CEA608: Handling XDS and TEXT modes #5807
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[Problem] There are 3 services / modes transported on line 21: - Captioning - TEXT (generally not program related) - XDS (eXtended Data Services) Bytes belonging to the unsupported modes are interleaved with the bytes of the captioning mode. See Chapter 7, Chapter 8.5 and Chapter 8.6 of the CEA608 Standard for more details. [Solution] Drop all bytes belonging to unsupported modes. [Test] - All streams containing only captioning services should not be influenced - Test all 4 CEA 608 channels with live over-the-air content and using all available TEXT and XDS streams.
| @@ -306,6 +330,38 @@ protected Subtitle createSubtitle() { | |||
| return new CeaSubtitle(cues); | |||
| } | |||
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| private boolean isCodeForUnsupportedMode(byte cc1, byte cc2) { | |||
| // Control codes from 0x01 to 0x0F indicate the beginning of XDS Data | |||
| if (0x01 <= cc1 && cc1 <= 0x0F) { | |||
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Section 8.5 C of the CEA608 Standard:
When any of the control codes from 0x01 to 0x0F is used to begin a control-character pair it indicates the beginning of XDS Data. This XDS data should be considered part of a different data channel and can be used to interrupt the Text data according to the FCC rules for other data channels.
| } | ||
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| switch (cc2) { | ||
| case CTRL_END_OF_CAPTION: |
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See C.16 of the CEA608 standard.
EDM or ENM command does not instruct the decoder to treat subsequent data as caption data
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There are six caption-specific commands which do terminate a Text Mode data stream: EOC, RCL, RDC, RU2, RU3, and RU4.
library/core/src/main/java/com/google/android/exoplayer2/text/cea/Cea608Decoder.java
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library/core/src/main/java/com/google/android/exoplayer2/text/cea/Cea608Decoder.java
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| } | ||
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| // 2 commands switch to TEXT mode. | ||
| if (((cc1 & 0xF7) == 0x14) // first byte must be 0x14 or 0x1C based on channel |
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Can you move (cc1 & 0xF7) == 0x14 to a private static method isModeSwitchCommand(cc1) for readability? And use the same method on L349 below. Then you only need to add the explaining comment once.
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I could not come up with a better name, but the suggested function signature could be misleading. Usually both bytes are needed to figure out what the command is doing, and there could be a long range of values this function returns true, although there is no mode switch expected.
| @@ -363,6 +419,16 @@ protected void decode(SubtitleInputBuffer inputBuffer) { | |||
| continue; | |||
| } | |||
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| if (isCodeForUnsupportedMode(ccData1, ccData2)) { | |||
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If I'm not mistaken, it seems that this code can move to handleCrtl? That is probably a better place to handle these transitions as it's part of "control code handling" in general.
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I do not think so. We need stop processing all data (characters included) in case we are not in CAPTION mode. Moving this down would add the characters of the TEXT and XDS modes to the buffers of our processing of CAPTION mode.
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Added the suggested improvements as a new commit, now the comments given to the first commit might point to locations that might be "Outdated" |
[Problem]
There are 3 services / modes transported on line 21:
Bytes belonging to the unsupported modes are interleaved with the
bytes of the captioning mode.
See Chapter 7, Chapter 8.5 and Chapter 8.6 of the CEA608 Standard for
more details.
[Solution]
Drop all bytes belonging to unsupported modes.
[Test]
using all available TEXT and XDS streams.
[Links]
https://www.law.cornell.edu/cfr/text/47/79.101
https://en.wikipedia.org/wiki/EIA-608#Channels