The trace survives the substitution
Under a complete replacement of the linguistic content, top-1 generator retrieval falls from 94.19% in-language to 91.02%, a gap of 3.17 points, while top-5 holds at 98.80% and MRR at 0.948. Against the analytical floor of 3.70% for an embedding carrying no generator information, the holdout result is 24.6x chance and retains 96.5% of the recoverable in-language performance. The near-saturated top-5 matters operationally: when the nearest neighbour is wrong, the correct generator is almost always still in the shortlist.
- 91.02%
- Holdout top-1
- 98.80%
- Holdout top-5
- 0.965
- Transfer index vs in-language
Geometry: position transfers, tightness does not
The rows of the results table do not degrade uniformly, and the pattern is the informative part. Inter-generator scale is essentially unaffected: d-inter falls 1.8% from validation to holdout, so generators do not collapse toward each other when the language changes. Intra-generator compactness degrades 10x. Clusters stay in the right places but become much looser, and that single term drives the separation ratio from 173.5 to 16.7 without any loss of inter-generator structure.
- 1.8%
- Drop in inter-generator distance
- 10x
- Inflation in intra-cluster spread
- 173.5 to 16.7
- Separation ratio, a misleading metric
Language is factored out, not merely tolerated
The language entanglement ratio is the across-language centroid spread of a generator divided by the inter-generator distance scale, so it says how far moving a generator to another language displaces it relative to the distance to its neighbours. On the holdout it halves over the analysed window, from 0.127 at epoch 6 to 0.064 at epoch 19, while holdout retrieval rises 3.1 points. Nothing in the objective ever saw those languages, so the reduction cannot come from fitting them.
- 0.127 to 0.064
- rho-lang on unseen languages
- 39%
- Reduction in absolute language spread
- +3.1 pts
- Holdout top-1 over the same window
Errors follow lineage, not language
Of the 351 generator pairs on the holdout, the great majority are well separated. The residual dark structure is organized by provenance: ChatGPT-TTS with OpenAI TTS-1 HD as the closest pair, MOSS-1.7B with MOSS-8B and Qwen3-0.6B with Qwen3-1.7B as tight same-family blocks, and a cluster of kokoro, RVC, griffin_lim and minimax_speech-02-turbo that plausibly share a waveform-generation stage. If the embedding were encoding language, confusions would cluster by which holdout language each system emits. They do not.
- 351
- Generator pairs on the holdout
- Vendor / family
- Axis the residual errors follow
- xtts v1.1 / v2
- Same lineage, still separated
Per-language difficulty is a gallery effect
Aggregate holdout accuracy conceals a 13.7-point spread across the six unseen languages. Thai reaches 97.2% and Vietnamese 96.0%, despite both being tonal and typologically remote from an Indo-European-heavy training set, while Russian sits at 85.5% and Korean at 83.5%. Typological distance does not predict forensic difficulty; the number of generators competing within each language does. The same effect is starker in-language, where Portuguese scores lowest at 84.7% despite sitting in the same data-volume band as languages that saturate near 99.8%.
- 13.7 pts
- Spread across holdout languages
- 97.2% / 83.5%
- Thai best, Korean worst
- Gallery density
- The confound, not data volume
What the study does not establish
The paper states its limits as plainly as its results. No trained ablations or independent baselines were run, so cross-language positives coincide with language robustness rather than being shown to cause it. The holdout shares its label space with training, so this says nothing about open-set attribution. All audio comes from one collection pipeline, so corpus-level factors are untested, and language remains confounded with speaker and content. One seed, one configuration, and the final epoch is not the best one: holdout top-1 peaks at 91.46% at epoch 17.
- 5
- Ablations named as required before submission
- Seen generators
- Label space of the holdout
- 1 seed
- No confidence intervals reported
