--- title: Emotional Text-to-Speech: Fix Flat Delivery with... description: Deepgram explains why voice agents sound robotic: it's usually the delivery register (pitch range and pacing), which stays fixed for a whole session, not the... canonical: https://daily.dev/posts/emotional-text-to-speech-fix-flat-delivery-with-expressivity-lnwmseqqu twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Emotional Text-to-Speech: Fix Flat Delivery with Expressivity | daily.dev og:description: Deepgram explains why voice agents sound robotic: it's usually the delivery register (pitch range and pacing), which stays fixed for a whole session, not the... og:url: https://daily.dev/posts/emotional-text-to-speech-fix-flat-delivery-with-expressivity-lnwmseqqu og:image: https://api.daily.dev/og/posts/LnWmsEqQU.png og:image:alt: Emotional Text-to-Speech: Fix Flat Delivery with Expressivity og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Emotional Text-to-Speech: Fix Flat Delivery with Expressivity **[Deepgram](https://daily.dev/sources/deepgram)** · 12 min read · 0 upvotes · 0 comments ## Summary Deepgram explains why voice agents sound robotic: it's usually the delivery register (pitch range and pacing), which stays fixed for a whole session, not the underlying model. Flux TTS exposes an expressivity parameter from -2 (calm) to 2 (animated), defaulting to 0, the only value validated for production; non-default values raise hallucination and mispronunciation risk. The parameter is set once per connection (query param on WebSocket/batch endpoints, or agent.speak.provider.expressivity in the Voice Agent API Settings message) and can't be changed mid-call - only mid-call speed adjustment is documented as forthcoming. Prosody carries across turns automatically. The parameter is beta and unavailable on Aura-2. Guidance maps calm values to support/IVR/healthcare use cases and animated values to consumer/outbound, while regulated flows should stay at default since only the default is validated. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### What does the expressivity parameter do in Deepgram's Flux TTS? It sets the delivery register of synthesized speech on a scale from -2 (calm, steadier pacing, narrow pitch) to 2 (animated, widest pitch range, brighter pitch), with 0 as each voice's tuned default. It is set once when a connection opens and holds for the entire session; only the default value is validated for production, since non-default values raise hallucination and pronunciation-error risk. _Configuring voice agent tone reliably is easier when you track TTS parameter changes like this on daily.dev._ ### Can I change the expressivity setting in the middle of a live voice agent call? No. Deepgram's own FAQ states expressivity cannot be changed mid-conversation because the value is fixed when the connection opens and holds for the session; changing it requires reconnecting, which also resets the prosody state carried across turns. A separate mid-call speed adjustment feature (0.85 to 1.15 in 0.05 steps) is documented as forthcoming but not yet shipped. _Developers building voice agents follow API limitations like this via daily.dev before they hit them in production._ ### Is the expressivity parameter available on Deepgram's Aura-2 model? No, expressivity only exists on Flux TTS; Aura-2's /v1/speak endpoint does not accept the parameter. In the Voice Agent API, expressivity requires setting agent.speak.provider.version to v2 and configuring agent.speak.provider.expressivity before opening the session. _Comparing TTS provider capabilities is simpler when developers surface details like this through daily.dev._ ## Similar posts on daily.dev - [Batch Text-to-Speech Deep Dive: From One Speech Call to a Produced Show](https://daily.dev/posts/batch-text-to-speech-deep-dive-from-one-speech-call-to-a-produced-show-mank6swid) · Deepgram · 1 upvotes · 0 comments - [Conversation Context for Voice Agents: Accuracy Guide](https://daily.dev/posts/conversation-context-for-voice-agents-accuracy-guide-nvai7bs4k) · Deepgram · 0 upvotes · 0 comments --- Tags: [#architecture](https://daily.dev/tags/architecture), [#deep-learning](https://daily.dev/tags/deep-learning), [#text-to-speech](https://daily.dev/tags/text-to-speech), [#voice-ai](https://daily.dev/tags/voice-ai), [#deepgram](https://daily.dev/tags/deepgram) [View this post on daily.dev](https://daily.dev/posts/emotional-text-to-speech-fix-flat-delivery-with-expressivity-lnwmseqqu) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Emotional Text-to-Speech: Fix Flat Delivery with Expressivity","url":"https://daily.dev/posts/emotional-text-to-speech-fix-flat-delivery-with-expressivity-lnwmseqqu","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/emotional-text-to-speech-fix-flat-delivery-with-expressivity-lnwmseqqu"},"datePublished":"2026-09-01T12:33:49.639Z","dateModified":"2026-09-01T12:34:19.001Z","description":"Deepgram explains why voice agents sound robotic: it's usually the delivery register (pitch range and pacing), which stays fixed for a whole session, not the...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/a044842c2877b34d46a0302d93ac3aea?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/a044842c2877b34d46a0302d93ac3aea?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Deepgram","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Deepgram","logo":"https://media.daily.dev/image/upload/s--e63hOwKU--/f_auto/v1716188454/logos/deepgram","url":"https://daily.dev/sources/deepgram"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/emotional-text-to-speech-fix-flat-delivery-with-expressivity-lnwmseqqu","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"architecture,deep-learning,text-to-speech,voice-ai,deepgram","timeRequired":"PT12M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Deepgram","item":"https://daily.dev/sources/deepgram"},{"@type":"ListItem","position":3,"name":"Emotional Text-to-Speech: Fix Flat Delivery with Expressivity"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/emotional-text-to-speech-fix-flat-delivery-with-expressivity-lnwmseqqu#faq","mainEntity":[{"@type":"Question","name":"What does the expressivity parameter do in Deepgram's Flux TTS?","acceptedAnswer":{"@type":"Answer","text":"It sets the delivery register of synthesized speech on a scale from -2 (calm, steadier pacing, narrow pitch) to 2 (animated, widest pitch range, brighter pitch), with 0 as each voice's tuned default. 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