Problem: MXF and MPG files (TV broadcast formats) often contain:
- Multiple audio streams (4-8 streams for different language tracks)
- Multichannel layouts (5.1, 7.1) instead of stereo
- Default ffmpeg behavior was -c:a aac without channel limit, which
meant multichannel got transcoded as multichannel AAC, overwriting
what should have been clean stereo
Solution:
1. get_audio_streams() helper probes all audio streams with ffprobe
- Returns codec, channels, sample_rate, language, layout for each
2. build_audio_args() picks best stream + downmix:
- Prefers first 2-channel stereo stream (usually main mix)
- Falls back to first stream if none are 2-ch
- Always: -ac 2 (force stereo downmix), -ar 48000, -c:a aac, -b:a 192k
- Bitrate raised from 128k to 192k for music quality
3. Smart trim path now detects broadcast formats:
- .mxf, .mpg, .mpeg, .ts, .m2ts, .mts → transcode (not stream copy)
- Standard MP4/MOV → stream copy (faster, lossless)
4. Pre-conversion step for broadcast files without trim:
- Even without --start/--duration, MXF/MPG get converted to MP4
- Same audio handling as trim path
5. Main render adds explicit -map 0✌️0 -map 0🅰️0? -ac 2 to ensure
only first video and first audio stream get encoded, with stereo
6. ACR recognize also gets -map 0🅰️0 -ac 2 for MXF compatibility
7. UI accepts: video/*,.mxf,.mpg,.mpeg,.ts,.m2ts,.mts
8. Upload limit raised: 2GB → 10GB (MXF files are large)
This means a TV broadcast MXF with [SLO/EN/DE language tracks] now
correctly outputs stereo MP4 with the main language track preserved.
