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16 bit, 24 bit and Noise Floor
botface
post Feb 28 2012, 20:13
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I'm involved in a discussion on a home recording forum. We are not discussing post processing, mixing or mastering just the simple capture of a single acoustic instrument.

I'm suggesting that with a recording environment that yields a noise floor of -60dbfs it makes no difference at whether you capture in 16 bit or 24 bit as you effectively only have 10 bits to work with and so any theoretical benefits of using 24 bit are lost

I'm getting arguments back like :
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Specifically 24 bit capture files will still provide the widest range of dynamic and the greatest resolution for capturing anything


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With 16 bit you have that many steps and with 24 you have that many more ... which means the more bits you have, the less grainy your resolution is


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To give an example: You have a scale of 0 to 100. If you print it on a 10cm piece of metall you barely have enough place to distinguish 1mm distances. If you print it onto a 1m bar, you can have plenty more subdivisions. Of course 100 is still 100 and zero is still zero, but you have many more subdivisions. Same with digital signal. Say you would use a 2 db resolution: then you would have just four different levels to represent the different signal levels which would have to be somehow digitalized onto these four steps.
With 16 bit you have 2^16 possible steps (65536) and with 24 bits 2^24 possible steps (16777216, quite a bit more...). Whether you believe them to be more useful to represent an audio signal I leave to your own ears.I'm not claming that the perceived distance between noise level and maximum level is bigger with 24 bits than with 16 bits. It's just that you have many more subdivisions in between, and that is audible.


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If the full theoretically possible dynamical span gets more quantizing levels per dB so must also the same dynamical span of let's say 60dB gain above recorded noise floor. I' can't see where this argument is wrong ... Maybe we're talking about different things?


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16 bit recordings have a theoretical dynamic span of 96dB; 24 bit recordings one of 144 dB. If I take the number of possible representations of level with 16 bit, then I have 2^16/96 = 682,7 (rounded) steps per 1dB. WIth a 24 bit recording I have 2^24/144 = 116508,4 (again rounded) steps per 1 dB. So it seems that the dynamical resolution is much finer with 24 bit.


QUOTE
The scale is the same, but with 24 bits we get bigger number of smaller steps than with 16 bit at any part of the scale which allows for better resolution.
You can want to increase the dynamic range by cutting off the noise, and stretching what is left down to the negative infinity level to fill the whole range, and higher resolution of 24 bit will become very handy compared to 16 bit.


I've given examples, I've worked through the arithmetic, I've provided analogies but nobody seems to think the noise floor of the recording environment has any impact on the ability of 24 bit to capture more detail. Can any of you provide any examples or analogies that you've found to work in the past.
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WernerO
post Feb 29 2012, 08:36
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QUOTE (botface @ Feb 28 2012, 20:13) *
I'm suggesting that with a recording environment that yields a noise floor of -60dbfs it makes no difference at whether you capture in 16 bit or 24 bit as you effectively only have 10 bits to work with


That is probably not quite correct. Assuming that your '-60dBFS' noise level is noise integrated over a meaningful bandwidth (for instance a reading on a level meter) and that you are in a real room with a real system, the spectral distribution of that noise signal is most likely skewed severely to the lower frequencies. That means that once above a few 100Hz you will need more than 10 bits to capture all, simply because the ambient noise there is much lower than the integrated meter reading wants you to believe.


QUOTE (saratoga @ Feb 28 2012, 21:30) *
perhaps a simple way to do that would be a few example files of pure tones in white noise with a given SNR quantized to different bit depths


4 bit demo, including music, here http://www.tnt-audio.com/sorgenti/dither_e.html
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knutinh
post Feb 29 2012, 11:00
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QUOTE (WernerO @ Feb 29 2012, 09:36) *
That is probably not quite correct. Assuming that your '-60dBFS' noise level is noise integrated over a meaningful bandwidth (for instance a reading on a level meter) and that you are in a real room with a real system, the spectral distribution of that noise signal is most likely skewed severely to the lower frequencies. That means that once above a few 100Hz you will need more than 10 bits to capture all, simply because the ambient noise there is much lower than the integrated meter reading wants you to believe.

A good point. As long as no precise spectral estimate of a)background noise and b)16-bit quantization error is provided and compared, it is difficult to be confident that 24 bits has exacly zero perceptual value.

-k
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Arnold B. Kruege...
post Mar 2 2012, 01:24
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QUOTE (knutinh @ Feb 29 2012, 05:00) *
QUOTE (WernerO @ Feb 29 2012, 09:36) *
That is probably not quite correct. Assuming that your '-60dBFS' noise level is noise integrated over a meaningful bandwidth (for instance a reading on a level meter) and that you are in a real room with a real system, the spectral distribution of that noise signal is most likely skewed severely to the lower frequencies. That means that once above a few 100Hz you will need more than 10 bits to capture all, simply because the ambient noise there is much lower than the integrated meter reading wants you to believe.

A good point. As long as no precise spectral estimate of a)background noise and b)16-bit quantization error is provided and compared, it is difficult to be confident that 24 bits has exacly zero perceptual value.


The general point to be made is that a dB for dB masking of a lower amplitude noise floor only happens when both noise floors have identical spectral shaping. However, just because the two noise floors have different spectral shapes does not mean that all bets are off.

The right thing to do is to subtract the two on a point-for-point, octave-for-octave or whatever relevant means of comparison you have available , and then apply an appropriate audibility weighting curve to the difference. Since the levels of noise floors are pretty low, the roll off of high and low frequencies will usually be pretty extreme. This tends to minimize the original differences in the shapes of the spectral content of the two noise floors.

Having measured a ton of noise floors over the years, i've seen just about every rule of thumb to fall apart. Probably the canonical noise floor of a room will be red or brown noise-shaped, but a heavy dose of turbulence in the air flow of the HVAC system can wash a lot of that out. Some microphones have noise floors that are shaped more like white noise, while other are shaped more like pink noise. Complementary equalization curves (e.g. RIAA) can distort the noise floors of electronics between the pre-emphais and de-emphasis networks. Finally, a 60 dB noise floor is almost a whopping 40 dB away from a 96 dB noise floor and that tends to wash our a lot of minor differences.

Also, while the noise floor of real world 16 bit systems is usually within a few dB of 96 dB, very few real world 24 bit systems come within 20 dB of 24 bits. Usually, going to 24 bits from 16 is only good for a 10-20 dB advantage and sometimes it is as little as 5 dB.

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Posts in this topic
- botface   16 bit, 24 bit and Noise Floor   Feb 28 2012, 20:13
- - saratoga   You are correct and those people you quoted do not...   Feb 28 2012, 20:17
- - greynol   As a simple experiment, you can create a pure tone...   Feb 28 2012, 20:18
|- - Ethan Winer   QUOTE (greynol @ Feb 28 2012, 14:18) As a...   Mar 1 2012, 19:59
|- - Glenn Gundlach   QUOTE (greynol @ Feb 28 2012, 11:18) As a...   Mar 2 2012, 07:36
|- - Garf   QUOTE (Glenn Gundlach @ Mar 2 2012, 07:36...   Mar 2 2012, 12:07
|- - Arnold B. Krueger   This is another one of those situations where your...   Mar 2 2012, 14:02
|- - 2Bdecided   QUOTE (Arnold B. Krueger @ Mar 2 2012, 13...   Mar 3 2012, 19:56
- - hlloyge   Oh, just work in 24 bits so you can have more prec...   Feb 28 2012, 20:21
- - DVDdoug   Let's look at the "ruler" example - ...   Feb 28 2012, 20:26
- - greynol   Please, before submitting any more abject analogie...   Feb 28 2012, 20:43
|- - saratoga   QUOTE (greynol @ Feb 28 2012, 14:43) Plea...   Feb 28 2012, 20:51
|- - Garf   QUOTE (greynol @ Feb 28 2012, 20:43) Plea...   Feb 29 2012, 15:48
- - KMD   greynol is right, sounds below the noise floor are...   Feb 28 2012, 20:46
- - greynol   You're right. Case in point: create a 1k tone...   Feb 28 2012, 21:22
|- - saratoga   QUOTE (greynol @ Feb 28 2012, 15:22) You...   Feb 28 2012, 21:30
- - mjb2006   Most explanations—even those for technical audienc...   Feb 28 2012, 21:22
|- - spindle   QUOTE (mjb2006 @ Feb 28 2012, 20:22) Most...   Mar 4 2012, 12:12
|- - saratoga   QUOTE (spindle @ Mar 4 2012, 06:12) Forgi...   Mar 5 2012, 07:53
|- - Arnold B. Krueger   QUOTE (spindle @ Mar 4 2012, 06:12) Forgi...   Mar 5 2012, 13:27
||- - spindle   QUOTE (Arnold B. Krueger @ Mar 5 2012, 12...   Mar 6 2012, 13:36
|- - 2Bdecided   QUOTE (spindle @ Mar 4 2012, 11:12) Forgi...   Mar 7 2012, 11:41
- - greynol   I should have led with my second example.   Feb 28 2012, 21:39
- - WernerO   QUOTE (botface @ Feb 28 2012, 20:13) I...   Feb 29 2012, 08:36
|- - knutinh   QUOTE (WernerO @ Feb 29 2012, 09:36) That...   Feb 29 2012, 11:00
|- - Arnold B. Krueger   QUOTE (knutinh @ Feb 29 2012, 05:00) QUOT...   Mar 2 2012, 01:24
- - icstm   your point about single tones vs white noise is al...   Feb 29 2012, 18:56
|- - Garf   QUOTE (icstm @ Feb 29 2012, 18:56) your p...   Mar 1 2012, 08:40
|- - botface   QUOTE (Garf @ Mar 1 2012, 07:40) The orig...   Mar 1 2012, 10:13
- - 2Bdecided   Try lossyWAV with --bitdist to see how many bits i...   Mar 1 2012, 10:51
|- - Garf   QUOTE (2Bdecided @ Mar 1 2012, 10:51) Try...   Mar 1 2012, 11:19
- - googlebot   A noise floor can be considered part of the signal...   Mar 2 2012, 20:59
|- - knutinh   QUOTE (googlebot @ Mar 2 2012, 21:59) A n...   Mar 2 2012, 21:09
|- - googlebot   As discussed earlier in the thread it is not trivi...   Mar 2 2012, 21:18
|- - Garf   QUOTE (knutinh @ Mar 2 2012, 21:09) QUOTE...   Mar 4 2012, 10:12
- - Notat   I wish I knew of such a reference. I'll take a...   Mar 5 2012, 05:45
- - bandpass   Here's a wood-working analogy Suppose that y...   Mar 5 2012, 15:16
|- - icstm   QUOTE (bandpass @ Mar 5 2012, 14:16) Here...   Mar 6 2012, 11:05
- - icstm   QUOTE (Garf @ Mar 2 2012, 11:07) As far a...   Mar 5 2012, 16:51
|- - Arnold B. Krueger   QUOTE (icstm @ Mar 5 2012, 10:51) QUOTE (...   Mar 5 2012, 17:34
|- - Arnold B. Krueger   QUOTE (icstm @ Mar 5 2012, 10:51) QUOTE (...   Mar 5 2012, 17:45
|- - googlebot   QUOTE (Arnold B. Krueger @ Mar 5 2012, 17...   Mar 5 2012, 20:52
|- - icstm   QUOTE (Arnold B. Krueger @ Mar 5 2012, 16...   Mar 6 2012, 10:53
|- - Arnold B. Krueger   QUOTE (icstm @ Mar 6 2012, 04:53) QUOTE (...   Mar 6 2012, 14:12
|- - 2Bdecided   QUOTE (icstm @ Mar 6 2012, 09:53) OK, but...   Mar 7 2012, 11:55
- - Wombat   What about a simple experiment? Create a silent 16...   Mar 5 2012, 21:15
|- - googlebot   QUOTE (Wombat @ Mar 5 2012, 21:15) What a...   Mar 5 2012, 21:18
- - Wombat   QUOTE (googlebot @ Mar 5 2012, 21:18) Tha...   Mar 5 2012, 21:22
- - icstm   I understand that. So I guess the problem is with ...   Mar 7 2012, 13:46
- - KMD   Noise "floor" is an bad phrase as signal...   Mar 7 2012, 13:53


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