With the emergence of the internet and the proliferation of screens and content platforms since then, closed captioning has become an essential part of program delivery, from traditional broadcast to streaming platforms. From the perspective of viewers, content creators and providers, and technology manufacturers, closed captioning has become a requirement. Audiences expect it will be available for nearly all content they consume, and content creators and providers expect any hardware or software tool they use for video to support it. 

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Closed captioning is the process of displaying text over an image to provide dialogue, language translation or interpretive information synchronized with program audio. Both closed captioning and subtitling are typically used as a transcription of the audio part of the program, sometimes including descriptions of non-speech elements such as sound cues and music. 

When transmitted, closed captions are encapsulated (“embedded”) into the video stream. This technique was initially standardized for broadcast television with CEA-608 and CEA-708, two closed captioning standards that remain widely used today. With them, captions are written directly into a video. In contrast, more recent standards like SMPTE ST 2110 take a different approach, storing closed captioning data as metadata separate from the video. During playback, this captioning information is synchronized and delivered alongside the video stream. Let’s take a closer look at each:  

SMPTE ST 2110 is a suite of standards developed by the Society of Motion Picture and Television Engineers (SMPTE) that defines how to transmit uncompressed video, audio, and ancillary data over IP networks. ST 2110-20 standardizes the transport of uncompressed video over IP networks, while ST 2110-30 does the same for audio. ST 2110-40 specifies the transport of ancillary (ANC) data over IP networks in professional media applications. Ancillary data passed through a ST 2110-40 stream not only supports closed captioning but also subtitles, timecode, and other forms of embedded data associated with the primary video signal. 

In addition to describing how ANC data is encapsulated and transmitted alongside video and audio streams, SMPTE ST 2110-40 maps ANC data packets into RTP packets and enables them to remain synchronized with the video and audio essence streams. Unlike traditional SDI, it allows for the transmission of ANC data as a separate stream from the video and audio, enabling independent manipulation of each element. By breaking ANC data into “essence streams” separate from the video, broadcasters and others working with professional media may more easily add or modify captions, subtitles, and other metadata at different points in the workflow. 

Before the development of SMPTE ST 2110-40, ANC packets – whether closed captioning or similar ancillary information – could not be separated from the video signal. Adding closed captioning to an SDI signal required you to send the video into a closed captioning encoder, add the captions, and then output the video again. To add another ANC service, you had to send the video into another device, add the service, and send the video out again. 

With SMPTE ST 2110-40, the video signal can be broken into separate video, audio, and ancillary data components, enabling each element to be routed, processed and managed separately on an IP network rather than being bundled together as a single stream like in traditional SDI systems. 

In a SMPTE ST 2110 IP environment, a production output may be sent to a workstation with an I/O card like KONA IP25 connected for seamless network integration. Software on the computer hosting the KONA IP25 card can generate the needed captions, and the KONA IP25 card can transmit a caption-filled ST 2110-40 stream (and if desired a ST 2110-20 video with captions burned into the active video) to join the original video and audio essences in the network. To add closed captioning or other ANC services, it’s no longer necessary to send the video through the closed captioning encoder. Instead, the captioner device creates captions and sends the SMPTE ST 2110-40 captioning packets into the network. Without touching the production ST 2110-20 video, captions have been created and all the essences can now be utilized to deliver the final product. 

OP-47 (for HD) and OP-42 (for SD) are closed captioning standards used in 50 Hz countries that allow text and rudimentary graphics to be carried in the vertical blanking interval (VANC) area at the top and bottom of the video frame. They transmit closed captioning data as numbered pages of data that can be called up using a remote control. Despite its circa-1970s origins and the rise of the internet, select television stations in Europe continue to provide teletext services, accessed most often through dedicated applications or web interfaces. However, it’s considered outdated by most and has largely been replaced by online information sources.  

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Closed captioning has specialized terminology that professionals need to understand. Continue reading for a glossary of terms and phrases, from subtitles and closed captioning to ANC packets and HDMI.  

• Subtitling versus Closed Captioning 
  While both consist of text displayed in sync with program audio, and while the terms are often used interchangeably in certain parts of the world, subtitling and closed captioning are not strictly the same thing. The difference lies in the audio content included. Think of subtitling as a transcription of words spoken during the program. Closed captioning represents a broader array of meaningful audio; in addition to spoken dialogue, it would denote applause, laughter, sound effects and music. 
   
• Closed Captioning Ancillary Data (ANC) Packets 
  A method of embedding non-video information within a digital video signal, ANC packets carry closed captioning, subtitles, timecode, teletext data, GPI closures and other forms of embedded data associated with the primary video signal. Closed captioning is only one metadata form of many that can be conveyed via ANC packets.  
  
  The term is used across different video standards. In a CTA-608 environment, for instance, captions are encoded in Line 21 of the video signal, whereas with a CTA-708 signal, closed captions are transmitted as data packets embedded in the “picture user data” section of the video signal. With SMPTE ST 2110, ANC packets that include closed captioning are transmitted as a separate essence associated with the primary video signal. 
  
  In an SDI signal, ANC packets are transmitted during either the vertical (VANC) or horizontal (HANC) blanking intervals of the video frame, allowing additional data to be carried alongside the primary video content without affecting the picture itself. The low-bandwidth data in the blanking interval is updated on a per-field or per-frame basis. Closed Captioning Data (CCD) and Active Format Description (AFD) are examples of metadata stored as VANC. SMPTE 291M describes the details of ancillary data packet structures and types. 
   
• HDMI (High-Definition Multimedia Interface) is a common audio/video standard for transmitting digital video and audio between sources and displays. However, HDMI signals do not carry closed captioning metadata. In this respect, technologies that can accept SDI or HD-SDI closed captions only option is to burn-in the closed captions to the active video on the HDMI output.

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Whether you are producing or delivering content for live or on-demand viewing, closed captioning metadata will likely be involved. This means all the various tools you use, from capture through postproduction and delivery, must support the ancillary data you need, such as closed captioning, embedded timecode, or SCTE triggers. Check out this roundup of key considerations: 

Outside of the AJA solutions already mentioned and included on the closed captioning solutions homepage, AJA offers a host of other products designed to support closed captioning needs across the production chain, from the HELO Plus H.264 streaming and recording device, to the Desktop and Mobile I/O families, and Developer Products. Dive deeper into each solution’s closed captioning functionality on each product’s respective page. 

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