September 6, 2013 - 10:31am

In our previous posts, we talked about equipment that supports CEA-608/708 and SCTE-20 as well as live visual confidence monitoring. Now that we know how to assemble an entire signal chain which supports closed caption protocols and live confidence monitoring, you might be asking the following questions…

1.    How can we guarantee that the captions and syntax were properly inserted/encoded into the stream in the first place?
2.    How would you verify if the closed captions were missing during live transmission?

Consider the following:
How can you guarantee that the captions and syntax were properly inserted/encoded correctly? What if a viewer complains that captions are missing letters when decoded by their set-top-box? How can you tell if the problem is with the set-top-box or the encoded content? There are so many variables that come into play with closed caption encoding that the only way to guarantee the accuracy of the delivered content is to analyze in depth. For this type of analysis, the Sencore CMA 1820 exceeds expectations.

When you’re troubled with garbled, out of place, or missing captions, its best to verify the caption data and syntax/format. When a caption issues is experienced, a simple stream capture can provide the necessary resource for the Sencore CMA 1820. Not only can you see full syntax of the transport stream and elementary streams, the CMA 1820 provides a full off-line reference decoder and syntax analysis engine. With the CMA 1820, you can verify that the stream contains proper caption data with positioning, color, display time, etc.  

Figure 1: Capture of CMA 1820. Decoded image shows the word “THERE IN”, missing the letter “E” (Arrow 1). The Caption buffer section of CMA 1820 shows that the encoded source also has a missing “E” (Arrow 2). This proves that the decoder was properly displaying the captions however the source was missing the letter.

If a viewer calls in to let you know that closed captions are missing from a program they are watching, how would you verify if the closed captions were missing during live transmission - and if they are missing what is the root cause?

When it comes to closed caption analysis with real time monitoring capability, nothing beats the Sencore TSM 1770. You can analyze ETSI TR-101 290 and SCTE-35 as well as have the ability to perform confidence monitoring for closed captions. Note the caption text populating in the CEA-608 and CEA-708 captioning section of Figure 2 below. With this option available you can verify that the captions coming in are present regardless of possible set-top-box failure.

With the TSM 1770 you also have the ability to see errors induced by improperly signaled Closed Captions. Within Figure 3 below, you can see errors caused by an encoder. The Closed Captions are being transmitted but are not defined in the stream PMT.

Figure 2: CEA-608 & CEA-708 captions populating in the TSM 1770

Figure 3: Error messaging on TSM 1770 showing closed caption signaling errors


Insert and encode captions in the right the right time!

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