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I've tried MP4Box -add part1.mp4 -cat part2.mp4 concat.mp4 and get:

[iso file] Incomplete box ...|
Unknown input file type
Error importing part1.mp4: Corrupted Data in file/stream

mencoder -oac copy -ovc copy -o final.mp4 part1.mp4 part2.mp4 produces a final.mp4 whose file size which is the sum of part1.mp4 and part2.mp4, but quality is reduced in that there are audio blips which are not present in the original individual files.

mencoder -oac pcm -ovc copy -o final.mp4 part1.mp4 part2.mp4 produces a very large file, but without most of the blips of the previous mencoder command. However, there is still an audible blip at the point in the video where the two originals were joined.

I'm using Kazam to record a screencast and need to be able to record segments which I later join together. (The final file needs to be H246/MP4.)

I also tried ffmpeg and avconv with various command line incantations from the docs and forums, all of which threw up errors.

mediainfo part1.mp4 shows the following:

General
Complete name                            : part1.mp4
Format                                   : MPEG-4
Format profile                           : Base Media / Version 2
Codec ID                                 : mp42
File size                                : 2.55 MiB
Duration                                 : 1mn 46s
Overall bit rate mode                    : Variable
Overall bit rate                         : 201 Kbps
Encoded date                             : UTC 2013-07-06 17:00:02
Tagged date                              : UTC 2013-07-06 17:00:02
Writing application                      : x264

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : Baseline@L3.2
Format settings, CABAC                   : No
Format settings, ReFrames                : 1 frame
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 1mn 46s
Bit rate                                 : 70.5 Kbps
Width                                    : 1 280 pixels
Height                                   : 800 pixels
Display aspect ratio                     : 1.600
Frame rate mode                          : Constant
Frame rate                               : 15.000 fps
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.005
Stream size                              : 914 KiB (35%)
Writing library                          : x264 core 120 r2151 a3f4407
Encoding settings                        : cabac=0 / ref=1 / deblock=0:0:0 / analyse=0:0 / me=dia / subme=0 / psy=1 / psy_rd=1.00:0.00 / mixed_ref=0 / me_range=16 / chroma_me=1 / trellis=0 / 8x8dct=0 / cqm=0 / deadzone=21,11 / fast_pskip=1 / chroma_qp_offset=0 / threads=1 / sliced_threads=0 / nr=0 / decimate=1 / interlaced=0 / bluray_compat=0 / constrained_intra=0 / bframes=0 / weightp=0 / keyint=150 / keyint_min=15 / scenecut=0 / intra_refresh=0 / rc=cqp / mbtree=0 / qp=15 / ip_ratio=1.40 / aq=0
Language                                 : English
Encoded date                             : UTC 2013-07-06 17:00:02
Tagged date                              : UTC 2013-07-06 17:00:02

Audio
ID                                       : 2
Format                                   : MPEG Audio
Format version                           : Version 1
Format profile                           : Layer 3
Codec ID                                 : 6B
Duration                                 : 1mn 46s
Bit rate mode                            : Variable
Bit rate                                 : 128 Kbps
Channel(s)                               : 1 channel
Sampling rate                            : 44.1 KHz
Compression mode                         : Lossy
Stream size                              : 1.61 MiB (63%)
Language                                 : English
Encoded date                             : UTC 2013-07-06 17:00:02
Tagged date                              : UTC 2013-07-06 17:00:02
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