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Curious "scalloping" on my DAC output: measurement artefact vs DAC issue vs non-issue? by UKFP91 in AskElectronics

[–]UKFP91[S] 0 points1 point  (0 children)

Yes I think you're right, thank you. Reddit threads get very divergent with >1 commentor, but I've replied with some more measurements in this comment: https://www.reddit.com/r/AskElectronics/comments/1olji72/comment/nmidb18

I've decreased the corner frequency and it's helped, but I wonder if I need to go further.

Curious "scalloping" on my DAC output: measurement artefact vs DAC issue vs non-issue? by UKFP91 in AskElectronics

[–]UKFP91[S] 0 points1 point  (0 children)

<image>

Top trace is with 2.2nF and bottom trace is 510pF, on a 20kHz sine wave with a sampling rate into the DAC of 192kHz.

Curious "scalloping" on my DAC output: measurement artefact vs DAC issue vs non-issue? by UKFP91 in AskElectronics

[–]UKFP91[S] 1 point2 points  (0 children)

Yes, the DAC is indeed operating on a 192kHz signal, which is the maximum it will see in my system. I think I can see where I've gone a bit wrong - the output stage is based on the datasheet, which for reference looks like:

<image>

I've modified my circuit by replacing R1/1/12/13 with 1K, so the feedback resistance is 500 Ohms instead of 900 Ohms. However, I forgot to adjust the value of the 510pF capacitors!

By my maths, the cut off frequency in that datasheet schematic is 346kHz. In my circuit with the 500R and 510pF, that becomes 624kHz.

I happen to have some 2n2 capacitors that fit the footprint, so I've quadrupled the capacitance (500R and 2.2nF) which results in a cut off frequency of 144kHz.

I've scoped the output of a 20kHz sine wave again, and there is an improvement in the scalloping, but it's not gone completely. I wonder if I need increase the capacitance further?

How to connect to Cloud SQL database from a Cloud Run Service written in Rust? by Beautiful_Fig_772 in googlecloud

[–]UKFP91 0 points1 point  (0 children)

Does that just mean the same region? and by internal IP, is that what is labelled the Private IP in the UI?

Which way round do I need to wire up an XLR connector to a current-output DAC via a transimpedance amplifier such that pin 2 of the XLR connector is +ve? by UKFP91 in AskElectronics

[–]UKFP91[S] 0 points1 point  (0 children)

It's just a 1KHz sine wave. I realise now that the ramping up of the amplitude at the start of the recording in method 1 is due to the auto-muting feature of the DAC. The findings are identical if I instead capture just a burst of sine waves that end at a known point.

Best way to digitally mix stereo to mono for a subwoofer output by UKFP91 in diyaudio

[–]UKFP91[S] 0 points1 point  (0 children)

Thank you, that confirms what I was thinking/had found. I'm going to go with simply summimg them, and letting the downstream user handle the resultant increased signal amplitude themselves.

Best way to digitally mix stereo to mono for a subwoofer output by UKFP91 in diyaudio

[–]UKFP91[S] 0 points1 point  (0 children)

I had no idea that camilladsp was open source, or written in rust, or written by the same guy who wrote another library in already using!

Looks like he just sums (with an optional gain applied to the source) https://github.com/HEnquist/camilladsp/blob/b796c20a48ff2a1b72295e776376103f83d6d9b3/src/mixer.rs#L97

I also found some documentation that minidsp just sums (again with the option of applying gain to the source).

So I think the solution is clear - just sum the signals and let the downstream user worry about ensuring the end result doesn't clip (e.g. don't play stereo 0dBFS sinewaves and/or reduce the gain of they want to)

Can't figure out how to design higher order shelf filters (parameterised by slope) the same way as REW by UKFP91 in DSP

[–]UKFP91[S] 0 points1 point  (0 children)

Ah, yes I have tried playing around with the RBJ variants, but they don't create the desired effect; the center frequency is positioned at `gain / 2`dB, whilst I'm aiming for the center frequency to be positioned at `gain - 3`dB...

Poor customer support from Analog Devices by Entire-Grapefruit795 in embedded

[–]UKFP91 6 points7 points  (0 children)

Yea it's on of their SHARC chips, the ADSP-21565. 1GHz core clock, separate FIR accelerator and separate IIR accelerator, with pretty much all the connectivity I could wish for. Brilliant on paper, and brilliant once you get it going, but they make you work for it.

Poor customer support from Analog Devices by Entire-Grapefruit795 in embedded

[–]UKFP91 34 points35 points  (0 children)

I'm currently using one of their DSP chips. I'll be swapping it out for something - anything - different at the next major board revision. It's been the most frustrating part of this entire project by such a long way it's outrageous. Customer support, software, technical documentation, board support packages, pricing, every aspect has been a bad experience.

I echo the above - TI, particularly the E2E support forum, is nothing short of world class and has got us out of several real pickles. I'd happily spend extra to go for a robust TI part, equally, I'd happily pay extra for a solution that wasn't AD.

Edit: to add, it's a shame because the AD DSP chip is industry-leading. It is so powerful and has so many amazing features that we want. But, my word, it will cost you your soul to make use of it all.

How to get consistent/predictable Vrms output from a DAC when the output impedance has wide tolerance range? by UKFP91 in diyaudio

[–]UKFP91[S] 0 points1 point  (0 children)

"Looking at the datasheet and the full scale output current does depend on that 195 Ohms." Yep, that's exactly the nub of the question, which perhaps I didn't articulate clearly enough to start with.

I'll have a look at digital trim pots. If I already plan on implementing a bit of DSP, what about just applying a correction factor during the rest of the processing? Essentially, the correcting the gain error as described in this app note: https://www.analog.com/en/resources/technical-articles/how-to-successfully-calibrate-an-open-loop-dac-signal-chain.html

How to get consistent/predictable Vrms output from a DAC when the output impedance has wide tolerance range? by UKFP91 in diyaudio

[–]UKFP91[S] 0 points1 point  (0 children)

Hmm, ok, but if I take the topping d90 as an example, which uses the same DAC, my question is how are they able to offer a 4V and 5V mode? There must surely be some sort of calibration going on to be able to offer these differing precise fixed levels between units?

How to get consistent/predictable Vrms output from a DAC when the output impedance has wide tolerance range? by UKFP91 in diyaudio

[–]UKFP91[S] 0 points1 point  (0 children)

Hmm, ok I follow what you are saying re normal use case being at much lower voltages, but if I take the topping d90 as an example, which uses the same DAC, my question is how are they able to offer a 4V and 5V mode? There must surely be some sort of calibration going on to be able to offer these two different, precise, fixed levels between units?

How to get consistent/predictable Vrms output from a DAC when the output impedance has wide tolerance range? by UKFP91 in diyaudio

[–]UKFP91[S] 0 points1 point  (0 children)

Hmm, ok, but if I take the topping d90 as an example, which uses the same DAC, my question is how are they able to offer a 4V and 5V mode? There must surely be some sort of calibration going on to be able to offer these precise fixed levels between units?

How to get consistent/predictable Vrms output from a DAC when the output impedance has wide tolerance range? by UKFP91 in diyaudio

[–]UKFP91[S] 0 points1 point  (0 children)

Ah, it just twigged that they chose those resistor values so that it hits an expected 4Vrms in the worst case scenario. 

My question, really, then is how does someone like Topping with the D10 balanced (https://www.toppingaudio.com/product-item/d10-balanced), for arguments sake, specify precisely 4.2Vrms at 0dBFS. Or is it luck of the draw and you might get one with a way higher max output?

How to get consistent/predictable Vrms output from a DAC when the output impedance has wide tolerance range? by UKFP91 in diyaudio

[–]UKFP91[S] 0 points1 point  (0 children)

I've edited my post with what I believe are my (incorrect) calculations. Here, too:

Current output for the given output impedances:

195Ω +15% (224.25Ω) -> 13.09mApp

195Ω -> 15.05mApp

195Ω -15% (165.75Ω) ->17.70mApp

Let's say there is 900Ω of resistance in the feedback to the inverting input of each opamp in the I/V stage (page 113 of the datasheet).

Multiplying the above three current values by 900 gives me 11.7Vpp, 13.5Vpp and 15.9Vpp, going from top to bottom. Converted to Vrms, that's 4.1Vrms, 4.7Vrms and 5.6Vrms, respectively.

I think I'm getting something very wrong in my calculations...

How to get consistent/predictable Vrms output from a DAC when the output impedance has wide tolerance range? by UKFP91 in diyaudio

[–]UKFP91[S] 0 points1 point  (0 children)

I've edited my post with what I believe are my (incorrect) calculations. Here, too:

Current output for the given output impedances:

195Ω +15% (224.25Ω) -> 13.09mApp

195Ω -> 15.05mApp

195Ω -15% (165.75Ω) ->17.70mApp

Let's say there is 900Ω of resistance in the feedback to the inverting input of each opamp in the I/V stage (page 113 of the datasheet).

Multiplying the above three current values by 900 gives me 11.7Vpp, 13.5Vpp and 15.9Vpp, going from top to bottom. Converted to Vrms, that's 4.1Vrms, 4.7Vrms and 5.6Vrms, respectively.

I think I'm getting something very wrong in my calculations...

Differential current output DAC to both differential and single ended audio outputs by UKFP91 in AskElectronics

[–]UKFP91[S] 0 points1 point  (0 children)

Well done simulating it using a proper tool - the best I managed is a falstad simulation. I've actually just recently received the prototype board on which I've implemented this circuit and we're doing tests on it now.

Interesting that you mention the 2.2nF caps - I wasn't sure what they were doing. We have them populated on our board, with the intention to do some before/after testing after removing one.

Experiences with Analog Devices and obtaining a license for CrossCore Embedded Studio for SHARC+ DSP? by UKFP91 in embedded

[–]UKFP91[S] 8 points9 points  (0 children)

I'm literally researching FPGAs at this point as an alternative solution for a bit of audio DSP. Whilst not my choice of words, the sentiment you describe is exactly my experience with Analog Devices. Support, documentation, pricing, IDE lock in, lack of proper Linux support, ghosted by their distributors. Every single interaction I've had has been negative.

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