dcx2496 attenuation

Hi all,

I've decided to go with the Behringer DCX2496 to do the sub EQ/delay/phase adjustments as I add a subwoofer to the 2-channel system in my home office. It's overkill for just the subwoofer, but it gives me the future expansion capability to bi-amp my main speakers - something I've been wanting to do for a long time.

I'm using 2-channel, non-home-theater gear. I'm planning on running the DCX in the pre-out/power-in loop on my integrated amp. I'll run one output to the sub, and two more to the poweramp inputs to drive the main speakers.

The outputs on the DCX are at pro levels, I've read that I should attenuate those by about 20db to get them back to consumer levels. I have some XLR to phono adaptors with some room in the shell to hold resistors, so my question is:

What value resistors should I use, and what topology? (One resistor in series, 5 in an H-pad, or something else?)


(BTW - I had some fun last weekend playing with REW and getting a baseline
reading on the room before I start adding treatments, EQ, etc. Extremely cool
software! I'll post some graphs when I am done.)

Best regards,
Ron

[tbrooke]

This may be more than you want to know but check out:
http://www.diyaudio.com/forums/showthread.php?threadid=63547

Tom

[brucek]

The outputs on the DCX are at pro levels, I've read that I should attenuate those by about 20db to get them back to consumer levels. I have some XLR to phono adaptors with some room in the shell to hold resistors, so my question is:

What value resistors should I use, and what topology?

Ron, you're a bit confused here. The DCX accepts pro levels and will pass them through as such. This would be a unity gain device. The maximum input level to the DCX is +22dBu. This defines its operating range for both input and output and as a result its dynamic range.

The level 22dBu translates to ~9.75vRMS. This means I can send the DCX any level from 0volts to 9.75vRMS and its output (with no filters etc applied) will swing from 0volts to 9.75vRMS.

You have a consumer device that has an output level that generally would have a maximum output level of ~+2dBV. This translates to about 1.26voltsRMS out. See the problem?

Your maximum level of 1.25volts will pass through the DCX just fine and output at the same level. The rub is that you are not taking advantage of even a quarter of the bits available and you'll have a very poor S/N ratio and dynamic range.

If you are going to mix pro and consumer level devices, you'll need to boost your consumer level output to a pro level before feeding the DCX. Most people use a CleanBox device, although there are others that will do the job.

Remember that all these devices in the mains chain add noise... and its cumulative.....

brucek

Thanks, I had not seen that post before. Looks like lots of fun stuff to go through.

Best regards,
Ron

brucek wrote:

Ron, you're a bit confused here.

Not too unusual, I'm afraid.

brucek wrote:

The DCX accepts pro levels and will pass them through as such. This would be a unity gain device. The maximum input level to the DCX is +22dBu. [...] ~9.75vRMS. [...] You have a consumer device that has [...] a maximum output level of ~+2dBV. [...] Your maximum level of 1.25volts will pass through the DCX just fine and output at the same level. The rub is that you are not taking advantage of even a quarter of the bits available and you'll have a very poor S/N ratio and dynamic range.

Ah, thank you for that explanation. That makes the issue very clear.

brucek wrote:

If you are going to mix pro and consumer level devices, you'll need to boost your consumer level output to a pro level before feeding the DCX. Most people use a CleanBox device, although there are others that will do the job.

Remember that all these devices in the mains chain add noise... and its cumulative.....

brucek

OK, thanks for the pointer to that device, as well as the caution on noise. Raising and lowering that level so many times sure runs it through a lot of opamps. I'll have to think about that.

Thanks,
Ron

[Ilkka]

brucek wrote:

Your maximum level of 1.25volts will pass through the DCX just fine and output at the same level. The rub is that you are not taking advantage of even a quarter of the bits available and you'll have a very poor S/N ratio and dynamic range.

I use this device in consumer level system without any S/N ratio or dynamic range problems. I'm sure they would get better with higher input level but IMO you are making it sound like a huge problem. I used to have an analog EQ before this so I know how my system should sound like.

I'm running my whole front end (LCR) and sub through it.

[brucek]

I use this device in consumer level system without any S/N ratio or dynamic range problems

Well, I can only look at specs and make suggestions, so I have to defer to what you say, since I don't own a 2496.

But, as I'm sure you know, the LSB (least significant bit) voltage is directly proportional to the maximum input full scale voltage. In this case it's +/-9.75 volts RMS. So, if the input level isn't high enough, the LSB will not be distinguishable from the noise. The noise floor for the ADC converter is fixed and a function of the bit resolution, which in this case is 24bits. It has a theoretical noise floor of 147dB.

Well, of course the device can't obtain that figure, but it does show an incredible noise floor spec of 112dB in relation to full scale of 22dBu. That's extremely good (almost unbelievable actually ). Well, 112dB translates to about 18.5 bits usable above the noise. Quite good.

This all assumes you have a full scale input signal with a swing of +/- 9.75 volts. As you decrease the input signal, the noise rises exponentially. It's the nature of the math. A full scale of 1.25 volts is only about 13% of the possible full scale allowed. I won't bore anyone with the math, but this translates to about 4 bits being used with a dynamic range of about 24dB. That's horrible.

What can I say.

brucek

[Ilkka]

I know the math but that doesn't really bother me since I know it works just okay in my consumer level system. I don't know how it does it but it doesn't sound any worse than my old analog EQ.

[Wayne A. Pflughaupt]

...but this translates to about 4 bits being used with a dynamic range of about 24dB. That's horrible.

Wow. Before I saw that I was going to say, considering receivers typical 90-95 dBa S/N ratings will be at least 20 dB or more worse than the DCX’s, there’s a lot of S/N to spare. But 24 dB! Obviously, that should be grossly audible, so something doesn’t add up. Maybe the math looks worse on paper than what it translates to in real life.

I don't know how it does it but it doesn't sound any worse than my old analog EQ.

Does that mean your analog EQ sounded bad? What kind was it?

Interesting that the extra A/D-D/A conversion doesn’t appear to be audible. Must be some really good converters. Still, I wish manufacturers would give us receivers with digital pre-amp outputs, and amps that would accept a digital signal. Maybe someday.

Regards,
Wayne

[Wayne A. Pflughaupt]

Probaby opening a can of worms here, but...

As you decrease the input signal, the noise rises exponentially. It's the nature of the math.

Well, what if we follow that to it’s obvious conclusion? With the unit idling – i.e. no input signal - the noise should be so loud it would be audible in the next room. But that isn’t the case, is it?

After a little ruminating and a little research, it seems the discrepancy comes from us commonly (and grossly) confusing S/N ratio with dynamic range. (I’ll include myself here – see my last post!! I’ve seen most of this material before, but somehow today it just clicked.)

The two are related, but they are not the same. S/N is a measurement of residual noise. Dynamic range is the ratio of the loudest undistorted signal compared to the quietest discernable signal – i.e. before it “disappears” into the residual noise.

It’s entirely possible to have a component - an analog component at least - with an inaudible noise floor, but limited dynamic range. In other words, it would clip with a 20-dB signal, but still be dead silent with no signal.

Likewise, it’s possible to have very poor residual noise characteristics with excellent dynamic range. Anyone here have any experience with guitar amps? They’ll play loud enough to make you deaf, but unplug the guitar and you can hear the hiss and noise from across the room.

I caught this tidbit from Rane’s Digital Dharma of Audio A/D Converters white paper:

With 20-bit high-resolution [A/D] conversion, low signal-level detail is preserved. The improvement in fine detail shows up most noticeably by reducing the quantization errors of low-level signals.

Notice, no mention of noise with the presence of a low-level signal.

And this:

Here is what is gained by using 20-bits [over 16]:

• 24 dB more dynamic range
• 24 dB less residual noise [emphasis theirs]
• 16:1 reduction in quantization error
• Improved jitter (timing stability) performance

It should be obvious that when it comes to digital audio electronics, “the math” is referring to dynamic range, not S/N. Indeed, “the math” is dependant on the presence of a signal. The math doesn’t function with no signal. Mathematical formulas don't have much to do when you start with a value of zero.

Getting back to our DCX, obviously it doesn’t make sense that its dynamic rage will be limited to 24 dB if used with a home system. All a lower signal level means is that you’re merely not fully utilizing its available dynamic range. But in no way does that mean you’re increasing its residual noise floor. As we see in the second quote above, that’s a function of the A/D converter: As long as the converter is sufficient, the residual noise specification is there. The dynamic range spec, by its very nature, requires a signal in order to be utilized.

In this regard analog and digital equipment are the same: The residual noise floor is determined by the quality of the component itself, not the level if the incoming signal. You don’t raise the inherent noise floor of a component by feeding it a low-level signal. If that were the case, the noise would come up to increasingly audible levels as we turned down the volume, and there’d be no such thing as “background music.”

This also answers “the great boosting debate” with the BFD: Boosting will decrease available dynamic range, because the incoming signal will have to be reduced. But it doesn’t increase residual noise levels. I imagine this is why no one has ever complained of noise problems appearing with boosted filters.

I suppose the BFD or any other digital EQ could increase noise at the adjusted frequency, in an amount equal to the level of boost, as you have with an analog equalizer, but I'm not altogether sure if even that's possible, given what Rane says, "It is important at the onset of exploring digital audio to understand that once a waveform has been converted into digital format, nothing can inadvertently occur to change its sonic properties." So I suppose any boosted noise would ultimately be residual that's present in the incoming analog signal.

Regards,
Wayne