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I used ffmpeg to transcode some files into new format and with certain parameters. After transcoding, I found some output file's metadata is not what I expected, the output value is not the same with I set in the cmd line.
Before transcoding I check the media info of the inputfile:
ffmpeg -i dz2015082000010.mpg
ffmpeg version 3.2.4 Copyright (c) 2000-2017 the FFmpeg developers
built with gcc 4.8.3 (GCC) 20140911 (Red Hat 4.8.3-9)
configuration: --enable-static --enable-memalign-hack --enable-libx264
--enable-gpl --enable-pthreads --enable-version3 --enable-avisynth --enable-bzlib --enable-iconv --enable-zlib --enable-nonfree --extra-cflags=-I/usr/local/include/ --extra-ldflags=-L/usr/local/lib --enable-debug=3 --disable-optimizations --enable-nonfree --enable-libmp3lame libavutil 55. 34.101 / 55. 34.101 libavcodec 57. 64.101 / 57. 64.101 libavformat 57. 56.101 /
57. 56.101 libavdevice 57. 1.100 / 57. 1.100 libavfilter 6. 65.100 / 6. 65.100 libswscale 4. 2.100 / 4. 2.100 libswresample 2. 3.100 / 2. 3.100 libpostproc 54. 1.100 /
54. 1.100 Input #0, mpeg, from 'dz2015082000010.mpg': Duration: 00:01:49.30, start: 0.685389, bitrate: 15723 kb/s
Stream #0:0[0x1e0]: Video: mpeg2video (Main), yuv420p(tv, top first), 1920x1080 [SAR 1:1 DAR 16:9], 15000 kb/s, 25 fps, 25 tbr,
90k tbn, 50 tbc
Stream #0:1[0x1c0]: Audio: mp2, 48000 Hz, stereo, s16p, 384 kb/s At least one output file must be specified
Next, transcoding with the cmd line:
ffmpeg -i dz2015082000010.mpg -vcodec libx264 -b:v 4000k -s 1920x1080 -r 25 -g 25 -vprofile main -acodec aac -strict -2 -b:a 128k -ac 2 -ar 44100 -y output.ts
After transcoding, I check the media info of the output file:
ffmpeg -i output.ts
ffmpeg version 3.2.4 Copyright (c) 2000-2017 the FFmpeg developers built with gcc 4.8.3 (GCC) 20140911 (Red Hat
4.8.3-9) configuration: --enable-static --enable-memalign-hack --enable-libx264 --enable-gpl --enable-pthreads --enable-version3 --enable-avisynth --enable-bzlib --enable-iconv --enable-zlib --enable-nonfree --extra-cflags=-I/usr/local/include/ --extra-ldflags=-L/usr/local/lib --enable-debug=3 --disable-optimizations --enable-nonfree --enable-libmp3lame libavutil 55. 34.101 / 55. 34.101 libavcodec 57. 64.101
/ 57. 64.101 libavformat 57. 56.101 / 57. 56.101
libavdevice 57. 1.100 / 57. 1.100 libavfilter 6. 65.100
/ 6. 65.100 libswscale 4. 2.100 / 4. 2.100
libswresample 2. 3.100 / 2. 3.100 libpostproc 54. 1.100
/ 54. 1.100 Input #0, mpegts, from 'full-2.ts': Duration:
00:01:49.30, start: 1.456778, bitrate: 4455 kb/s Program 1
Metadata:
service_name : Service01
service_provider: FFmpeg
Stream #0:0[0x100]: Video: h264 (Main) ([27][0][0][0] / 0x001B), yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], 25 fps, 25 tbr,
90k tbn, 50 tbc
Stream #0:1[0x101]: Audio: aac (LC) ([15][0][0][0] / 0x000F), 44100 Hz, stereo, fltp, 4 kb/s At least one output file must be
specified
I don't know why the audio bitrate is changed to 4 kb/s after transcoding, I set the value with -b:a 128k before, anybody can help me? BTW, the output file sounds all right.

The native encoder won't waste bits on silent portions. And it doesn't do strict CBR. If you really need an output to be around the target bitrate, you can mix in a very low level of noise.