantizer is used to turn the feature values into transferrable data.^[3]

Support

Implementations

Google's implementation is available on GitHub under the Apache License.^[1]^[7] Written in C++, it is optimized for 64-bit ARM but also runs on x86, on either Android or Linux.^[4]

Applications

Google Duo uses Lyra to transmit sound for video chats when bandwidth is limited.^[1]^[5]

References

^ ^a ^b ^c ^d ^e ^f ^g Buckley, Ian (2021-04-08). "Google Makes Its Lyra Low Bitrate Speech Codec Public". MakeUseOf. Retrieved 2022-07-21.
^ ^a ^b ^c ^d ^e ^f "Lyra: A New Very Low-Bitrate Codec for Speech Compression". Google AI Blog. 25 February 2021. Retrieved 2022-07-21.
^ ^a ^b ^c "Lyra V2 - a better, faster, and more versatile speech codec". Google Open Source Blog. Retrieved 2023-04-26.
^ ^a ^b ^c ^d "Google Duo uses a new codec for better call quality over poor connections". XDA. 2021-04-09. Retrieved 2022-07-21.
^ ^a ^b ^c Levent-Levi, Tsahi (2021-04-19). "Lyra, Satin and the future of voice codecs in WebRTC". BlogGeek.me. Retrieved 2022-07-21.
^ Kleijn, W. B.; Lim, F. S.; Luebs, A.; Skoglund, J.; Stimberg, F.; Wang, Q.; Walters, T. C. (April 2018). Wavenet based low rate speech coding. 2018 IEEE international conference on acoustics, speech and signal processing (ICASSP). IEEE. pp. 676–680. arXiv:1712.01120.
^ Google (2021). "Lyra: A Very Low-Bitrate Codec for Speech Compression". GitHub. Retrieved 21 July 2022.