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How to measure the latency of wireless video transmitting system?

Views:27     Author:Site Editor     Publish Time: 2019-06-10      Origin:Site

Introduction


For wireless microwave video transmission system, latency is a an important parameter. For developers, if the system delay can be reduced by 3ms, the cost may be increase by 10%. Today we will introduce you to a practical latency measure method.

 

First of all, let’s introduce two concepts:


 

  • End to end latency: the whole time required from image captured from camera, wireless transmitted to receiving side and then displayed on monitor. As shown below:

    end to end latency diagram

  • Camera Latency: Time required for the process that camera outputs video to monitor directly by HDMI cable connection. As shown below:

Camera Latency Diagram

Measure the camera latency


  • Prepare all the stuffs that you will need for this step:

 

Items

Qty

Remark

Go pro camera

1Unit


Computer

1Unit

Install OnlyStopWatch software for timing

Wireless Transmitter(FIM-2410)

1Unit

2.4Ghz 10km UAV video transmitter

Wireless Receiver(FIM-2410)

1Unit

2.4Ghz 10km UAV video receiver

HDMI Cable

2Pcs


HD Monitor

1Unit


Power Source

3Pcs



  • Measure steps:

 

  • 1. Open the Only Stop Watch software and start timing

  • 2. Connect Gopro camera to the HD display via a HDMI cable

  • 3. Point the Gopro camera at the timer on computer screen. Simultaneously, you can get the timer on the display.

  • 4. Adjust the position of the computer screen and monitor, so that you can capture two screen images at same time with mobile phone and take multiple photos in succession

 

The difference between two timers in the photo is the delay of GO Pro camera, which is recorded as X. As shown in FIG, go pro camera latency is 69ms. Based on many times calculating, the latency is about 65-120ms.



Secondly let’s measure the end to end latency

 

We need to repeat the above process  however the difference is Gopro camera need to be connected to the transmitter through HDMI, and the receiver will be connected to the monitor. Please refer to the figure below:


Ultra low latency of video transmitter

As follows end to end latency is 277ms (640-363)


Wireless Video Transmitter Latency Testing


At present, most of wireless high-definition digital image transmission system on the market have undergone encoding, modulation, transmission, demodulation, decoding, etc. Delays of different schemes will also be different.

 

The factor that has the greatest impact on latency is the video codec. Let’s take P60 (60 frames) image as an example. Time to encode and decode per frame is 16.7 ms. In order to ensure video smoothness, currently the popular encoding scheme is to first receive 1~3 frames for caching and then extract images from the cache and encode them. The decoding also buffers 1~3 frames and then outputs. So the codec time is the main part of the delay.

 

With the development of technology, some new technologies have emerged. Such as block coding, which can encode images while receiving. This way the cache delay can be saved. In addition, making use of FPGA parallel processing combined with block coding technology for image coding can much greater short the latency.




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