Digital Volume Control v. Analogue Attenuation

Discussion in 'Computer Audiophile: Software, Configs, Tools' started by Lyer25, Apr 24, 2017.

  1. Lyer25

    Lyer25 Old at heart, young in other places

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    Hey all,

    Right, so this is my first new thread since I know asking questions that have already been addressed in the forum is a great way to get shit all over my face/is strongly discouraged. I'm of an awkward-nervous disposition, so it took a bit of convincing myself to write this. Trawled through the forums and didn't find an answer, so hoping this goes over well.

    I've come across a number of anecdotes on how analogue is supposedly superior to DVC due to loss of signal info in the latter, but upon further reading off-site I found analogue pots has its own set of drawbacks. To draw on my own limited experience with audio gear, the only solid example I can come up with is the ridiculous channel imbalance issues plaguing the Micro iDSD when the pot is set below 10 o'clock. There are also reports of how there is a good amount of noise in analogue implementations at lower listening levels, which I have heard, though I cannot be sure whether it's a problem unique to my gear or not.

    I do sometimes get to attend headphone meets and get to try out really nice stuff, but brief impressions in areas with moderate background noise are hardly accurate.

    DVC, on the other hand, loses resolution with enough dB taken off. I don't listen to music at particularly loud volumes primarily because it aggravates my mild tinnitus, an ever-present reminder of many functions I've attended where the sound crew has bad enough hearing that they feel the need to blast everything at ~120dB. One standout event took place back in 2015 when I had to cover a product launch here in the Philippines; the crew did a sound check and I had to go near the stage to interact with the notables present, right next to obnoxiously loud speakers that had my ears ringing for a good few hours afterwards.

    I should sue my old company for exposing me to crap like that. Agh.

    But I digress.

    With 8 bits roughly covering 48dB in leeway, I'm told some implementations of DVC (using 24 bit) are essentially superior to analogue solutions as they're more cost-effective and do not eat into the signal unless one turns it down to nearly-inaudible levels; it seems to me analogue pots struggle to match this, and higher-end implementations thereof seem to cost quite a bit more. I do prefer smooth pots to stepped attenuators though, since it takes a bit of doing to get just the right level for me, or so it seems anyway.

    I'd like to ask the members' personal experiences with analogue and digital volume control since it may help me decide on future purchases. Of course, I also hope having this thread as a resource may prove useful down the line, but I admit my reason for posting this thread is ultimately self-satisfaction.



    Cheers,

    Kevin
     
  2. Torq

    Torq MOT: Headphone.com

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    If, when you say "digital volume control" you're talking about software-based attenuation (I'm assuming so, since you're talking about bit-depth), then the instant you apply any attenuation you've lost your bit-perfect output. The result might be audibly transparent, but bit-perfect it won't be.

    The bit-depth of the implementation (many modern implementations are 64-bit, never mind 24-bit) is also only one part of the equation - the actual math employed to reconstruct the attenuated signal varies between implementations. Some are better than others - and the variations there can be entirely audible. Without knowing more about those implementations than is generally disclosed by manufacturers, I'd err towards analog attenuation.

    Digitally controlled analog attenuation is another way of going about things and generally eliminates channel-balance issues, without the issues that arise from changing the bit-level data in the audio stream that an algorithm/software implementation would have.

    Anyway, while in theory digital volume control might be audibly transparent (and it's certainly not something I'd bet on being able to identify in back-to-back A/B/X testing), I do find that fatigue sets in much faster whenever I've used it ... to the point where I run systems that do their attenuation in the analog domain and my DACs all get fed the unmolested source data.
     
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  3. Lyer25

    Lyer25 Old at heart, young in other places

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    I do tend to think that most people's hearing isn't quite so good as we might be led to believe barring people who've invested effort into training their ears or have perfect pitch, so I was primarily concerned about whether or not there might be any audible differences to the larger music fan audience as opposed to better-trained audiophiles; my own listening ability is far from good despite my high frequency hearing still being mostly intact. Also, would the reluctance of manufacturers to disclose their implementations be perchance due to the whole "trade secret" culture? Understandable, but I feel it's consumers who ultimately lose out since it gives snake oilsmen a lot of leeway. Which implementations of DVC would you say are better than most, just for reference?

    The onset of fatigue could be expectation bias or because the signal is getting effed in all sorts of weird ways, then? Hmm, might be fun to borrow higher-end gear from friends and try ABX-ing.

    And I think I misinterpreted how digitally-controlled analogue implementations work. I was under the assumption that it was essentially like your run-of-the-mill analogue potentiometer except that there's an extra digital component turning the knob, so to speak. Given the same component on the analogue side is being used, wouldn't digitally-controlled analogue then be subject to the same imbalance and noise issues?

    Thanks for the enlightening answer, Torq! Much appreciated.


    P.S.
    I understand that there's a negative connotation attached to the term "audiophile" in certain circles, but my usage is in no way ironic nor pejorative. It just seems to me a good catchall phrase to describe a demographic who aim to pursue good sound in contrast to how I believe "audiophools", who spend money on quartz rocks and attach them to speaker cables, are perceived.
     
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  4. JeffYoung

    JeffYoung Almost "Made"

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    Even within digitally-controlled analog attenuation there are sub-categories.

    The least expensive is to choose a rung on a resistor ladder using transistors. But transistors aren't noise-free.

    The more expensive way is to choose a rung with relays, and use the transistor control to energise the relays. Less noise, as the audio signal only goes through the relays, and as a bonus you get the cool clickety-clickety sound when you adjust the volume.

    Metrum's Adagio (and presumably the new Amethyst) have an interesting strategy. Assuming I understand it right, they employ two 16-bit DACs: one for high-order bits and one for low-order bits. They then apply a fixed attenuation to the low-order-bits output before combining the two. By modifying how the 24-bit signal is split into two (16/8, 15/9, 14/10, etc), you get a volume control with a single (analog) attenuation component. Which strikes me as pretty clever.
     
    Last edited: Apr 25, 2017
  5. Torq

    Torq MOT: Headphone.com

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    It's more a case of it being something that very few people would understand at all, let alone properly. Those not already skilled-in-the-art would, for example, would get lost with the basic description of simple "divide-by-two attentuator"* (which would simply shift everything one bit to the right each time it was applied, filling the MSB with a 0). More complex schemes are a complete non-starter. So any descriptions tend to be more in terms of marketing prose than actual technical relevance.

    Most of the popular audio-player applications that offer digital volume control "do it properly". By which I mean they use suitable numeric precision and scaling, before applying the raw attenuation function**. But, for example, compare the OS-level volume control/mixer behavior in Windows XP with that in Vista (or later) and you'll very likely be able to tell the difference without much trouble (and won't need exotic gear to do so).

    It could be.

    More often it's simply a function of forgetting to flip the software volume control off (it defaults to on in a surprising number of applications), and once I am getting fatigued (which is pretty rare with my rigs) I go looking for the cause - and find I've not applied that setting.

    Not really ... while there are many possible approaches, most are closer to a digitally-switched stepped-attenuator than a conventional pot. How they are switched varies, and whether you have a resistor ladder, strings or a combination, they're much more closely toleranced than any regular pot.

    --

    *You'd never do this in an actual audio implementation - it's simply to illustrate that most people won't grok something that is actually trivial.
    **No matter how precise, it's still no longer bit-perfect - by definition.
     
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  6. Kattefjaes

    Kattefjaes Mostly Harmless

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    <Mutters about Sony's funny hybrid DAC-ish/class D output DAPs, which are an interesting edge case>
     
  7. Lyer25

    Lyer25 Old at heart, young in other places

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    Gah, forgot about this thread. Belated thanks, @Torq and @JeffYoung ! Enjoyed reading through your responses, and learned quite a bit to boot. Not much for EE jargon (if it even is EE, I'm that unfamiliar) since my studies were in a completely different direction, haha. I'll admit I had to Google quite a bit to fully comprehend, but it was fun in its own way.

    EDIT: @Kattefjaes , I just looked up the different amplifier classes to refresh my memory, but I'm still somewhat confused. Usually happens in conversations like this, haha. Anyway, would your referring to the output of Sony's new DAPs being DAC-ish mean that they're somewhat line-level? Something something Class D amps have 90%+ efficiency and other stuff I'm not prepared to digest at 1AM in the morning. How might this be different from conventional DAP outputs?
     
    Last edited: Jun 27, 2017
  8. Mshenay

    Mshenay Barred from loaner program. DON'T SEND ME GEAR.

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    Good to know that, my Audio GD is Digitally controlled analog attenuation, it does indeed clicky click when I adjust the volume

    good thread as well, when I was exploring a portable amp to replace my iBasso PB2 this question was brought up a lot. For headphones, it seemed the benefits of Digitally controlled analog were most appreciated on really sensitive iems or cans, for something without as much sensitivity the analog method was preferred.

    I have analog controls on both Ember II and my PicoPower, and at lower volumes they aren't 100% balanced, though I've accepted listening a just above that imbalance point. An my HE 4 is the only headphone that's running out of the Amp on my Audio GD any more these days,

    great thread to read though, I'm starting my EET Program in a few months, so I'm sure I'll dig through this again soon!
     
  9. bilboda

    bilboda Acquaintance

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    My dac1321 has digital volume control, as do the other soekris dacs.

    If I understand Soren correctly, the signal gets upsampled and it's in this extra bit space where he applies attenuation, leaving the original bits intact. He considers this bit perfect.

    It does come in handy when my passive pre-amp just can't get loud enough with the varying inputs and out puts.

    eh? just a lil late in my response, was looking for another thread and saw this.
     
  10. Torq

    Torq MOT: Headphone.com

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    You cannot have "bit perfect" digital/software volume control, by definition.

    You can have "audibly transparent", which is really all that matters; but it is not the same as "bit perfect". The minute you change the data going to the DAC, then you're no longer "bit-perfect".
     
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  11. bilboda

    bilboda Acquaintance

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    Hmmm....I think I added the bit. I think he wrote he considers it perfect..
     
  12. msommers

    msommers High on Epipens

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    From a personal standpoint, I thought the DSJr volume attenuator was pretty good. But honestly after throwing Saga in the mix, I have to eat my words...I can't go back
     
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