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I've been kicking around the idea of low-cost, low-latency HD video streaming over WIFI or LAN for the past few weeks, and would love some input on what strategies can be used to minimize latency in digital video transmission.

The idea is to design a low power, fairly low cost video transmitter and receiver hardware set to broadcast video over a WIFI bridge. This has been implemented by others with a few Raspberry Pis and GStreamer, with latency as low as 250ms.

My question is, what is the absolute minimum latency achievable over such a connection? Commercial units such as the Connex HD downlink achieve negligible latency, but clearly use sophisticated hardware to do so.

If I stick to H.264 compression and use a hardware encoder and decoder chip, with minimal buffering, how low can latency be? I'm not afraid to get a few custom PCBs made, or write the required code, but want to get some idea of the theoretical minimum latency for H.264 encoding/decoding is before I start.

adamweld
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    Unfortunitly I think you have posted this in the wrong site; It might get better attention at [superuser](http://superuser.com/) – hoss Jul 29 '15 at 05:16
  • I don't think you have to use a hardware encoder. In fact, maybe you can even tune a software H.264 better for low latency encoding (ffmpeg e.g. has a built-in option for low latency). This answer discusses a few things about low latency streaming, mentions the existence of <100ms systems, and even recommends using H.264 for low latency streaming. http://stackoverflow.com/questions/12020430/look-for-fastest-video-encoder-with-least-lag-to-stream-webcam-streaming-to-ipad/12085571#12085571 – Bastian35022 Jul 30 '15 at 11:37

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Personally I'd just say buy something. There are a ton of products that can do this quite well and a great cost and if you purchase or ask for a demo unit and do some testing you can see how well they perform and base your opinion on the results.

Recently we used an off the shelf pair of products (encoder and decoder) that were connected via a 5GHz point to point 802.11 100Gb connection. When transmitting 1920x1080p30 had about a 10 frame delay 10 frame delay

I'm say the majority of the latency (maybe 10 - 15ms per frame) comes from the compression. If you could manage to find a fat enough network pipe to transmit uncompressed video then you could get the latency down to a frame or so. However a Raspberry Pi is not the hardware that will give you the best performance.

hoss
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  • Thanks for your response. I'm hoping to use this setup for a mobile streaming video system for FPV flying of model aircraft, so it needs to be small and have low power consumption. I wonder if there is a "good enough" compression algorithm built specifically for super fast compression; maybe just high compression single JPGs so there is less lag from frame comparisons. – adamweld Jul 30 '15 at 06:35
  • Well anything that is temporally compressed (Photo-JPEG, ProRes, etc...) will (de)compress faster than spatial compression like H264. The only issue with temporal compression is the bit rate tends to be higher since each frame is compressed on it's own. You should mention in your question your application and restrictions (power, weight) so you others can answer. Additionally, you should probably post on Super user since this is a programming form. – hoss Jul 30 '15 at 11:55