Any good A/V sound guys here?

Are there any good forums where the smart A/V guys get to help the pathetically sound challenged such as myself? We have a pair of D-Link DVC-1100 video phones used for institute classes and other things. They work fine except we aren't passing good audio into it. The only input is a 1/8th inch microphone plug similar to that of a PC sound card microphone input.

We need a microphone for the instructor of the class such that when he is speaking, we hear him and nothing else. We also need some sort of microphone we can place in the middle of the room or on a table or something and pick up all the members of the local class. At the remote site we want to be able to hear the opposing class participants when they are called upon to answer questions or provide comments, but at the same time we don't want to hear them bumping the table, opening their scriptures, coughing, or being disruptive. Essentially whenever the instructor is speaking we want everything else muted.

I picked up a Biamp Advantage 301e Mic/Lin mixer off e-bay. It is supose to have this feature to give priority to one channel over the others. I think that part of it works. However, the output is line level. There was an anchient looking adapter discovered in our building that (i think) is suppose to drop the line level back down to microphone level. But, the output it gives is one of the big 3-prong XLR plugs like you would use on the building sound system. I think this gadget is due for replacement, but never the less when I try using it in conjuction with other adapters to get me down to an 1/8th inch input, the sound is terrible. I get a loud humming noise and it sounds like it is overmodulated or highly distorted.

What exactly is the best adapter I should buy to go from the screw terminals on the back of the Biamp mixer, to the 1/8th PC soundcard style microphone input on this DVC-1100 video phone? Are there any doo-dads to put in this place that will reduce the noise level?

Also one time a class member was absent and we tried to record the class on a video tape. Even though the video has a picture-in-picture view to show both ends simultaneously, what occurred to us is that the sound that plays through to the TV is only the sound from the remote end in either direction. Is there any way to combine the local microphone input with the line level audio output that comes out of the video phone, and feed both of these into the VCR--without feeding the microphone into the local TV speaker which would cause feedback? This really isn't all that important, but it would kind of be nice to be able to record the class if we wanted to. Primarely I'm interested in just fixing the live sound quality problems though.

Thanks,
Jeff Phillips

This topic started as an off-shoot from a different thread. I'll move RussellHltn's previous reply over to here:

RussellHltn wrote:First step is use a good quality microphone so the sound is clear. What is the noise? Can it be minimized? Perhaps using a different mic with a different pattern can help. Are students having to go to a fixed area so that the camera can pick them up or all of them calling out from their seats? If they have to go to a "speakers spot" then it's probably just a matter of a good directional mic on a stand. Failing that, I think you're looking at multiple mics and a automatic mic mixer to select the one closest to the speaker. That's just the thoughts off the top of my head. Details would help.

Typically the main class uses the stake center's high council room. We place the video phone on top of one of the TV roller carts at the back end of the room and aim it at the instructor in the front. The local students there sit on either side of the big high council set of tables.

Because of smooth and interactive flow of discussion between the instructor and various students, it is cumbersome for them to pick up a microphone to pass around -- typically people are only interjecting a brief few words. By the time the microphone is passed to them the class has already moved onto other responses to what they have suggested.

We have some omnidirectional microphones, I don't know what they are called... They have a flat base and sit on the table. Kind of trapezoidal shaped... pretty low profile and slanted to sort of aim toward the group of students. These seem to pick up *everything* to the point where we can't hear the instructor well.

We also have a wireless lapel microphone that the instructor wears. It works quite well except when the class members are sliding things across the table. The mixer we obtained seems to fix that problem but then introduce its own. Now I'm trying to figure out how to get good sound out of this thing and into the microphone jack on the back of the D-link video phone.

-Jeff

The first thing is to find out what the pin out of that jack is. PC mics and small tape recorders use the same size jack, but it's not wired the same way.

The other is to find out what kind of line out that mixer has. That is, what reference (if any) does it have to ground. Otherwise you can create problems when trying to connect it to other things.

RussellHltn wrote:The first thing is to find out what the pin out of that jack is. PC mics and small tape recorders use the same size jack, but it's not wired the same way.

D-Link has been less than forthcoming as far as what the exact specifications are of their microphone input jack. I've asked about this before. I have a pretty good hunch that it is setup like a PC soundcard microphone in jack, and not like a tape recorder.

The other is to find out what kind of line out that mixer has. That is, what reference (if any) does it have to ground. Otherwise you can create problems when trying to connect it to other things.

Here is a link to the manual for the Biamp Advantage 301e mixer. I'm not sure what I'm looking for on here???

http://www.biamp.com/discontinued_produ ... prod&tid=6

"Main Output: These three terminals provide an electronically balanced Main Output from the mixer, wired with high to (+_, low to (-), and ground to (COM). For unbalanced output, connect hight to (+) and ground to (COM), leaving low (-) unconnected. Main output signal is a combination of the various channel signals, as well as signal from Stack Input (7) and any signal processing applied at Patch. This output is for connection to the input of sound system amplifers, tape recorders, etc."

So I gather I need unbalanced to go into this video phone. But, what is the best thing to put inbetween?

-Jeff

OK, the first thing is pay attention to what the mixer manual says about connecting unbalanced - Connect the shield to "Com" and the signal wire to "+". Do not use the "-" terminal.

The D-Link uses a mono mic connector, not the PC mic style. So we won't have to worry about phantom power and a few other issues.

You'll need some kind of attenuating device. Something like this. If you go with this particular cable, you'll need to find or build a female RCA to wire end cable to connect to the back of the mixer. Also plug the video unit and the mixer into the same outlet or power bar to avoid ground problems. Don't run the levels on the mixer too hot, because it's what I call a "professional line level" output and it's a bit hot for "consumer line level" devices. Not a optimal situation, but not too ugly.

Ohhhhhh. I just came back from eBay. Someone has a Shure Automatic Mic mixer cheap. I think I played with those just before I bailed out of the conference audio business years and years ago. Neat units when you can find the special mics that go with them. Not sure how well they work with standard mics. I think they are cheap because everyone has gone DSP/Programmable units now.

Disclaimer: Commercial links and/or references are not an endorsement by the Church. Opinions reflected are my own.

RussellHltn wrote:You'll need some kind of attenuating device. Something like this. If you go with this particular cable, you'll need to find or build a female RCA to wire end cable to connect to the back of the mixer.

Our local radio shack dealer doesn't sell that perticular cable. The one they sold me is 1/8th inch on both ends, which I can deal with. What bothers me is that the package says it is "for connecting between speaker-out and mic-in jacks when recording". Of course I'm not using a speaker out jack, I'm using (what you call) a "professional line level" output. This one is 90dB attenuation. Is that too much? Too little?

The website doesn't say what the attenuation amount is of either this one or the one you posted the link to. This one I bought is CAT # 42-2152A. The website rejects the "A" on the end, and #42-2512 says online that it is used to, "Record from a line-level output with our 6.56-foot attenuating dubbing cord. It connects the earphone jack from a radio, cassette or CD player to a recorder's input or mic jack via 1/8" phone plugs on each end."

Isn't an earphone jack speaker level? Isn't that different from line level? Why would the package say speaker level and the website say line level? I'm confused.

Also plug the video unit and the mixer into the same outlet or power bar to avoid ground problems.

The video phone doesn't have a grounded outlet, just a regular two-prong AC/DC adapter. Should I attempt to ground it somehow?

Don't run the levels on the mixer too hot, because it's what I call a "professional line level" output and it's a bit hot for "consumer line level" devices. Not a optimal situation, but not too ugly.

Hmm. Is it better to have the microphone channel levels high and the master out low, or vise versa? Or does it not make any difference? I'm never sure which one I'm suppose to be adjusting, or which one is being adjusted. It's especially difficult to tell as I am 55 miles away from the end that has the microphones when people are actually present to be talking into them. Oh sure, the "test, 1, 2, 3" guy always speaks loud and clear into the microphone, so we turn it down while testing and then can't hear any real normal speech during class.

Thanks,

-Jeff

jeffphil wrote:The one they sold me is 1/8th inch on both ends, which I can deal with. What bothers me is that the package says it is "for connecting between speaker-out and mic-in jacks when recording". Of course I'm not using a speaker out jack, I'm using (what you call) a "professional line level" output. This one is 90dB attenuation. Is that too much? Too little?

Ehhhhh, hard to say. It depends on how much you crank up the speakers. A bigger issue may be what impedance the cable presents to the mixer. The mixer won't want to load into anything less then 300 ohm. Higher is ok, lower is lower then designed.

jeffphil wrote:Isn't an earphone jack speaker level?

That's a loaded question. Not too long ago most earphones where 8 ohm speakers you stuck into your ear. Needless to say, you didn't want them to be the same level as your speakers - that would be painfully loud. Now many earphones are about 40 ohm and less of a problem that way. I wouldn't worry about connecting them. It's not off enough to blow things up, but it may not work out that well. My thought is that it's designed for small electronics such as tape recorders and radios and stuff. For them, speaker, earphone and maybe line out are all "close enough" that the mic input can handle it.

You might call around the other stores and see if they have the one I pointed out. Since it has an RCA jack, I'd assume it's more like what we're looking for.

jeffphil wrote:Why would the package say speaker level and the website say line level? I'm confused.

<sigh> I'll refrain from comments that might create legal issues. :rolleyes: I do see where the on-line site says it both ways.

jeffphil wrote:Hmm. Is it better to have the microphone channel levels high and the master out low, or vise versa?

In general you try to shoot for having them around the setting. Don't spend a lot of effort to do that, but you don't want either of them really high while the other is really low.

We're in the middle of our stake basketball playoffs, and I had a couple of people that wanted to participate but lacked transportation. Giving them a ride provided a good excuse for me to get up to the stake center and tinker with this mixer project during their games.

I took both video phones out there this time so I could hear the effects while playing with the mixer. (The 55 mile distance between the microphones and the speaker has been a significant pain in terms of getting this all setup and adjusted right.) I wasn't able to get this 90 dB attenuating cable to do diddly squat. When I hooked it up between the mixer's line-out and the video phone's mic-in, all we heard was silence--as though it wasn't hooked up to anything.

I tried hooking the line-out up directly to the mic-in without anything in between to reduce the level. This actually worked better than anything else we've tried. The only thing is, we have to turn the volume way down low on the TV to avoid it being heard at an excessive volume. Also there is a humming sound in the background that is rather annoying.

On the bright side, the audio is understandable. It doesn't sound like it is being distorted or over modulated as it was before. I've concluded that the 20 year old transformer that we found in some storage box upstairs in the branch intended for connecting line-output devices to the building PA system's microphone inputs was either complete junk, or not the appropriate thing to use in this project. A simple cord does a much better, although still not all that great of a job.

I guess I should try the other attenuating cable you recommended? Unless it is the same as this 90 dB one didn't work. What's the difference between an attenuating cable, verses using a transformer or a "pad" as I've heard some people make reference to? What can produce the best sound?

-Jeff

jeffphil wrote:When I hooked it up between the mixer's line-out and the video phone's mic-in, all we heard was silence--as though it wasn't hooked up to anything. I tried hooking the line-out up directly to the mic-in without anything in between to reduce the level. This actually worked better than anything else we've tried.

Makes me wonder if the mixer is OK. Are the jumpers in place for the in/out patch and the remote? Does the -20 light come one reliably with any sound and the peak come on occasionally?

jeffphil wrote:I've concluded that the 20 year old transformer that we found in some storage box upstairs in the branch intended for connecting line-output devices to the building PA system's microphone inputs was either complete junk, or not the appropriate thing to use in this project.

If it's what I think it is, it's designed for connecting unbalanced line outs to balanced mic in. Your going from balanced line out to unbalanced mic in. Lots of places to go wrong there.

jeffphil wrote:What's the difference between an attenuating cable, verses using a transformer or a "pad" as I've heard some people make reference to? What can produce the best sound?

A pad is an adjustable attenuator while an attenuating cable is a fixed attenuator. A transformer is more appropriate in cases where you need electrical isolation or impedance matching.

RussellHltn wrote:Makes me wonder if the mixer is OK. Are the jumpers in place for the in/out patch and the remote? Does the -20 light come one reliably with any sound and the peak come on occasionally?

Yes, the -20 light comes on when we speak into the microphone at a reasonable volume on either channel, and the limit light flashes at loud noises when we turn the levels up high. The mixer is mixing, as its ducking effect is clearly heard through to the TV at the remote end of the call. It's just that there is a hum in the background probably as I don't have it hooked up at the appropriate levels -- right now I'm just going straight out of the line level and into the mic level input with nothing in between. The volume is much louder than it should be this way.

If it's what I think it is, it's designed for connecting unbalanced line outs to balanced mic in. Your going from balanced line out to unbalanced mic in. Lots of places to go wrong there.

Ah, that sounds right. Perhaps I won't through this 20 year old transformer away just yet after all. Maybe it works just fine for another purpose.

A pad is an adjustable attenuator while an attenuating cable is a fixed attenuator. A transformer is more appropriate in cases where you need electrical isolation or impedance matching.

Oh that makes sense! Thanks for the explanation. So when an attenuator is sufficient, a transformer isn't going to do anything any better in this sort of application?

-Jeff

jeffphil wrote:So when an attenuator is sufficient, a transformer isn't going to do anything any better in this sort of application?

Correct. In fact it can make things worse. Transformers have a limited frequency response. Not really an issue for this application, but it would for high fidelity audio.

Is there any kind of filter or gizmo that will reduce the background "hiss" or "hum" that is always there? I imagine deleting such sound would distort musical pieces, but for just people speaking I am wandering if there is anything that can make it go silient in between words/sylables instead of that constant hissing noise. I think a small part of the hiss is inately part of the video phones themselves.

Thanks for all your help! You've been awesome! (I really need to sit down and get more up on audio gear.)

-Jeff

Generally you shouldn't hear any hiss or hum. That suggests that the levels aren't set right. I'm still wondering why you are getting mic levels out of a line level out. Just the normal background of a line level out is going to have a fair amount of noise because the signal is supposed to be much higher. Did you get a chance to check the two jumpers in the back and make sure they were set the way the book said they should be?

Either that, or try the attenuator cable again. The attenuator will take everything down and then you can turn up signal.

RussellHltn wrote:Generally you shouldn't hear any hiss or hum. That suggests that the levels aren't set right. I'm still wondering why you are getting mic levels out of a line level out. Just the normal background of a line level out is going to have a fair amount of noise because the signal is supposed to be much higher. Did you get a chance to check the two jumpers in the back and make sure they were set the way the book said they should be?

Either that, or try the attenuator cable again. The attenuator will take everything down and then you can turn up signal.

Well I don't know why the attenuator cable didn't work. But the other day one of the Radio Shack guys recommended a 1:1 matching transformer. I tried that, and now it seems to be working a lot better. I haven't had a chance to fully test it from the remote end though.

-Jeff

jeffphil wrote:1:1 matching transformer

A 1:1 will provide isolation to take care of ground problems. An impedance matching transformer will bring the volume level up and may make things clearer.

RussellHltn wrote:A 1:1 will provide isolation to take care of ground problems. An impedance matching transformer will bring the volume level up and may make things clearer.

I'll guess much of the improvement was because of grounding issue, being as how the video phone doesn't even have a grounded outlet even though everything was hooked through the same power strip.

Are you saying I'd be better of with a different (non 1:1) transformer? How do I know what ratio to get? The one I'm using now is R.S. CAT # 273-1374:

http://www.radioshack.com/product/index ... age=search


-Jeff

jeffphil wrote:Are you saying I'd be better of with a different (non 1:1) transformer?

Depends on if you need impedance matching. (low impedance connected to high impedance or the other way around.) Impedance matching will give you high power transfer which results in a higher volume - perhaps even overload requiring attenuation depending on what's going on.

Such transformers are not rated by turns ratio, but by the impedances they are designed to match.