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Old July 9th, 2005, 12:16 PM   #1
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Beachtek DXA-8

Hi!

I am problably going to buy a Beachtek DXA-8 and wonder how good the limiters are, do you "hear" the volume goes up/down when it limits high signals?

I have bad experience from cameras with Auto Gain, that one can hear how it compensates for high volumes, it "pumps" the volume up and down.

/Roger
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Old July 9th, 2005, 01:41 PM   #2
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Hi Roger,

The CXA-8 limits only; there is no auto gain involved. The limiter is actually pretty good, I've really never noticed any of the pumping you've mentioned.

Of course, when you start to limit heavily, its time to consider turning down the input level ;-)

Cheers,

-Matt
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Old July 9th, 2005, 01:41 PM   #3
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Quote:
Originally Posted by Roger Averdahl
Hi!

I am problably going to buy a Beachtek DXA-8 and wonder how good the limiters are, do you "hear" the volume goes up/down when it limits high signals?

I have bad experience from cameras with Auto Gain, that one can hear how it compensates for high volumes, it "pumps" the volume up and down.

/Roger
Don't have any experience with the Beachtek so I might get corrected by someone with specific info if that model works in a non-standard manner In general lmiters and AGC work in different ways.

A true limiter sets an upper bound to the signal being output - the audio gets louder and louder until it hits that setpoint and then won't go any farther, gain being reduced in step with any further increases in incoming signal. As the incoming signal gets lower, gain increases until the signal drops below the limiter set point and then it stays constant.

AGC, OTOH, reduces gain when the signal is hot but increases it when the signal is low, trying to hold it constant at some midpoint. This means that during quiet episodes it cranks up the gain trying to hear signal that isn't there and ends up bringing up the noise instead. When the desired signal reappears it lowers it again but it takes a moment to clamp it back down. The results is the "quiets" are noisy while sound entrances are initially loud and then drop to the desired level, audibly changing when you're listening.

Limiters are a good thing when needed but IMO AGC is worthless except in those situations where just getting something - anything at all - on tape is preferable to getting nothing and you simply don't have the time, manpower, or hardware to properly set up and monitor the audio recording. A single operator shooting unpredicatable breaking news isn't going to have time to twiddle with audio levels while his eye is glued to the viewfinder as he shoots (if he can even reach the controls) and AGC might make the difference in getting the story or not, but for much beyond that manual control with limiting to avoid accidental overload is the best way to go IMHO.
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Old July 10th, 2005, 05:58 PM   #4
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I've used my Beachtek DXA-8 extensively with my GL2 and the action is very smooth. Like Matt, I never hear any surging at all.

I've done some extensive bench measurements on the the DXA-8 performing with the GL2. Here are the hightlights:

1. The Beach's limiter LEDs light dimly at about 4 milivolts, but no limiting takes place (i.e., the output equals the input) until they're at full brigtness, or about 9 milivolts.

2. The output is clamped at 9-10 milivolts as the input climbs as high as 160 milivolts (LEDs remaining brightly lit, of course).

3. As the input climbs from 160 to 230 milivolts, the output climbs from about 10 to about 14 milivolts.

4. Above that I didn't care, because I could see (on an oscilloscope connected in parallel to the headphones) and hear the GL2's input start clipping between 14 and 15 milivolts. Presumably the GL1's input behaves the same.
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Last edited by David Ennis; July 10th, 2005 at 06:40 PM.
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Old July 11th, 2005, 04:35 PM   #5
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Also note that the limiters will not prevent distortion from over-voltage. I have to use attenuators because my mic can produce higher voltages than the Beachtek can handle when the going gets really loud ("trumpets" and mellophones at 100+ dB) at 50 feet.
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Old July 12th, 2005, 12:43 PM   #6
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Hi all!

Thank you for all your input, it really helps! :)

I have borrowed a DXA-8 and have been testing it and am very pleased with it. I am going back to my dealer and pay for it tomorrow.

/Roger :)
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Old July 15th, 2005, 12:45 PM   #7
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Update:
I found out that the DXA-8 can not be used when the camera is set to use AGC. Sometimes the AGC does a pretty good job, thats why i want to have the opportunity to choose between AGC and manual gain control.

So therefore i decided to buy the DXA-6 instead. The downside right now is that all three units i have tried reverses the phase of one channel and thus makes the stereo signal out of phase. I have been in contact with Harry Kaufmann at BeachTek and he confirmed that the units are faulty and BeachTek are going to fix them! :)

But the DXA-6 works like a charm with either AGC or manual gain control and made it the clear winner for me. I miss the DXA-8's limiter though... ;)

(AGC=Auto Gain Control)

/Roger
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Old July 15th, 2005, 04:38 PM   #8
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Quote:
Originally Posted by Roger Averdahl
...I found out that the DXA-8 can not be used when the camera is set to use AGC...
Why? What happens when you try?
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Old July 15th, 2005, 04:44 PM   #9
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Dear Roger,

I have a DXA-8 and I love it.

To obtain the maximum benefit of the DXA-8, I recommend that you set the camera to manual gain control and set the gain (in the camera) to a minimum. Then you use the very quiet preamplifiers in the DXA-8 to provide the proper signal levels.

On the XL1s, the preamplifiers are noisy, and this completely eliminates the problem (no more hiss).

If you can not set manual gain, I would still buy the DXA-8.

If you got your information from Beacktek, then I would go with it. However, I highly recommend the DXA-8, especially if your camera preamplifers are noisy.
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Old July 15th, 2005, 06:11 PM   #10
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Quote:
Originally Posted by Dan Keaton
...I recommend that you set the camera to manual gain control and set the gain (in the camera) to a minimum. Then you use the very quiet preamplifiers in the DXA-8 to provide the proper signal levels...
One caution. Turning down gain on cams so equipped then turning up the amplitude of the feed to get a good recording level indication invites clippling at the input. Recording level indicators don't care what kind of mess is being recorded. Headphone monitoring is always essential, but especially if you're taking this approach.

If you don't have a noise problem, I'd say adjust the DXA-8 until the limiter LED flashes frequently but dimly. When the LED is dim it's putting out about 4 milivolts (-48 dB), which is comfortably in the nominal range for most cams. Then adjust the cam's gain controls for an average of about -12 dB level meter reading. You'll probably find that the cam's gain controls are not very far from mid range. How far the Beach's control is from mid range depends on the mic sensitivity and sound pressure level.
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Old July 16th, 2005, 07:56 AM   #11
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Quote:
Originally Posted by Fred Retread
Why? What happens when you try?
When using AGC on the camera the hiss is so loud so that you can actually see it clearly on the VU meters on the camera when recording, even when no microphones are connected and the volume controls on the DXA-8 is turned all the way down.

The DXA-8 is absolutely useless when using AGC. The manual clearly states that the AGC must be turned off when using the DXA-8. (The DXA-8 works perfect when AGC is turned off though.)

/Roger
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