LambdaAudio v6

LambdaAudio v6 is a Japanese automatic speech recognition (ASR) model that converts speech into Japanese text. It combines a Zipformer audio encoder with an LFM2.5 language decoder, both adapted to Japanese audio or transcripts before joint training.

On Japanese subset of Google FLEURS ASR benchmark, it achieves a character error rate (CER) close to Whisper large-v3-turbo (6.05% vs. 5.83%) while transcribing audio 2.3× faster.

Model details

Field	Value
Developer	Keisuke Miyamoto
Task / language	Speech-to-text / Japanese (`ja`)
Model type	Encoder-decoder with an audio adapter and gated cross-attention
Model size	350M parameters
Audio encoder	KeisukeMiyamoto/zipformer-cpt
Text decoder / tokenizer	KeisukeMiyamoto/lfm2.5-230m-cpt
Input	16 kHz audio, up to 30 seconds per segment
Output	Japanese transcription text
Audio feature interval	20 ms after the encoder; 40 ms after the adapter

Architecture

The Zipformer encoder extracts features from 16 kHz audio. An audio adapter reduces the sequence length and projects these features into the decoder's hidden dimension. Gated cross-attention in the LFM2 full-attention blocks lets the decoder use audio features to predict the next text token.

Training data

Stage	Dataset	Sources and labels
Encoder continued pretraining	Whisper Cluster Audio	Internet Archive, publicly available podcasts, and radio; 500 cluster labels derived from Whisper large-v3-turbo encoder features
Decoder continued pretraining	Japanese Audio Transcripts	Japanese transcripts from NHK archives and technology podcasts
Joint ASR training	Lambda Audio Corpus	NHK archives, technology podcasts, and ReazonSpeech

Benchmarks

Each cell shows CER (%) / total transcription time (seconds). Lower values are better.

Model	ReazonSpeech	Common Voice 25 Japanese	FLEURS Japanese
LambdaAudio v6	10.75 / 121.55	23.87 / 167.84	6.05 / 20.75
Whisper large-v3-turbo	11.42 / 354.04	26.40 / 592.25	5.83 / 47.82
Kotoba-Whisper v2.2	11.70 / 322.53	22.89 / 541.39	7.35 / 46.15
ReazonSpeech wav2vec2	14.01 / 49.35	31.43 / 68.22	15.04 / 9.22
Parakeet tdt_ctc-0.6b-ja	10.62 / 55.10	19.99 / 82.28	5.94 / 9.74
Cohere Transcribe 03-2026	6.45 / 176.00	21.58 / 227.42	4.36 / 30.77