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16 bit, 24 bit and Noise Floor
googlebot
post Mar 2 2012, 20:59
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A noise floor can be considered part of the signal in many situations - and be worth capturing as is.

To add another bad analogy: I always prefer MPEG2 BluRays, because they preserve film grain and noise much more conservatively than certain H.264 encoders, which add some form of softening even at very high bitrates. One could claim that this is a feature, because the latter encoder prefers allocating bandwidth to even more detailed signal than noise. But sometimes I want the noise.
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knutinh
post Mar 2 2012, 21:09
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QUOTE (googlebot @ Mar 2 2012, 21:59) *
A noise floor can be considered part of the signal in many situations - and be worth capturing as is.

To add another bad analogy: I always prefer MPEG2 BluRays, because they preserve film grain and noise much more conservatively than certain H.264 encoders, which add some form of softening even at very high bitrates. One could claim that this is a feature, because the latter encoder prefers allocating bandwidth to even more detailed signal than noise. But sometimes I want the noise.

I know of this preference within the cinema crowd.

I have never heard of such a thing within audio. I understand that many object to the artifacts associated with noise reduction, but preserving the crackles of vinyl or air-conditions of some recording venue does not seem to appeal to most music-lovers.

-h
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googlebot
post Mar 2 2012, 21:18
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As discussed earlier in the thread it is not trivial to decide, what kind of noise can be safely drowned by taking away bits. It's easy to decide for certain well defined types of noise, but you seldom find them as exactly that in the analog domain. There is brownish, pinkish, whitish, but not brown, pink, white. Especially when further processing is involved, you might want to preserve the original floor as good as possible.
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2Bdecided
post Mar 3 2012, 19:56
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QUOTE (Arnold B. Krueger @ Mar 2 2012, 13:02) *
This is another one of those situations where your mileage varies all over the map depending on the details.
There's no magic though. Just look at the spectrum. That tells you what you need to know. We stop hearing the signal where the noise power falling into a critical band is about the same as the signal power. You can get stranger results if you modulate the noise.

Cheers,
David.
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Garf
post Mar 4 2012, 10:12
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QUOTE (knutinh @ Mar 2 2012, 21:09) *
QUOTE (googlebot @ Mar 2 2012, 21:59) *
A noise floor can be considered part of the signal in many situations - and be worth capturing as is.

To add another bad analogy: I always prefer MPEG2 BluRays, because they preserve film grain and noise much more conservatively than certain H.264 encoders, which add some form of softening even at very high bitrates. One could claim that this is a feature, because the latter encoder prefers allocating bandwidth to even more detailed signal than noise. But sometimes I want the noise.

I know of this preference within the cinema crowd.

I have never heard of such a thing within audio.


A noisy signal will have less distortion introduced (noise modulation iirc?) when its quantized heavily as compared to a less noisy signal. That's why using dither is recommended.
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spindle
post Mar 4 2012, 12:12
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QUOTE (mjb2006 @ Feb 28 2012, 20:22) *
Most explanations—even those for technical audiences—of the effect of sample rate and bit depth in sampling generally boil down to the idea of "capturing more detail". I think this is a big part of the problem; everything that people learn about digital audio gets integrated with this basic tenet. You can talk until you're blue in the face about noise, Nyquist, and everything else, but you're still going to run up against this perception: there is a squiggly line which is being segmented along the X and Y axes with a certain degree of precision in order to represent it as a series of numbers, and if you crank up the knobs that control the precision along one axis or the other, you're going to get a better representation of the original squiggly line, so why not do that. When someone's thinking that way, what can you do? You can tell them that if all the music is between 0 and 5, making a knob that goes to 11 instead of 10 is not going to give you "more detail". But they just aren't convinced. So I'm not sure what you can really do. There's still a need for a sampling primer for audiophile myth-busting purposes.


Forgive me but, through ignorance, I fall pretty well into that category and I certainly can't follow the technicalities of this thread. Could you possibly point me at such a primer (for numpties) that will help me understand why 'finer granularity' is not necessarily better and is not the complete story?

TIA
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Notat
post Mar 5 2012, 05:45
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I wish I knew of such a reference. I'll take a quick crack at it. If you trust the mathematics, it mostly has to do with the limits of human hearing. I appreciate that trusting the mathematics is easier said than done. Sampling and dither theory are not exactly intuitive.

Improving resolution in the time dimension (higher sample rates) has the effect of allowing digital audio to represent higher frequencies. It is pretty well established that we can't hear above 20 kHz so increasing resolution in the time dimension beyond 48 kHz sample rate is only of use to dogs.

Improving resolution in the amplitude direction (higher bit depth) has the effect of reducing the amount of noise introduced by the process of converting from analog to digital or vise-versa. In most cases, the microphones and other electronics are generating their own noise and so bit resolution is not the weakest link in the recording or playback chain. A 24-bit recording has a 144 dB range. Our range of hearing intensity from quietest detectable sound to pain is 130 dB.
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saratoga
post Mar 5 2012, 07:53
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QUOTE (spindle @ Mar 4 2012, 06:12) *
Forgive me but, through ignorance, I fall pretty well into that category and I certainly can't follow the technicalities of this thread. Could you possibly point me at such a primer (for numpties) that will help me understand why 'finer granularity' is not necessarily better and is not the complete story?


For every bit you add, theres 2x the levels to chose from, and thus on average 1/2 the error for each sample. Intuitive right? Put your levels 2x as close together and you'll be wrong by half as much on average. Simple.

Well if you know the noise on your recording is at least 60dB below peak at every frequency, all you have to do is make sure that the average error is a little bit below 60dB. Then the error will be smaller then the noise. At that point making the error even smaller doesn't help very much. So you can keep adding more bits, but all you're doing is making something too weak to be heard weaker.






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Arnold B. Kruege...
post Mar 5 2012, 13:27
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QUOTE (spindle @ Mar 4 2012, 06:12) *
Forgive me but, through ignorance, I fall pretty well into that category and I certainly can't follow the technicalities of this thread. Could you possibly point me at such a primer (for numpties) that will help me understand why 'finer granularity' is not necessarily better and is not the complete story?


This might help:

Good Resolutions (HFN)
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bandpass
post Mar 5 2012, 15:16
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Here's a wood-working analogy smile.gif

Suppose that you have a staircase with wooden banister spindles; these things:

Suppose that one is broken and that you are going to make a copy of an intact one as a replacement.

For measuring the original, you have at your disposal: plastic vernier callipers with 0.01" resolution, and a steel micrometer with 0.00005" resolution (a factor of 200 'better').

Using the micrometer does not result in a replacement spindle that anyone could distinguish from one made using the callipers. Though it has ~8 bits more resolution, (for this application) the extra bits are in the noise. The same is true of 24-bit audio.
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icstm
post Mar 5 2012, 16:51
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QUOTE (Garf @ Mar 2 2012, 11:07) *
As far as I know, you can't hear tones more than about 6dB below the noise floor.
so what is the noise floor then, if i can hear below it? I am confused unsure.gif


QUOTE (googlebot @ Mar 2 2012, 19:59) *
A noise floor can be considered part of the signal in many situations - and be worth capturing as is.


I do not understand

QUOTE (knutinh @ Mar 2 2012, 20:09) *
QUOTE (googlebot @ Mar 2 2012, 21:59) *
A noise floor can be considered part of the signal in many situations - and be worth capturing as is.

To add another bad analogy: I always prefer MPEG2 BluRays, because they preserve film grain and noise much more conservatively than certain H.264 encoders, which add some form of softening even at very high bitrates. One could claim that this is a feature, because the latter encoder prefers allocating bandwidth to even more detailed signal than noise. But sometimes I want the noise.

I know of this preference within the cinema crowd.

I have never heard of such a thing within audio. I understand that many object to the artifacts associated with noise reduction, but preserving the crackles of vinyl or air-conditions of some recording venue does not seem to appeal to most music-lovers.

-h


Isn't that because the orginial format in Cinema includes the grainy picture. In audio, the orginal is a live or studio recording with no noise. In cinema, the orginal was NOT the live performance on stage.

QUOTE (Notat @ Mar 5 2012, 04:45) *
A 24-bit recording has a 144 dB range. Our range of hearing intensity from quietest detectable sound to pain is 130 dB.
So 24bit recording COULD add something to the the listening experience?
(though I assume that this is theoretical, as you and I and others have said the rest of your equipment is unlikely to provide such a range or (without wanting to confuse matters) is unlikely to do so accurately)
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Arnold B. Kruege...
post Mar 5 2012, 17:34
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QUOTE (icstm @ Mar 5 2012, 10:51) *
QUOTE (Garf @ Mar 2 2012, 11:07) *
As far as I know, you can't hear tones more than about 6dB below the noise floor.
so what is the noise floor then, if i can hear below it? I am confused unsure.gif


The noise floor that we speak about technically is something that we measure. For example a piece of audio gear might have a noise floor of 1 millivolt or 1/1000 of a volt. The loudest signal that can be handled at the same time might have an amplitude of 10 volts.


The interesting thing is that we can hear pure tones at midrange frequencies whose amplitude is 1/10 of a millivolt.


QUOTE
QUOTE ( @ Mar 2 2012, 19:59) *
A noise floor can be considered part of the signal in many situations - and be worth capturing as is.


I do not understand


Imagine a microphone sits in a room that has both music being played and droning on in the background is noise from the HVAC (heating) system. The following signals are coming out of the microphone:

(1) The music.
(2) The noise from the HVAC
(3) The noise from the microphone's electronics

In that order of sizes. The signal from the microphone appears to be music plus a noise floor, but that noise floor has contributions from the HVAC and the microphone itself. The noise from the HVAC is a noise floor that is superimposed on the noise floor from the microphone.



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Arnold B. Kruege...
post Mar 5 2012, 17:45
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QUOTE (icstm @ Mar 5 2012, 10:51) *
QUOTE (Notat @ Mar 5 2012, 04:45) *
A 24-bit recording has a 144 dB range. Our range of hearing intensity from quietest detectable sound to pain is 130 dB.


So 24bit recording COULD add something to the the listening experience?


Let's put it this way. The noise floor of your living room is maybe 40 dB SPL. In order to have a sound that is 144 dB above the noise floor in your living room (1OW 184 dB SPL), we'd probably have to set off some explosives and not just a little.

The loudest car stereo at national "Crank It Up" contests might be able to get that loud but trust me nobody sits inside of them during the contest. The windows have to be specially made and held in place!

Listening to sounds this loud would seriously injure you or even flat out kill you on the spot.

The flight deck of an aircraft carrier launching planes (where everybody wears full-body protective gear) is maybe 130-140 dB SPL.

Back in the real world the loudest sound in the seats at the symphony might be 110 dB when they are playing the loudest passage of the loudest musical work they play. The noise floor of the room with musicians and audience, but no music playing might be 35 dB. That gives you a dynamic range of 75 dB.
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googlebot
post Mar 5 2012, 20:52
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QUOTE (Arnold B. Krueger @ Mar 5 2012, 17:45) *
Let's put it this way. The noise floor of your living room is maybe 40 dB SPL. In order to have a sound that is 144 dB above the noise floor in your living room (1OW 184 dB SPL), we'd probably have to set off some explosives and not just a little.


That's maybe a little too simplified. I think I could differentiate a playback of brown from white noise at 40 dB SPL played back in an environment with room noise at 40 dB SPL.

IMHO icstm's questions are valid and have not been answered thoroughly. If both noise and signals below noise can be heard, how is the poster supposed to understand, that it would be sufficient to capture just the range above it?

Music isn't all rock far above noise. In classic and Jazz it alters my experience significantly, if at all, and what kind of noise is present, when no instrument plays. Having near digital silence is not the best matching choice for every performance. That's an artistic choice and no technical one. Although 96 dB should be enough for everybody, you cannot argue by math and our threshold of hearing that 96 dB is provably enough in any case. You just need to live in a quiet countryside and crank up your volume to 126+ dB SPL max. wink.gif

This post has been edited by googlebot: Mar 5 2012, 21:15
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Wombat
post Mar 5 2012, 21:15
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What about a simple experiment? Create a silent 16bit file and turn up the volume of your system until you can barely hear the noisefloor. Now press play with an average loud song.
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googlebot
post Mar 5 2012, 21:18
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QUOTE (Wombat @ Mar 5 2012, 21:15) *
What about a simple experiment? Create a silent 16bit file and turn up the volume of your system until you can barely hear the noisefloor. Now press play with an average loud song.


That advice is evil! smile.gif Your DAC may output nothing in this case. Better play back a file with the LSB flipping randomly.
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Wombat
post Mar 5 2012, 21:22
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QUOTE (googlebot @ Mar 5 2012, 21:18) *
That advice is evil! smile.gif Your DAC may output nothing in this case. Better play back a file with the LSB flipping randomly.

Or you can combine that with a noise-shaping dithernoise test. Choose a curve your hearing won´t catch the noise that easy and will allow even a louder playback.
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icstm
post Mar 6 2012, 10:53
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QUOTE (Arnold B. Krueger @ Mar 5 2012, 16:45) *
QUOTE (icstm @ Mar 5 2012, 10:51) *
QUOTE (Notat @ Mar 5 2012, 04:45) *
A 24-bit recording has a 144 dB range. Our range of hearing intensity from quietest detectable sound to pain is 130 dB.


So 24bit recording COULD add something to the the listening experience?


Let's put it this way. The noise floor of your living room is maybe 40 dB SPL. In order to have a sound that is 144 dB above the noise floor in your living room (1OW 184 dB SPL), we'd probably have to set off some explosives and not just a little.
OK, but if I look back at your other post, you are saying I can hear below the floor. So for 144dB of dynamic range I do not need 184dB SPL (i think). unsure.gif
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icstm
post Mar 6 2012, 11:05
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QUOTE (bandpass @ Mar 5 2012, 14:16) *
Here's a wood-working analogy smile.gif

Suppose that you have a staircase with wooden banister spindles; these things:

Suppose that one is broken and that you are going to make a copy of an intact one as a replacement.

For measuring the original, you have at your disposal: plastic vernier callipers with 0.01" resolution, and a steel micrometer with 0.00005" resolution (a factor of 200 'better').

Using the micrometer does not result in a replacement spindle that anyone could distinguish from one made using the callipers. Though it has ~8 bits more resolution, (for this application) the extra bits are in the noise. The same is true of 24-bit audio.

Having slept on this I am not sure this is quite right.

The extra bits are not NOISE as you say.

You have a 30cm banister. To fit this, you have a TOLERANCE. If this tolerance can be RESOLVED with the vernier callipers then the micrometer is not needed. To all intents and purposes the callipers provided an ACCURATE measurement. The micrometer would have also provided an accurate measurement, but with great PRECISION. This precision (afaik) is not in the noise, as it was and can be resolved, however:
1) In normal use, you cannot make out this difference (ie mk 1 eyeball when comparing the other spindles)
2) In normal use any difference can be catered for in the fixings within the arm rail (ie where the tolerance lies)

As I understand it for it to have been noise, I would be saying that these PRECISE measurements were in fact not ACCURATE due to some other reason (namely noise) and thus could well vary should the measurement be conducted again.

Does that make sense?
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spindle
post Mar 6 2012, 13:36
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QUOTE (Arnold B. Krueger @ Mar 5 2012, 12:27) *
This might help:

Good Resolutions (HFN)


Thank you (and indeed to the others who have tried to educate me). The link and the other pages on that site are enough for me to chew on for now.

However, I don't know whether to be relieved or perplexed that even my 'numpty' question is the cause of continuing discussion. I'll continue to lurk and learn.
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Arnold B. Kruege...
post Mar 6 2012, 14:12
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QUOTE (icstm @ Mar 6 2012, 04:53) *
QUOTE (Arnold B. Krueger @ Mar 5 2012, 16:45) *
QUOTE (icstm @ Mar 5 2012, 10:51) *
QUOTE (Notat @ Mar 5 2012, 04:45) *
A 24-bit recording has a 144 dB range. Our range of hearing intensity from quietest detectable sound to pain is 130 dB.


So 24bit recording COULD add something to the the listening experience?


Let's put it this way. The noise floor of your living room is maybe 40 dB SPL. In order to have a sound that is 144 dB above the noise floor in your living room (1OW 184 dB SPL), we'd probably have to set off some explosives and not just a little.
OK, but if I look back at your other post, you are saying I can hear below the floor. So for 144dB of dynamic range I do not need 184dB SPL (i think). unsure.gif


The numbers that one sees for hearing below the noise floor vary, which only makes sense because there are a number of dependencies. However, I can't think of a real world situation where you would hear music 20 dB below the noise floor. Even with pure tones, 6 dB below the noise floor can be hard to hear. It surely wouldn't be your preferred music listening situation!

In reality, real world listening to music below the noise floor relates to things like reverb tails which are the echoes that follow music that ends abruptly. Another situation is fade-outs at the end of songs that are artificially put there during the production process. As a practical matter, it is far more important that reverb tails and fade outs end without creating bad noises, than that they seem to go on forever. With even reasonably high levels of background noise, these effects can be very musically satisfying.

In most cases in real world recordings, if you crank your system up to hear fade outs and reverb tails better, you'll hear them disappear into a mass of rumbling HVAC noise.
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2Bdecided
post Mar 7 2012, 11:41
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QUOTE (spindle @ Mar 4 2012, 11:12) *
Forgive me but, through ignorance, I fall pretty well into that category and I certainly can't follow the technicalities of this thread. Could you possibly point me at such a primer (for numpties) that will help me understand why 'finer granularity' is not necessarily better and is not the complete story?
Imagine you're measuring it with a ruler.

"Finer Granularity" means more (closer) tick marks on your ruler. There's always some discrepancy between the distance you're measuring, and the nearest mark to it on your ruler.

The difference between the real value and the nearest tick mark is an error.

You know the maximum error in any case: it's half the distance between the rick marks - because that's the greatest amount you'll have to round up (or down) to hit a tick mark.


In audio, when the audio signal you've stored isn't quite what you started out with, the difference is noise*. So now we know how much noise has been added due to "measuring" - a noise level equal to half the distance between the "tick marks". Calculate this, and that noise turns out to be nearly 100dB below the maximum signal level.

Given the sound levels in real life, and the ones our ears can cope with, that's enough. Pushing the measurement noise further down by increasing the number of bits does give close tick marks = "finer granularity", but the improvement is inaudible. It's also tricky to go much beyond 16-bits because basic electronics have ~20-bits of noise at best - and everything, from microphone through mixing desk to amplifier needs electronics - not to mention the digital/analogue converters.


* - it could be noise or distortion, but we'll use dither to ensure it's never distortion and always noise. We'll probably also use noise shaping to push the noise level further down in the most audible range, at the expense of slightly more noise at the frequency extremes where our ears aren't so sensitive.

Cheers,
David.
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2Bdecided
post Mar 7 2012, 11:55
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QUOTE (icstm @ Mar 6 2012, 09:53) *
OK, but if I look back at your other post, you are saying I can hear below the floor.
You (and most people!) are confusing measurements of total signal power with measurements of the amount of signal at a given frequency.

A pure 1kHz tone (a beep) only contains one frequency, so the measure of total signal power, and the amount of signal at that one frequency, will be exactly the same for this signal.

Whereas noise can contain a spread of frequencies, with the power varying by frequency. If you have a noise signal where the power is spread evenly across all useful audible frequencies (say, 20Hz-20kHz), then for a particular total power, it'll have a lot less signal at any one frequency.

So a -100dB FS pure tone sticks up above -100dB FS noise - because at the specific frequency of the pure tone, the level of the noise is 10s of dB lower than the level of the tone.

Whether you can see/hear a tone in some noise depends on the level of the tone, the level of the noise, and how you split the signal out into difference frequency bands to try to "see"/hear details like this.

The ear has the equivalent of about 24 logarithmically spaced filters (compare this with the typical 5 filters on a classic graphic equaliser). This means you can hear a tone that measures 20-30dB lower than the total power of the noise around it - because most of that noise falls into different filter bands and can be ignored by your ears.

Cheers,
David.
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icstm
post Mar 7 2012, 13:46
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I understand that. So I guess the problem is with the defn. of a noise floor, as it only really sence in a given range and 20-20k is maybe too big a range to define a floor?? just a thought.
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KMD
post Mar 7 2012, 13:53
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Noise "floor" is an bad phrase as signals don't go over or under the noise thay are added with it. It is best avoided and use the term noise, and the context, instead.
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