The problem of AV sync

[docferdie]

Funny how a poorly encoded music video of britney spears in her DVD audio can get one thinking.
The speed of sound is about 344 m/s at room temperature while the speed of light is about 300,000,000 m/s. This translates to an audio delay of about 7.2 ms per 2.5 meters of distance travelled.
From I theoretical standpoint I fail to see the benefits of audio receivers that have an audio delay feature to achieve resync. Having video that is playing ahead of the audio can not be fixed by the receiver as this would mean that you would essentially have to skip the first few parts of the audio. What we really need is a TV with video delay seeing as how light is much faster. I don't know if movie studios build in a video delay to compensate for this phenomenon into DVDs.
Does anyone have any idea what magnitude of audio delay becomes perceiveable to most people?

 

[av_phile]

I have encountered a few concert DVDs that do exhibit this sync problem. I think it's a real problem that must be corrected either at the DVD production point or from home playback systems. If not mistaken I've come across an article in this forum about future receivers that will have this feature.

 

[Unregistered]

The speed of sound is about 344 m/s at room temperature while the speed of light is about 300,000,000 m/s. This translates to an audio delay of about 7.2 ms per 2.5 meters of distance travelled.
From I theoretical standpoint I fail to see the benefits of audio receivers that have an audio delay feature to achieve resync. Having video that is playing ahead of the audio can not be fixed by the receiver as this would mean that you would essentially have to skip the first few parts of the audio. Does anyone have any idea what magnitude of audio delay becomes perceiveable to most people?

It is the video that is delayed due to all the processing it undergoes, thus the audio appears to be running ahead of the video. The audio delay feature is an attempt to make them match. I believe the minimum perceptible delay is 6ms.

 

[JohnA]

small correction

Speed of Light= 299,792,458 m/s (metres per second)

Speed of Sound= V(sound in air)= 331.4 +0.6Tc m/s or at 70 degrees Celsius (room temperature) it is 1,130 feet per second.

 

[Unregistered]

...and most people just round up to 300,000,000 m/s.

 

[Unregistered]

If you ask me,it should be up to the engineers to get it right before they go on sale.Not the consumer hardware.

 

[JoeE SP9]

I have been piping an audio signal from video through my rig since the early seventies. I have noticed the same sync problem with broadcast tv. It is particularly bad when the station is broadcasting a satellite fed signal.

 

[Mark Duncan]

This is all just a poor job of lip syncing done by Brittney Spears...

Mark

 

[docferdie]

at 70 degrees Celsius (room temperature) .

Man it must be really hot where you live. What are your air conditioning bills like?

 

[Mark Duncan]

Sound's like JohnA is "runnin' with the Devil"...

 

[Unregistered]

Speed of Light= 299,792,458 m/s (metres per second)

Speed of Sound= V(sound in air)= 331.4 +0.6Tc m/s or at 70 degrees Celsius (room temperature) it is 1,130 feet per second.

I think you mean to say room temp is 70 degrees F. not C
This puts room temp down to 21.1 c for a result of 1129 feet per second

 

[Rob Babcock]

It's not always the engineers or producers who get it "wrong." Many people have outboard or even inboard scalers, including lots of PCHT & RPTV owners. The producer has no way of knowing this, or just what video processing the end user may subject the video to. That's the main value of the snych feature.

BTW, it may not have been her producer: Britney may have been off a bit while she lip synched her songs!

 

[dontsleep]

hehe,I see TX.

 

[JohnA]

My bad

Thanks for the correction it is 70 degrees F and not C. Sometimes the brain goes into auto