[Elphel-support] Problems to detect timestamps

Sat Jan 9 11:07:53 PST 2016

Hello Jennifer,

Unfortunately we did not had enough resources to get into the videosteramer code - we do not use it ourselves because of the multiple limitation of the streaming. It strips frames of the Exif headers that we use not only for the time stamping (with 1 microsecond resolution), but for other metadata too (like IMU or GPS data). Additionally videostream is designed for just synchronous streaming, while camera has a large videobuffer and it is possible to control it precisely, for example acquiring higher resolution/quality using longer time and then "catching up" either skipping frames completely or requcing resolution/quality.

Can you please describe your overall application, it may be possible to achieve the same goals differently, using approach similar to what we use ourselves?

There are several tools available for capturing and synchronizing images:
1. camogm (http://wiki.elphel.com/index.php?title=Camogm ) allows to record sequences of images, each precisely timestamped and having optional GPS data (if GPS is connected). Multiple cameras can be synchronized together with microsecond precision, so each camera sequence of images will have frames marked with the absolutely the same timestamps.

2. imgsrv ( http://wiki.elphel.com/index.php?title=Imgsrv ) - optimized image server available at http://<camera_ip>:8081 - it has a mini-help page when you open this URL without additional parameters. This is the fastest way to grab images over the network that fully utilizes 19MB buffer in the camera, can be used with the client-based scripts or the in-camera PHP code. Among the operation modes it has multi-part JPEG (/mimg suffix in imgsrv request) that is supported by multiple browsers - it is very easy to decode as it is just a sequence of individual JPEG images with complete Exif headers.

3. Event logger (http://wiki.elphel.com/index.php?title=Event_logger) is the FPGA-based system that combines image data timestamping (from multiple cameras), GPS data (including precise absolute timing) and high-speed IMU (we use 2.5K samples/s) that allows absolute timestaping of each image with the accuracy not possible with just internal camera oscillator. Additionally the logger accepts external signals over the dedicated input and/or over the network (with network the timing uncertainty of the logged event will be higher - around 2 ms).

Andrey

---- On Sat, 09 Jan 2016 01:12:24 -0800 Jennifer Valle <jvs1192 at gmail.com> wrote ---- 

Good Morning,

 A month ago I wrote a message about the problems I had with timestamps that obtained with the RTSP protocol live555. It is a project of validation of cameras in space-time and the synchronization is very important. I would be very grateful if you could solve these problems before February, when I present the final degree project.

Kinds regards,

Jennifer Valle

2015-12-01 22:32 GMT+01:00 Oleg <support-list at support.elphel.com>:
Hello Jennifer,

Were you able to solve your problem?

I think there has been a confusion, when I referred to the "original" code live555, I was referring to the latest version without changing code. The problem is that when I connect to the camera with the instruction openrtsp -m, I get files with the timestamp (PresentationTime) in the filename and these values are incorrect.

video-JPEG-1-1448298848.836308 
video-JPEG-1-3534277304.245330
video-JPEG-1-3534277304.281407
video-JPEG-1-3534277304.317484
video-JPEG-1-3534277304.353561
video-JPEG-1-3534277304.389627

The first file has a correct timestamp, but in the following files have incorrect "times" (year 2081).

I was able to reproduce your problem. Installed live555 version (dated 2015-11-09).

The first correct timestamp is the timestamp of the PC. 
Might be something with the RTSP server implementation in the camera. To find out I need to have a closer look at the code: http://elphel.cvs.sourceforge.net/viewvc/elphel/elphel353-8.0/apps/astreamer/
Will take some time - busy with other things.

Best regards,
Oleg Dzhimiev
Electronics Engineer
phone: +1 801 783 5555 x124
Elphel, Inc.

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