Extremely troubling DCX2496-DEQ2496 interface problem - diyAudio
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Old 14th June 2004, 09:35 PM   #1
Vadim is offline Vadim  Canada
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Default Extremely troubling DCX2496-DEQ2496 interface problem

I have been ‘playing’ with both units lately. I have them connected through the AESBU link. Well, I noticed an extremely troubling issue here. While looking at the output signals from the DCX on the oscilloscope, I noticed that there a significant signal shoot-through right across the output channels. Here are the details.

I am running a 3-way crossover and EQ on the DCX for my dipole system. I use the DEQ for the RTA duties and some extra EQ. Also the DEQ has SPDI/F input that I use with my CD Player along with a more user-friendly analog input level, courtesy the 12-22 dB switch.

So, my crossover frequencies are set at 70 and 450 Hz with 24-dB Linkwitz-Ralley alignment. The problem arose when I plugged in my signal generator into the DEQ analog input. While testing with 20 Hz sinusoidal signal I noticed that the output LED’s on the high band (450 Hz and up) also lit up. That was very strange and it was not supposed to happen.

I then used my scope to ‘look’ at the high band channel. To my horror I saw ‘very fast’ voltage spikes reaching as high as 8 Volts. They were 20 Hz periodic! Interestingly this phenomena disappears completely when the signal reaches approximately 80 Hz, or when the analog input signal level drops down to about 3-4 Volts. So asked myself, what the h*** is going on here?

I proceeded to check out each unit separately with analog input signals from my trusty signal generator. So far I noticed no problems when units are used separately and with analog inputs only! I had no way to check the DEQ with SPDI/F input as I do not have this capability. The problem only appears when the digital connection is used between the DCX and the DEQ. So at this point I see a definite problem if you feed the DCX with a AESBU signal.

I also discovered that this problem is a function of a sampling rate. In fact, if instead of 96 kHz sampling rate the 44.1 kHz sampling rate is used, then this problem is no longer there. This definitely points out to a design issue and not a problem with my particular DCX2496. I say this because the DCX uses a software Asynchronous Sample Rate Converter. In some ways it is actually preferable to use a 44.1 kHz sampling rate with your typical CD Player, because the CD player will output the 44.1 kHz data.

On the other hand, I have no idea how the anti-imaging filters are implemented on the DCX, so I fear that if those filters are a switch-capacitor deal, and are optimized for the 96 kHz sampling rate, - then we may have a problem with aliasing while using the 44.1 KHz sampling. All the more reasons to modify the beast!

Well, regardless of the possible aliasing issue the voltage shoot-throw is by far a more objectionable problem, so I stick with 44.1 kHz sampling for now.

I wonder if anyone here would like to confirm what I see and post the results.

Vadim
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Old 14th June 2004, 11:15 PM   #2
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I understand you have an analog input problem, not a digital input problem.

I have not looked at the analog input, but it seems strange to me that there is resampling on the analog input.

On the digital input, there is hardware async resampling.

You have of course tried different input signal levels?

Petter
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Old 15th June 2004, 12:05 AM   #3
Vadim is offline Vadim  Canada
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Petter,

Well, I think the problem is with the digital input. And, yes, I did try different analog input levels. Also I do not see this problem if I simply input the analog signal into the DCX.

In fact, I just upgraded the DCX to the software Ver. 1.16 and I observe the same problem, although, now if I switch from 96 kHz sampling rate to 88.2 KHz sampling rate the problem disappears.

I think I must make it clear, that the DCX does not give the user any control over the sampling rate. It is set at 96 kHz. That is to say that any signal, be it digital or analog will be sample-rate-converted to 96 kHz.

Now, the DEQ2496 has the Sample Rate control option. So, when I use that sample rate control option on the DEQ2496, I observe the changes I talked about. It is my understanding that the user can change the sampling rate of the input signal to the DEQ2496, - and so the DIGITAL output signal from the DEQ2496 will have the same (changed!) sampling rate.

The signal from the DEQ is then fed into the DCX via the digital link, - and I observe the voltage shoot-through problem only when the sampling rate, as set on the DEQ, is 96 kHz. This tells me that any 96 kHz digital input via the AESBU interface to the DCX will have this problem. In fact it seems to me that when the DCX’s sample rate converter ‘looks’ at the digital input, the converter has a problem dealing with 96 kHz input.

Vadim
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Old 15th June 2004, 01:28 PM   #4
tiroth is offline tiroth  United States
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Just for the sake of argument, couldn't it be the case that the DEQ is sending bad data to the DCX? Can you try another 96kHz source to rule that out?

I've observed behavior like you are seeing before...when there was some kind of signal integrity issue in a DAC that was causing double clocking...resulting in full-magnitude digital excursions 1-cycle wide. These looked like little blips on the scope. There was audible 'clicking' though.
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Old 15th June 2004, 03:55 PM   #5
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What an odd dilemma!

So, let me get this straight: when you feed a 96khz-sampled 20hz sine wave into the AES input of your DCX, it misrepresents it horribly? If you sample it at a different frequency, the problem goes away?

I don't think you can conclude, as yet, that the problem is in the DCX--try recording the 20hz sine wave into your computer through AES (if you have that capability) and see if the sine wave is misshaped.

Good luck!
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Old 20th January 2007, 06:56 PM   #6
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Question Have you resolved the DEQ2496 96khz sampling impluse problem?

Hi,

I got the same problem as your DEQ2496 sampling problem when using 96KHZ.

My configuration is SRC2496 -> DEQ2496 -> DCX2496

When I used 96KHZ sampling at the SRC, the sudden impulse or audioable "Clicking' come up from the amplifier and it was very large and can damage the speaker.

So my question, did you find any crue to for this problem? or it has to avoid the 96khz sampling at DEQ2496?

Gary
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Old 20th January 2007, 07:23 PM   #7
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Quote:
When I used 96KHZ sampling at the SRC, the sudden impulse or audioable "Clicking' come up from the amplifier and it was very large and can damage the speaker.
I had a similar problem using a deq. I also run src-deq-dcx.

This problem occured suddenly, I had no problem previous, running the units for over three years.

It did not matter in the end if I ran the unit at 96, 88.12 or 44.1.
I switched from the digital connection to the analog connection between deq and dcx - the problem vanished at any sampling rate.
The problem also did not occur when using the analog input of the src.
It only was audible when using the spdif input of the src.

I finally replaced the deq, and he new unit was fine, except on some cd's at 96 khz some small click sometimes was still audible. I would say on about 5% of cd's i listened to. This also happened only when feeding digital direct from cd player.

I contacted behringer, and their argument was that somehow the high resolution of the deq at 96kHz lays bare flaws on some cd's - funny enough - I never had the problem previous to the end of last year.
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