Strange, Video bitrate is changed after trim the video with ffmpeg

[reko]

Original Video is mkv file :
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L4.1
Format settings, CABAC : Yes
Format settings, ReFrames : 5 frames
Codec ID : V_MPEG4/ISO/AVC
Duration : 2 h 11 min
Bit rate : 9 173 kb/s
Width : 1 920 pixels
Height : 808 pixels
Display aspect ratio : 2.40:1
Frame rate mode : Constant
Frame rate : 24.000 FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.246
Stream size : 8.44 GiB (86%)
Writing library : x264 core 159 r2991 1771b55
Encoding settings : cabac=1 / ref=5 / deblock=1:-3:-3 / analyse=0x3:0x133 / me=umh / subme=9 / psy=1 / psy_rd=1.00:0.00 / mixed_ref=1 / me_range=16 / chroma_me=1 / trellis=2 / 8x8dct=1 / cqm=0 / deadzone=21,11 / fast_pskip=0 / chroma_qp_offset=-2 / threads=24 / lookahead_threads=2 / sliced_threads=0 / nr=0 / decimate=1 / interlaced=0 / bluray_compat=0 / constrained_intra=0 / bframes=3 / b_pyramid=2 / b_adapt=1 / b_bias=0 / direct=3 / weightb=1 / open_gop=0 / weightp=2 / keyint=240 / keyint_min=24 / scenecut=40 / intra_refresh=0 / rc_lookahead=50 / rc=2pass / mbtree=1 / bitrate=9173 / ratetol=1.0 / qcomp=0.60 / qpmin=0 / qpmax=69 / qpstep=4 / cplxblur=20.0 / qblur=0.5 / ip_ratio=1.40 / aq=1:1.00
Language : English
Default : Yes
Forced : No

Then I encode it with ffmpeg :
ffmpeg -i "Parasite.mkv" -c:v libx265 -preset ultrafast -acodec ac3 -b:a 448k -map 0 ParasiteReduced.mkv

The result (at least for me) still watchable on 42" TV although the video bitrate is only 342 kb/s from 9173 kb/s
ParasiteReduced.mkv Media Info :
Video
ID : 1
Format : HEVC
Format/Info : High Efficiency Video Coding
Format profile : Main@L4@Main
Codec ID : V_MPEGH/ISO/HEVC
Bit rate : 342 kb/s
Width : 1 920 pixels
Height : 808 pixels
Display aspect ratio : 2.40:1
Frame rate mode : Variable
Original frame rate : 24.000 FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Writing library : x265 3.0_Au+10-74a8672ea4f7:[Windows][GCC 8.2.1][64 bit] 8bit+10bit
Encoding settings : cpuid=1111039 / frame-threads=3 / numa-pools=8 / wpp / no-pmode / no-pme / no-psnr / no-ssim / log-level=2 / input-csp=1 / input-res=1920x808 / interlace=0 / total-frames=0 / level-idc=0 / high-tier=1 / uhd-bd=0 / ref=1 / no-allow-non-conformance / no-repeat-headers / annexb / no-aud / no-hrd / info / hash=0 / no-temporal-layers / open-gop / min-keyint=24 / keyint=250 / gop-lookahead=0 / bframes=3 / b-adapt=0 / b-pyramid / bframe-bias=0 / rc-lookahead=5 / lookahead-slices=5 / scenecut=0 / radl=0 / no-splice / no-intra-refresh / ctu=32 / min-cu-size=16 / no-rect / no-amp / max-tu-size=32 / tu-inter-depth=1 / tu-intra-depth=1 / limit-tu=0 / rdoq-level=0 / dynamic-rd=0.00 / no-ssim-rd / no-signhide / no-tskip / nr-intra=0 / nr-inter=0 / no-constrained-intra / strong-intra-smoothing / max-merge=2 / limit-refs=0 / no-limit-modes / me=0 / subme=0 / merange=57 / temporal-mvp / no-weightp / no-weightb / no-analyze-src-pics / deblock=0:0 / no-sao / no-sao-non-deblock / rd=2 / early-skip / rskip / fast-intra / no-tskip-fast / no-cu-lossless / no-b-intra / no-splitrd-skip / rdpenalty=0 / psy-rd=2.00 / psy-rdoq=0.00 / no-rd-refine / no-lossless / cbqpoffs=0 / crqpoffs=0 / rc=crf / crf=28.0 / qcomp=0.60 / qpstep=4 / stats-write=0 / stats-read=0 / ipratio=1.40 / pbratio=1.30 / aq-mode=1 / aq-strength=0.00 / cutree / zone-count=0 / no-strict-cbr / qg-size=32 / no-rc-grain / qpmax=69 / qpmin=0 / no-const-vbv / sar=1 / overscan=0 / videoformat=5 / range=0 / colorprim=2 / transfer=2 / colormatrix=2 / chromaloc=0 / display-window=0 / max-cll=0,0 / min-luma=0 / max-luma=255 / log2-max-poc-lsb=8 / vui-timing-info / vui-hrd-info / slices=1 / no-opt-qp-pps / no-opt-ref-list-length-pps / no-multi-pass-opt-rps / scenecut-bias=0.05 / no-opt-cu-delta-qp / no-aq-motion / no-hdr / no-hdr-opt / no-dhdr10-opt / no-idr-recovery-sei / analysis-reuse-level=5 / scale-factor=0 / refine-intra=0 / refine-inter=0 / refine-mv=0 / refine-ctu-distortion=0 / no-limit-sao / ctu-info=0 / no-lowpass-dct / refine-analysis-type=0 / copy-pic=1 / max-ausize-factor=1.0 / no-dynamic-refine / no-single-sei / no-hevc-aq / no-svt / qp-adaptation-range=1.00
Language : English
Default : Yes
Forced : No
Statistics Tags Issue : mkvmerge v7.4.0 ('Circles') 64bit built on Dec 12 2014 12:19:56 2020-01-07 17:45:36 / Lavf58.26.101
FromStats_BitRate : 9168071
FromStats_Duration : 02:11:48.839000000
FromStats_FrameCount : 189812
FromStats_StreamSize : 9063594840

But then after I trim the ParasiteReduced.mkv :
ffmpeg -ss 01:52:00 -i "ParasiteReduced.mkv" -t 00:03:00 -c copy output.mkv

The Media Info of output.mkv video bitrate is going up from 342 kb/s into 1253 kb/s

Video
ID : 1
Format : HEVC
Format/Info : High Efficiency Video Coding
Format profile : Main@L4@Main
Codec ID : V_MPEGH/ISO/HEVC
Duration : 3 min 1 s
Bit rate : 1 253 kb/s
Width : 1 920 pixels
Height : 808 pixels
Display aspect ratio : 2.40:1
Frame rate mode : Constant
Frame rate : 24.000 FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Bits/(Pixel*Frame) : 0.034
Stream size : 27.1 MiB (72%)
Writing library : x265 3.0_Au+10-74a8672ea4f7:[Windows][GCC 8.2.1][64 bit] 8bit+10bit
Encoding settings : cpuid=1111039 / frame-threads=3 / numa-pools=8 / wpp / no-pmode / no-pme / no-psnr / no-ssim / log-level=2 / input-csp=1 / input-res=1920x808 / interlace=0 / total-frames=0 / level-idc=0 / high-tier=1 / uhd-bd=0 / ref=1 / no-allow-non-conformance / no-repeat-headers / annexb / no-aud / no-hrd / info / hash=0 / no-temporal-layers / open-gop / min-keyint=24 / keyint=250 / gop-lookahead=0 / bframes=3 / b-adapt=0 / b-pyramid / bframe-bias=0 / rc-lookahead=5 / lookahead-slices=5 / scenecut=0 / radl=0 / no-splice / no-intra-refresh / ctu=32 / min-cu-size=16 / no-rect / no-amp / max-tu-size=32 / tu-inter-depth=1 / tu-intra-depth=1 / limit-tu=0 / rdoq-level=0 / dynamic-rd=0.00 / no-ssim-rd / no-signhide / no-tskip / nr-intra=0 / nr-inter=0 / no-constrained-intra / strong-intra-smoothing / max-merge=2 / limit-refs=0 / no-limit-modes / me=0 / subme=0 / merange=57 / temporal-mvp / no-weightp / no-weightb / no-analyze-src-pics / deblock=0:0 / no-sao / no-sao-non-deblock / rd=2 / early-skip / rskip / fast-intra / no-tskip-fast / no-cu-lossless / no-b-intra / no-splitrd-skip / rdpenalty=0 / psy-rd=2.00 / psy-rdoq=0.00 / no-rd-refine / no-lossless / cbqpoffs=0 / crqpoffs=0 / rc=crf / crf=28.0 / qcomp=0.60 / qpstep=4 / stats-write=0 / stats-read=0 / ipratio=1.40 / pbratio=1.30 / aq-mode=1 / aq-strength=0.00 / cutree / zone-count=0 / no-strict-cbr / qg-size=32 / no-rc-grain / qpmax=69 / qpmin=0 / no-const-vbv / sar=1 / overscan=0 / videoformat=5 / range=0 / colorprim=2 / transfer=2 / colormatrix=2 / chromaloc=0 / display-window=0 / max-cll=0,0 / min-luma=0 / max-luma=255 / log2-max-poc-lsb=8 / vui-timing-info / vui-hrd-info / slices=1 / no-opt-qp-pps / no-opt-ref-list-length-pps / no-multi-pass-opt-rps / scenecut-bias=0.05 / no-opt-cu-delta-qp / no-aq-motion / no-hdr / no-hdr-opt / no-dhdr10-opt / no-idr-recovery-sei / analysis-reuse-level=5 / scale-factor=0 / refine-intra=0 / refine-inter=0 / refine-mv=0 / refine-ctu-distortion=0 / no-limit-sao / ctu-info=0 / no-lowpass-dct / refine-analysis-type=0 / copy-pic=1 / max-ausize-factor=1.0 / no-dynamic-refine / no-single-sei / no-hevc-aq / no-svt / qp-adaptation-range=1.00
Language : English
Default : Yes
Forced : No
Statistics Tags Issue : mkvmerge v7.4.0 ('Circles') 64bit built on Dec 12 2014 12:19:56 2020-01-07 17:45:36 / Lavf58.26.101
FromStats_BitRate : 9168071
FromStats_Duration : 00:03:01.612000000
FromStats_FrameCount : 189812
FromStats_StreamSize : 9063594840

3 Minutes Sample HEVC Video - 38 Mb - Video Bitrate 1253 kb/s (http://www.indonesiangems.com/x265/output.mkv)

I wonder how is the code to trim it with the same video bitrate 342 kb/s ? (Also wonder, why the trimmed video bitrate is going up to almost 4 times than the original video bitrate).

Any kind of help would be greatly appreciated.
Thank you in advanced.

[StainlessS]

I aint no expert [nor hardly a newbie],
but should the -c copy not be
-vcodec copy
[and if suitable] -acodec copy
I dont know what -c copy is supposed to do.

[excellentswordfight]

The bitrate is varaible, the segment you have trimmed just has more bits spent for that segment.

[reko]

Hi StainlessS,
thank you for your respond.

I aint no expert [nor hardly a newbie],
but should the -c copy
The copying a portion of ParasiteReduced use -c copy, StainlessS.

not be
-vcodec copy I don't have -vcodec copy :).

[reko]

The bitrate is varaible, the segment you have trimmed just has more bits spent for that segment.

Hi excellentswordfight,
Thank you for your explanation.

Make sense.
So, because the source file (ParasiteReduced.mkv) has a variable bitrate, and the portion I choose (quite many movement - fast camera panning) consequently this portion get more bits spent.
That's why the output.mkv also has more kbps.

BTW, I see that the ParasiteReduced.mkv has a variable bitrate.
I wonder why the output.mkv turn to constant bitrate ?

Thank you once again, excellentswordfight.

[excellentswordfight]

Hi excellentswordfight,
Thank you for your explanation.

Make sense.
So, because the source file (ParasiteReduced.mkv) has a variable bitrate, and the portion I choose (quite many movement - fast camera panning) consequently this portion get more bits spent.
That's why the output.mkv also has more kbps.

BTW, I see that the ParasiteReduced.mkv has a variable bitrate.
I wonder why the output.mkv turn to constant bitrate ?

Thank you once again, excellentswordfight.
Are you sure you are not looking at "Frame rate mode"? If the source file has variable bitrate (and it does) then the trimmed stream copy surly does so as well as you do not touch the video.

As to why Frame rate mode is reported as variable in your first re-encode and not the trimmed copy of it, no idea.

Hi StainlessS,
thank you for your respond.

The copying a portion of ParasiteReduced use -c copy, StainlessS.

I don't have -vcodec copy :).
You are misunderstanding him, I'm pretty sure that he means that you should use -vcodec copy instead of -c copy. But its fine, -c copy works (and it did work). AFAIK -c copy will just try to copy every stream/component, most users usually specify this per stream type e.g. -vcodec/acodec.

[tonemapped]

The bitrate is varaible, the segment you have trimmed just has more bits spent for that segment.

+1

I often trim segments using avidemux (still works great) and the output depends on the segment, even with 3-pass CBR. On a test 7,500 kbps encode, one segment came out with an average bitrate of under 4,000 kbps, and another at over 9,000 kbps.