Keyboard Magazine "In The Studio"
Article # 23
by Bobby Nathan
This months column will deal with the facts concerning Levels. The correct levels for recording as well as gain staging your mixing console.
MEASURING SOUND Sound level or volume is measured in units known as decibels (abbreviated DB). You may be well aware of sound level readings and their importance if you ever threw a party in your backyard and the town police came by with a sound meter to check if you were over the 65 DB limit, and thus upsetting your neighbors. I've put together a little chart for you to understand the DB levels of certain environments.
160 Jet Engine, Close up 150 140 The Who - Live Threshold of Pain 130 Pneumatic Hammer 120 Jimi Hendrix Thunder 110 Power Tools
100 Subway 90 Jazz Trio Heavy Truck Traffic 80 Average Factory 70 Busy Street Small Orchestra 60 Average Conversation
50 The Office at Keyboard Magazine
40 Subdued Conversation
30 Quiet Office
20 Dominic Milano's Living Room
10 A quiet Studio at Unique Recording
0 Threshold of Hearing
Using the above chart be advised that all references are approximate, especially in reference to Dominic's living room. 0 DB corresponds not to absolute silence, but to the lowest sound pressure level that the listener had hear. Now it is also important at this stage to make you aware that obviously O DB on a sound level meter is not the same as O DB on a VU meter of a recording console or tape recorder. I will explain this as we go on.
VU METERS VS PEAK METERS
Now in the studio, sound is measured not in decibels but rather in Volume Units and the meter that measures Volume units is known as a VU meter. A Volume Units are in reality a reference of Decibels (DB) against voltage or commonly referred to DBV. 0 dbv is referenced to 0.775 volts. Now to understand the VU meter we must look at the Voltmeter. A voltmeter was designed to look at voltages just like the VU meter but voltages on a technicians bench are usually steady tones and do not fluctuate as those levels associated with live sound. Therefore the VU meter was designed to have different ballistics. VU meter ballistics were designed so that the meter scale movement would approximate the response of the human ear. The VU meter looks at the average level of the program material. The VU meter does not register instantaneous peaks above the average program level. For instance a snare drum has loud transients or instantaneous peaks that don't really register on a VU meter. The VU meter will register higher than the average level, but much below the actual level of snare drum's peak level. For the most part let's think of the average level on a VU meter as being 0 VU. Looking at the diagram below of a VU meter notice that 0 VU is just before the red indications which stop at +3 of the VU meter. Notice that the lowest reading is -20. If we were recording a synthesizer playing a chord pad part we would try to keep the level at around 0 on the VU meter. If we were recording Snare drum we would be happy with a reading of only -3 and that is because the snare's peaks could be well over +12 to +18 VU.
Now some VU meters have a single LED (light emitting diode) that is usually red. Red to indicate trouble! If you have a piece of gear that has a Peak indicator you will notice that it will blink well before the VU meter even seems to move in many cases and it will blink even though the VU meter is only reading -6 VU. This is because of the way the VU meters ballistics are programmed. They are programmed to be slow, where as Peak Indicators and Meters are programmed to be lightning fast! If you consult your manual, there will be a notation as to was level the peak indicator represents.
Peak meters have faster ballistics to allow more accurate tracking the attacks of high level transients. Their ballistics were tailored to have a more gradual fall-off, meaning the meter drops in level more slowly than it rises for ease of reading levels. Now even though Peak Meters are much more accurate, most recording engineers prefer VU meters over Peak Meters because Peak meter ballistics can be fatiguing after long hours behind the board.
+4 VS -10
When a piece of gear is said to be +4 DB in level it is called "Pro" and means that 0 VU on the VU meter has been calibrated to really be +4 DB. That is 0 VU equals +4 DB. On the other token when a piece of gear is said to be -10 DB it is called "Semi-Pro" and in reality 0 VU = -10. So the difference between a -10 DB piece of gear and a +4 DB piece of gear is 14 DB Now interfacing the two can cause problems. If you insert a -10 piece of gear into a +4 input is will be very low and by the time you've made up the level (if you have that much gain make-up available), the difference of 14 DB alone added a significant amount of noise to the original signal! On the other hand if you insert a +4 DB piece of gear into a -10 DB input, severe distortion can occur. Luckily most -10 DB pieces of gear had a Pad. A pad cuts down the amount of input level without adding any noise. So it can work to plug a Pro +4 DB piece of gear into a Semi-pro piece of gear.
Now I'm not saying that a 10 DB piece of gear cannot yield great results. You just have to match levels correctly.
ALIGNING YOUR MIXER TO YOUR TAPE RECORDER
Before doing any recording, it is most important to make sure the meters on your mixing console are correctly aligned to your tape recorder. Early mixers or consoles didn't even have meters. Meters where added to make it easy for the recording engineer. Now here is the procedure to set up your meters. Use a 1K tone (if you do not have a sine wave oscilator, set up one of your synths with a sine wave tone and use C two octaves above A 440). If your mixer has a separate meter for each bus out then feed the 1K tone to all buses so that all meters read 0 VU. If they don't all read the same with the same levels sent to each then chances are your consoles meters need alignment. To align your meters you will have to use a Voltmeter that can measure DB Volts. Connect the meter to any of the busses, send level until your voltmeter reads .775 volts for 0 db (or whatever your console calls for, i.e. 1.226 volts for +4, etc.). Then inside the mixer there are Meter Trims that will adjust the VU meter amplifier so that when the bus is outputting .775 volts/ 0 db, the meter trim can be adjusted to read 0 VU. Never adjust the needle balance screw on the front bottom of the VU meter. This screw is a mechanical adjustment for the meter's needle in the "at rest" position (usually -20 VU).
After you are sure that your consoles meters are set correctly, then we can proceed to align the tape recorder's meters. But, we have to also check the recorder's VU meters with a voltmeter to make sure they are aligned correctly too. After they are trimmed (as in the above mentioned procedure) then the recorder's input control (or monitor level) will adjust the meter's on the tape machine to read 0 VU also. Do not adjust record gain! this will not adjust the meters. Note, in some recorder's the input level has to be set after the record alignment has been done. Always check your owners manual. After the console and Tape recorder are aligned you can depend solely on the mixer's meters and prevent a stiff neck! Do not attempt any of the above meter calibrations unless you are capable of the given alignments. If not, get a professional to make a house call or bring your gear to a pro to have the levels set correctly. After your gear is correctly aligned your should not experience any distortion from your mixer or from saturating the tape!
SIGNAL TO NOISE RATIO
If you've ever looked at the specs. on any given piece of gear, there is always a reference given as to the amount of "signal to noise ratio". Signal to noise ratio is the ratio of signal voltage (or volume) in relation to Noise voltage. Signal to noise is usually measured in decibels. In your mixer there is noise that is always preset due to the electronics. Proper gain staging is essential so that your signals exceed the noise floor (the loudest level at which noise exsists). For instance, if your mixer was designed so the 0 VU equals +4 and the signal to noise ratio was measured by the manufacturer to be -65 db. The difference betwwen the two is 69 db. This means that a signal passing through your mixer at 0 VU is 69 db louder than the noise that the mixer's electionics make.
LEVELS ON TAPE
Tape, like your mixer has a signal to noise characteristic too. You may have heard the expression, "This tape was recorded at +6". Well to understand this we have to know that standard levels we set in the recording industry. Originally 0 VU equaled 185 nanowebers per meter (nWb/m). A nWb/m measures how many lines per meter. These lines are detected by the Playback head of the Tape recorder and are then converted into output level. Then as tape manufacturers created new formulas that could handle hotter levels to raise the signal to noise ratio, the reference was changed in accordance with different tape manufacturers specs. Now there are two new standards they are : +3 equals 0 VU at 250 nWb/m and +6 equals 0 VU at 370 nWb/m. Well recording at these elevated levels, a playback tape is used to calibrate the recorders output to yield a hotter signal on tape and also adjust the tape machines VU meters some that they are calibrated to the hotter levels. If this subject is of interest, I'll devote a entire column to understanding this better. HEADROOM
All gear is rated in DB as to its maximum Headroom. The is the maximum amount of level that can be passed thru a piece of gear before distortion occurs. In pro gear levels of +24 to +28 DB are acceptable. Now remember that snare drum we were talking about? Well if it's transients exceeded the maximum amount of headroom allowed from our mixer then the snare would exit the mixer with distortion added. Tape machines and Tape have maximum amounts of head room too! Get to know your gear. Remember if you have only VU meters you can easily send more that +28 out of you mixer and not even now it!
Keep a level head ......See ya next month.
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