Is there a delay in digital recordings?

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Offline thedoc

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Is there a delay in digital recordings?
« on: 12/06/2014 19:30:01 »
Muhammad Aqeel  asked the Naked Scientists:
I would like to know if  in digital pictures (i.e televisions, CCTV, cameras etc), is the picture 1st recorded then shown on screen, or is it just a delay in transmission or no delay at all (it is shown together as its happening, in other words like a reflection in water or mirror).                          

What do you think?
« Last Edit: 12/06/2014 19:30:01 by _system »


Offline alancalverd

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Re: Is there a delay in digital recordings?
« Reply #1 on: 12/06/2014 22:50:12 »
There is an essential delay in transmission as the signal has to be encoded, and another delay in reception as it is decoded. This has rendered the BBC "time pips" useless as a time standard on their digital service, and the variable delay between sound and vision processing requires the use of an "audio sync" control on most TV digital sound bars.

Interestingly, the sound bar is only necessary because the modern flat screen TV is too thin to accommodate a decent loudspeaker system, so all the technological wizardry that has gone into producing HD images, has destroyed the inherent and rigid lip sync that came with the first talking movies!   
helping to stem the tide of ignorance


Offline evan_au

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Re: Is there a delay in digital recordings?
« Reply #2 on: 14/06/2014 09:38:46 »
There is often an intentional delay inserted in "live" broadcats, so the producers can bleep out any accidental or intentional content which is against the broadcast regulations - a delay of around 7 seconds is often used.

There are delays inherent in digital encoding:
  • For sound, there is a delay of 3ms for every meter between the sound source and the microphone. Around 5-20ms of speech from the microphone is recorded at a high rate like 8000-40000 samples per second. This is then compressed, which takes another 5-20ms.
  • For video, 16-20ms of video is sampled, and then processed by comparing the latest image to previous images, a process that takes another 20ms
  • Both sound and video must be transmitted over a network, which typically introduces a delay of 5ms per 1000km. If it goes via satellite, this introduces an extra 70,000km.
  • Then it must be transmitted to your home, which could take up to another 20ms.
  • If it is transmitted over the internet with variable delays, the receiver must implement a "jitter buffer", which can introduce delays of 100ms-5 seconds.
  • Then the TV must display it, which could take 20ms-1s. The sound experiences an extra delay of 3ms for every meter between the TV loudspeakers and your ears.
  • It takes your brain about 100ms to process the sound and 300ms to process the images.
« Last Edit: 14/06/2014 09:47:20 by evan_au »