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Really stupid question: I thought I've used a 24bit DAC, on some high end sound equipment. Why is this special/noteworthy?

by loxias 5y ago

LeoPanthera 5y ago
I believe this chip was first available in 1989. It's old technology.
- loxias 5y ago
  Ahh, so this is a "remember this?" post. Got it. Thanks :)
  - sitkack 5y ago
    old technology is not bad technology.
    - loxias 5y ago
      I enthusiastically and overwhelmingly agree. Possibly "over"-agree.
magicalhippo 5y ago
Not all DACs are equal. Even average soundcards sport a 24bit DAC. However they're designed for a dynamic output so tend to have quite crap DC precision. What matters to a soundcard is producing a precise waveform, not a precise DC voltage. This part is designed for DC level setting, ie a programmable reference voltage. Typically it won't change that often (compared to a soundcard), but it needs to be precise. For example, compare the AD5791[1], a 20-bit DAC with 1LSB DC accuracy costing $40 in 1000 qty, to a 24bit audio DAC like the AD1955[2] costing about $7. The datasheet for the audio DAC doesn't even specify the DC precision, only a 6% gain error from the output buffer. [1]: https://www.analog.com/en/products/ad5791.html https://www.analog.com/en/products/ad5791.html [2]: https://www.analog.com/en/products/ad1955.html https://www.analog.com/en/products/ad1955.html
- mastax 5y ago
  On the other hand if all you care about is DC precision you can use a very simple, though not necessarily easy, circuit. You need a stable voltage derived from a stable voltage reference, and a stable clock to PWM that stable voltage. Then you "just" low-pass filter the heck out of the PWM to get an adjustable DC value, and calibrate out any offsets. I've seen this used in adjustable voltage standards (forget which) and the venerable Fluke 5700A Multifunction Calibrator[0]. [0]: https://www.youtube.com/watch?v=qhr6h7wuqo0 https://www.youtube.com/watch?v=qhr6h7wuqo0
makomk 5y ago
Most "24 bit" audio DACs have a lot less than 24 bits of real usable data due to the noise floor effectively burying the lowest bits in garbage, and I think the noise increases at less perceptually noticable frequencies too. 24 bits is used for audio mainly because it's just convenient to standardise on that. I think they also have a lot more non-ideal behaviour in general than this, and they certainly don't have a 0.1% precision voltage reference because the precise output voltage range doesn't really matter for audio.
jeffbee 5y ago
I doubt anyone ever shipped this DAC for digital audio applications. You're right that on sheer width there have been 18-bit or better DACs on the market since the 1980s. First one that comes to mind is the Burr-Brown (now Texas Instruments) PCM58. ADI themselves have been marketing 18-bit or better audio DACs for ages, since at least the AD1860. Compare the datasheet of the AD1860 to this one to see the difference between a precision DC instrument and a dynamic audio DAC. This one has .01% gain error while an audio DAC is rated for 2% gain error (because nobody cares).
- intricatedetail 5y ago
  These industrial DACs are actually brilliant for audio - They retain low-end and transients much better, but it also means need to be calibrated properly so that DC don't destroy speakers. It's easier and cheaper to just add HPF though, but sounds "worse".
VLM 5y ago
You can specify 144 dB of dynamic range but in practice: 1) Loudness war means pro sound engineers will mix everything to be full scale all the time so music has 0 dB of dynamic range ideally 2) Almost no one owns equipment capable of rendering 144 dB of dynamic range even if they have a bit stream specifying it. So if a quiet near silent room is 30 dBA where you hear your own heartbeat and OSHA permissible exposure limit is 90 dBA its only 60 dB of dynamic range between "too quiet to hear" and "so loud you legally need earplugs" so you'll be throwing out about 84 dB of theoretical dynamic range anyway even if you owned gear capable of rendering it. Note that its very hard to buy audio amplifiers capable of more than 90 dB or so SNR even if just hooked up to test equipment, nothing on the market capable of 144 dB, so either the bottom end will be lost in the hiss or the high end will be distorted into unrecognizability. 3) Then you get into topics like power supply rejection ratio. A "bad" amp would pass noise from the power supply thru. A "better" amp rejects more noise from the power supply. A good way to look at it is the hummmmm and crackle and stuff that you hear on a bad old tube amp could be modeled as a very large value attenuator directly connecting the power supply to the output. With 1970s tech and capacitors, 80 dB was considered pretty good PSRR. The very top end newest class D amps can just barely exceed 100 dB PSRR under idealized testing conditions. So at full blast (which would just blow your eardrums out anyway) everything more than 100 dB down from peak will merely be power supply noise passed thru the system. Its kind of like I can spec a piece of wood to 1.5875mm thick. And then convert it to imperial inch measurements. If the best measurement tool I have to measure wood, is a carpenters tape measure ruled to eighths of inches, the best measurement I can do to render those five digits of precision is "about half a division" on that tape measure. The five digits of precision in the mm figure imply I should be using a calibrated micrometer to measure that thickness, but if you assume technology hasn't invented them yet, then there's little point in spec a digital calculated measurement to five digits of mm precision. Likewise I can make a string of binary 1 and 0 that theoretically can be understood or mathematically proven to imply 144 dB of sound precision, but even in 2021 electronics hardware doesn't exist to reliably repeatedly and provably render those 1 and 0 into sound, at least not at audio rates. Note that 2 to the power of 24 is a eight digit number, implying you're operating at tens of ppb. NIST has really nice new Josephson junction standards capable of reliably operating at tenths of ppb. So you're very optimistically trying to field gear operating at only a hundred times less accurate than the best lab in the entire world can currently measure. Good luck with that. That's why nobody sells voltmeters with more than 8 or so digits of precision, if the world standard calibration system only has ten digits on its best days, no point building voltmeter hardware displaying 12 or 15 digits LOL. We don't really have the technology to do 24 bit accurate voltages out in the field right now. So if we can't build or calibrate testing gear significantly better than 24 bits, like to 32 or so bits perhaps, there's no way to actually measure and tell if 24 bit gear is accurately precisely repeatedly working to 24 actual bits of precision. Maybe your audio gear is distorting in a subtle fashion and only operating to 22 noise free bit and the last two bits are essentially a RNG or stuck on/off or otherwise no relationship with reality; very few electronics labs in the entire world have the gear to prove that claim true or false.

Really stupid question: I thought I've used a 24bit DAC, on some high end sound equipment. Why is this special/noteworthy?

by loxias 5y ago

Really stupid question: I thought I've used a 24bit DAC, on some high end sound equipment. Why is this special/noteworthy?

LeoPanthera 5y ago

I believe this chip was first available in 1989. It's old technology.

loxias 5y ago
Ahh, so this is a "remember this?" post. Got it. Thanks :)
- sitkack 5y ago
  old technology is not bad technology.
  - loxias 5y ago
    I enthusiastically and overwhelmingly agree. Possibly "over"-agree.

magicalhippo 5y ago

Not all DACs are equal. Even average soundcards sport a 24bit DAC. However they're designed for a dynamic output so tend to have quite crap DC precision. What matters to a soundcard is producing a precise waveform, not a precise DC voltage. This part is designed for DC level setting, ie a programmable reference voltage. Typically it won't change that often (compared to a soundcard), but it needs to be precise. For example, compare the AD5791[1], a 20-bit DAC with 1LSB DC accuracy costing $40 in 1000 qty, to a 24bit audio DAC like the AD1955[2] costing about $7. The datasheet for the audio DAC doesn't even specify the DC precision, only a 6% gain error from the output buffer. [1]: https://www.analog.com/en/products/ad5791.html https://www.analog.com/en/products/ad5791.html [2]: https://www.analog.com/en/products/ad1955.html https://www.analog.com/en/products/ad1955.html

mastax 5y ago
On the other hand if all you care about is DC precision you can use a very simple, though not necessarily easy, circuit. You need a stable voltage derived from a stable voltage reference, and a stable clock to PWM that stable voltage. Then you "just" low-pass filter the heck out of the PWM to get an adjustable DC value, and calibrate out any offsets. I've seen this used in adjustable voltage standards (forget which) and the venerable Fluke 5700A Multifunction Calibrator[0]. [0]: https://www.youtube.com/watch?v=qhr6h7wuqo0 https://www.youtube.com/watch?v=qhr6h7wuqo0

makomk 5y ago

Most "24 bit" audio DACs have a lot less than 24 bits of real usable data due to the noise floor effectively burying the lowest bits in garbage, and I think the noise increases at less perceptually noticable frequencies too. 24 bits is used for audio mainly because it's just convenient to standardise on that. I think they also have a lot more non-ideal behaviour in general than this, and they certainly don't have a 0.1% precision voltage reference because the precise output voltage range doesn't really matter for audio.

jeffbee 5y ago

I doubt anyone ever shipped this DAC for digital audio applications. You're right that on sheer width there have been 18-bit or better DACs on the market since the 1980s. First one that comes to mind is the Burr-Brown (now Texas Instruments) PCM58. ADI themselves have been marketing 18-bit or better audio DACs for ages, since at least the AD1860. Compare the datasheet of the AD1860 to this one to see the difference between a precision DC instrument and a dynamic audio DAC. This one has .01% gain error while an audio DAC is rated for 2% gain error (because nobody cares).

intricatedetail 5y ago
These industrial DACs are actually brilliant for audio - They retain low-end and transients much better, but it also means need to be calibrated properly so that DC don't destroy speakers. It's easier and cheaper to just add HPF though, but sounds "worse".

VLM 5y ago

You can specify 144 dB of dynamic range but in practice: 1) Loudness war means pro sound engineers will mix everything to be full scale all the time so music has 0 dB of dynamic range ideally 2) Almost no one owns equipment capable of rendering 144 dB of dynamic range even if they have a bit stream specifying it. So if a quiet near silent room is 30 dBA where you hear your own heartbeat and OSHA permissible exposure limit is 90 dBA its only 60 dB of dynamic range between "too quiet to hear" and "so loud you legally need earplugs" so you'll be throwing out about 84 dB of theoretical dynamic range anyway even if you owned gear capable of rendering it. Note that its very hard to buy audio amplifiers capable of more than 90 dB or so SNR even if just hooked up to test equipment, nothing on the market capable of 144 dB, so either the bottom end will be lost in the hiss or the high end will be distorted into unrecognizability. 3) Then you get into topics like power supply rejection ratio. A "bad" amp would pass noise from the power supply thru. A "better" amp rejects more noise from the power supply. A good way to look at it is the hummmmm and crackle and stuff that you hear on a bad old tube amp could be modeled as a very large value attenuator directly connecting the power supply to the output. With 1970s tech and capacitors, 80 dB was considered pretty good PSRR. The very top end newest class D amps can just barely exceed 100 dB PSRR under idealized testing conditions. So at full blast (which would just blow your eardrums out anyway) everything more than 100 dB down from peak will merely be power supply noise passed thru the system. Its kind of like I can spec a piece of wood to 1.5875mm thick. And then convert it to imperial inch measurements. If the best measurement tool I have to measure wood, is a carpenters tape measure ruled to eighths of inches, the best measurement I can do to render those five digits of precision is "about half a division" on that tape measure. The five digits of precision in the mm figure imply I should be using a calibrated micrometer to measure that thickness, but if you assume technology hasn't invented them yet, then there's little point in spec a digital calculated measurement to five digits of mm precision. Likewise I can make a string of binary 1 and 0 that theoretically can be understood or mathematically proven to imply 144 dB of sound precision, but even in 2021 electronics hardware doesn't exist to reliably repeatedly and provably render those 1 and 0 into sound, at least not at audio rates. Note that 2 to the power of 24 is a eight digit number, implying you're operating at tens of ppb. NIST has really nice new Josephson junction standards capable of reliably operating at tenths of ppb. So you're very optimistically trying to field gear operating at only a hundred times less accurate than the best lab in the entire world can currently measure. Good luck with that. That's why nobody sells voltmeters with more than 8 or so digits of precision, if the world standard calibration system only has ten digits on its best days, no point building voltmeter hardware displaying 12 or 15 digits LOL. We don't really have the technology to do 24 bit accurate voltages out in the field right now. So if we can't build or calibrate testing gear significantly better than 24 bits, like to 32 or so bits perhaps, there's no way to actually measure and tell if 24 bit gear is accurately precisely repeatedly working to 24 actual bits of precision. Maybe your audio gear is distorting in a subtle fashion and only operating to 22 noise free bit and the last two bits are essentially a RNG or stuck on/off or otherwise no relationship with reality; very few electronics labs in the entire world have the gear to prove that claim true or false.