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Audio time delay - How exactly does it work? Limits?

post #1 of 6
12/19/09 at 7:49am
Thread Starter 
Just wondering how audio time delay works and if there are any limits to how far you can have say a subwoofer from the rest of the system.

Specifically if one puts a sub at say 30 feet and other 7 speakers at say 10 feet from the sweet spot how does the time delay technically work? Is their a limit to the number of feet that the sub distance can be increased to?

Currently my sound is sent to my Lexicon MC12B preamp via 5.1 analog inputs and my video goes direct to my projector (I am using a Sony 550 Blu Ray player).

Is the sound recorded much "earlier", on a Blu Ray disk, than the video to facilitate audio time delay function as some sort of standard? If yes what is the difference in time of sound being recorded vs video?
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post #2 of 6
12/19/09 at 7:56am
There are several methods.

Analog is a series of all-pass filters that each provide a phase delay. So, not exactly a time delay, but a phase delay you set to just happen to be the correct time for the crossover region. The phase delay on many subs is just a single stage variable all pass. For a good description, look on Linkwitz's site.

Second is the now obsolete and unavailable "bucket bragrade" chips. Not sure how they worked, but sounded like garbage anyway.

Lastly is in the digital domain. It can be done either by shift registers or by DSP methods. it can be very accurate in time delay, synthesize phase delay, or whatever. (my DCX works this way). Of course, you have the A to D, and D to A.
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post #3 of 6
12/20/09 at 7:53am
Quote:
Originally Posted by Frank D View Post

Just wondering how audio time delay works and if there are any limits to how far you can have say a subwoofer from the rest of the system.

Specifically if one puts a sub at say 30 feet and other 7 speakers at say 10 feet from the sweet spot how does the time delay technically work? Is their a limit to the number of feet that the sub distance can be increased to?

Currently my sound is sent to my Lexicon MC12B preamp via 5.1 analog inputs and my video goes direct to my projector (I am using a Sony 550 Blu Ray player).

Is the sound recorded much "earlier", on a Blu Ray disk, than the video to facilitate audio time delay function as some sort of standard? If yes what is the difference in time of sound being recorded vs video?

All the channels on a BluRay are recorded in sync with each other. On playback, it is the job of the processor in pre/pro to "sync" all the channels acoustically in the room. The "distance" control is used to do this. In an auto setup system, the acoustic distance is measured and set appropriately. This takes into account the actual physical distance PLUS any latency in the system caused by processing in the pre/pro or in the subwoofer, (through crossovers or EQ's.) Often, due to latency, the acoustic distance is longer than the physical distance. The processor will set the distance further away, whch causes the subwoofer to fire "first" and delays the speakers to sync them back with the sub.

Your pre/pro doesn't look to have an auto setup. Therefore, you should start with inputting the actual physical distance. Then experiment with slightly longer distances and see if you can improve the response.

Why are you considering setting the sub so far away though? 30 ft. away from the sweet spot will not only be a problem for the distance setting, but the level will need to be turned up considerably to calibrate the sub to the speakers, compromising headroom. You would be much better off with a closer placement, especially somewhere "within" the speakers.

Craig
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post #4 of 6
12/20/09 at 10:00am
Thread Starter 
Thanks for the response guys,

Craig,

Just for the record my sub is not 30 feet physically away from the rest of my system. I have an AS EQ1 sub EQ and its processing adds about 8.5 feet to the distance. My sub is actually only 11 feet away for a total distance of 19.5 feet.

You say that the all the channels on the Blu Ray disc are in sync with each other. I guess the video is also in sync with the audio too? Or is the audio actully recorded a bit earlier to allow for audio delay later?

I am also interested in the theory behind audio delay and how it interacts with video. I guess video normally takes much longer to get to the screen than audio (video has much higher bandwidth etc.) so one normally will delay the centre channel to sync up to the video and then adjust the rest of the channels to compensate. This is kind of automatically done by measuring the distance in say feet from each speaker to the sweet spot. Now for a subwoofer to be say 30 feet back it must need to get the sound about 30 ms early (1foot is equal to bout 1.1 ms I believe). How is the sub able to get this sound that early? Makes me think that all preamps get the sound early and just hold onto it (according to some standard?) so that the video syncs up. Then one has available room to adjust audio delay even more if need be. If yes then how early, on average, is the preamp getting the sound?
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post #5 of 6
12/20/09 at 11:19am
Quote:
Originally Posted by tvrgeek View Post

Second is the now obsolete and unavailable "bucket bragrade" chips. Not sure how they worked, but sounded like garbage anyway.

A "train" made up of of capacitors. The clock moved the analog charge onto the next capacitor in line. Of course not all the charge was moved and the signal degraded from one stop to the next hence the rather high distortion.

The same principle as used in CCD camera chips.
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post #6 of 6
12/21/09 at 3:54am
Quote:
Originally Posted by Frank D View Post

...You say that the all the channels on the Blu Ray disc are in sync with each other. I guess the video is also in sync with the audio too? Or is the audio actully recorded a bit earlier to allow for audio delay later?...

Yes because there can be no universal delay. Someone using nearfield's or headphone's would require no delay. Since video and audio go through different processors it is actually a challenge keeping them in sync.
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