I have a project that I would like to do but I'm not exactly sure of the correct tools I would need for it. The main part of the project is testing and comparing the USB input on audio dacs. The second part of the project is the accuracy of some of these dacs. If anyone could recommend what tools I would need so measure and analyze USB and DACs it would be much appreciated. I have an Oscope, some USB breakout boards, DC variable power supplies, and a multimeter. Any extra input would be awesome. Thanks!
-r
Tool recommendation please?
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adafruit_support_mike
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Re: Tool recommendation please?
In a perfect world, you'd want a spectrally pure sine wave generator and a spectrum analyzer.
In a world where you don't have $10,000 to spend on those two machines, you'll want a good signal generator and an oscilloscope that can do FFTs.
The basic idea of measuring signal fidelity is to send a known signal into the system, then compare the output to the input. Digital audio gives you the luxury of knowing that every device is getting exactly the same input, but presents you with the challenge of making sure your digital source is worth using.
What you want is a digital file that represents a pure sine wave at some frequency.. 1kHz is normal. Feed that into the DAC, then display the DAC's output with one channel of an oscilloscope. Have the scope's other channel connected to a frequency generator producing a 1kHz sine wave, then have the scope do math to the two signals.
The first measurement is to subtract the reference signal from the DAC's output. If the DAC reproduces the waveform perfectly, the difference will be zero. If the DAC distorts the waveform, the difference between the two signals will show that distortion. If you can do an FFT of the difference between the two signals, that will show you the frequency components of the error.
Getting the two signals to line up will be a bit of a challenge, but good tools will make the job easier. Higher-quality oscilloscopes allow you to adjust the phase of an input, sliding it left or right on the screen relative to the other trace. If you don't have that, a good signal generator will let you change its output frequency by small amounts (my B&K goes to millihertz), and you can use that to bring a pair of signals in sync. If your scope has cursors that track the peak of the input, those will make it easier to see when the two signals are in phase with each other.
You'll want to take those kinds of measurement for multiple frequencies, but once you can generate the digital source for one frequency doing more should be fairly easy.
You'll probably want a range of speakers and headphones too. The inductance of the speaker will serve as a low-pass filter for the DAC's output, and you'll need to include that in your measurements.. especially if the DACs have class-D amplifiers on their output.
In a world where you don't have $10,000 to spend on those two machines, you'll want a good signal generator and an oscilloscope that can do FFTs.
The basic idea of measuring signal fidelity is to send a known signal into the system, then compare the output to the input. Digital audio gives you the luxury of knowing that every device is getting exactly the same input, but presents you with the challenge of making sure your digital source is worth using.
What you want is a digital file that represents a pure sine wave at some frequency.. 1kHz is normal. Feed that into the DAC, then display the DAC's output with one channel of an oscilloscope. Have the scope's other channel connected to a frequency generator producing a 1kHz sine wave, then have the scope do math to the two signals.
The first measurement is to subtract the reference signal from the DAC's output. If the DAC reproduces the waveform perfectly, the difference will be zero. If the DAC distorts the waveform, the difference between the two signals will show that distortion. If you can do an FFT of the difference between the two signals, that will show you the frequency components of the error.
Getting the two signals to line up will be a bit of a challenge, but good tools will make the job easier. Higher-quality oscilloscopes allow you to adjust the phase of an input, sliding it left or right on the screen relative to the other trace. If you don't have that, a good signal generator will let you change its output frequency by small amounts (my B&K goes to millihertz), and you can use that to bring a pair of signals in sync. If your scope has cursors that track the peak of the input, those will make it easier to see when the two signals are in phase with each other.
You'll want to take those kinds of measurement for multiple frequencies, but once you can generate the digital source for one frequency doing more should be fairly easy.
You'll probably want a range of speakers and headphones too. The inductance of the speaker will serve as a low-pass filter for the DAC's output, and you'll need to include that in your measurements.. especially if the DACs have class-D amplifiers on their output.
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rwink
- Posts: 20
- Joined: Sun Oct 06, 2013 6:43 pm
Re: Tool recommendation please?
Thank you very much for this detailed reply. I wasn't expecting so much information. This definitely saved me a few hours and possibly a few evenings of research to get me started on my project. I commend you and Bill. Both have been very helpful to me and many others, Thank you for that!
Now I just need to get the signal generator :)
Now I just need to get the signal generator :)
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adafruit_support_mike
- Posts: 67485
- Joined: Thu Feb 11, 2010 2:51 pm
Re: Tool recommendation please?
Signal generators are like oscilloscopes: quality has a pricetag, but digital signal processing has finally gotten fast enough and good enough to be competitive.
Take a look at the BitScope: https://www.adafruit.com/products/2053
I won't pretend it's as good as a rack of instruments that each cost ten times as much, but the other truth of test gear is that price and performance increase exponentially. Each additional digit of speed or resolution is an order of magnitude improvement, and costs about ten times as much. By that standard, the BitScope gives you a surprising amount of bang for the buck.
Take a look at the BitScope: https://www.adafruit.com/products/2053
I won't pretend it's as good as a rack of instruments that each cost ten times as much, but the other truth of test gear is that price and performance increase exponentially. Each additional digit of speed or resolution is an order of magnitude improvement, and costs about ten times as much. By that standard, the BitScope gives you a surprising amount of bang for the buck.
Please be positive and constructive with your questions and comments.