What is speech synthesis and how is it used?

Speech synthesis uses complex al­gorithms to output texts as spoken words using a simulated voice. The benefits of speech synthesis include better ac­cess­ib­il­ity and dis­sem­in­a­tion of in­form­a­tion, a per­son­al­ised user ex­per­i­ence and more efficient in­ter­ac­tions.

What is speech synthesis?

Speech synthesis, often referred to as text-to-speech (TTS), is a tech­no­logy that turns written text into spoken language and outputs it using a simulated voice that closely mimics natural human speech. TTS tech­no­logy uses stored speech segments to generate an ar­ti­fi­cial voice that re­pro­duces texts as acoustic signals, so that it sounds as authentic and natural as possible. While earlier TTS tech­no­lo­gies still strung together fixed strings of words or sentences, modern speech synthesis is able to achieve different lin­guist­ic nuances and emphases and in­tel­li­gently combine speech segments to create original content.

Speech synthesis is ideal for cost ef­fect­ively conveying texts, messages and in­form­a­tion without human speakers and op­tim­ising com­mu­nic­a­tion, ac­cess­ib­il­ity and reach. For this reason, speech synthesis is used in various in­dus­tries and for various purposes, both com­mer­cially and in areas such as education, service or nav­ig­a­tion.

How does speech synthesis work?

The speech synthesis process usually begins with inputting written content such as messages, texts, ad­vert­ising messages or emails. The software then converts the text into simulated, natural-sounding speech using different tech­no­lo­gies like al­gorithms, pre-recorded speech signals, neural networks, ar­ti­fi­cial in­tel­li­gence and machine learning. In order to achieve an output that sounds as natural as possible, the tone of voice, in­ton­a­tion and style of speech are adapted as closely as possible to a human way of speaking.

In the early days of speech synthesis, canned speech was used, i.e., pre-recorded words and sentences that were strung together to create familiar robotic voices. Nowadays, TTS software is able to draw on a large database of speech signals and segments to ensure flexible and natural speech gen­er­a­tion, even for un­fa­mil­i­ar texts.

In addition, tech­no­lo­gies such as acoustic models, formant synthesis, ar­tic­u­lat­ory synthesis and overlap add are used to break down text into audio signals and syn­thes­ise spoken word sequences, speech rate, prosody and in­ton­a­tion as naturally as possible.

How is speech synthesis used?

Speech synthesis can be used for a broad spectrum of use cases, including:

• Ac­cess­ible tech­no­lo­gies: Speech synthesis software makes it possible, among other things, for people with visual impair­ments to have texts read out. With screen readers, blind and visually impaired people can navigate computers in­de­pend­ently, access in­form­a­tion, produce trans­la­tions and even display syn­thes­ised speech in Braille.
• Education and training: Speech synthesis software can be used to make re­cord­ings and tran­scrip­tions of lectures, teaching materials or con­fer­ences ac­cess­ible. It also allows for efficient dis­tri­bu­tion of these materials. Authors and editors can also check texts for errors and com­pre­hens­ib­il­ity by listening to them read aloud.
• Podcasts, audio blogs and audiobook pro­duc­tion: For popular audio formats such as podcasts, audio blogs or au­diobooks, speech synthesis enables fast, cost-effective and high-quality pro­duc­tion. Instead of finding voice actors, pro­fes­sion­al audio content can be produced cost ef­fect­ively and to a high standard using TTS. It can be output as MP3 files or in streaming formats.
• Telephone an­nounce­ments and customer service: Whether for automated telephone and loud­speak­er an­nounce­ments or customer service systems, in the business world, speech synthesis enables efficient support for customers and fast inquiry pro­cessing.
• Nav­ig­a­tion systems: Speech synthesis plays an important role in the field of nav­ig­a­tion systems and is used in GPS devices and nav­ig­a­tion apps. It provides better service, modern auto­ma­tion and greater safety in public transport through traffic in­form­a­tion, route and driving in­struc­tions and automatic stop an­nounce­ments.
• En­ter­tain­ment and media: In en­ter­tain­ment media such as video games, animated films, doc­u­ment­ar­ies or other in­ter­act­ive formats, speech synthesis enhances immersive gaming ex­per­i­ences and gives ar­ti­fi­cial char­ac­ters realistic, lifelike speech.
• Automated voice services and voice as­sist­ants: Thanks to speech synthesis, you can enhance virtual as­sist­ants and enable functions with spoken voice output or control, whether for voice search SEO, voice search op­tim­isa­tion, voice as­sist­ants, chatbots or gen­er­at­ive AI.

With TTS, you can not only use pre­defined neural voices but also create your own neural voices or simulate real voices through re­cord­ings. This means that ar­ti­fi­cial voices can be adapted to product and company brands, ad­vert­ising campaigns, voice apps or even content such as audio books and podcasts.

What’s the dif­fer­ence between speech synthesis and speech re­cog­ni­tion?

Speech synthesis trans­forms written content into spoken language by using computer-generated voices to reproduce texts acous­tic­ally. Speech re­cog­ni­tion, on the other hand, is designed to un­der­stand spoken language and convert it into written text by con­vert­ing the acoustic ut­ter­ances into digital char­ac­ters. In short, speech synthesis is the coun­ter­part to speech re­cog­ni­tion as it converts text into spoken language, while speech re­cog­ni­tion converts spoken language into written text.

Speech synthesis and speech re­cog­ni­tion are often closely linked and are fre­quently used together in voice as­sist­ance systems. Speech synthesis is used to provide users with answers in spoken form. Speech re­cog­ni­tion is re­spons­ible for ensuring that the system un­der­stands the requests and responds ac­cord­ingly. These tech­no­lo­gies com­ple­ment each other perfectly, con­trib­ut­ing to improved human-machine in­ter­ac­tion.

Other types of speech synthesis

In addition to pure text-to-speech software, speech synthesis offers other speech systems such as:

• Speech pros­thes­is: Speech pros­theses help people with physical or speech dis­ab­il­it­ies to produce natural speech using computer-generated speech systems and minimal input. They are designed to promote ac­cess­ib­il­ity and fa­cil­it­ate com­mu­nic­a­tion and access to computers.
• Mul­timod­al speech synthesis: Mul­timod­al speech synthesis, also known as au­di­ovisu­al speech synthesis, uses syn­thes­ised speech in com­bin­a­tion with animated faces to sup­ple­ment speech with visual signals and facial ex­pres­sions such as smiling or shaking one’s head. In this way, the ex­press­ive­ness, live­li­ness, nat­ur­al­ness and nuance of speech synthesis can be improved.