24bits 192KHz oversampling digital input for the DCX2496

September 2007

new improved digital input

Level 2

Theoritical considerations

The Behringer DCX2496 is plagued with a faulty digital input : the Cirrus Logic sample rate converter choosen by Behringer is not able at all to work properly @ 96KHz although Behringer called it's flagship crossover 2496 which stands for 24bits sampled at 96KHz. The reason of this situation is very easy to understand as Cirrus themselves announced it in a technical bulletin (see below) mentionning - to say the less - the sample rate converter erratic behaviour.

Of course most of the audiophiles will live with this flaw as our current digital audio sources, not to mention CDs themselves, are using a 44,1KHz sampling rate. However an increasing number of users are introducing oversampling devices in their systems, and some of them are using 192KHz digital sources which the DCX2496 "out the box" is unable to deal with. Hence this page which should learn you how to make your digital crossover 24/192KHz compatible and, over all, get rid of the scratchy noises you suffer with some audio CDs when using a Fs other than 44,1KHz.

Let's first consider the famous Nyquist-Shannon's theorem : "Exact reconstruction of a continuous-time baseband signal from its samples is possible if the signal is bandlimited and the sampling frequency is greater than twice the signal bandwidth" which for us music lovers means that 44,1KHz should be more than adequate to reproduce the full 20KHz audio bandpass. For those who want to go further in signal transmission theories a Google search will help. In a few words, main advantages of signal oversampling are the following :

antialiasing : in simple words it is the smoothing of the sinewave which is nothing else than a succession of rectangular values (think about a pixelized digital image, and now imagine the same image with smaller samples). By increasing the number of samples a smoother sinewave form is achieved, closer the ideal form of the original signal.
easier analog to digital and digital to analog conversion in the audio chain.
improved signal to noise ratio in the digital domain

These are the reasons why many digital devices are using oversampling circuits, you'll discover more into details by reading their technical brochures.

The DCX2496 hidden flaw

Let's get back to the DCX2496, which should be able in theory to sample digital signal up to 96.000 times per second. In theory yes, but practically not at all ! The reason is that the DCX2496 is plagued with a badly designed sample rate converter, I have named the Cirrus Logic CS8420 choosen by Behringer engineers to sample the input signal. The bug is truly engraved in the chip and it is responsible for the SRC's impredictable and erratic behaviour.

Should the SRC go up the wall, and believe me it will one day or another, you will notice it immediately. Only solution then is to reboot the chip, in other words power your crossover off and on again as suggested in the CS8420 datasheet. Sometimes I wonder if Microsoft has secret links with Cirrus, but this is another story...

Here's the technical description of the problem, CS8420 chip's datasheet page 49 :

Anyone who has experienced this phenomenon will be able to decribe it as (choose from list)

- dull sound

- darkened highs, muddy mids

- frying eggs or frying bacon sound on hevalily loaded musical program

and the list goes on...

This is the result of what Cirrus describes as "notches occuring in the frequency response and spurious tones being generated in response to some input frequencies". It is easy to make your DCX surrender if you own a Behringer SRC2496 : switch it to 96KHz and enjoy the result !

Yes, there is a solution !

After a thourough search accross the web I remembered that a German audiophile fellow had found an elegant solution to this issue but simply getting rid of the culprit : he had used with succes a new chip, the Cirrus Logic CS8416. This one having been launched after the CS8420 it is a modern and efficient design and the dreaded bug has been removed. However, this German audiophile had choosen, in my opinion, a touchy way by "lifting" some legs of the CS8420 in order to feed the DCX printed circuit board with the new chip installed on a separated board. Those chips are SMT devices with tiiiiiiny legs - actually fly legs ! - and I wasn't too happy with the risk of destroying the fragile PCB pads.

I discussed this with my Selectronic friends and after studying the problem in details they came back to me with a ready to fit board. Not only it's a plug-and-play unit but it has many more advantages :

no dangerous surgery is needed in the DCX2496.
a high precision / low jitter temperature controlled oscillator is embeded on the board, insuring enhanced performance of the digital section of the DCX, same clock as this one.
a supplementary pad is provided of a second TCXO low jitter clock to replace to cheap original oscillator installed by Behringer in the DCX2496. This 2nd is optionnal and can be installed anytime very easily (see second page).
digital audio signal interfacing with the source is made by the means of a wonderful Scientific Conversion transformer offering choice between SP/DIF (75 ohms) or AES/EBU (110 ohms) to the user. Your DCX will be fully and truly compatible with these audio standards after installation of the new board. This transformer is a very efficient weapon againts jitter as described in this document published by AES.
the circuit board traces have been optimized to avoid the very long connections between the input XLR socket and the sample rate conversion circuitry. As a result, almost 1ft (300mm) of tortuous cabling has been deleted and replaced by the shortest possible trace made of thick copper for the sake of delicate signal flow.
the kit is sold as ready to fit unit. No component or chip has to be installed but the connecting flat wire.
de-emphasis wasn't managed by the CS8420. The CS8416 deals with de-emphasis automatically. Of course, most modern recordings are made without pre-emphasis but some of us have old recordings from the beginning of the CD era which a pre-emphasis matsred indeed.
last, but not the least : the Cirrus Logic CS8416 is able to decode signal from a DVD player, as mentionned in this brochure including DTS encoded record. Your DCX can be plugged directly on your DVD player from now on !

Basic version (without the additionnal low jitter clock)

notice provision for the optionnal TCXO module on the left

Scale (compare with CD)

Part 2 : installation of the board in the DCX2496