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Closed Captioning

The Makito X4 encoder supports the capture of CTA-608 and CTA-708 closed captions from the HD/SD-SDI video input and transports it within the video elementary stream of the MPEG-2 Transport Stream.

The CTA-608 and CTA-708 captions are encapsulated in a Caption Description Packet (CDP), as described in SMPTE-334-2, within the VANC as mentioned in SMPTE 334-1. The Caption information is multiplexed and transported as per ATSC A/72 Part 1:2008 (ATSC A/53 Part 4:2007) in a CC SEI NAL. The CDP consists of a sequence of bytes that can hold the CEA-708 DTV caption data, CEA-608 caption data, caption service information, and SMPTE 12M-1 timecode. 

The CDP also includes the Caption Service Descriptor (CSD) (as defined in ATSC A/65). The caption services stored in the CSD are announced in the MPEG-2 TS Program Map table (PMT).


EIA-608, CEA-608 or CTA-608?

The 608/708 closed captioning specifications were developed by the Electronics Industry Association (EIA). EIA later delegated the closed captioning specifications to the "Consumer Electronics Association" (CEA), and the EIA standards documents were renamed to CEA.

CEA later became the Consumer Technology Association (CTA), so these specifications are now referred to as CTA-608 and CTA-708. 

CTA has an affiliation with the American National Standards Institute (ANSI), so the latest versions of these specifications are:

Line 21 Data Services (ANSI/CTA-608-E S-2019)
Digital Television (DTV) Closed Captioning (ANSI/CTA-708-E R-2018)

The following block diagram shows the workflow for video and CC data into the encoder.

With the support of CTA-708, ATSC A/72 mandates a dedicated link bitrate of 9600 bits/sec (1200 bytes/sec or 20 bytes per frame for a p60 SDI signal). Of the 9600 bits/sec, 960 bits/sec are reserved for CTA-608 captions. 8640 bits/s are dedicated to 708-CC. This is a dedicated channel, so it has to be filled with filler data if no captioning information is present.

Supported resolutions include 1080p, 720p, 1080i, 480i, and 576i.


Analog CTA-608 Line 21 Closed Captioning is only supported on the Makito X4 Rugged encoder (Composite input). See Makito X4 Rugged Closed Captioning.

Makito X4 Rugged Closed Captioning

The Makito X4 Rugged supports capture, multiplexing and transport of Closed Captioning (CC) and other Line 21 information over Composite (with no 7.5 IRE setup on Line 21), as well as HD/SD SDI input interfaces.

Closed Captioning data, as defined by the CTA-608-B standard, includes the following services over Line 21 Fields 1 and 2 of an NTSC analog video signal: CC1, CC2, CC3, CC4, Text1, Text2, Text3, Text4, and XDS (see following figure). The Makito X4 Rugged encoder supports transport of all these services. [Note that Closed Captioning as specified by CTA-608-B does not exist over PAL.]

Line 21 of each field can contain up to 2 bytes of information, which can be used by any of the above services (not simultaneously but rather successively). With NTSC video at 30fps, the maximum CTA-608-B mandated throughput represents:

2 bytes x 2 Fields x 30fps x 8 bit/byte = 960 bps

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