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Old 3rd October 2012, 09:33 AM   #41
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Hi ds23man,


I thanks for your suggestions re Cobranet and connecting remote DAC's /DSP, very interesting.

Hi Twest,
Thanks, good points. I like Mini DSP stuff but like the DEQX, the Mini DSP DAC's and power supply are not great. In order to run a full 3 way active set up you have to buy two of their DSP only units and you still lose out on performance and features compared to DEQX.
Its still better value than DEQX but not best performance and still mid market price.
I have been waiting a long time for Bruno to release his Hypex solution, he has the potential to really shake things up, I hope he does!

All the best
Derek.
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Old 3rd October 2012, 11:07 AM   #42
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Derek, is your assessment of the miniDSP DACs and PSU by chance based on ABX or similar testing? Unfortunately I don't have access to either a 2x4 or 2x8 for ABX testing so I'm limited to speculation here but third party measurements of the 2x4 posted in the miniDSP forum here on DIYA are clean. I've never measured better than 70dB SNR in a recording environment and miniDSP is getting the ADAU1701's -90dB THD and 104dB DNR typ. So I'd expect the 2x4 to be fine with the usual 16 bit 44.1 material provided the downstream gain structure allows good bit depth utilization in the DAC---I could see a problem if using a power amp with the typical ~28dB gain for 50-60dB SPL listening levels but amps with a gain around unity ought to be OK if linear phase is one's thing (Analog doesn't spec the ADAU1701's group delay, which usually indicates linear phase antialiasing). For the 2x8 I haven't seen measurements but---if it is using Cirrus's -100dB THD/114dB DNR block---the only turnkey solutions I know of with higher headline specs and near minimum phase responses are the CS4398 (-107dB THD, 120dB DNR) and certain configurations of the WM8741 and 8742 (-100dB THD/123-125dB DNR). If one finds that extra bit is make or break it's probably not a bad idea to look into adjusting the playback channel's gain structure for better bit depth utilization in the DAC.

For example, I've gone through that exercise with CS4272s (one of Cirrus's -100dB/114dB DNR parts) and amps ranging from 0dB to 28dB gain (all of comparable THD, slew rate, and so on) and the results were kind of surprising. In the test configuration the ~27dB gain amps measured around -50dB acoustic THD for conversational listenling levels due to hitting the DAC floor. Switching to amps with around 10dB gain resolved the speakers -55 to -60dB THD but the ABX results from critical listening were 100% for the lower gain amps. Not by a small margin either; it was more like "why are we bothering to test this?"

As for power supplies, do you have specific concerns with the 2x8? (The 2x4 doesn't seem to be power supply limited.) The eval boards and reference designs for Cirrus's -100dB THD/114dB DNR all use a +5V lab supply with LC filters that corner around 3.5kHz. Statistically speaking that pretty much says it's an LM317 for the regulator but 3-4kHz is a typical of the regulation bandwidth of linear IC regulators. So the filtering is well chosen and should play nicely with pretty much any reasonable choice. That includes switchmode; the LC filters Cirrus uses hit peak rejection in the same range as the harmonics of the usual ~500kHz switchers. One might be able to wring a few dB more performance out of the DAC with some kind of super regulator but---given there's usually a few dB of performance variation from one chip to another due to process tolerances in the fab---the merits of that are debatable. Everything in Cirrus's design collateral suggests the performance limit is the DAC itself. Which is to be expected since manufacturers have quite a bit of incentive to set up their eval boards for the best headline specs possible.

I'm using a similar LC filter approach on my board with NCP1117s, which are ON's take on the LM1117. Great parts. The TDK MLZ inductors I'm using sim a little better than the ones Cirrus uses due to their lower ESR, too. But it's not until one gets everything assembled and ABXes it that one has a meaningful result. I am quite sure, however, that I'll get zero audiophile street cred from using a 48 cent regulator and 14 cent inductors even if the solution should be perfectly adequate to yield speced DAC performance---some exotic shunt regulator that didn't actually work as well would probably be perceived as better.
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Old 4th October 2012, 08:51 AM   #43
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Five pages of text and I don't think that anyone has expressed an opinion
about what's the worlds best dsp chip yet. If you would like to contribute don't
mind the cost, availability and so on. My idea with this thread wasn't to find an
easy or affordable solution. Simple question, wish one would you choose no
matter what ?

Wikipedia gave some background (modern dsp's).

http://en.wikipedia.org/wiki/Digital_signal_processor

Last edited by dahlberg; 4th October 2012 at 08:56 AM.
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Old 4th October 2012, 12:42 PM   #44
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That's because it doesn't really matter. Microcontrollers tend to be a bit more performant for IIR and DSP architectures a bit more performant for FIR but pretty much any current chip supports Q31x63 and finding DSCs or DSPs with more than enough processing power for most things isn't at all difficult (though sometimes one ends up with a multi-chip solution). You'll find various preferences about various toolchains and hardware details---Abraxalito's open source DSP XO thread in this forum has quite a bit of discussion about that---and I've already described the core tradeoffs between ARM Cortex M4, SigmaDSP, SHARC, and Blackfin on this thread. Plus there's a great deal of discussion about PC crossovers in this forum, the PC based forum, and even the multi way forum.

As usual, there's no one best; it depends on the application one has in mind and what the design constraints are. Post 1 in this thread is, after all, one of those very broad questions that can be good for stimulating discussion but usually are not so good at producing specific answers. But, as you might guess from my last several posts in this thread, my choice of the moment for embedded applications is NXP's LPC4300 family. You can find details of my PC crossover here.
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Old 4th October 2012, 01:47 PM   #45
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Quote:
Originally Posted by dahlberg View Post
Five pages of text and I don't think that anyone has expressed an opinion
about what's the worlds best dsp chip yet. If you would like to contribute don't
mind the cost, availability and so on. My idea with this thread wasn't to find an
easy or affordable solution. Simple question, wish one would you choose no
matter what ?

Wikipedia gave some background (modern dsp's).

Digital signal processor - Wikipedia, the free encyclopedia
picking the dsp is only one small part of the equation. You then have to design the hardware, layout the board, get boards made, build up prototypes and then write code for it

Even if you could use an off the shelf board that had everything you wanted on it you would still have to write the software. Again that is a major undertaking !!

Not for your average diyer I'm afraid.

Last edited by Trevor White; 4th October 2012 at 01:49 PM.
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Old 4th October 2012, 03:43 PM   #46
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just wondering here guys. all that talk about being on the same board, I didnt mention it at the time, but its worth the query. wouldnt it simply be better to hold the clock with the dac/s and isolate the DSP? it just seems that the DSP brings with it high slew rates, simultaneous switching noise and ripple, that to me has the potential to create more noise and limit the performance of the dac output more than a small amount of jitter from having the DSP on another module installed in the same case

have you done any testing on that twest820? it seems to me you would need to go to great trouble with multilayer boards to isolate the DSP, while going to efforts to keep it on the same board....
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Old 5th October 2012, 03:36 AM   #47
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Depends on the quality of the power supply implementation and the specifics of one's jitter concerns. Cirrus's LC approach I mentioned in post 42 is a good default. It's capable of providing 100+dB of isolation between analog and digital planes tied to the same regulator from a few kHz to a few hundred MHz.

Below 40kHz doesn't worry me much and that's where audible baseband jitter would occur. The lowest frequency noise source from the DSP that I know of is load pulsation with the sample rate---when a sample comes in the processor will be active for some duration depending on the amount of DSP to be done and then idle until the next sample arrives. In simple stereo processing implementations the fundamental of this supply loading is four times the Nyquist rate and therefore out of band pretty much by definition. Batch processing of samples due to loop unrolling or such can reduce it to frequencies below the Nyquist rate. But that's within or near regulator bandwidth and most parts have decent noise rejection in the kHz to a few tens of kHz range due to high PSRR and Avol.

From perhaps 100MHz up I'm less confident in what happens. Models get rather sensitive to what parasitics one decides to plug in and I don't have USD 50k lying around to buy a good scope with a few GHz of bandwidth to measure what's going on. It is, after all, a bit tricky to implement filters when everything's an inductor. Probably a decent rule of thumb is the supply filters need to be effective out to the GBP of the analog components so digital twitches don't blow into the op amps and burn all the slew rate and gain margin---in most cases the MCLK oscillator won't be any faster than the op amps so it's protected by this too. I'm using 50MHz typ op amps in the output buffers and I'm simming 120dB supply isolation at 300MHz so I figure things ought to be more than fine. Gotta build it and see, though.

Something to note about Cirrus's eval board schematics is the filter between planes is a CLCLC so supply noise sees two full LC stages no matter which direction it's moving in addition to the local bypassing for the chip creating the noise. A significant detail is each capacitor is implemented as an electrolytic in parallel with an MLCC. In the hundreds of MHz the MLCCs provide about 20dB of isolation each. If the layout's tight.
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Old 5th October 2012, 05:15 AM   #48
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but you still have shared ground, or am I missing something? check here for what i'm talking about in comparison. now this is a 2 channel version, something similar for multichannel is what I would call ideal. the reclock stage sits on the dac side, everything else including the connected PC and i2s buffer remains on the other side, so the only jitter that remains is that of the clock itself, the clock buffers, the interconnect and the dac itself.

its necessarily complicated and not cheap to implement, but effective. the difference between his and my setup currently is I have u.fl and w.fl connectors natively on the ackodac, so no adapters needed. this setup is confirmed bitperfect at 32bit and in my world this man knows how to make a PCB. the rest of the thread, or just the manuals linked in his sig will give you an indication of how the rest of the design works. with boards designed to work together it could be quite neat.

I know what you mean though, not saying its not possible, but there are certainly a fair few boxes to tick and its very difficult to really know whats going on at high speeds, as you say, due to the models becoming a bit bunky up there and measurement gear prohibitively expensive
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Old 5th October 2012, 07:47 AM   #49
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well, ideal really would be if it was all on the one board, but I think I would still like to see the dac, i2s buffer and clock side side isolated from the DSP and PC. couldnt agree more about gain structure, particularly when you take the traditional crossover out of the question, its surprising how little gain one actually needs

you might look at the Murata EMIFIL CLCLC and CRCLCRC integrated filter parts

Last edited by qusp; 5th October 2012 at 07:56 AM.
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Old 6th October 2012, 02:11 AM   #50
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Cirrus floods ground on both sides of the board and routes power in traces; have a look at appendix E of the CS42528+CS43900 reference design. The performance isn't speced but it's fairly safe to assume the board yields the codec's datasheet performance---reference designs which don't deliver this have a way of making large customers talk about cancelling contracts and switching to different vendors, so they tend to get fixed pretty quickly. The layout style is used consistently across the other Cirrus eval boards and reference designs I've looked at, including the CS4398, so it would appear to be rather not broken and therefore in no real need of fixing.

The Focusrite Saffire 40 I use in my PC crossover uses a similar approach to layout with CS4272s and NJM4565s. It's also a mixed signal system and is DC grounded to a laptop via 1394. It measures out at the CS4272's datasheet performance on loopback tests with a power supply that's a small EI trafo, two LM317s, and an LM337. I use it for recording as well as for VST hosted XO and EQ on playback and it does a fine job of satisfying the pro audio design basic of sound out = sound in. What limits sound quality is the recording which, in my case, is mostly constrained by the mics I have access to. I think I might able to expose some limitations of the system if I ever get ahold of a set of Earthworks QTCs but, comparing my own instruments to the best professional recordings I have in my collection, any technical limitations of the rig in playback are decidedly smaller than the variation in recording quality among, oh, I dunno, the top 10% of CDs in terms of production values. (I've also done what I can to ensure the speakers and amps are as high fidelity as I know how to get them; see link in post 44.)

As such, I feel I've a handle on what's good enough to achieve subjective transparency. At least in my living room for my ears, anyway. These subjective results are in agreement with analytic design workback; every time I end up running the maths I end up concluding one wants good time domain behavior and THD of -100dB or below and 100+dB DNR in each analog stage in order to keep the electronics' IMD level and noise floor low enough playback performance limits on the phase modulation distortion floor of the drivers. If one's aggressively managing levels the way you do on a mixing desk then one can get away with less performant gear. If one is underutilizing a power amp with a lot of gain (an unfortunately common problem) then probably more performant stuff will be helpful.

Cirrus's -100dB THD/114dB DNR lineup is nicely positioned for this. The parts are good enough. The parts are cheap. The antialising filters are well chosen. And you sweat the design work considerably less than if you're trying to hit the performance limits of one of ESS's -120dB THD/129+dB DNR DACs. Reducing a noise floor down into the nV range has a way of being brutal---I've discovered I really like not having to worry about how to manage Johnson noise from a 2k resistor---but if a few uV is OK then things become much more plug and play.

Thanks for the EMIFIL suggestion. I'd found the parts a while back, actually, and ended up rolling my own filters as the EMI market is aimed at frequencies rather higher that what one wants for audio (one or two of the parts come close but they're non-stock at Digikey). Something that gives me confidence in my sims is they show the same sort of behavior as Murata's BNX insertion loss graphs. I suspect the primary reason I'm getting significantly higher rejection is I'm designing for ~100mA rather than 10+A and can therefore use 0805 chip inductors. Those have lower parallel capacitances the inductors in the BNX parts and I have the freedom to chose higher ESRs than the BNX current ratings allow (there's something of a sweet spot around a couple hundred milliohms where the filtering's good and supply sag is minimal).
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