# BFD Guide Question?



## Testpattern (Jun 6, 2007)

Excellent write up. It helped me to get things moving forward and answered most of my initial questions. Much appreciated. 

I did have one question concerning the following statement from the guide:

"NOTE: Something that you should remember to do when you are setting up your BFD is add a foot to the value you enter for sub distance in your pre/pro or receiver set up. The 1 msec DSP processing delay in the BFD would account for approximately a foot in distance."

Shouldn't you "subtract" a foot to make your receiver delay input accomplish the objective? If you sit 15 feet from the action and tell your receiver to delay the acoustic response 15 feet to correspond with sound arriving from the mains. If you add a foot, the delay will now be 16 feet - plus the processing delay of 1ms which essentially adds another foot. This gives you 17 feet of delay when the objective was 15 feet.

On the other hand, if you subtract 1 foot and enter 14 feet into the receiver, you will get 14 feet of delay plus the 1ms processing time which brings the total to 15 feet - the initial and desired objective.

I'm raising this question as a point of curiosity since most of us couldn't tell the difference one way or the other most likely.

Does this make sense or have I missed something... which is usually the case?


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## brucek (Apr 11, 2006)

> Shouldn't you "subtract" a foot to make your receiver delay input accomplish the objective?


When you add a foot to the actual distance, the receiver will advance the signal 1 msec sooner to make up for the loss of 1 msec in the BFD....

brucek


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## Testpattern (Jun 6, 2007)

That would explain it. I thought the consumer electronics worked with delays only. I didn't realize they had the capability to advance the sound of a given channel.

Thanks,


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## JohnM (Apr 11, 2006)

Well they don't advance it as such, they delay all the other channels. The only channel which has no delay is the one that has the highest distance, everything else is delayed to match up with that according to their relative distance settings. If all the channels are set to the same distance there are no delays for any of them.


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## Guest (Jan 24, 2008)

What I've always been curious about is, how does this effect they synching to lips while watching a movie?

If the receiver is delaying all the audio to line up with the speaker at the greatest distance... all the sound is now being delayed that much from when the picture hits the screen.

I guess when comparing the speed of light to the speed of sound, the sound is already running behind the picture and a few more msec doesn't make that big of a difference.


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## brucek (Apr 11, 2006)

> and a few more msec doesn't make that big of a difference


That's right. The distances in an HT room are too short to worry about lip sync....

brucek


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