when would I use Shure A15AS attenuator?

[Chris Gorman]

I'm thinking about returning this attenuator which I got along with the Shure A15LA attenuator and some connection adaptors. Before returning it I'm wondering if there might be situations where I would want to have this switchable attenuator. It switches to 15, 20 25, and female xlr in and male xlr out.

[Steve House]

Originally Posted by Chris Gorman

I'm thinking about returning this attenuator which I got along with the Shure A15LA attenuator and some connection adaptors. Before returning it I'm wondering if there might be situations where I would want to have this switchable attenuator. It switches to 15, 20 25, and female xlr in and male xlr out.

You would use it to drop the level of a signal, say a hot mic signal, down to the level a more sensitive input needs to see without driving it to clipping. Lets say your mic puts out -30db and the input sensitivity on your camera's mic connection is -55db. Insert the attenuator into the line and set the switch to 25db (cut). The result will be a mic signal at -55db, properly matched to the camera.

[Chris Gorman]

[QUOTE=... Lets say your mic puts out -30db and the input sensitivity on your camera's mic connection is -55db. Insert the attenuator into the line and set the switch to 25db (cut). The result will be a mic signal at -55db, properly matched to the camera.[/QUOTE]

Thanks. I still have several questions - some just kind of basic dumb questions. (1) Do I look at the sensitivity of everything in the chain? eg., Mic, the mixer/board it's going through, my Sony UM1 transmitter (audio input level = -60dBV to -39 dBV), Sony UM1 wireless receiver (audio output level = -58 dBm), Sony Z1U camcorder Audio signal = 327 mV at load impedance 47 kilohms, Input impedance more than 47 kilohms.

I just copied those specs from the manuals but don't a clue how to translate that info into any useful practice.

I don't see any specs listed for input sensitivity for my iRiver Mp3 voice recorder - which I might decide to someday hook up to a DJ's output if it can be done safely without blowing up the iRiver. Of course I'd first have to get some custom cable made - or maybe just an adapter to go from the line stereo mini plug into whatever.

(2) I'm dealing with locations where I don't know anything about the mic at the source or the equipment where the sound is being passed through because it's whatever is there at a concert, convention, wedding reception, etc. They'd give me a mic or line feed from an xlr, rca, or 1/4", but I have no way of knowing signal strength, and usually they don't either.

(3) besides the technical part of my question, I guess what would also really be helpful is hearing from other shooters working in similar locations and what attentuators, etc., they have found useful.

My goal is to at least have in my bag the things I might need to attach at various locations.

[Steve House]

Well, regarding the iRiver, you just stumbled on one of the problems that sometimes surfaces when you use consumer equipment - it's sometimes hard to know just what the manufacturer was thinking of or what choices they made.

Your first question was whether you would ever have a use of the attenuator and IMHO it's like a screwdriver or soldering iron, something you need to have in your kit just in case you run into a situation where you need it. Things like that are like having a parachute on an airplane test flight - it's better to have one and not need it than it is to need one and not have it. It's purpose is to give you a selectable reduction in signal strength for anytime you need to drop a signal level by 10, 15, or 25 db. That could be in plugging a hot mic into your camera's xlr mic inputs or it could be reducing a hot line level or any number of other scenarios. Like in my first example, looking at the specs in the manul the Z1U is rated a -60dBu at the XLR mic input. Audio Technica's AT897 shotgun mic is rated at -40dB sensitivity. If you plugged that mic into your camera's XLR inputs you would have to reduce the recording level control a lot to keep it from clipping and loud sounds might send a strong enough signal to overload the camera's mic preamps. So you'd use the attenuator between the mic and the camera to reduce the signal by, say, 15db or 20db to reduce the chance of overload.

So not only would I not send it back, I'd suggest you even get another one because if you're recording stereo you have two channels, L & R, to worry about and if you need to pad the signal in one of them you'll probably need to pad it in the other as well.

When setting recording levels you do need to look at everything in the chain. It's called "gain staging" and it basically means adjusting the levels at each step of the process so everything is running at its best settings. But things like the attenuator only directly affect the signal in the one link of the chain where it's inserted. So if the signal is clipped because you're taking a feed off the house sound board and the house audio guy doen't know how to set levels properly on the mic input to his board, there's nothing you can do to fix it in the line from the board to your camera. On the other hand, if he's sending you a signal thats too hot for your camera inputs, an attenuator is one of the possible ways to correct it.

Incoming signal too hot for the input? Problems with overload, distortion, and clipping. Incoming signal too low? Problems with excessive circuit noise, hiss, intereference picked up by the cables, etc due to having to turn up the gain to get acceptable recording levels. Like Goldilocks, you gotta find the mix that's just right.

If you're getting sound from the house board you really don't need to worry about what mics are being used etc. All you care about is what levels are being handed to you from the sound board. A "mic level signal" is typically in the range of -55dBu and that's also pretty close to the output of your Sony wireless receiver and what your ZiU expects on the XLR connectors when they're set for mic level. With line level it's a bit more complcated since "0 VU" on a meter, the normal operating level in other words, is measured with two standards, +4Bu for professional gear and -10dBv for consumer gear. That means a professional mixer whose meters are reading 0 VU output, the start of the red arc at the top of the meter scale, will be sending a signal that is actually almost 12dBu hotter than the same meter reading on a consumer tape deck or CD player or on a mixer set to output the consumer -10dBv standard. Many sound boards can be set so the "0" in their meters represent either standard and so you'll need to ask the board operator how he has it set up. Your Z1U's XLR inputs expect the professional level where +4dBu represents "normal 0VU" signal when you set it for line rather than mic input.

Hope this helps - it is a bewildering topic.

[Chris Gorman]

Thanks for the great explanation and your patience - much appreciated.. I hope I won't wear out my welcome by asking for yet a bit more clarification. Is there an actual danger to my equipment if I accidently input a signal way too strong, or is it safe to plug it in and listen for distortion before making a decision to attenuate?

If I knew in advance the board was giving me a pro line level for my Sony wireless, I guess I should automatically put the 50 dB attenuator in the line between the board and transmitter.

The numerical markings on the switchable A15AS attenuator barrel still are confusing. Next to the switch are numbers 15, 20, 25. Above that a diagram with cryptic markings and numbers seems to show 10, 20, 30 repectively for each setting, if I read it correctly.

Now I see where in the manual you're reading the audio specs for the Z1U. What do the 2 lines under that mean +40 dBU, and 0 dBu=0.775 Vrms?

Should I even be thinking about he Z1U's menu settings where you can set trim levels (trimming what?) for each audio input. Also in the menu you can set audio output to 2Vrms (default) or 1Vrms - what does that mean?

[Steve House]

Originally Posted by Chris Gorman

Thanks for the great explanation and your patience - much appreciated.. I hope I won't wear out my welcome by asking for yet a bit more clarification. Is there an actual danger to my equipment if I accidently input a signal way too strong, or is it safe to plug it in and listen for distortion before making a decision to attenuate?

If I knew in advance the board was giving me a pro line level for my Sony wireless, I guess I should automatically put the 50 dB attenuator in the line between the board and transmitter.

The numerical markings on the switchable A15AS attenuator barrel still are confusing. Next to the switch are numbers 15, 20, 25. Above that a diagram with cryptic markings and numbers seems to show 10, 20, 30 repectively for each setting, if I read it correctly.

Now I see where in the manual you're reading the audio specs for the Z1U. What do the 2 lines under that mean +40 dBU, and 0 dBu=0.775 Vrms?

Should I even be thinking about he Z1U's menu settings where you can set trim levels (trimming what?) for each audio input. Also in the menu you can set audio output to 2Vrms (default) or 1Vrms - what does that mean?

Don't go plugging the mic input of your camera or iRiver into the AC mains but there's probably not much danger of letting its supply of magic smoke escape with normal audio levels.

The 1 volt or 2 volt setting on the audio output is to allow you to pick a comfortable volume when you plug external speakers into the audio output. You might use the 1 volt setting if you're using amplified speakers and the 2 volt setting for plain speakers.

The input sensitivity of a piece of equipment is whatever signal at the input causes it to deliver its rated output or otherwise perform normally.

The audio trim setting is an adjustment of the sensitivity of the mix level input. I don't have a Z1 so I'm just reading the manual here but it appears the Audio Trim is a coarse setting for the Audio Recording Volume control, adjusting for the different levels of input signals that it might be presented. I can't find anything to tell me if it is a pre-preamp attenuator or if it controls the mic preamp's gain - maybe someone else knows for sure and will chime in - nor do I know whether like setting it to "+6db" makes the input 6db more sensitive or 6db less sensitive - you'll have to experiment and see if it gets louder or softer. But its function is to adjust the sensitivity of the mic level input on the XLR connectors so you can use various mics (the actual output levels with different mics can vary widely depending on the type of mic it is and the sound levels - not all mics output at standard "mic level", if that makes any sense).

I think the "+40" is a misprint and should read "+4." Decibels are ratios and they express degrees that something is bigger or smaller than something else. So when we say a signal's level is "+4dBu" that means it's 4 db higher than some arbitrarily defined "0db" level. With equipment intended for professional and broadcast audio, the standardized 0dB level is 0.775 volts RMS (Root Mean Square - think of it as the average loudness of a sine wave) and the nominal operating line level is +4dBu. For consumer gear, the 0db reference voltage is 1 volt RMS and the nominal operating line level is -10dBv. (Notice the "u" and "v" - they are slightly different sized units.) When the dust settles, the nominal line level or "0VU" level in pro gear is approximately 12db stronger that the 0VU level in consumer gear.

Don't know what the 10, 20, 30 refers to - the pad is selectable for 15, 20, or 25 db of signal reduction.

[Chris Gorman]

Thanks, this is quite an education on the math side of it, it's still sinking in, hope it sticks.

Now that I look more closely I see on p.68 of the manual they give an explanation that makes me wonder now if this serves the same purpose as the external attenuator.

My on board shotgun tends to output a weak signal so I'm remembering now that I had actually adjusted the trim on that input to attempt to accommodate during an early test and then forgot all about it.

Uh,oh, now with re-reading about the audio Limit, p.65 - only valid when audio select "switch" is set to manual. Near those switches on the body are the dials for audio levels - which are apparently only for internal mic. Nothing indicates if the "switch" is for internal or external, or both.

Wish they had a chapter in the "Hands On HDV" DVD on the details of the audio settings.

[Steve House]

Originally Posted by Chris Gorman

Thanks, this is quite an education on the math side of it, it's still sinking in, hope it sticks.

Now that I look more closely I see on p.68 of the manual they give an explanation that makes me wonder now if this serves the same purpose as the external attenuator.

My on board shotgun tends to output a weak signal so I'm remembering now that I had actually adjusted the trim on that input to attempt to accommodate during an early test and then forgot all about it.

Uh,oh, now with re-reading about the audio Limit, p.65 - only valid when audio select "switch" is set to manual. Near those switches on the body are the dials for audio levels - which are apparently only for internal mic. Nothing indicates if the "switch" is for internal or external, or both.

Wish they had a chapter in the "Hands On HDV" DVD on the details of the audio settings.

The camera's audio recording level dials apply to both internal and external mics. They control whatever audio input has been set as the active source, be it internal mic or external. The notation "int mic" next to the dial for channel 1 is a reminder that the ch1 dial adjusts both ch1 and ch2 when you use the internal mic.

[Chris Gorman]

There must be pros/cons of setting the switch(s) to auto vs. manual. Here I'm talking about situtations where you're working alone and can't ride levels once the action begins, sometimes don't even have the chance to do a sound check of the actual action in advance.

So far I've done pretty good using the auto setting on the cam body, but not sure if this is the best practice.

I always keep the Limiter set to ON in the menu which seems to me to be a good safeguard in case I need to switch one or both xlr inputs to manual in a hurry.

There are so many variables I not sure if there would be situations (keeping mind no assistant) where it would be preferable to switch to Manual on the camera, use the wheel(s), with the "safety" of having the Limiter set to ON. Maybe if the source was really low despite other optimized pre-sets, I'd use the wheel to dial it up at bit - I guess risking a worse SN ratio?

My goal is get to the point that I'm so familiar with all the Z1U audio settings and best options, that I can spontaneously in the field do the right thing whatever the circumstance.

First I want to make sure I'm starting with the correct understanding, so this discussion is very helpful. Any additional comments you might have about my guess work here are appreciated.

[Steve House]

What you risk with the Auto setting is increased noise and "pumping." Imagine you're recording someone who's saying " See,......when we were still in base camp ...... we could see the weather moving in ..... but ...um... we thought we could get back...um... before the storm broke." Conversation interrupted with long pauses. During the pause when he's not speaking the camera thinks the signal is still there but very soft and so it increases the gain "listening" for a voice that really isn't there. Background sounds, circuit noise and mic hiss, air conditioning, airplanes flying overhead, camera handling noises (if the mic is on the camera) get louder and louder. Then the subject speaks but now the gain is turned way up too high so the first syllable is very very loud but then the AGC catches it and clamps the gain back down and it drops back down to normal. Then he pauses again and we repeat the whole cycle over again.

[Seth Bloombaum]

Ditto what Steve said.

Also, depending on the output level of the mic, auto gain can really mess with music recording. I'll go out on a limb here and say that auto gain should *never* be used with music. Blindly compressing the dynamic range of music and pumping up the quiet sections can make it sound very un-musical...

[Chris Gorman]

Thanks, excellent point. In fact, that's how I've always worked with my older camcorder used with a Beachtek adapter, cam audio set to "Manual" and then Beachtek dials set. I must have gone brain dead or something dealing with all the Z1U audio options because I somehow forgot about how important it is to avoid auto in any case. Thanks for the wake-up.

I'm packing up now to head out for a shoot - actors stage performance in a tiny theatre, so I get to see how my audio choices work. This is a spur of the moment volunteer gig, with the option to sell the footage to any of the the actors who might want their performance on dvd, tape.

Very unique challenge on this one - actors voices not going into any sound board, only music through the board. At most - actors voices just out to some speakers or maybe not even that. Monologues, one after another, so I can't switch my lav(s) from one actor to another, which could be done with an assistant backstage. Looks like I'll have to put my lav somewhere near the speakers, and maybe get ambient with my on camera shotgun.