VibeVoice-ASR-BitNet — LiteRT export

LiteRT (TFLite) conversion of microsoft/VibeVoice-ASR-BitNet, so the model runs on Android and Linux with no ggml / llama.cpp dependency.

The decoder is ternary (BitNet I2_S). LiteRT has no ternary kernel, and a normal export turns it into an int8 matmul — keeping the values, discarding the packing. Packing is the performance: batch-1 decode is memory-bandwidth-bound, so a 1.31 B-parameter decoder moves ~328 MB per token at 2 bits versus ~1310 MB at int8. These files keep the 2-bit packing, and the runtime consumes it through a custom op.

Engine, kernel and export scripts: vieenrose/LiteRT · samples/asr/vibevoice

Contents

The decoder's weights ship outside the graph because LiteRT's dispatcher will not hand constant tensors to a custom kernel; the head's are baked in because it uses no custom op. Weights are mmap'd at load, so they stay clean file-backed pages.

Measured

Boox Tab Mini C — Snapdragon 662, Cortex-A73, ARMv8.0 without dotprod, roughly worst case for int8 SIMD. Back to back against the ggml build of the same model:

Parity on speed at half the unevictable memory, which is the figure that decides whether an Android app survives memory pressure.

Those memory figures are decode only. Through the full pipeline on Android, peak RssAnon is 906 MB: 173 MB of runtime/harness baseline, +55 MB compiling the encoder, +231 MB compiling the head, +46 MB for the decoder graphs and their 330 MB of weights (fully zero-copy — 330.1 MB mmap'd, 0.0 MB copied), and +399 MB of encoder activations. The front end, not the BitNet decoder, dominates memory as well as time.

Note also that XNNPACK's weight_cache_file_path fails silently if the path is not writable — no error, no warning, weights packed into anonymous memory instead, and peak RssAnon goes 906 MB to 1552 MB. Check the file exists after compiling.

End to end through an Android app, 60 s of English in 10 s windows, every pair measured back to back on one device:

2.8x, transcript byte-identical at every step. Only the last row processes less audio; the rest is the same work done properly.

Per 10 s window the budget is encode 14.6 s, prefill 10.3 s, decode 7.7 s. The audio front end — not the BitNet part — is the largest single cost. Dropping one of its two tokenizer encoders would halve it and does not work: the acoustic and semantic branches are nearly orthogonal (cos 0.0435) and comparable in magnitude, so each carries a large share of the summed features the decoder was trained on.

BitNet shrinks the decoder's weights 4x, but on an ARMv8.0 core with no dotprod the ternary kernel is compute-bound, so that reduction buys memory rather than speed. Decode runs 212-249 ms/token against a ~124 ms floor set by streaming 328 MB of 2-bit weights; the LM head is only 42-46 ms/token of it.

Fidelity

The int8 front end is more accurate than the ggml build it replaces, because per-channel scales beat the single per-tensor scale I8_S uses.

Caveats

• The encoder window is fixed at 10 s; longer audio must be windowed by the caller. Convolutions are causal, so a window needs left context and no lookahead. Widening it is not the free win it looks like: 30 s windows cut prompt tokens 25% and the encoder is linear in window length, but its activations are not free — peak RssAnon went 937 MB to 1739 MB and a 3.7 GB device killed the process mid-decode.
• The decoder graphs are context-specific — a ctx=128 prefill cannot be paired with a ctx=512 decode.
• The chat template is mandatory. Feeding audio features without the surrounding system/user/assistant turns produces fluent nonsense rather than a transcript.
• Transcript quality has been spot-checked, not benchmarked. WER against the reference implementation is not yet measured.