Monday, 7 May 2012

Post Audio Sync and Playback.

Here's a thing - working with music, I often have to synchronise the visuals from a  "Live" performance, in a recording studio or concert, with the final master audio from, say, a final production master or CD. Then a client wants to view the edit, and doesn't think the audio is spot on - why? 

Computer screens have two kinds of delay, one is called the panel response time, given in milliseconds, it tells you how long it takes the panels pixels to transition from black to white and black again - typically many panels would take 25 ms to transition - this is a long time, creating smeared movement, especially on panning or horizontal moving objects. So, the faster the response time the better - now we see many panels around 5ms which is great.

The other delay in picture output is created by the processing delay - how long it takes for the panel and its associated electronics to process the incoming signal and present it to the viewer. Typical values are 25 - 50 ms - now this is a lot and very noticeable, the other aspect to this is that almost no manufacturers  add this information to their spec. sheet. 

High end audio amplification equipment often has an audio delay which is user adjustable to sort this out, but of course laptops and the like do not, so you need to employ a video player which 1) allows you to adjust the audio delay and 2) A test signal to set this up. Media Player Classic is a great free player for two reasons, not only can you easily set the required delay, but it also has V sync settings and a "tear" testing set up to rid you of those awful horizontal tear lines on computer 60Hz screens when viewing 50Hz frame rate material. See the images below that show settings and there's a link to a test file which consists of white flashes and audio beeps to help align the sound and picture on your system.

There is a small .wmv sync test file here: Sync Test File.  If you opt for an external audio delay unit, I'd highly recommend this one as it is optical audio fitted with toslink connectors - it is also reasonably cheap at £79  Audio Delay unit

What HiFi say: "Few things in AV are as annoying as lip-sync errors. It makes all movies seem badly dubbed. And if your amp hasn't got the facility to fix it, you're stuck. Unless you get this JS Tech digital audio delay unit. It's a simple box with just three connections: an optical in / out plus a power supply input. The unit is easy to use: the two buttons on the front adjust the delay up to 340ms."

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