[FFmpeg-user] Setting -output_ts_offset always adds 2 frames

[Julien De Luca]

Hello all,

I'm trying to set the initial pkt_pts_time using output_ts_offset. On
streams encoded using ffmpeg, it always sets the offset + 0.08s, which is 2
frames. Here is the command I use :

```
ffmpeg -i input.ts -acodec aac -vcodec libx264 -f mpegts -preset veryfast
-filter:v scale=640:-1 -framerate 25 -profile:v main -b:v 1200000 -level
3.1 -b:a 96000 -g 75 -bf 3 -output_ts_offset 10 -muxdelay 0 -muxpreload 0
output.ts
```

And the output from ffprobe for the first video frame :
```
{
  "media_type": "video",
  "stream_index": 0,
  "key_frame": 1,
  "pkt_pts": 907200,
  "pkt_pts_time": "10.080000",
  "pkt_dts": 907200,
  "pkt_dts_time": "10.080000",
  "best_effort_timestamp": 907200,
  "best_effort_timestamp_time": "10.080000",
  "pkt_duration": 3600,
  "pkt_duration_time": "0.040000",
  "pkt_pos": "1128",
  "pkt_size": "4872",
  "width": 640,
  "height": 360,
  "pix_fmt": "yuv420p",
  "sample_aspect_ratio": "1:1",
  "pict_type": "I",
  "coded_picture_number": 0,
  "display_picture_number": 0,
  "interlaced_frame": 0,
  "top_field_first": 0,
  "repeat_pict": 0,
  "chroma_location": "left"
}
```

The input is a ts file from a segmented stream created using hls options of
ffmpeg.

On another input created using AWS Mediaconvert, pkt_pts_time is set
correctly, at exactly 10.

Is there any known reason this may happen ?

Thank you !