July 11, 2017

Current video stream latency and a way to reduce it

by Oleg Dzhimiev

Fig.1 Live stream latency testing

Recently we had an inquiry whether our cameras are capable of streaming low latency video. The short answer is yes, the camera’s average output latency for 1080p at 30 fps is ~16 ms. It is possible to reduce it to almost 0.5 ms with a few changes to the driver.

However the total latency of the system, from capturing to displaying, includes delays caused by network, pc, software and display.

In the results of the experiment (similar to this one) these delays contribute the most (around 40-50 ms) to the stream latency – at least, for the given equipment.


 

Goal

Measure the total latency of a live stream over network from 10393 camera.
 

Setup

  • Camera: NC393-F-CS
    • Resolution@fps: 1080p@30fps,  720p@60fps
    • Compression quality: 90%
    • Exposure time: 1.7 ms
    • Stream formats: mjpeg, rtsp
    • Sensor: MT9P001, 5MPx, 1/2.5″
    • Lens: Computar f=5mm, f/1.4, 1/2″
  • PC: Shuttle box, i7, 16GB RAM, GeForce GTX 560 Ti
  • Display: ASUS VS24A, 60Hz (=16.7ms), 5ms gtg
  • OS: Kubuntu 16.04
  • Network connection: 1Gbps, direct camera-PC via cable
  • Applications:
    • gstreamer
    • chrome, firefox
    • mplayer
    • vlc
  • Stopwatch: basic javascript

 

Notes


Table 1: Transfer times and data rate

Resolution/fps Image size1, KB Transfer time2, ms Data rate3, Mbps
720p/60 250 2 120
1080p/30 500 4 120

1 – average compressed (90%) image size
2 – time it takes to transfer a single image over network. Jitter is unknown. t = Image_size*1Gbps
3 – required bandwidth: rate = fps*Image_size

Camera output latency calculation

All numbers are for the given lens, sensor and camera setup and parameters. Briefly.

Sensor
Because of ERS each row’s latency is different. See tables 2 and 3.
 
Table 2: tROW and tTR

Resolution tROW1, us tTR2, us
720p 22.75 13.33
1080p 29.42 20
full res (2592×1936) 36.38 27

1 – row time, see datasheet. tROW = f(Width)
2 – time it takes to transfer a row over sensor cable, clock = 96MHz. tTR = Width/96MHz
 
Table 3: Average latency and the whole range.

Resolution tERS avg1, ms tERS whole range2, ms
720p 8 0.01-16
1080p 16 0.02-32

1 – average latency
2 – min – last row latency, max – 1st row latency

Exposure

tEXP < 1 ms – typical exposure time for outdoors. A display is bright enough to set 1.7 ms with the gains maxed.

Compressor

The compressor is implemented in fpga and works 3x times faster but needs a stripe of 20 rows in memory. Thus, the compressor will finish ~20/3*tROW after the whole image is read out.

tCMP = 20/3*tROW

Summary

tCAM = tERS + tEXP + tCMP

Since the image is read and compressed by fpga logic of the Zynq and this pipeline has been simulated we can be sure in these numbers.
 
Table 4: Average output latency + exposure

Resolution tCAM, ms
720p 9.9
1080p 17.9

Stopwatch accuracy

Not accurate. For simplicity, we will rely on the camera’s internal clock that time stamps every image, and take the javascript timer readings as unique labels, thus not caring what time they are showing.
 

Results

Fig.2 1080p 30fps

Fig.3 720p 60fps

 
GStreamer has shown the best results among the tested programs.
Since the camera fps is discrete the result is a multiple of 1/fps (see this article):

  • 30 fps => 33.3 ms
  • 60 fps => 16.7 ms

 

Resolution/fps Total Latency, ms Network+PC+SW latency, ms
720p@60fps 33.3-50 23.4-40.1
1080p@30fps 33.3-66.7 15.4-48.8

 

Possible improvements

Camera

Currently, the driver waits for the interrupt from the compressor that indicates the image is fully compressed and ready for transfer. Meanwhile one does not have to wait for the whole image but start the transfer when the minimum of the compressed is data ready.

There are 3 more interrupts related to the image pipeline events. One of them is “compression started” – switching to it can reduce the output latency to (10+20/3)*tROW or 0.4 ms for 720p and 0.5 ms for 1080p.

Other hardware and software

In addition to the most obvious improvements:

  • For wifi: use 5GHz over 2.4GHz – smaller jitter, non-overlapping channels
  • Lower latency software: for mjpeg use gstreamer or vlc (takes an extra effort to setup) over chrome or firefox because they do extra buffering

 

Links

 

Updates

 
Table 6: Camera ports

mjpeg rtsp
port 0 2323 554
port 1 2324 556
port 2 2325 558
port 3 2326 560

GStreamer pipelines

  • For mjpeg:

~$ gst-launch-1.0 souphttpsrc is-live=true location=http://192.168.0.9:2323/mimg ! jpegdec ! xvimagesink

  • For rtsp:

~$ gst-launch-1.0 rtspsrc is-live=true location=rtsp://192.168.0.9:554 ! rtpjpegdepay ! jpegdec ! xvimagesink

VLC

~$ vlc rtsp://192.168.0.9:554

Chrome/Firefox

Open http://192.168.0.9:2323/mimg


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