Originally published on hexisteme notes .

While evaluating Qwen3-TTS for narration, I ran a small A/B to see whether an instruction could raise the speaking rate. It couldn't, I wrote that down, and speed control moved to post-processing. The next day a different experiment turned up a line of code that made half of that A/B meaningless: on the smaller model, the instruction never reached the model. The call returned normal audio, the transcript was perfect, and nothing in the measurements looked wrong.

Before comparing settings of a control knob, show that the knob moves the output at all. Otherwise the comparison measures nothing.

The A/B

The goal was fast narration, around 230 words per minute. The setup was Qwen3-TTS's CustomVoice models through the qwen_tts 0.1.1 package on torch with the MPS backend, at two sizes: 0.6B and 1.7B. generate_custom_voice has no speed or rate parameter. Its only lever is instruct , a free-text instruction.…