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Why Would The CD Be Louder Than A DVD?

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When I put in a CD, it's SO MUCH LOUDER than a DVD- Why does this happen? Am I doing something wrong?

Explaining this actually means a basic trip into the technical world of digital audio. Are you ready? Here we go...


Digital audio has a hard maximum ceiling for audio. This level is known as "0". All sound in the digital realm is then measured in a scale using ZERO as maximum and working downward (-10, -20, -30 etc). Sound cannot be written to a digital audio format that exceeds this 0 maximum. So think of 0 as the absolute speed limit in the digital audio world.


Now- movie soundtracks are designed to be dynamic. They want to give you all those big loud booms and hushed whispers....

The idea of "dynamic" is simply that there is a big difference between the loudest sound and the quietest sound. But, like we said before- no matter what we're encoding- digital has a very hard limit of the maximum level something can be... So, in order to have room for dynamics- you can't make the loud louder- so what do you do? Well, you have to make everything else softer!

So- movies are created to have their average overall sound level be low. If the average level is kinda low then this gives them plenty of room to get loud before they hit that digital maximum level (they call this headroom-- its the amount of room you have to go up before you hit your head on the ceiling- cute huh?).

Movies are designed to have their average dialog level be about 25 or 30 steps below the maximum level available. That way, when they want to have some big dynamic special effect- they have room to go louder... So between the dialog (average level) and the loudest sound (max level) they have 30 steps of dynamics to use.


Video games and CDs are often designed in a different way. Instead of being dynamic- they are squashed. Almost the entire signal is squashed down into a tight package- and the whole thing is just "loud" all the time. The average CD uses just the top 3-6 steps of available volume all the time. In other words- where movies have the ability of having 30 steps of dynamics to use-- CDs only use about 1/10th that much! They push the entire signal all the way up to those top 3 levels below the max- and thus it seems significantly louder than a DVD.

If you could look at the waveform for a digital movie soundtrack, it would look something like this:

Notice the average level is much lower- but has an occasional dynamic peak that hits the max level.

If you could look at the waveform for a digital music soundtrack, it would look something like this:


 

The same audio "squashing" process is used for TV broadcasts, Radio Broadcasts and VHS (although for slightly different reasons)-- you'll find they will also be "louder" on your system. The case isn't really that one is louder than the others (like I said above, they all have relatively the same maximum level)-- it just appears "louder" because all the signal is squished as close to the top as possible... while others set the average level lower to have room for dynamics.

In closing, there is nothing wrong with your receiver or equipment. The difference in volume is normal and exactly how it should be given the nature of the different audio types.

Article written by Vince Maskeeper of www.musicianassist.com . Thanks also to www.HomeTheaterForum.com.

Copyright 2002 Home Theater Forum. All rights reserved. Reprinted by permission.

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