Dolby Digital Question

[SopRage]

Hello!

I'm brand new to the forums (and the audio world) and had a question about Dolby Digital, particularly in its application in games.

When DVDs are encoded in Dolby Digital, as I understand it, the information is encoded such that when the DVD is played back at reference level in a properly calibrated system, each channel can only put out 105 decibels of total sound.

My question is whether this is how it works for Xbox 360 games as well. I'm curious, because I'm planning on purchasing an amplifier soon and want to know how much "headroom" I need.

Is it possible for a game to output more than 105 decibels per channel? Are the games louder than movies? Do I need to worry about additional headroom for games? Do the games, like movies, have the same 105 decibels-per-channel limit?

Any ideas or information would be great!

- Jared

 

[OttoMatic]

Hi there Jared,

Well, the max SPL output (in decibels) of the system will be dependent on a number of things, including how powerful your amp is (watts) and the sensitivity of your speakers (written like "89dB/1W/1m" meaning 89dB with 1W of power from the amp and measured at 1 meter from the speaker.) Program material will also play into it (if you had something that were recorded at a very low level, you could never get all that loud even if it were turned up all the way).

I don't know anything about Xbox, but I suspect it puts out some type of DD signal. My friend's kids brought some game system like that over and it was rocking! Anyway, it was sending a DD signal when it was here. I don't know what its maximum dynamic range is, or how it relates to a calibrated value of 75 dB. But I'm not sure that really matters. Also, if you want it to be loud, you can continue to raise the volume beyond the calibrated range. From there, maximum SPL (decibels) will depend on your amp and your speakers.

I would think that any one channel could get over 105 dB. But it wouldn't really be likely for all channels to be running at that level at the same time. If so, and if for very long, I would think that a 5.1 system running at that max (or near that max) would be excruciatingly loud.

So, what is it that your are looking for exactly? You want to have a rocking gaming system?

What do you have now, how big is your space and what is your budget?

 

[MDS]

When DVDs are encoded in Dolby Digital, as I understand it, the information is encoded such that when the DVD is played back at reference level in a properly calibrated system, each channel can only put out 105 decibels of total sound.

Not to be flippant, but a DVD isn't encoded at any absolute SPL. Take the DVD out of the case and hold it up to your ear - it will be silent.

As Otto already mentioned, the absolute SPL your system will achieve is dependent on a lot of factors and you can easily exceed the 105 dB MAX that is the so-called Dolby Reference Level. To be compliant with reference level you must calibrate the system so that PEAKS achieve 105 dB (and you won't be able to stand that level for long, at least my ears can't).

Briefly...a DVD encoded as Dolby Digital is digital...a sequence of numbers. Those 'samples' range from all zeros (negative infinity dB, aka silence) to the max bit depth (all ones, aka '0 dB'). The average level of dialog is typically around -30 dB which, not coincidentally, is the average level of the receiver's test tones.

Remember, 0 dB is the MAX. When you calibrate so that an input level of -30 dB yields 75 dB of output SPL, you have reference level because when that signal peaks (all ones or 0 dB), you have 75+30=105 dB. The signal does not peak at all times (obviously) because if it did the waveform would be a straight line and be pure excruciating noise.

So...just for kicks say you go through the calibration process and find that you can achieve the 75 dB level with all of the channel trims set to 5 (just an example). The range is typically -12 dB to +12 dB. If you were to bump up each of those channel trims you would be 'out of spec' because your peaks would exceed 105 dB when the sample value peaks.

 

[SopRage]

Hey guys,

I think I need to clarify.

What I'm really trying to find out is the upper-most power requirements for my system.

For what it's worth, my setup is:

- Yamaha RX-V663
- Ascend Acoustics CMT-340 SE mains
- Ascend Acoustics HTM-200 SE center and surrounds
- HSU VTF-2 MK3 Subwoofer

It's all been calibrated via DVE and a Radio Shack SPL meter, and I'm currently sitting about 10 feet from my speakers.

I plan on having kids in the near future, and am looking to set the max volume on my receiver to a value that will prevent them from accidently frying the speakers into oblivion.

With 95 watts per channel, I figure I should be able to sit comfortably at -10 dB on the receiver with watts to spare (for movies), but I want to make sure that my Xbox 360 won't be, at that volume, requesting more power than my receiver can muster.

I hope that makes some sense?

 

[OttoMatic]

OK, so you've already got the rocking gaming system!

When you say you're planning on having kids in the near future does that mean that someone's going to visit you for a night, and they're bringing some kids with them, or does that mean that your spouse/significant other (or perhaps you yourself, if you're a lady) is actually going to bring a new child into this world? If the latter, then you probably won't actually have to worry about this for a while. Actually, when my daughter was at the age of crawling and exploring, I put an expandable gate around my system, speakers, and TV -- from one side of the room to the other. As she approaches 3, she does not know how to use the remote and just brings it to me when she wants something.

Either way on the kids, you want to know where to set your max level on your volume knob. I don't think you can count on any particular recording or game being at such and such a level, to the point where we can just do some math and figure out a good maximum. I think the easiest way to do this is to just play the games and figure out what you think is good max. Perhaps even set it a little low and see if you find yourself wanting to turn it up. That's how I would do it.

 

[Phil Taylor]

If you plan on having kids in the future you may want to keep your SPL under 125db to avoid sterility.