Hi there,

I recently installed the digitool mx in a court room and I am having some noise problems that I can't for the life of me figure out. The set up consists of mics via xlr into the digitool and out into a recorder unit called the Dars. When I take a mic straight into the recorder it sounds crystal clear, but when going through the digi there is noticable noise. With each output connected the noise gets louder.. and the end result is definatly not good enough for this application.

Also, without any input to the digi and just a single output (doesn't matter what channel) the noise is present.

I've reduced the noise by gainstaging to the max.. Mic levels are now set at 50db and the outputs are relatively low.. enough so there is no clipping at the dars input.

I bypassed all filters and processors except a low pass filter at both the input and output set around 8k which helped a little.

Also all channels are set to 24dbu
I would have tried upgrading firmware but its already the newest version.
I also tried connecting the ac to another outlet but that did nothing.
The inputs to the usb recorder are xlr.. the input settings are set to condenser mics (which they are).
I've checked all wire connections.

Its weird I installed another digitool recently and everything worked out great.

Any ideas or suggestions?

You can download the project file here -->http://www.divshare.com/download/8946385-ddd
It is the second preset. The only difference between it and the present settings is that all the outputs now match channel 1's.


Kind Regards

Well my friend I downloaded your preset and took a look and it is apparent that you have set the gain structure incorrectly.

You have the Output scale set to +24 dBu and yet your are driving the XLR inputs to your recording media. XLR inputs suggest that this device is looking for Mic or preamp level signal, and you have the output scale on your Digitool MX set for the maximum line level of more than 12 Volts (+24 dBu). You are boosting the noise floor unnecessarily by more than +18 dB.

Try setting the output scale to the lowest ouput level of +6 dB.

You will then have to go back and adjust the trims on your input Mics, but you should have much better results with the lower output setting.


Marty

Thanks for the Reply!

I will definately switch to +6dbu. I think when I tried that last time,
it brought the volume down and when I brought the level back up in the recorder the noise was still present. Although I did not switch the mic input settings and didn't spend much time at all on this setting b/c I figured it was supposed to be 24dbu.

Cheers

The inputs and outputs are scaled in 6 dB blocks. If you would look at the Output LED arrays you will see that the internal level in terms of 0 dBFS (O dB Full Scale) does not change when you change the output scale. The object is to optimise bit management. If you are not using enough bits then you will be closer to the digital noise floor, and if you then have the scale at +24 dB you are essentially amplifying the noise floor unnecessarily.
The trick is to use enough bits without running out of digital headroom.

The way to balance the A/D and D/A bit structure is to use the input/output gain scale to get optimum signal transfer for the intended application. In your case it needs to not be scaled to be a line level device, because your recording media is expecting preamp levels.

I am positive now that it is not a problem with the digitool, but compatibility btw digitool and dars - dmx8 mixer.
Continue reading if you feel like it.. I tried a completely different gain structure, and tried another one as recommended by someone else. I then brought in a digitool that I successfully installed with no issues to the courtroom and.. same problem. So I know the digitool works fine. The usb recorder works fine (when set up straight mic in). But when the digitool is hooked up by a very short xlr cable to the dbx8, noise intereferance is introduced. Last night I was reading through the dmx8 manual and it says something along the lines of "all microphone cables should be directed away from unit and not passed by any computers or electrical sources that can cause interferance". Well it is impossible to have the cables pointed away from the unit when they're both in the same rack. An interesting thought.. the Digitool is a computer. I've used loads of professional recording soundcards and I don't ever recall such specific intructions about keeping the mic cables pointed away from the soundcard and away from computers.. heck my soundcard at home used to sit directly on the tower and I never had any problems. So ya thats where I'm at now. You can delete this if u want.

Hello Ben,

I think I may have had an ah ha moment. I Googled DMX-8 Mixer. I did not realize that this is a digital recording medium. It maybe simply a matter of incompatibility. This problem is occuring more in more often as we see more digital based product in the market place.

It is never a good idea to chain multiple digital audio devices together. There are artifacts of the digital noise floor that do not do well when multiple A to D and D to A filters are cascaded. I call it "dithering the dither."

Some digital products can be cascaded with less problems than others, but there are so many potential variables in how the math is performed that some methods are better than others, the ones with problems will use truncated math techniques the leave digital artifacts that become a source of trouble when redundant A/D and D/A converters are linked.

Does the DMX-8 have the ability to pad the inputs? It may still be possible through gain stage manipulation within both platforms to still end up with a usable system.

I recently had a guy who was cascading 4 different digital products together and he had serious noise problems. Sometime in the future all of these products should have digital inputs and outputs.
There is an AES/EBU digital standard, but it was nearly obsolete when they voted on it some years ago.