Over-Sampling DAC and Behringer

[PrestonTom]

I am considering picking up a Behringer Ultracurve 2496 (or something like that) and perhaps using it to fool around and employ some steep crossover filters, time alignment etc.

There has been much arguing about the quality of a Behringer and I do not want to stir that up again. However, it seems that some of the problems folks have had are due to (or may have been due to) using the analog inputs. Specifically, not feeding a sufficient voltage to use all the bits on the Behringer's ADC. This and other problems would be obviated if one used the digital input.

My question: My Denon CD player (model 280) has 1) a digital coax output and 2) empolys 8 times over-sampling to get rid of aliased images. There was no mention whether there was a reconstruction filter (I suspect not) or whether this was delta / sigma modulation.

Specifically the question is: Would an 8 times over sampled digital signal be coming out the Denon's coax? How would the Behringer's processing etc deal with this. In other words is this going to be a problem?

I know some of you have familiarity with the Behringer. Your thoughts please

PS: I should know this already, but the more I thought about it the more confused I became.....

Thanks,

-Tom

[sfogg]

Tom,

The oversampled signal is fed to the Denon's DACs only. The digital

output would be at native rate... ie 44.1kHz for CDs. The Behringer can

handle that fine.

However something like the DCX2496 I believe resamples digital inputs to 96kHz anyway to keep the DSP coding easier.

Further after the data is being fed to its DACs it likely oversamples again there anyway.

Remember... if you feed the Behringer a digital signal and are using it

as a crossover you need some way to control the volume of all the

outputs on the Behringer. For a stereo pair of three way speakers that

means a 6 channel pre-amp is needed to go between the Behringer and the

amps.

If you setup your levels in/out of the DCX2496 the analog inputs are

fine. You need a pre-amp that can drive it though... it takes around 8v

of input to reach full scale... and you will get roughly 8v of output

so unless you amps have input level controls you are going to need to

add attenuation between the DCX and your amps. Two resistors per output

will get the job done.

See this thread for some details on what needs to be done to optimally setup levels...

http://forums.klipsch.com/forums/thread/613138.aspx

Shawn

[DrWho]

I forget who wrote the standard (AES or IEEE) but all digital outputs

behave the same. In other words it will work just fine with your

Behringer digital input.

The 8x oversampling has to do with whatever processing your denon is

doing on the inside (though I can't imagine why a CD player would need

oversampling.....it's not doing any DSP, or at least shouldn't be).

Anyways, is it labeled as a SPDIF out? If so, then you're all good.

[Daddy Dee]

On Behringer quality, I don't have personal experience. But I have taken note of an engineer friend and one of the Klipsch engineers who use Behringer at home and they are pleased with the results.

[sfogg]

"I forget who wrote the standard (AES or IEEE) but all digital outputs behave the same. "

S/PDIF

The S/P... Sony/Philips.....

"(though I can't imagine why a CD player would need oversampling....."

All CD players in the last decade or more have performed oversampling. It lets the analog filter be placed further away from the audio band then if the DACs didn't perform oversampling.

Shawn

[PrestonTom]

Thanks guys,

To clarify a couple of points: the digitial out on the Denon is coax. This would be s/pdif and not AES format. However, newer Pro units (although set up for accepting AES) can frequently also accept S/PDIF as an input (the Behringer FAQ supports this). I am not sure how this is done since the voltages are different, but more importantly the word size is different. But appearently this has been solved.

As for the level control, that is a difficult, although solvable. Three pair of 2-ch preamps would not be a great idea since setting the listening level would require turning three sets of knobs. I assume one could make a box that would be a "passive preamp" - essentially a 6 gang pot with an audio taper (if these exist). Of course this would require the Behringer output to be relatively low impedance, the output to be more than a 1 volt RMS and the input impedance on the amp to be relatively high (a Carver TFM has this and it also has trim pots).

Regarding why the Denon has overampling: they mention some "noise shaping". I assume they are oversampling in order to send the sampling image to a very high frequency and then just using a capacitor (if at all) for the signal reconstruction filter. I supect there is no serious DSP going on.

As you see I am going to some lengths to avoid an extra step of DAC from the CD player to the preamp followed by ADC from a preamp to the Behringer. Again this is not for HT, rather it is for 2-channel music. The Behringer could be fun for some time-alignment of the driver, help (but not solve) some room acoustics, and provide crossovers with steeper slopes.

I may change my mind on this (re: the Analog vs digital input to the Behringer) once I roll up my sleeves and get into it.

Good Luck,

-Tom

[sfogg]

Tom,

"but more importantly the word size is different. "

Word size is the same (32bits per word), the main difference between AES/EBU (pro format) and consumer format is in the pre-amble bits. What each bit signifies and where the bit is is different between the two.

"I assume one could make a box that would be a "passive preamp" - essentially a 6 gang pot with an audio taper (if these exist). "

There is a company that makes a 6 channel pot so this could be done though I think that pot is at least several hundred dollars as it is a 6 channel stepped attenuator. Another option would be something like the Welborne Labs remote controlled passive pre-amp kit and add 4 more channels to it which can be done.

"the output to be more than a 1 volt RMS "

Since you wouldn't be using the full balanced output the voltage level would be cut in half... but you should still see around 4v on the output so it should be OK.

"As you see I am going to some lengths to avoid an extra step of DAC from the CD player to the preamp followed by ADC from a preamp to the Behringer."

Don't kill yourself over this. If you get the levels into the Behringer good (which will take more then a passive pre-amp) it sounds fine and any slight possible degradation of the AD/DA (which runs at 96/24) will pale in comparison to the advantages the unit offers. If you can go digital though and deal with the volume control after the Behringer go for it.

One additional thing to consider... how you are going to handle your center channel?

If you go digital to the Behringer you could maybe use the L/R analog outputs of the Denon and sum them for the center channel. The timing will be thrown off though because of the additional latency of the Behringer.

Are you planning on bi-amping or tri-amping with the Behringer? If you are only bi-amping you can actually use the Behringer to generate a center channel output for you and then could use the delay in the Behringer to time align the center channel. If you tri-amp though the Behringer will be out of outputs so you can't have it sum a center channel.

If you do decide to go with a Behringer let me know and I'll cook up so elliptic crossovers for you if you wanted to give them a try to hear what *steep* crossovers does for you.

BTW, I'm in Ma. if you are ever up this way let me know and I'll show you the Behringers in operation. They are pretty slick. The biggest problem with them right now is simply finding one. Behringer ran into a supplier problem and production has been stopped on them for awhile and stock is depleted most everywhere.

Shawn

[bmxfelon420]

All CD players in the last decade or more have performed oversampling. It lets the analog filter be placed further away from the audio band then if the DACs didn't perform oversampling. Shawn

just curious, would a dvd player also oversample? Id imagine so, since it is similar compression.

[sfogg]

"just curious, would a dvd player also oversample?"

The digital filters in the DACs almost certainly would. In addition some DVD players actually will perform oversampling earlier on in the chain too... they call it different names but usually something along the lines of 'remastering' or whatever. Kind of pointless (good marketing point though) since the DAC is going to do it anyway.... and will usually oversample to a higher frequency.

Shawn

[bmxfelon420]

that does make sense, as it would make no sense for a manufacturer to use a different dac for dvd players and cd players if they didnt have to. Not to mention dvd's are similar to cd audio anyways. Interesting stuff.

[PrestonTom]

Thanks Shawn,

The issue of the center channel is yet another concern. If I keep it, then I would certainly need the time delay. As you point out, to compensate for the DSP delay but also center channels are physically closer than the L & R (in my case a few msec "closer"). I was thinking of using two Behringer units, but I am not sure if I would be able to synchronize the clocks between the two units.

I am still in a quandry about the pre-amp. If I use it between the CDs output (DAC) and the Behringer's input (ADC) in order to get the ADC to use all its bits over the entire voltage range, then it would not make a very good volume control. In other words the Quantization error would be inversely proportional to the listening level. Once you are at 15 bits or more, then the difference is probably not detectable. However, the dynamic range (of the volume control) is not very good (6dB for every bit lost). How do folks get aound this. Do they use a 6gang pot on the amps? Certainly they don't adjust three (or four) sets of knobs on the amps every time to set the listening level.

Again, I appreciate your thoughts since you have obviously solved these problems,

-Tom

[sfogg]

Tom,

"How do folks get aound this. Do they use a 6gang pot on the amps? Certainly they don't adjust three (or four) sets of knobs on the amps every time to set the listening level."

Some use 6 channel pre-amps between the Behringer and the amps or use a 6 channel input into something like the Panasonic digital receivers for an all in one solution.

Or they use volume control ahead of the Behringer.

"I am still in a quandry about the pre-amp. If I use it between the CDs output (DAC) and the Behringer's input (ADC) in order to get the ADC to use all its bits over the entire voltage range, then it would not make a very good volume control."

People stress out about this thinking about it in isolation.

Two questions....

A) Roughly... what is the loudest you listen at?

Roughly... what is the noise floor in your room/system?

A-B = Your systems dynamic range... IE the resolution of your system.

(A-B)/6 = the number of 'bits' of resolution your room and listening habits support.

Now... consider the case of doing volume control ahead of the Behringer.

The Behringer is rated to I think about 113dB SNR.... almost 19 bits of resolution.

Say the peak level one listens to are 105dB and the noise floor in the room is 50dB....55dB of dynamic range/SN/Resolution of the system.... just over 9 'bits'.

Setup the input levels into the Behringer such that your max listening levels are within say 3dB of clipping the inputs on the Behringer. On peaks the Behringer then has 110dB SNR.

Now... consider what happens in the system when you use volume control ahead of the Behringer.

If a person turns it the volume up as loud as they go (105dB in room) the Behringer has 110dB SNR while the rest of the system has about 55dB SNR. (A-

Now a person turns down the volume 5dB such that the loudest in room SPL is 100dB.

What happens?

The Behringers resolution has dropped by 5dB since the max signal into the unit has dropped 5dB in level.

However... the resolution of the rest of the system has ALSO dropped by 5dB too... down to 50dB SNR (A-. Max level has dropped but the noise floor in the room has not changed.

Now... set the max volume such that you are listening at 80dB max... the Behringers SNR has dropped by 25dB down to about 85dB SNR. Now the max volume is only 30dB above the noise floor.... the system/room itself is limited to 5 bits of resolution yet the Behringer still has about 14 bits of resolution through it.

Now... set the max volume such that you are listening at 60dB max... the Behringers SNR has dropped by 45dB down to about 65dB SNR.... almost 11 bits of resolution. However the signal in the room is only 10dB above the noise... the system doesn't even have 2 bits of resolution available to it.

Moral of the story..... set the input levels in/out of the Behringer properly and in almost all cases the loss of resolution through the Behringer by putting a volume control ahead of it is fairly meaningless when considering the SNR of the entire system which includes the room. Screw up the levels in/out of the Behringer and that can change of course.

My room is *quiet*... it supports more resolution then most peoples rooms. I run volume control ahead of the Behringers and the system sounds very good at all levels.

Shawn

[PrestonTom]

Shawn,

That is an interesting perspective on introducing the volume knob prior to the ADC on the Behringer and not needing to worry about introducing quantization noise when the volume knob is cut back. I went back and checked and it looks like your argument is even more powerful than you realize.

The Behringer claims to have 24 bit resolutuion (I am assuming that all 24 bits would be used on the ADC - this assumption is not always true for some oddball reasons & I will need to check that out). That said, then I could easily have close to 48 dB of range on my volume knob (24-16 = 8 bits or 48 dB). You had quoted a 117 dB noise floor on the Behringer, but I believe you were assuming the noise floor was quantization noise (which, of course would be 144 dB re: peak). It looks like the noise floor results from other sources (not surprising). A 48 dB range is plenty for a volume knob.

This would allow me to use a 2-Ch preamp prior to Beringer's ADC. I assume 18 dB gain from a preamp is sufficient to "fill up" all the ADCs bits (on the Behringer). After the Behringer's DAC output (and prior to the amps) I could simply use some resistors as voltage dividers so as not to present too much voltage to the amps

[sfogg]

Tom,

"The Behringer claims to have 24 bit resolutuion (I am assuming that

all 24 bits would be used on the ADC - this assumption is not always

true for some oddball reasons & I will need to check that out)."

Yes, it uses 24 bit ADC/DACs however no ADC or DAC actually has full 24

bits of resolution. IOW... none of them have 144dB of SNR. The best are

in the 110-120dB range... which is where the Behringer falls into

place. I was using the actual SNR of the Behringer, not the theoretical

one.

"A 48 dB range is plenty for a volume knob."

Exactly, as long as you setup the in/output levels properly.

"After the Behringer's DAC output (and prior to the amps) I could

simply use some resistors as voltage dividers so as not to present too

much voltage to the amps"

That is exactly what I did to my amps, works fine. You want to setup

the voltage dividers to both set the max volume level you want to

listen at (with full input/output from the Behringer) as well as to use

them to handle most of the balancing of levels between drivers.

That way you don't need to turn down the outputs on the Behringer

(which are done in the digital domain) very much at all to maximize the

DACs resolution too. I got each driver to within a dB or so of where I

wanted them then used the level controls on the Behringer to do the

final tweaks. I started out with 20dB of attenuation on the outputs

(along with going to single ended) and that was a decent starting

point. It took a couple of iteractions of voltage dividers to get it

all in place though. That was with amps with about 25dB of gain in them.

If you have amps with input level controls on them it will make

balancing the drivers and overall output levels part of the setup that

much easier. In my case the amps I used (Teacs) had input level

controls but they were fairly coarse (not stepped) so I didn't think

I'd be able to match them very well between three amps (9 channels). So

I removed the pots and replaced them with fixed attenuation (2 resistor

voltage dividers) so that I could precisely match the attenuation

between the L/C/R speakers drivers.

If it isn't obvious... setting up a system like this isn't a plug and

play sort of project. It takes some work. But if you go in

understanding what needs to be done and tackle the project in a logical

manor you will come through OK.

Hope this helps,

Shawn

[PrestonTom]

Tom,

"The Behringer claims to have 24 bit resolutuion (I am assuming that all 24 bits would be used on the ADC - this assumption is not always true for some oddball reasons & I will need to check that out)."

Yes, it uses 24 bit ADC/DACs however no ADC or DAC actually has full 24 bits of resolution. IOW... none of them have 144dB of SNR. The best are in the 110-120dB range... which is where the Behringer falls into place. I was using the actual SNR of the Behringer, not the theoretical one.

"A 48 dB range is plenty for a volume knob."

Exactly, as long as you setup the in/output levels properly.

"After the Behringer's DAC output (and prior to the amps) I could simply use some resistors as voltage dividers so as not to present too much voltage to the amps"

That is exactly what I did to my amps, works fine. You want to setup the voltage dividers to both set the max volume level you want to listen at (with full input/output from the Behringer) as well as to use them to handle most of the balancing of levels between drivers. That way you don't need to turn down the outputs on the Behringer (which are done in the digital domain) very much at all to maximize the DACs resolution too. I got each driver to within a dB or so of where I wanted them then used the level controls on the Behringer to do the final tweaks. I started out with 20dB of attenuation on the outputs (along with going to single ended) and that was a decent starting point. It took a couple of iteractions of voltage dividers to get it all in place though. That was with amps with about 25dB of gain in them.

If you have amps with input level controls on them it will make balancing the drivers and overall output levels part of the setup that much easier. In my case the amps I used (Teacs) had input level controls but they were fairly coarse (not stepped) so I didn't think I'd be able to match them very well between three amps (9 channels). So I removed the pots and replaced them with fixed attenuation (2 resistor voltage dividers) so that I could precisely match the attenuation between the L/C/R speakers drivers.

If it isn't obvious... setting up a system like this isn't a plug and play sort of project. It takes some work. But if you go in understanding what needs to be done and tackle the project in a logical manor you will come through OK.

Hope this helps,

Shawn

Shawn,

Your comments have been very helpful & I realize it is not "plug & play". Actually, it is sort of like some of the problems that I solve at work. But it can be satisfying.

At some point in time I will take you up on your offer to see your system. I live in SE CT, so I am not too far away.

It looks like my next step will be to try and find a vendor that actually has the Behringers in stock. But this will have to wait until I get done with my Kitchen re-modeling (a 5 month project that was supposed to take 5 weeks). It will be done pretty soon.......

BTW do you (or anyone else) have any experience with cabling between a consumer digital output (S/PDIF coax) to a pro digital input (AES). Specifically, do I need to insert a transformer in the patch cord to take care of the impedance mismatch?

Good Luck,

-Tom

[sfogg]

Tom,

"It looks like my next step will be to try and find a vendor that actually has the Behringers in stock. "

That may be the hardest task of all at this point. There is a guy in

Japan (apparently there are still some on the shelves over there) that

has been shipping universal voltage models to people all over the

place. Not sure what those are selling for. I don't know who else has

them at this point. I bought mine right as the supply was starting to

dry up.

Other option is to watch ebay. They pop up there from time to time, if

they have a BIN they go fast. If they don't have a BIN they sometimes

go for quite a bit more then they sold for originally.

Not sure what is needed to go from coax to AES/EBU. Checking the huge

Behringer thread on DIYAUDIO would probably have the answer.

Shawn

[Rivendell61]

Tom,

AES is typically 5V, 110 Ohms, vs SPDIF at .5V, 75 Ohms. That said, the AES should read the S/PDIF with no problem. If the cable is over 2m you might need an active converter (to convert to AES at the SPDIF end). For a shorter cable you might want one of the in/line XLR/BNC--110/75 Ohm transformers--$30 for the Canare version from Benchmark:

http://www.benchmarkmedia.com/photos/500-06800-000.jpg

Mark

[PrestonTom]

Tom,

AES is typically 5V, 110 Ohms, vs SPDIF at .5V, 75 Ohms. That said, the AES should read the S/PDIF with no problem. If the cable is over 2m you might need an active converter (to convert to AES at the SPDIF end). For a shorter cable you might want one of the in/line XLR/BNC--110/75 Ohm transformers--$30 for the Canare version from Benchmark:

http://www.benchmarkmedia.com/photos/500-06800-000.jpg

Mark

Thanks,

-Tom

[turntable3]

Some years ago I owned a behringer ultra-curve dsp8024 which was connected to a cd-

player via an adaptor-cable S/PDIF to AES/EBU. I used a 75 ohms cable an it worked fine.

Unfortunately the forum does not let me insert a picture...

so please look at:

http://www.behringer.com/DSP8024/index.cfm?lang=eng

Then choose manual and go to page 31 where you find the schematics of the adaptor-cable.

In my case the cable worked perfectly with a length of 75cm.

Jack

[turntable3]

trying to attach picture....

this will work with deq2496 too