Recording video

The video sample I received from Harry Lieben did not contain any real-time recoding information, which means I can’t determine the moment at which it was recorded. This unfortunately means that manual intervention is needed to synchronise the GPS with the video.

You will need either the start-time of the recording, or the end-time of it. Since the GPS stores time in UTC, some transformation is needed. It is assumed that the time you enter is local time, and that the recording took place in the same timezone as the location of your computer. That is, the time-shift of your current location is sub-tracted from the time you enter to obtain the start-time in UTC.

Given the stress at the start of a race, you probably want to use the time you turn off recording a video. Use a piece of paper to write down this time, preferably from the display of the GPS itself.

Video file format

The MPEG-2 video that Harry Lieben recorded is not the most practical format out there. Because of the way the file is organised internally, chances as that you’ll loose the sound at some stage of the processing. I haven’t managed to produce an export to another format that didn’t loose the sound using QuickTime Player. I know that sounds like a bug, but I don’t see a fix coming.

Luckily, I found MPEG Streamclip. This application can convert the MPEG video file to a variety of other formats, after which the audio is stored such tat QuickTime can more easily keep track of it. The quickest way to do this, is to open the MPEG-2 file in MPEG Streamclip, and choose “Demux to M2V and AIFF…” from the “Demux” submenu in the “File” menu. This method is fast, and does not convert the video at all, it just splits the two streams. This will produce two files, one with video and one with sound, but it you open the video in QuickTime Player, it will keep the sound when saving to another format, for instance MP4 after adding the text track. Video conversion can be computationally expensive, so you may want to run that step overnight.

There are some options to automate this: the creation time of the original file on the SD card may be an option, and other video camera’s may include this piece of metadata. I’d be willing to look into this if you provide a sample.
Be careful with this if you record a trip in the UK, and run my tool back here in the Netherlands. You are likely to get off-by-one-hour errors. The command-line tool itself requires a time in UTC.
You may even consider painting the edge of your velomobile with blackboard-paint, and using a piece of chalk.