RLHF stands for reinforcement learning from human feedback. It is the training technique that turned raw language models into usable assistants: instead of only predicting the next word, the model is additionally trained to prefer responses that human reviewers judged better. When people say a model was aligned or instruction-tuned, RLHF (or one of its successors) is usually the machinery they mean.
This guide explains what RLHF means, how the training actually works, the human-generated data every stage depends on, the newer alternatives like DPO, and how the same feedback loop applies to voice AI.
What RLHF means
A pretrained model has read enormous amounts of text and learned to continue it plausibly. Plausible is not the same as helpful, safe, or honest. RLHF closes that gap by adding a second training signal: the judgments of human reviewers who compare model outputs and mark which one is better. The model is then optimized to produce the kind of output humans prefer. The insight that made RLHF work at scale is that people are much more reliable at comparing two answers than at writing a perfect answer or scoring one in isolation.
How RLHF works: the three stages
1. Supervised fine-tuning (SFT). Human writers produce demonstrations: high-quality example responses to prompts. The pretrained model is fine-tuned on them so it learns the basic shape of a good answer.
2. Reward model training. The model generates several responses to each prompt, and human reviewers rank them. Those preference comparisons train a separate reward model that learns to predict which outputs humans would prefer, turning scattered judgments into a reusable scoring function.
3. Reinforcement learning. The main model is optimized against the reward model, typically with an algorithm such as PPO: it generates, gets scored, and adjusts toward higher-scoring behavior, with constraints that stop it drifting too far from the SFT model. The result is a model that behaves the way the human preferences pointed.
The human data behind every stage
Strip away the reinforcement learning and RLHF is a human data pipeline. Its quality ceiling is set by three datasets: the demonstrations (how good are the example answers), the preference comparisons (how consistent and well-guided are the rankings), and the adversarial prompts used to probe failure modes. The practical quality levers are the ones any data collection project has: clear rating guidelines, calibrated and diverse reviewer pools, agreement measurement between raters, and honest handling of ambiguous cases. Thin or inconsistent preference data does not just cap performance; it teaches the reward model the raters' confusion. These are the same disciplines covered in our data quality guide, applied to judgments instead of transcripts.
DPO and the successors
RLHF's reinforcement learning stage is complex and unstable to run, so newer methods simplify it. Direct preference optimization (DPO) trains the model on the preference pairs directly, skipping the separate reward model and the RL loop. RLAIF replaces some human judgments with judgments from another model, trading cost for fidelity to actual human preference. The detail that matters for anyone budgeting a project: every one of these methods still consumes human preference data. The optimization machinery changed; the dependence on well-collected human judgments did not.
RLHF for voice AI
The feedback loop is not text-only. Voice AI teams use exactly the same pattern: listeners compare two text-to-speech renditions and pick the more natural one, reviewers rank transcript candidates for readability, and testers rate whether a voice assistant's spoken answer was appropriate. Preference data like this is how synthetic voices get natural and how spoken assistants get judged, and collecting it well requires the same calibrated human pools as any RLHF project, plus listeners who match the deployment language and accent. Spirelight can assess a spoken-response evaluation brief. The TTS training data guide covers the voice side; participant sourcing, calibration, QA, rights, deliverables, schedule, and price are confirmed through the data services brief.