Using FFMPEG to Concatenate and Embed Subtitles

I recently upgraded my drone to a DJI Mavic Air 2. Among other things, it can create h.265 videos directly. It still uses the MP4 container format and the separate SRT format for storing video subtitles, the flight data. Following most camera standards, it creates video files that are individually smaller than 4 GB, which works out to be about five minutes in 4k video.

If I want to upload a longer raw video to social media, the video files need to be concatenated before uploading.

Concatenating the video with FFMPEG has been something I’ve known how to do for a long time using either of two methods. Today I learned how to properly embed the subtitles in either the MKV or MP4 container format.

The MP4 format is more widely supported than the MKV format, but is less flexible as to what it can contain.  The MKV (Matroska Multimedia Container) container format can hold almost any type of media, and so I’m able to copy the SRT format directly. The MP4 (MPEG-4 Part 14) container format only supports a limited selection of subtitle formats, so I’m required to have FFMPEG convert the SRT stream to a MP4 compatible stream.  If you are interested in video container formats, these tables are very helpful.

I’ll give several examples using the two video files and their associated subtitle files created by the drone named DJI00001.MP4, DJI00002.MP4, DJI00001.SRT, and DJI00002.SRT. The method I’m using should work for any number of files, up to the largest filesize you can store on your filesystem.

To simply concatenate the video files, create a text input file (I’m using mp4files.txt) with the contents as follows

file DJI00001.MP4
file DJI00001.MP4

then use the ffmpeg command to create a new concatenated file.

ffmpeg -f concat -safe 0 -i mp4files.txt -c copy ConcatenatedVideo.MP4

If you want to embed the subtitles, you need to create a second text file, do some stream mapping, and specify what format the subtitles should be. In this case I’m using srtfiles.txt

file DJI00001.SRT
file DJI00002.SRT

My FFMPEG command to create an MP4 file gets a lot more involved because now I’m specifying multiple inputs and have to specify the subtitle format.

ffmpeg -f concat -safe 0 -i mp4files.txt -f concat -safe 0 -i srtfiles.txt -map 0:v -map 1 -c:v copy -c:s mov_text ConcatenatedVideo.MP4

The FFMPEG command to create an MKV command is only a tiny bit different, and the resulting file is only a tiny bit smaller.

ffmpeg -f concat -safe 0 -i mp4files.txt -f concat -safe 0 -i srtfiles.txt -map 0:v -map 1 -c:v copy -c:s copy ConcatenatedVideo.MKV

When playing the ConcatenatedVideo files on my local machine, I can now enable or disable the closed caption track properly in the player for either format. Unfortunately in my initial testing with YouTube, neither format maintains the second stream of subtitles.

This is not all a waste of time and effort, because an advantage of embedding the subtitles into the container format is that the timing has been matched to the video, and can now be extracted in a concatenated form for use with YouTube.

ffmpeg -i ConcatenatedVideo.MKV -c copy ConcatenatedVideo.SRT

You can exclude the “-c copy” when extracting the subtitles and FFMPEG fill run it through its subrip codec and produce nearly identical results. It will only work with the MKV file because the subtitle format stored in the MP4 file is not easily converted to a SRT file.

Using the -f concat option invokes the concat demuxer in FFMPEG, which has the limitation that the format needs to be exactly the same for each file. If there are any changes between files you want to concatenate, you must use a more involved command invoking the concat filter. In a different project I ran into a command issue with the concat filter command when the command got to be much over 900 characters long.

 

FAA Drone Questionnaire

I got an email from the FAA asking me to participate in a questionnaire about recreational drone use and information the FAA can provide.

One of the questions was How long do you typically fly? My answer was 60 minutes.  I usually have three fully charged batteries, each of which will fly for approximately 28 minutes. I like to land and swap batteries with a few minutes to spare, giving me a total flight time per session of close to an hour.

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The form doesn’t like any answer more than 30 minutes. One more example of how out of touch the FAA is with the rapidly advancing technology.

SD Card Holder

I’d been traveling with several microSD cards for both GoPro and Drone usage for the past year when I finally got around to buying this inexpensive plastic card to organize them.

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Before buying it, I had read several reviews, and worried that cheap plastic might either make it brittle, or let the flash cards fall out of the enclosure. For $6.95 it was worth a try.

What I really found out was that I should have been using it for at least a year before I got it. The fact that I can carry 10 micro SD cards and they are all in order is a complete boon for organization.

20170922_185055016_iOSBefore using this, I’d often have two identical looking SD cards in my hands and forget which one was full and which one was empty. I’d also take a card out of my device and set it down, and nearly lose it.  Now I take a card out of my camera and immediately put it into this holder. I fill this holder from one end with empty cards and from the other with full cards and never have to remember which cards have something on them any more.

It’s essentially the same size as a credit card, but is quite a bit thicker. I wouldn’t carry it in my wallet, nor would I use the hole for a key ring, but having it in a pocket in my drone bag keeps all the memory cards in one place in a known order.

Being a responsible drone pilot

I try to be a responsible drone pilot. I use the FAA B4UFly App on a regular basis to see what it has to say about locations that I’m interested in flying. Unfortunately, it lists every uncontrolled heliport as an airport and reports “Action Required” so often as to be nearly useless.

Yesterday I was sitting on the beach at Golden Gardens Park in Seattle, watching sailboats race offshore. I thought it would be good to check to see what the app would show. I was sitting at sea level, with a hill near me that would be at least 200 feet high. The rules for drones say that you are not allowed to fly over 400 feet above the surface without permission from the controlling authority. Aircraft are not allowed to fly below 700 feet without similar permission.

GoldenGardensVFR

Golden Gardens is located about where the “E” in KENMORE sits on the map above. It’s under the Class B Airspace that starts at 3,000 feet, going up to 10,000 feet.

While understanding VFR charts  may be more than the average drone pilot should be expected to recognize, especially for a dense area like Seattle, the B4UFly App’s tendency to show warnings is similar to the boy who cried wolf. It’s impossible to recognize when an action is really appropriate.

AMA Membership and BestBuy

An interesting thing I found in the drone aisle in BestBuy was a card to join the Academy of Model Aeronautics.


I’ve been a member since I got involved with drones. I had to become a member to join the rc flying club I frequent. It’s good to see that it’s being easy to recognize in consumer stores, as opposed to the complete hobby level word of mouth that I’ve seen in the past.

Best Buy and DJI Accessories

I don’t shop too much locally for technology, because it always seems that what I want is not available locally. Yesterday I was wandering around killing time and went into the local Best Buy.  Instead of just looking at the normal things I might go for in a best buy, I looked around to see what was new. They now have an aisle dedicated to drones. There were complete drones from DJI, Yuneec, and several other manufacturers. More interesting to me was that they carried accessories for those drones as well.  I knew that they sold drones on-line, it just never occurred to me to look at them in person in the store.

I bought a Mavic Pro directly from DJI soon after they were released. I got the Fly-More package that entails several items including spare propellers. I’ve not needed to replace a propeller in my first 7 hours of flight time but it’s nice to know that propellers are available locally. DJI charges slightly less, $9.00, but they charge for shipping, and you have to wait for the item to arrive.

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DJI Battery Monitoring

DJI charges a significant amount of money for their batteries and calls them Smart Batteries. I’ve seen the statistics reported of how many times the battery has been power cycled, as well as details of how much flight time is available. This was a new and interesting feature to me.

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I had left the drone in the back of my car overnight. The temperature had probably gotten into the high 30s, and was still in the mid 50s with the sun shining. The warning message “Battery Temperature Too Low. Warm battery to at least 15 degrees Celcius before flying” came up on my screen and would not let me initiate a takeoff.

I have used batteries in cold climates in the past. I know how temperature affects both current output from batteries and future usability. I’m impressed that DJI has built in this feature to their firmware.