Thursday, 25 July 2013

Lip-sync test...

The most important thing for you, when watching a movie, is to believe! 

The art of cinema is about making you believe that what is displayed is the real life, and that you are a part of it.

To achieve this, the cinema addresses our two main senses, the ones that are permanently stimulated in the real life, the most trustworthy ones: Seeing and Hearing (Sight and Sound).

If what we see and what we hear is converging information, then we believe it’s real, even if the other senses are not stimulated. If it is incoherent, the information is diverging, and intuitively we question its reality.

So, succeeding in providing the Suspension of Disbelief relies on this coherence between the sound and the image. To give you an example, if you hear someone saying “teeth” when you see his/her mouth rounded like to pronounce “o”, you will immediately detect that something is flawed.

From a technical standpoint this translates into at least two criterions that have to be met if you want the audience to believe:

1)    The time criterion

You can only believe that the image of the action on the screen is reality if the sound reaches your ear exactly at the right time. It is obvious that if the sound is too early or too late, it cannot be reality.

This has been so evident to everyone in the cinema industry that the simple tool used to allow the sound engineer to synchronize the sound and the image –the “clapperboard”- has become an acknowledged symbol of the whole cinematic industry.

This clapperboard gives the sound engineer the information about when the sound is synchronized with the image (when the upper part touches the main board). On movies, this technique has been mastered for decades, and the synchronisation is perfectly achieved.

However, when using a digital copy of a movie printed on a blu-ray or any convenient support, a new problem arises…

Digital recording and processing of video, because of a larger bandwidth, takes significantly more time than processing audio. This typically induced a time delay between audio and video, which can be anything from 20 ms to 100 ms, or even more.
Such time delay is easily perceptible, and flaws the cinematic experience.

To solve this problem, you need to delay the sound by using an internal function named “lip-sync” or Time Delay Adjustment in your AV processor.

Note: This should not be mistaken with the HDMI ‘automatic’ lip-sync correction, which modifies the time alignment in a way that is not known by the user and may vary according to the selected inputs.

Now, you have accessed to the lip-sync adjustment function, which is asking you by how many milliseconds you want to delay the sound. Very good question indeed! 

Well, the answer is simple: The sound should reach your ears when you feel it should.

In practice, doing a proper adjustment by watching a movie – not showing a clapperboard – is just a nightmare. Unless of course you are using a synchronisation tool that has been designed for video.

Here is the tool that shows sound and image in sync.

Connect your laptop to the projector and run this movie file to assist you while making adjustments to your AV processor.

Sometimes this VIDEO is out of synch on the BLOG -  Click here to see actual YouTube Posting

2)    The location criterion

There is much to say about this, but in brief, our hearing is (fortunately!) capable to point out the direction of the sound with an accuracy depending on the person.

Obviously, if the sound does not come from where it is supposed to come (according to what is happening on the screen), you will immediately detect the flaw. The larger the screen, the more obvious it will be.

As in most movies the dialog comes from a character that appears located at or near the centre of the screen, achieving this coherence led the cinema installers to place a loudspeaker behind the screen, firing through it.

This has created the need for Acoustically Transparent screens, which have been around since “talkies” have been produced before WW2.

Video as a technology to bring cinema in the home does not change this crucial need of a proper location of at least the centre speaker behind the screen. It cannot be met with TV, as the sound cannot travel through glass. But TV screens being “small” (well, they used to be small. Today we are facing a new problem), this was not a sever issue.

However, when using a large screen and a projector, if you want to achieve the magic of a cinematic experience-the Suspension of Disbelief-, you need an A.T. screen and a centre loudspeaker placed behind it, as near as possible to its middle.

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