bolo-torch

The PyTorch engine plugin for bolo, the torch-free speech server.

bolo never imports PyTorch. When a catalog entry says engine: plugin:torch, bolo starts this package in a separate process (python -m bolo_torch) and talks to it over the plugin protocol (docs/plugins.md): it loads models there, streams synthesis back sentence by sentence, restarts the process if it crashes and stops it when it has been idle. Everything else (queues, traces, metrics, the console, the OpenAI-compatible API) works as for bolo's own engines.

What it runs

Catalog id	Task	Model	Extra	Licence	Gated	Tested here
indic-parler-tts	TTS	AI4Bharat Indic Parler-TTS, 69 named voices, 21 languages	parler	Apache-2.0	yes	CPU, see below
parler-tts-tiny-v1	TTS	Parler-TTS Tiny v1, English	parler	Apache-2.0	no	CPU, see below
indic-f5	TTS	AI4Bharat IndicF5, voice cloning from a reference clip, 11 languages	f5	MIT	yes	CPU, see below
f5-tts-v1-base	TTS	SWivid F5-TTS v1 Base, English and Chinese voice cloning (bolo's reference clips or voices_dir)	f5-original	CC-BY-NC-4.0 (non-commercial)	no	no
indic-speak	TTS	Bodhan AI Indic-Speak, 3B Llama + SNAC + Vocos, 22 Indian languages and English	indic-speak	Indic Open Model License v1.0 (conditional)	yes	no
veena	TTS	Maya Research Veena, 3B Llama + SNAC, Hindi and English	orpheus	Apache-2.0	no	no (only the SNAC decoder)
svara-tts-v1	TTS	Kenpath Svara v1, 3B Llama + SNAC, 19 languages, 38 voices	orpheus	Apache-2.0	no	no (only the SNAC decoder)
sepformer-whamr16k	separation	SpeechBrain SepFormer, WHAMR!, 16 kHz, 2 speakers	sepformer	Apache-2.0 (WSJ0 data, see below)	no	CPU, see below
sepformer-whamr	separation	SpeechBrain SepFormer, WHAMR!, 8 kHz, 2 speakers	sepformer	Apache-2.0 (WSJ0 data, see below)	no	CPU, see below
pixit-ami	separation	pyannote PixIT speech-separation-ami-1.0, diarization + separation	pyannote	MIT	yes (two repositories)	CPU, see below
dicow-v3.3	STT, diarization-conditioned	BUT-FIT DiCoW v3.3: Whisper large-v3-turbo told who talks when	dicow	CC-BY-4.0	no	CPU, see below
se-dicow	STT, diarization-conditioned	BUT-FIT SE-DiCoW: DiCoW with self-enrolment	dicow	Apache-2.0	no	CPU, see below

The separation models are bolo separation models in "blind" mode: bolo uses them for separate_overlaps=true on diarized transcription and on POST /api/v1/diarize, and assigns their tracks to its own diarized speakers. They can also be run without bolo: bolo-torch separate mix.wav -m sepformer-whamr16k -o out/.

The DiCoW models are bolo stt models that are speaker-conditioned: in a diarized transcription (diarize=true or response_format=diarized_json) bolo diarizes the recording (with nemotron-3-diarization unless the request names another diarizer), then sends the whole recording to the plugin with the speakers' frame activity, and the model transcribes it once per speaker. Overlapping speech is transcribed for each speaker instead of being cut at turn boundaries. Without diarization they work as an ordinary (slow) Whisper. See "DiCoW" below and docs/models.md, "Diarization-conditioned recognition".

The catalog also lists sravaani-1.0 and indicconformer-600m-official for this plugin. bolo-torch 0.1.0 does not implement them; loading one answers 400 invalid_request saying so.

How each family runs

• Parler-TTS (parler-tts 0.2.3): the voice is a description. A voice id from the catalog (divya, rohit, thoma, ...) becomes "Name's voice is clear and natural, at a moderate pace, in a very close recording that almost has no background noise. Very clear audio."; a catalog voice can carry its own description. Indic Parler-TTS reads the description with its text encoder's tokenizer (google/flan-t5-large, downloaded once, tokenizer files only). speed above 1.2 or below 0.8 adds "The speaker talks fast/slowly." Audio streams one sentence at a time (bolo also splits text into sentences before it reaches the plugin). The option stream_seconds turns on Parler's own streamer inside a sentence; on CPU it did not start audio sooner and almost doubled the real-time factor (measured below), so it is off by default.

• F5-TTS v1 Base (layout: f5-tts): f5_tts.infer.utils_infer.load_model builds the v1 DiT from F5TTS_v1_Base/model_1250000.safetensors and its vocab.txt, the Vocos vocoder comes from the pinned charactr/vocos-mel-24khz files, and generation is the package's infer_batch_process (as for IndicF5). Catalog voices with reference: <bolo sample id> use bolo's built-in synthetic clips and their transcripts.

• Indic-Speak: the gated repository ships its own inference.py; the loader imports it from the model directory and calls TTS(<dir>)(text, speaker=…, style=…) as the model card documents (24 kHz). Voice ids are the speaker names (amit, kavya, …), optionally <speaker>:<STYLE>; unknown names are refused because the model would silently average them.

• IndicF5: the loader builds the DiT from the f5_tts copy that ships in the model repository and loads model.safetensors from the local directory. The repository's model.py is not used: it looks its vocabulary up as a Hub repository id (which fails for a local directory), wraps the models in torch.compile and reads audio with torchaudio.load, which needs torchcodec in torchaudio 2.9+. Only Vocos's config.yaml (charactr/vocos-mel-24khz, MIT) is downloaded; the vocoder weights are in the IndicF5 file. A voice is a reference clip plus its transcript: pan_f_happy (the Punjabi prompt in the repository, transcript from the IndicF5 README) and any <name>.wav + <name>.txt pair in BOLO_TORCH_VOICES_DIR (or the voices_dir option). Option nfe_step (default 32) sets the number of flow steps.

• Veena and Svara (Orpheus-style): the language model writes SNAC codec tokens, 7 per 85 ms frame. Prompts follow the Veena model card (<spk_kavya> text) and the Svara inference code (Hindi (Male): text, with the <|audio|> token). Audio is streamed while the model generates: every 4 frames (option stream_frames) the new frames are decoded with 3 frames of left context and one frame of look-ahead held back. The SNAC decoder (hubertsiuzdak/snac_24khz, MIT, pinned) was run here and gives 2048 samples per frame as the code expects; the 3B language models were not (7.6 GB and 13.2 GB, and they need a GPU in practice).

• SepFormer: SpeechBrain's SepformerSeparation. bolo sends 16 kHz audio; the 8 kHz model gets it resampled to 8 kHz and returns its tracks resampled to 16 kHz.

• PixIT: pyannote.audio Pipeline.from_pretrained on the pipeline configuration bolo downloaded. The pipeline then downloads pyannote/separation-ami-1.0 (gated), microsoft/wavlm-large and speechbrain/spkrec-ecapa-voxceleb itself (2.6 GB together). One track per speaker the pipeline finds. pyannote.audio 4.0.7 calls SpeechBrain with arguments SpeechBrain 1.1.2 no longer takes as keywords (token, revision, huggingface_cache_dir) and a torch.device where it wants a string; the loader moves them into SpeechBrain's FetchConfig before loading, otherwise the pipeline does not load. The pinned revision of the ECAPA embedding was not honoured in that path (the current main was downloaded). Option leakage_removal overrides the pipeline's setting (default true), which zeroes a track wherever its speaker is judged silent.

• DiCoW (BUT-FIT, BUT-FIT/DiCoW_v3_3; Whisper large-v3-turbo with frame-level diarization-dependent transformations, FDDT). For each diarized speaker the plugin builds the four-class STNO mask (silence / only the target / only others / target in overlap) every 20 ms from the activity bolo sends, with the formula of BUT's inference pipeline (BUTSpeechFIT/DiCoW, pipeline.py, Apache-2.0), and runs the model's own generation code (Whisper long-form decoding in 30 s windows, greedy, all speakers of a recording in one batch, up to the option batch, default 4). The repository's Python files are imported as a package (what trust_remote_code runs): transformers 4.55 cannot load them from a local directory because it copies only the modules modeling_dicow.py imports directly. Segment times come from the generator's own segments. A speaker with less than 0.2 s of speech (option skip_silent_s) is not decoded. On a CPU with AMX or AVX512-BF16 the weights are loaded in bfloat16 unless dtype is set; otherwise float32. Output is lower-case without punctuation and with contractions spelled out ("we are", "do not"), the normalised style of its training references. SE-DiCoW uses the same loader and also passes each speaker's self-enrolment window (the 30 s where that speaker talks alone the most, chosen as BUT's pipeline does).

Install

bolo-torch is on PyPI and installs bolo (the bolo-speech distribution) with it:

# 1. PyTorch. The CPU-only build is much smaller than the default (CUDA) build on Linux:
pip install torch torchaudio --index-url https://download.pytorch.org/whl/cpu
# 2. the plugin, with the model families you want:
pip install "bolo-torch[parler,sepformer]"

From a checkout of the bolo repository:

# 1. PyTorch. The CPU-only build is much smaller than the default (CUDA) build on Linux:
pip install torch torchaudio --index-url https://download.pytorch.org/whl/cpu
# 2. bolo and the plugin, with the model families you want:
pip install ./server "./plugins/bolo-torch[parler,sepformer]"

Extras: parler (Parler-TTS), f5 (IndicF5), f5-original (the original F5-TTS checkpoints, on the f5-tts package, which pins its own libraries: give it its own environment), indic-speak (transformers 5 and SNAC), orpheus (Veena, Svara), sepformer, pyannote (PixIT), dicow (DiCoW, SE-DiCoW: transformers 4.55 and pandas). DiCoW uses the plugin protocol's speakers request (bolo 0.2.0 or later). Installed in bolo's environment, the plugin is found through its entry point (bolo.engines: torch) and bolo doctor lists plugin:torch. Check the plugin's own environment with bolo-torch doctor.

Some extras do not mix. parler-tts 0.2.3 pins transformers==4.46.1, while pyannote.audio[separation] needs transformers>=4.48.3 and DiCoW's model code needs transformers 4.55. Put PixIT in a second environment and point bolo at it (a configured command wins over the entry point):

python -m venv /opt/bolo-torch-pyannote
/opt/bolo-torch-pyannote/bin/pip install torch torchaudio --index-url https://download.pytorch.org/whl/cpu
/opt/bolo-torch-pyannote/bin/pip install ./server "./plugins/bolo-torch[pyannote,sepformer]"
BOLO_PLUGINS="torch=/opt/bolo-torch-pyannote/bin/python -m bolo_torch" bolo serve

That is how PixIT was run here. One plugin name serves all plugin:torch entries, so the environment the plugin runs in must hold every family you want to use at the same time.

The parler extra also pins sentencepiece<0.2.1: Parler's audio library pins protobuf<5, and sentencepiece 0.2.3 cannot be imported with protobuf 4 (cannot import name 'runtime_version'), which broke loading Indic Parler-TTS here until it was pinned.

GPUs

• bolo passes its own device setting (default cpu) to the plugin. To use a GPU set BOLO_TORCH_DEVICE=auto (CUDA if available, then Apple MPS, else CPU) or cuda, cuda:1, mps. A model's catalog option device also works. Asking for cuda with a CPU-only torch answers 400 with that explanation.
• dtype is automatic: float32 on CPU, bfloat16 on CUDA when supported (else float16), float16 on MPS. Override with BOLO_TORCH_DTYPE or the option dtype (float32, float16, bfloat16).
• For CUDA install the default torch wheel (pip install torch) or the one for your CUDA version from https://pytorch.org/get-started/locally/, then the plugin.
• No GPU was used in testing. The GPU, MPS and half-precision paths are untested.

Settings

Environment variable	Meaning
BOLO_TORCH_DEVICE	auto, cpu, cuda, cuda:N, mps; wins over bolo's device
BOLO_TORCH_DTYPE	auto, float32, float16, bfloat16
BOLO_TORCH_THREADS	torch intra-op threads (default: bolo's threads setting)
BOLO_TORCH_CACHE	where the plugin's own downloads go (default: the Hugging Face cache, HF_HOME)
HF_TOKEN / BOLO_HF_TOKEN	Hugging Face token for gated repositories
BOLO_TORCH_VOICES_DIR	extra IndicF5 voices: <name>.wav + <name>.txt

Catalog options the plugin reads: device, dtype, seed, max_chars (sentence length limit), and per family description_tokenizer / description_template / stream_seconds / max_new_tokens (Parler), nfe_step / voices_dir (IndicF5), prompt_format / temperature / top_p / repetition_penalty / max_new_tokens / stream_frames / context_frames / codec_repo (Veena, Svara). Without a model directory from bolo, the option repo (and revision) makes the plugin download the model itself.

Gated models

Indic Parler-TTS, IndicF5 and PixIT are gated on Hugging Face. To use them:

1. Sign in to Hugging Face and accept the conditions on each model page: ai4bharat/indic-parler-tts, ai4bharat/IndicF5, and for PixIT both pyannote/speech-separation-ami-1.0 and pyannote/separation-ami-1.0.
2. Create a read token at https://huggingface.co/settings/tokens and set HF_TOKEN (or BOLO_HF_TOKEN) in the environment bolo runs in; the plugin inherits it.

Without access, bolo's own download answers 401 model_gated; a download the plugin makes (PixIT's second repository, a tokenizer) answers the same, with the repository to accept, instead of a generic 503.

Measured here

One 4-vCPU Xeon (2.1 GHz) virtual machine, no GPU, torch 2.11.0+cpu, Python 3.12, 2 threads, 2026-11-13. The machine was shared with other jobs (load average 2-8), so these are pessimistic and only roughly comparable. TTS figures are through bolo's HTTP API (/v1/audio/speech, response_format=pcm, streamed); load time is POST /api/v1/models/{id}/load and includes starting the plugin process when it was not running. RTF = processing time / audio duration.

Model	Load	Text	First audio	Total	Audio	RTF
parler-tts-tiny-v1 (voice jon)	7.4 s	42 chars, 2 sentences	4.6 s	13.6 s	2.9 s	4.8
		101 chars, 2 sentences	11.5 s	24.9 s	5.5 s	4.5
same, in process, stream_seconds=1.0	6.2 s	42 chars	4.9 s	24.2 s	3.0 s	8.0
indic-parler-tts (voice divya, Hindi)	16.1 s	20 chars	64.3 s	64.3 s	3.0 s	21.1
		47 chars, 2 sentences	39.4 s	95.8 s	4.3 s	22.5
indic-parler-tts (voice thoma, English)		42 chars, 2 sentences	20.8 s	75.9 s	3.6 s	21.1
indic-f5 (voice pan_f_happy, Hindi)	11.9 s	20 chars	235 s	235 s	1.5 s	154
		47 chars, 2 sentences	211 s	446 s	3.7 s	120

All outputs were transcribed back with faster-whisper-tiny (English) or faster-whisper-tiny-hindi and read as the input text, with small errors ("ताज़ा जाएंगी" for "बाज़ार जाएँगे" once). On CPU these models are far slower than real time; IndicF5 and Indic Parler-TTS are GPU models in practice.

Separation of a synthetic two-voice mix (bolo's samples en-long.wav + hi-short.wav overlapping from 6.0 s at 0 dB, 11.8 s; scored by SI-SDR over the overlap, permutation-invariant):

Model	Load	Separate 11.8 s	RTF	Female voice	Male voice
sepformer-whamr16k	2.6 s	43.6 s	3.7	0.5 → 5.5 dB	−0.5 → −0.3 dB
sepformer-whamr (8 kHz)	2.3 s	17.0 s	1.4	0.5 → 5.8 dB	−0.5 → −1.8 dB
pixit-ami, pipeline defaults	10.8 s	67.2 s	5.7	3 speakers found; the whole mixture on one track, the others silent
pixit-ami, leakage_removal false, 2 speakers				0.5 → 1.9 dB	−0.5 → −1.0 dB

SepFormer pulled the female voice out but did not improve the male one on this mix; PixIT, trained on real AMI meetings, did little on these two synthetic TTS voices. Neither result says much about meetings or calls: the test is one synthetic clip.

Through bolo (POST /api/v1/diarize with separate_overlaps=true), separating the one 7.7 s overlap window, the whole request (diarization included) took 36.0 s with sepformer-whamr16k, 21.0 s with sepformer-whamr and 33.8 s with pixit-ami (load 10.7 s). In each run bolo kept only speaker A's track (speaker B, who talks only during the overlap, had 0.5 s of clean speech, less than the 1 s bolo needs for a voice print). With sepformer-whamr16k that track scored −10.7 dB against the male reference, worse than the −0.9 dB of the plugin's own track for the same window (pixit-ami: −9.9 dB). That was bolo's track assignment as of the afternoon of 2026-11-13; it was reworked afterwards (it now also weighs each track's share of the speakers' solo speech) and checked with perfect tracks, so this single observation may no longer apply. The word-error evaluation of both separators through the current bolo is in docs/models.md, "Overlapping speech".

DiCoW

2026-11-14, same machine type (it has AMX, so DiCoW ran in bfloat16), bolo and the plugin at 2 threads, the machine idle for the speeds. Full numbers in docs/models.md, "Diarization-conditioned recognition", and docs/measurements/dicow-conditioned.json.

• Against the reference. On two AMI excerpts with Nemotron's speaker turns, the plugin's STNO masks were identical to those of BUT's pipeline.py and every speaker's transcript was word for word the reference pipeline's (6 of 6 speaker streams, DiCoW v3.3; the same for SE-DiCoW, including its self-enrolment windows).
• Accuracy through bolo (cpWER, diarized transcription):

Audio	Files	pyannote + whisper-base (bolo default)	Nemotron + whisper-base	Nemotron + dicow-v3.3	Nemotron + se-dicow
AMI far-field excerpts (45 s)	24	69.4 %	61.2 %	26.0 %	25.3 %
Synthetic two-voice mixes (16 and 8 kHz)	44	55.4 %	25.4 %	6.6 %	7.9 %

• Speed (diarization included, 45 s excerpts with 3-4 speakers): RTF 0.45 at 2 threads, 0.26 at 4 threads; 1.41 at 2 threads in float32. Load about 4.5 s; the plugin process holds about 3 GB in bfloat16. SE-DiCoW: RTF 0.59 at 2 threads.
• Known failure: SE-DiCoW once repeated one word for the last 6 s of a speaker's stream (a 16 kHz Hindi mix); decoding is greedy without Whisper's temperature fallback, as in BUT's pipeline.

Not tested

• Veena and Svara end to end: the language models were not downloaded (7.6 GB / 13.2 GB, no GPU here). The prompt layouts, token-to-SNAC mapping and streaming are unit-tested with fakes; the SNAC decoder ran for real.
• PixIT on real meeting audio, and with pyannote.audio 3.x (it ran with 4.0.7 and the shim above). Before pyannote/separation-ami-1.0 was accepted for the test account, loading it came back through bolo as 401 model_gated naming that repository.
• GPU (CUDA, MPS), float16, bfloat16 other than DiCoW on CPU, Python 3.10, 3.11 and 3.13 for the plugin (it ran on 3.12), macOS.
• DiCoW on whole meetings (only 45 s excerpts and ~30 s mixes), on real telephone calls, on Hindi beyond one synthetic voice pair, and on single-speaker audio against plain Whisper large-v3-turbo; on CPUs without AMX / AVX512-BF16 only float32 speed was measured (forced on this machine). DiCoW gives no word timestamps.
• Telephone (8 kHz) recordings with the 8 kHz SepFormer; real meeting audio with any separator.
• sravaani-1.0 and indicconformer-600m-official: not implemented. spark-tts-0.5b, maskgct and diarizen-wavlm-large-s80-md-v2 are catalogued but not implemented (no packaged inference code; the plugin protocol has no diarization request).
• f5-tts-v1-base and indic-speak: unit-tested with fake backends only; the weights (1.3 GB / 7.1 GB, Indic-Speak gated) were not downloaded, and Indic-Speak's inference.py (gated) was not read.

Licences

The plugin's code is Apache-2.0. Model weights and their training data carry their own terms; read each model card before commercial use.

Model	Weights	Notes
Indic Parler-TTS	Apache-2.0	Gated (accept conditions).
Parler-TTS Tiny v1	Apache-2.0	Text encoder from google/flan-t5 (Apache-2.0).
IndicF5	MIT	Gated. Trained on Rasa, IndicTTS, LIMMITS and IndicVoices-R; those datasets' terms were not reviewed here. Uses Vocos (MIT).
Veena	Apache-2.0	Training data "proprietary" per its card.
Svara v1	Apache-2.0	Base canopylabs/3b-hi-ft-research_release (Llama 3.2 lineage); trained on SYSPIN, RASA, IndicTTS, SPICOR. SNAC codec MIT.
SepFormer WHAMR! (8 kHz, 16 kHz)	Apache-2.0	Trained on WHAMR!, built from WSJ0 (LDC licence, research use) and WHAM! noise (CC-BY-NC-4.0): a provenance risk for commercial use.
DiCoW v3.3	CC-BY-4.0	Attribution to BUT Speech@FIT (cite the DiCoW and SE-DiCoW papers, model card). Fine-tuned from Whisper large-v3-turbo (MIT) on AMI and NOTSOFAR-1 (CC BY 4.0), LibriMix and synthetic LibriSpeech mixtures (LibriSpeech CC BY 4.0; LibriMix's noisy variants use WHAM! noise, CC-BY-NC-4.0, and which variants were used is not stated: unverified). The model repository's Python code is under the repository's licence; BUT's inference code (BUTSpeechFIT/DiCoW) is Apache-2.0.
SE-DiCoW	Apache-2.0	Same base model and training-data notes as DiCoW v3.3.
PixIT speech-separation-ami-1.0	MIT	Gated (both repositories). Downloads speechbrain/spkrec-ecapa-voxceleb (Apache-2.0) and microsoft/wavlm-large (no licence stated on its model card; check before commercial use).

No model here is marked non-commercial by its publisher; the SepFormer training-data caveat above is the one to weigh for commercial use. Nothing is downloaded unless you ask for that model.

Development

pip install torch --index-url https://download.pytorch.org/whl/cpu
pip install -e ./server -e "./plugins/bolo-torch[dev]"
cd plugins/bolo-torch
ruff check src tests && ruff format --check src tests && mypy && pytest -q

The tests use fake model backends and run the real plugin server as a subprocess through bolo's plugin host; they need torch but no model libraries and no downloads.