Lyra (codec)

Lyra (codec)

Filename extension	.lyra
Developed by	Google
Initial release	2021 (2021)
Latest release	1.3.2 December 20, 2022; 22 months ago (2022-12-20)
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.

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

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Resampled to 16 kHz

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Lyra at 3 kbps

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Opus at 6 kbps

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Speex at 3 kbps

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Problems playing these files? See media help.

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]

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]

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]

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

• Lyra: A New Very Low-Bitrate Codec for Speech Compression Google blog post with a demonstration comparing codecs

• Satin (codec), an AI-based codec developed by Microsoft
• Comparison of audio coding formats
• Speech coding
• Videotelephony