# [Note] 20210723 Unknown MP4 MIME Type
Source
- ffmpeg course slide:
http://slhck.info/ffmpeg-encoding-course
- 一樣的 Bitrate，解析度高的畫質會比較好嗎？
https://annkuoq.github.io/blog/2020-10-25-bitrate-vs-resolution/
>同 Bitrate，解析度越大 -> 畫質越差
同解析度，Bitrate 越大 -> 畫質越好
(但 Bitrate 大到一定的階段，畫質也不會一直無限好上去)
一樣的 Bitrate，不論解析度多大，檔案大小都一樣
- Manipulating audio channels
https://trac.ffmpeg.org/wiki/AudioChannelManipulation
### Issue: The MIME type of the MP4 files uploaded from a customer is unusual
files : https://drive.google.com/drive/folders/1GplZxQS6jGNLRv2tJLT_f5yGbwswJ7ut?usp=sharing
- How to check the MMIE type?
```bash=
$ file --mime-type <MP4_FILENAME>
```
- The type is `application/octet-stream`
According to [MDN_WEB_DOCS](https://developer.mozilla.org/zh-TW/docs/Web/HTTP/Basics_of_HTTP/MIME_types)
>"Generic binary data (or binary data whose true type is unknown)"
- How to solve this?
- Related discussion:
https://stackoverflow.com/questions/51059736/why-some-of-the-mp4-files-mime-type-are-application-octet-stream-instead-of-vid
- Use ffmpeg to convert them
```bash=
$ffmpeg -i C0081.MP4 -pix_fmt yuv420p -crf 18 N_C0081.MP4
```
- After converting, detect the MIME type again
---
## Analysis the format change before & after
>File: C0081.MP4
>File Size: 88.1MB
### Check the metadata of the C0081.MP4
```bash=
$ ffprobe -i C0081.MP4
```
```text
Metadata:
major_brand : XAVC
minor_version : 16785407
compatible_brands: XAVCmp42iso2
creation_time : 2021-07-22T07:23:29.000000Z
Duration: 00:00:13.02, start: 0.000000, bitrate: 54147 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709/bt709/iec61966-2-4), 1920x1080 [SAR 1:1 DAR 16:9], 51462 kb/s, 59.94 fps, 59.94 tbr, 60k tbn, 119.88 tbc (default)
Metadata:
creation_time : 2021-07-22T07:23:29.000000Z
handler_name : Video Media Handler
vendor_id : [0][0][0][0]
encoder : AVC Coding
Stream #0:1(und): Audio: pcm_s16be (twos / 0x736F7774), 48000 Hz, 2 channels, s16, 1536 kb/s (default)
Metadata:
creation_time : 2021-07-22T07:23:29.000000Z
handler_name : Sound Media Handler
vendor_id : [0][0][0][0]
Stream #0:2(und): Data: none (rtmd / 0x646D7472), 491 kb/s (default)
Metadata:
creation_time : 2021-07-22T07:23:29.000000Z
handler_name : Timed Metadata Media Handler
timecode : 11:18:06:14
```
### Check the the metadata of the N_C0081.MP4
> N_C0081.MP4 is prodiced by this command
> File Size: 16.4 MB
> $ffmpeg -i C0081.MP4 -pix_fmt yuv420p -crf 18 N_C0081.MP4
```bash=
$ ffprobe -i N_C0081.MP4
```
```text
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf58.76.100
Duration: 00:00:13.02, start: 0.000000, bitrate: 10101 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709/bt709/iec61966-2-4), 1920x1080 [SAR 1:1 DAR 16:9], 9961 kb/s, 59.94 fps, 59.94 tbr, 60k tbn, 119.88 tbc (default)
Metadata:
handler_name : Video Media Handler
vendor_id : [0][0][0][0]
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 128 kb/s (default)
Metadata:
handler_name : Sound Media Handler
vendor_id : [0][0][0][0]
```
### Copy the original video part & re-encode the audio part with other audio codec?
Why should we do this? why shouldn't we copy it directly?
We couldn't find a `pcm_s16be` with the video container.
```bash=
ffmpeg -y -i C0081.MP4 -vcodec copy -acodec copy copy.mp4
```
Result:
```text
[mp4 @ 0x7f8e3e00d800] Could not find tag for codec pcm_s16be in stream #1, codec not currently supported in container
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
Error initializing output stream 0:1 --
Stream mapping:
Stream #0:0 -> #0:0 (copy)
Stream #0:1 -> #0:1 (copy)
Last message repeated 1 times
```
Ref:
- [FFMPEG Could not find tag for codec pcm_s16be with MP4](https://stackoverflow.com/questions/58286743/ffmpeg-could-not-find-tag-for-codec-pcm-s16be-with-mp4)
>Apparently the MP4 norm does not include the pcm_s16be codec and hence ffmpeg refuses to create such a file.
### Method: Copy the video and re-encoded the audio part.
This is s script to get the bit rate of the audio stream and set the output with the bit rate.
```bash=
#!/usr/bin/env bash
AUDIO_BITRATE=`ffprobe -v error -select_streams a:0 -show_entries stream=bit_rate -of default=noprint_wrappers=1:nokey=1 $1`
echo $AUDIO_BITRATE
ffmpeg -y -i $1 -acodec aac -b:a ${AUDIO_BITRATE}k -vcodec copy new-$1
```
> -y : Overwrite output files without asking
> -i : input file url
> -acodec aac : set the audio codec with `aac` format
> -b:a :set the audio bit rate of the output file
> -vcodec copy : copy the video codec
- Usage
```bash=
$ chmod u+x script.sh
$ ./script.sh C0081.MP4
```
It would export to a file name `new-C0081.MP4`
- Check the exported file
Size: 83MB, Near to the original size
file: https://drive.google.com/drive/folders/1GxiwamGg-acFHvTQujls5YbmIUBPfNJd?usp=sharing
```bash=
$ ffprobe -i new-C0081.MP4
```
- meta data
```text
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf58.76.100
Duration: 00:00:13.02, start: 0.000000, bitrate: 51832 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709/bt709/iec61966-2-4), 1920x1080 [SAR 1:1 DAR 16:9], 51462 kb/s, 59.94 fps, 59.94 tbr, 60k tbn, 119.88 tbc (default)
Metadata:
handler_name : Video Media Handler
vendor_id : [0][0][0][0]
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 365 kb/s (default)
Metadata:
handler_name : Sound Media Handler
vendor_id : [0][0][0][0]
```
- mime type
```bash=
file --mime-type new-C0081.MP4
```
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