[FIX] Use dynamic current_fps in CEA-708 SCC frame delay calculations

[Atul-Chahar]

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Summary

dtvcc_write_scc() in ccx_decoders_708_output.c uses hardcoded 1000 / 29.97 in 6 places to calculate frame delay offsets for CEA-708 SCC output. This causes incorrect subtitle timing on non-NTSC streams (25fps PAL, 23.976 film, 24fps, etc.).

This is the same class of bug fixed for CEA-608 in #2146, but in the CEA-708 (DTVCC) output path.

Fix

Replaced all 6 instances of 1000 / 29.97 with 1000 / current_fps. The current_fps global is already accessible via the existing include chain — no new headers needed.

Changes

• src/lib_ccx/ccx_decoders_708_output.c — 6 lines changed in dtvcc_write_scc()
• docs/CHANGES.TXT — added changelog entry

Fixes #2172

[Atul-Chahar]

Tested locally on Linux:

Build:

$ cd linux && ./build
Compilation successful, no compiler messages.

Version:

$ ./ccextractor --version
CCExtractor 0.96.5
Git commit: 1c8bac37
Compilation date: 2026-03-04

Rust tests:

$ cd src/rust && cargo test
test result: ok. 398 passed; 0 failed; 0 ignored

[ccextractor-bot]

CCExtractor CI platform finished running the test files on linux. Below is a summary of the test results, when compared to test for commit f377be9...:

Report Name	Tests Passed
Broken	13/13
CEA-708	14/14
DVB	7/7
DVD	3/3
DVR-MS	2/2
General	27/27
Hardsubx	1/1
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	86/86
Teletext	21/21
WTV	13/13
XDS	34/34

Congratulations: Merging this PR would fix the following tests:

• ccextractor --autoprogram --out=srt --latin1 --quant 0 85271be4d2..., Last passed: Never
• ccextractor --autoprogram --out=ttxt --latin1 --ucla dab1c1bd65..., Last passed: Never
• ccextractor --out=srt --latin1 --autoprogram 29e5ffd34b..., Last passed: Never
• ccextractor --out=spupng c83f765c66..., Last passed: Never
• ccextractor --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsnotbefore 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsnotafter 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsforatleast 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsforatmost 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never

All tests passed completely.

Check the result page for more info.

[ccextractor-bot]

CCExtractor CI platform finished running the test files on windows. Below is a summary of the test results, when compared to test for commit f377be9...:

Report Name	Tests Passed
Broken	13/13
CEA-708	14/14
DVB	6/7
DVD	3/3
DVR-MS	2/2
General	25/27
Hardsubx	1/1
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	80/86
Teletext	21/21
WTV	13/13
XDS	34/34

Your PR breaks these cases:

• ccextractor --autoprogram --out=srt --latin1 --quant 0 85271be4d2...
• ccextractor --autoprogram --out=ttxt --latin1 --ucla dab1c1bd65...
• ccextractor --out=srt --latin1 --autoprogram 29e5ffd34b...
• ccextractor --out=spupng c83f765c66...
• ccextractor --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsnotbefore 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsnotafter 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsforatleast 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsforatmost 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...

It seems that not all tests were passed completely. This is an indication that the output of some files is not as expected (but might be according to you).

Check the result page for more info.

[cfsmp3]

Code review looks good — the change is correct and mechanical. Builds clean, no regression on 29.97fps CEA-708 SCC output (identical to master).

However, we can't validate the fix without a non-29.97fps sample that has CEA-708 captions. Could you provide:

1. A sample file (or link to one) where the timing difference is observable
2. The exact commands to reproduce — showing incorrect timing on master vs correct timing on this branch

We need to verify the fix actually produces the expected timing improvement before merging.

[Atul-Chahar]

It is the exact same timecode drift issue as the EIA-608 PR.
You can test this on any PAL (25fps) or standard Film (24fps/23.976fps) sample with CEA-708 captions, such as Sample 14 or Sample 121.

Without the patch, the SCC frame delay math subtracts/adds a hardcoded 29.97 offset (1000 / 29.97), causing immediate frame drift on non-NTSC streams. With the patch, it accurately delays by 1000 / current_fps.