strange crackling?

has anybody recognised a vert tiny crackling whic appears in very vey low volume (in both monitors and headphones) and which dissapears when you play the track in a mid or high volume?
lately i am trying to edit a track and it is very clear in mid and high levels but when i am listening to it in very low levels (almost inaudible) i cal listen to a very tiny crackling....

any help here?

 

Crackling, or hiss? You say "very very low". It's sounds like you're running into the noise floor on your DACs. I'm guessing that you're playing at low volume on your DACs, and amplifying massively afterward. If so, then here's what's happening. Your DACs have 16 bits of fixed precision (20 if you're running a high-quality device). But that's at maximum volume. The quieter the signal is when it's converted from digital to analog, the fewer bits you're using on your DAC, since the high-order bits will be zero. Consider the case where you're playing a signal that's 60db down from maximum level. That means you have 10 bits of zeros in each sample, and 6 bits of actual signal. And because you have 6 bits of actual signal, you'll hear quantization errors at about -30db (6db * 6 bits) relative to the audio signal.

On most audio adapters things can be significantly worse. Cheap audio adapaters will pick up noise from the computer system, which can be audible at as much as -40db. Microsoft *requires* -90db signal to noise ratio on "AC97-compliant" audio adapaters, but neglected to specify that this s/n ratio must be acheived while the system is actually doing something like updating the display, or reading from disk. Smart AC97 chip manufacturers, would of course run the tests on motherboards with particularly stable power supplies, and make sure that the test system is not running software other than the test harness (which of course would not draw anything on the screen). And smart motherboard manufacturers are quite happy to save $3 in parts at the price of inferior audio, because their audio chips are designated as AC97 whether the audio chip is integrated properly or not. In the real world, most internal audio adapters leak audible noise from operating devices on the system (mostly graphics cards and disk drives), because of noise on the power rails of the system. External adapters do much better. Consumer devices (like Soundblasters) often do much worse than rated spec, because of this issue.

 

thank you very much for your detailed reply.
i edit-produce in logic and i have a motu ultralite as an audio interface.
iy might be hiss, its a little dirtyness i am hearing in low volumes. i can check it out exactly in what level it appears and when it dissapears... i am pretty dissapointed because i worked a lot in that track to make it sound good and now it is ok but i listen that when i play it in low volume.
what could you suggest in order to avoid that anyway? how can i edit it now in your opinion?

thanx again

 

I'm thinking a Motu adaptor should be ok for system noise.

I don't really have a good understanding of *how* you're reducing the volume.

If it's a DAC s/n problem, then the best way to avoid it to not reduce volume with Logic's master fader; the VU on the master outputs should be solidly in the yellow. If you need to listen to things quietly, attenuate the volume with an analog mixer on the outputs of the Motu (or maybe volume knobs on the motu if it has such a thing). At any rate, *within* logic you want to be as hot as possible.

The same is true of other playback methods, fwiw. Any attenuation of the signal in digial-world is eating up s/n ratio when the digital data is converted to analog at the Motu DACs. If you knock 50db off a digital signal before feeding it to the DACs, you are going to get audible noise no matter how good your audio adpaters are.

In my case, the volume controls of choice are the headphone volume knob on my saphhire adpapter, and the volume controls on my powered speakers. Everything else is mixed to go out at more-or-less full volume. (I do have a little behringer submixer too, which is used to mix in the default AC97 windows outputs and the sapphire pro-audio outputs, so that the kids don't have to mess with cables if they're watching videos, and so that I don't blow my eardrums out with Windows system sounds).

Good grounding helps. I've had problems with laptop grounding before. If you're mixing on a laptop, try running on battery power, to see if that cures the problem. (If it does, then there's a whole other series of issues to deal with).

 

ok, since you mention some things about analog mixers..
my full set up consists of a powerbook g4, a motu ultralite, a mackie ontx 1220 and a bcr 2000 for internal controlling.
so i do reduce the volumes by both using the mixer and the controller, but never the motu. in the interface the volume is set to maximum and i do never touch it...
this thing you mentioned about grounding seems interesting. I will try it out. I edit little bit the track with an exhausting eq procession, but i have got against it again in the past.
Also a very important thing is that i very much use the automation option in logic.
I

 

ok, since you mention some things about analog mixers..
my full set up consists of a powerbook g4, a motu ultralite, a mackie ontx 1220 and a bcr 2000 for internal controlling.
so i do reduce the volumes by both using the mixer and the controller, but never the motu. in the interface the volume is set to maximum and i do never touch it...
this thing you mentioned about grounding seems interesting. I will try it out. I edit little bit the track with an exhausting eq procession, but i have got against it again in the past.
Also a very important thing is that i very much use the automation option in logic.
I would do the same with my speakers, but they are crap and their frequency response starts from 58 hz when i often use frequencies around 40. So no chance of amblifying them. My only help comes from 2 different kind of headphones (hi-fi and studio).
In this case though i recognised the problem by reducing the knob of the headphones in Mackie and by reducing the knob of the headphones volume in Motu ( i connected both pairs ).
Thank you again , i hope this info is more helpfull...

 

Sure. Here's the laptop thing. (I currently have this problem with my Toshiba laptop). When I have the ground pin on the laptop power cord connected, *any* device that's share ground through any connected cable starts spitting out massive amounts of noise. If I run on battery power, no noise. More interesing, if I disconnect ground pin on the power cord (via a ground life), I get no noise. It's major too. Not a little, a lot. And it happens even if the audio channels going to or from the laptop aren't active. Oddly, lifting ground on all the other devices doesn't cure the problem.

Standard caveat: disconnecting the groundpin on a laptop power plug doesn't seem like a very smart idea, if not an outright dangerous idea.

Anyway. I don't know if it's really relevant to you or not. But, in my personal experience, lifting ground on a laptop power adapter *may* improve noise in the rest of the system.