Old 09-01-2014, 12:16 AM   #1
clepsydrae
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Default The -18 dBFS rule -- myth or reality?

(this is a continuation thread from this thread starting at post #34 -- didn't want to be OT any more over there. Please read that thread for background before posting here.)

Quote:
Originally Posted by karbomusic
I had to raise the preamp ~10dB higher than necessary to just so I could get the raw guitar signal to -9 RMS. Why do I need to do any of that when recording @24 bit more than allows the gear sending the signal into the DAW to run within its prescribed range? And when some VSTs expect it; not for quality reasons but because of my statement below concerning incoming signals and where they naturally fall?
As i said in my other posts, i see the merit of the "levels convenience" issue: for some (many? most?) the way the levels work out through their signal chain, into and out of the digital world, etc, works out better for them at -18. I record into the computer and mix ITB, and for me recording hot is easier to see on the meters, works more conveniently with the plugins i use (primarily iZotope Alloy 2 these days, but others as well), and so forth. Signals peaking at -18 dBFS RMS are slightly harder to handle everywhere in my workflow... I end up boosting them immediately anyway to make them more useful. Others' mileage varies, clearly, to each their own, etc etc.

In terms of gain-staging, I would ideally send a hot signal into my interface to get as close as is comfortable to 0 dBFS without risk of clipping, if possible to do so without amplifying anything. In reality i don't even worry that much about making it hot, since the added noise I get coming in at -18 RMS is inaudible under normal conditions, but I certainly wouldn't make a point to record at -18.

In terms of the gain on the pre-amp, I see your point: in theory, no reason to boost the signal at the pre-amp (and add more noise) just to get closer to 0 dBFS. But for whatever reason of design, the two interfaces I've tested (both PreSonus) seem to add a non-variable amount of noise somewhere in the internal chain, the end result being that if I boost the preamp knob to send a hotter signal to the internal ADC, I end up with a signal with less noise than sending at a level that results in -18. Not a big deal at all, but that's what happens. Maybe my interface sucks, no idea.

But the noise issue is beside the point, which is that there is a notion floating around that for most interfaces recording -18 dBFS RMS peak will make the recorded material itself sound better, and not just because of convenient levels or because it's a handy rule of thumb to avoid digital clipping, and that specific assertion is what I'm skeptical about. Your post in the other thread referred to "those poor converters you are trashing on the way in" and said "-18 db is +18 outside the box and near the max most converters/pres can reliably handle"... maybe I misunderstood and you were only referring to clipping, but it sounded like you expect perceptible differences in audio quality. And even if you don't, there are plenty out there who do and who post all time about how recording at -18 made their audio sound so much better, more punchy, more clear, changed their lives completely, cured their cat's illness, etc...

If the -18-for-sound-quality thing is for real, then I accept that my three tests on two interfaces are certainly inadequate to disprove it, and that my very limited experience with other interfaces hampers my perspective; but to me the onus is on anyone who advocates for -18 as a sound quality issue to demonstrate it with some audio files. I've searched the internet high and low and find no examples or people willing to provide them. Some people scoff at ABX testing and disregard math/statistics. All of that makes me a little more suspicious about it being an urban legend. :-)

I'm very willing to be proven wrong.

(Incidentally, my previous test in the other thread included a metering error on my part which meant that the "-18 RMS peak" files were peaking at more like -12 or -14; i've reposted the corrected audio files; no change in the results that i can hear.)
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Old 09-01-2014, 12:29 AM   #2
reapercurious
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two words that changed my mixing and shut down a lot of my inner chatter about the zillions of contradictory things i have read

Master. Fader.

and to be fair, and sort of address what I think you're saying. if you want to mix quiet, then crank up your monitors till your ears almost bleed, then its easy to leave headroom.
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Old 09-01-2014, 02:38 AM   #3
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clepsydrae, in general, I agree with what you are saying here, and I certainly agree with your want to hear something concrete, rather than taking someone else's word for it. For myself (after doing my own experimentation), recording levels with an interface are much more of a practical concern than a quality concern. Take di guitar, for example, I have concluded, through experience and testing, that as long as I give myself a good safety margin for avoiding clipping, and that I'm not having to fiddle around the input/output levels on the amp sim too much, the quality of the incoming signal level will be fine.

For the example which I posted in the other thread, the -9dbfs RMS sound clip was too hot, in that I had to avoid playing too hard so as not to clip. But I wanted to prove a point: that tracking at a higher RMS level than the 'rule' will not absolutely destroy the incoming signal. Does it sound a little different? Yes. Might one person prefer a lower level based on sound quality alone? Yes. Was the sound destroyed? No. In practice, though, I never track that hot (for practical reasons), so the difference in sound between -18dbfs RMS and -9dbfs RMS doesn't come into play.
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Old 09-01-2014, 04:57 AM   #4
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Never heard of this rule. Seems a bit nonsensical to me.

When recording, I play hard into my interface, get it just to the point of clipping and then back it off a bit.

As far as the level that is captured in REAPER, I don't see how it matters. The digital level is relative so the s/n ration is the same whether you are recording at -12dB, -18dB or -30dB RMS.

I like to record with my peaks around -6dB because it is practical from a listening standpoint.

When mixing, I do normalize to -18dB RMS, but this is because of plugin input gain staging.

Does not apply to recording though.

This is all my take on it anyway. I wanted to join the conversation as I am interested in seeing where it goes. I will be happy to be corrected on anything.

Phew, glad I've never heard this rule before because I am quite impressionable.




...doh!
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Old 09-01-2014, 05:15 AM   #5
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I remember Kenny Gioia saying in one of his tutorials, when he was recording/tracking vocals that the average Input Level should be be between -18 and -12 db and peaks should not exceed - 7 dB.
He refered to the LED of track level meter in Reaper (w/o fx!).

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Old 09-01-2014, 06:07 AM   #6
karbomusic
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Quote:
The -18 dBFS rule -- myth or reality?
^Neither, it's a rule of thumb and it's RMS not peak, this confuses things unnecessarily by using the wrong terms.

Shame on you for leaving out the "of thumb" part which I have painstakenly defined and used multiple times now. It's the appx level your signal would land in the DAW if you were at unity externally, that's what makes it sensical. However, this is the only post I'll make because it is a....

RULE OF THUMB

Quote:
A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination.
You also omitted RMS again which I also bolded multiple times in the other thread for very good reason, it is NOT accurate when omitted. And some of your assumptions are projections. I'll own the embellishment of "trashing the converters" but the rest is just plain common sense.

So please, while you guys hash this out without me, would you please do me the simple favor of actually thinking instead of seeing the word "rule" then allowing your head to explode before understanding what comes after? Pretty please? With sugar on top?

Quote:
When recording, I play hard into my interface, get it just to the point of clipping and then back it off a bit.
Guess what general range that ends up landing in as RMS give or take a couple DB?

Most sound cards (as in rule of thumb) fall into the -18 RMS range when striking the best compromise between 0 dbFS and the analog side of their hardware hence the reason someone can follow this and be fairly comfortable they are within the specs of their gear:

Quote:
Analog levels - http://en.wikipedia.org/wiki/DBFS

dBFS is not to be used for analog levels, according to AES-6id-2006. There is no single standard for converting between digital and analog levels, mostly due to the differing capabilities of different equipment. The amount of oversampling also affects the conversion with values that are too low having significant error. The conversion level is chosen as the best compromise for the typical headroom and signal-to-noise levels of the equipment in question.

Examples:

EBU R68 is used in most European countries, specifying +18 dBu at 0 dBFS
In Europe, the EBU recommend that -18 dBFS equates to the Alignment Level
European & UK calibration for Post & Film is −18 dBFS = 0 VU
UK broadcasters, Alignment Level is taken as 0 dBu (PPM4 or -4VU)
US installations use +24 dBu for 0 dBFS
American and Australian Post: −20 dBFS = 0 VU = +4 dBu
The American SMPTE standard defines -20 dBFS as the Alignment Level
In Japan, France and some other countries, converters may be calibrated for +22 dBu at 0 dBFS.
BBC spec: −18 dBFS = PPM "4" = 0 dBu
German ARD & studio PPM +6 dBu = −10 (−9) dBFS. +16 (+15)dBu = 0 dBFS. No VU.
Belgium VRT: 0dB (VRT Ref.) = +6dBu ; -9dBFS = 0dB (VRT Ref.) ; 0dBFS = +15dBu.
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Old 09-01-2014, 08:47 AM   #7
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The real point of the whole thing is to run your analog gear at sensible levels to avoid adding analog distortion on the way into the box. This usually means keeping your average levels around 0dbVU = +4dbu.

The first part of this that gets fun is that so many folks nowadays don't actually have an analog chain with meaningful meters. You've got an all in one interface which might have a green light to tell you you've "got signal" (whatever that means) and then a red one to tell you you're clipping. Many don't even give you that courtesy. Most folks are not looking at a nice big analog needle, or even LED tree that tells them where the nominal level is, so they are pretty much forced to gauge it by the meters in Reaper, and that leads to the second fun part...

Every interface is calibrated differently. If you actually did set up say an analog mixer feeding your line level inputs, and set your levels on the mixer to 0dbVU, you could see in Reaper anything from -24 to maybe even -12dbfs. If you've got one calibrated at -24, and you push your average levels up to -18dbfs with peaks hitting around -6dbfs, then your mixer would be averaging at +6dbVU with peaks at +18dbVU, and that could be pushing the limits of many analog stages. How do you know how yours is calibrated? Either test it, or look at the spec sheet.

But that's the third fun part: Every spec sheet is different. Some call it headroom, some just list a maximum input level. Sometimes it's listed in straight db relative to the nominal level, sometimes it's listed as dbu, or dbV, and that's not always even the same scale that they used to specify nominal level. Some even just list it as a voltage, so that you have to go find a volts>db calculator.

Does it sound better? That's kind of subjective isn't it? It completely depends on the behavior of the analog chain. Some analog stages might sound kinda good when pushed up into distortion, some just sound like crap, and some folks might like the overdrive from this one or the other one, and some folks might thing it sounds better if it comes in as clean as possible. I'm not talking about "clipping" here. As mentioned above, it's quite possible to distort your analog chain without ever coming close to 0dbfs and incurring "digital clipping", and a lot of analog gear will start to get non-linear well before it even hits its own maximum output level.

The only right answer to this is to learn your gear. Learn to read the spec sheets, interpret and interpolate all the different ways that these things are presented. More importantly, test your gear, run various signals through at various levels and listen to the results, because in the end that's all that really matters.
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Old 09-01-2014, 09:07 AM   #8
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ashcat_lt, that's 99% what I wanted to write as well.

Of course the OP isn't sure about his monitoring situation - that makes the whole process a little difficult. But once you've got your room acoustics and monitors right, it's a pleasure to decide where to hit the AD in order to get the "best" signal for the situation.

[on a side note: tape was so much more fun when it comes to finding the "sweet spot", oh my... ]
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Old 09-01-2014, 09:29 PM   #9
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Quote:
Originally Posted by ashcat_lt View Post
The real point of the whole thing is to run your analog gear at sensible levels to avoid adding analog distortion on the way into the box. This usually means keeping your average levels around 0dbVU = +4dbu.
I've said it before and I'll say it again... all of this crap is really for consumers. People with outboard gear that transitioned from analog recording set their levels on their preamps on VU meters. With a correctly aligned system everything downstream from the outboard takes care of itself, all by itself, because that's exactly what the scale was designed for, to accommodate everything up to high headroom pro gear.

Everything else is people using cheap gear with 3 stage led's and not knowing what level to record at and obsessing over it... while recording with $200 USB devices who's converters are actually at -15 (because they never bothered to check or don't know how to check) and making -18 a rule, when it's not.
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Old 09-02-2014, 02:43 PM   #10
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Whatever works for you, according to your ears, I say. I'm all for running little experiments (I have done lots of them to satisfy my own curiosity), but people should be doing this for themselves on their own gear, if there is any personal concern on incoming levels. There is nothing complicated or difficult about it. Set Reaper's master meters to RMS, record some stuff at different levels, and listen. Use what you hear as a personal guideline.

Then again, if you like to geek out on specs and abide by rules, for whatever reasons, do so.
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