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Author Topic: Levels - headroom, operating level etc.  (Read 2267 times)

John Klett

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Levels - headroom, operating level etc.
« on: April 26, 2004, 07:16:45 pm »

Here's a private mail Q and A - names have been changed to protect the innocent.

Here's a technical opinion... this may be informative or totally misleading depending on what you believe.


At 3:38 PM -0500 4/26/04, Phred wrote:
"1) I'm trying to figure out what to set my "jumpers" for the HDCD 2 for.
They go any where from +12db to +24db....
They are factory set at +18 db."

Phred

It depends on what you are feeding into the converter.  Full Scale is that level where, when it is exceeded, the converter digitally clips or produces an "over".

In analog you have a clip point or maximum headroom level that is based on the maximum RMS level where a sine wave can be transmitted without clipping or exceeding some defined distortion amount.  This can sit anywhere from +18dBu to +32dBu in professional gear and is often lower in HiFi and Semi-Pro gear.  Most pro gear with active differential outputs can output +28dBu.  This is based on +/-15VDC power supplies which are most common in IC based gear.

Leave your converter set at +18dBu for now.  That sets you up (probably) so that the converter will clip at +18dBu and most all equipment can drive it to Full Scale without itself clipping.

Converter input levels are often referenced relative to Full Scale "FS" and you'd set either your operating level, 0VU, or 0dBu to be x number of dB below full scale.  I have seen any number of permutations of confusion about exactly what level is should be set to equal to what number of dB below full scale.  Alesis MasterLink's -18FS may not be what Sony's -18FS is so I end up measuring what FS really is when the converter is set to some number of dB FS.

In your case the Pacific Micro book is quite good and should define all this...  somewhere.

I should set you up with some stepped attenuators.

If you want to run your gear up really hot for effect and then convert it right there into the digital domain you would patch through the attenuators and step the level down to hit the converter where you want to.  With 24 bit converters I tend not fill up the meters.  You "had" to do that with 16 bit but 24bit has more room and I think things sound a little better when you stay a couple dB below FS...  Mastered "mass music" is still going to run right up to FS and have no dynamic range and just a few overs because A&R people who are deaf demand it...  that's why I don't don't do mastering engineering work all the time.



OKAY that is the "short" answer to your question but I feel a digression coming on...  hopefully this will somehow relate to your question.

I know you probably like clearly define standards but...

Your maximum analog headroom level really is going to be less than what you might think it should be based on the power supply rails or what you'd measure with a mid-band sine wave and a distortion analyzer.  Most amplifiers can't swing all the way to the power supply rails.  It's almost always at least a half volt less (a diode drop).

At high frequencies there is often enough capacitive loading in the wire and at the destination (especially at converter inputs I find) that the amplifier can't drive transients (peaks with high frequency content).  Signal with lots of high frequency content in general can have problems.  This is high frequency loading.

Here most amplifiers essentially current limit - I relate these to amplifier characteristics I refer to as current slew or current dump but nobody else that I know mentions those - I do when I talk to myself or maybe someone I work with on a regular basis who understands my tech talk.  These characteristics have driven me to designing in discrete transistors - at least on the outputs of those amplifiers driving the outside world.  I can swing wider peak to peak voltages and deliver (source or sink) more current using discrete transistor amplifiers.

Older Trident IC based consoles (POST A and B range) i.e. TSM and the original Series 80 desks had a discrete transistor follower (a pair) on the output amplifiers that boosted the current these outputs can deliver and, to me, they sounded sound a little better than later ones where those parts were pulled out and the TLO part was swapped out with a 5534.

One of the discrete amplifiers I am using now is like a little power amplifier.  It swings on (up to) +/-32 volt rails and have two output pairs to keep the dissipation down in those devices.  It seems to be able to drive a real load pretty well across a wide bandwidth.  We're still working on it.

With a perfectly ideal linear high impedance loads and perfectly ideal, no capacitance, no inductance wire there is no difference in current demand across the frequency band and the amount of current an amplifier needs to drive such a load is small...  nearly zero if the load impedance is nearly infinite.  None of that is real so you have mic preamps and other gear that "can't drive a wire" and so on.

so...  coming back, slowly, to the point...

We use RMS and other averaged levels because those are easier to measure.  So all the levels I am mentioning here are based on RMS level measurements of sine waves.

You have to choose an operating level which, in pro gear, is referenced to 0dBu or .775 volts RMS (1 Volt in consumer gear).  Most pro gear is set to an operating level of +4dBu and from there it can have anywhere from 14dB of headroom (+18dBu) to 28dB of headroom in gear that can deliver that kind of level.

You may choose any operating level and set that as a standard for your studio.  You need to look at the maximum output level for whatever pieces you would feed in to the converter.  Call it the maximum level you can run where program material still sounds good (you'd have to do this empirically) or go by some other standard.

So...  say you have a studio set to a standard operating level of +4dBu and reflected as 0 VU on your meters.  This is what you'll have by the way.  As you use the studio over time you'll get a sense of where and how much you can push your levels and where you have to drop down to keep the program clean.

Essentially you set the converter to whatever it needs to be set at so you hit it the way you want to.

You can evaluate whether or not +4dBu is a good operating level for you.  The "right" operating level is going to be some level where you have enough operating headroom and where you are operating with a satisfactory signal to noise ratio.  It always a compromise... its a pain in the ass to re-calibrate all your metering to any level that is not +4dBu.  I've seen people do it but you are really going to decide how you are going to "fill" your analog meters.  Operating level is only a benchmark.

It used to mean more, especially in broadcast and telephones where our "modern" recording technologies originated...

Our tube console was designed around an operating level that related directly to a standard 85dB-SPL monitoring level at one meter from the speaker(s).  There was NO LEVEL CONTROL for listening level - actually that's wrong - it can switch up or down by +4 or -4dB and when you switch to one of those a big light come on right in the middle of the console that says, in German, "UNCALIBRATED".  It looks ruder in German.

I think api consoles sound best when driven it right up to the edge.  My maxi Q's have absolutely no headroom so that always requires some sort of work around.  In the end you really have multiple operating levels in a studio and it proves quite handy to to have some line amplifiers and attenuators kicking around on patch so you can adjust "operating" level from one thing to the next.  Those go on the same bay with that balancing transformers you need to throw between active balanced outputs and unbalanced insert returns...  and to flip phase of unbalanced signals.




Phred: "2) Do you have a cable preference for mics? Mogami or Canare?"

Mogami 2549

I dislike Canare and other Quad cables... they pick up less electronic noise but don't sound good to me


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John Klett / Tech Mecca
http://www.technicalaudio.com

Kev

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Re: Levels - headroom, operating level etc.
« Reply #1 on: April 27, 2004, 10:42:53 pm »

Good stuff John
and yes it probably is,
" ..informative or totally misleading depending on what you believe."

" In the end you really have multiple operating levels in a studio ... "

Explaining this to die-hards can be very difficult.

John doesn't like Canare Quad  Sad
Rolling Eyes I do
but then I haven't spent much time with the Mogami John mentioned.  I might like that better.
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ted nightshade

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Re: Levels - headroom, operating level etc.
« Reply #2 on: April 27, 2004, 11:55:45 pm »

you said a mouthful, John.
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Ted Nightshade aka Cowan

There's a sex industry too.
Or maybe you prefer home cookin'?
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