digital oscilloscope sampling for audio work

[BillyBoy]

Hi,

I'm thinking about getting a digital scope, but I'm worried about something and thought I'd ask about your experience with them. 

It seems like most affordable digital scopes take low resolution samples (typically 8bit) at a very high rate, which is probably geared for use on high speed digital circuits. If you are using an 8-bit digital scope for audio work, do you find the granularity of the data values to be acceptable? I don't mean "not bad for the price". I'm trying to understand, cost aside, how well 8bit scopes work for audio analysis. I'm not worried about small display resolutions, since you can typically zoom in until you hit your data resolution.  I'm specifically asking about the sample granularity, not the sampling rate or bandwidth. Decent consumer grade audio recording gear normally gets 24bit samples at 96K or 192K so it sounds good to your ears. 8bit sampling seems crude by comparison.  For a +/-5V AC signal, that's a 10v range divided into 256 values, or about 0.039v resolution in your data.  24bit sampling would give you about 0.0000005v resolution.

Can you clearly see what you can clearly hear, like the difference in a circuit if you swap values of caps in a fuzz feedback loop? Or two different types of clipping diodes in a distortion pedal? If you make small component changes to your pedal, would you expect to be able to see the result on the scope? That sort of stuff.

I'm trying to decide if I get a digital scope, whether 8bits is good enough or I should instead save up for a 16bit model.   I would think 16 bits would be OK.  I hardly ever have pedal stuff over 18v, so let's round up to 20v AC, or a 40v range.  Dividing that by 65536 gives resolution of around 0.0006v. 

If you are happy with your ability to see details with your digital scope, what model are you using?

Thanks!

[alanp]

Don't do what I did. Buy a digital scope based on a lot of net reviews, and then get told that it's crap.

[BillyBoy]

Yeah, the world is full of useless reviews and uninformed commentary.  That's why I'm trying to get info from people that use these scopes to actually do something like I want to do.  I don't have enough money to buy a scope, see if it is good enough, then get another to see if it is better.  I use my money to do that with pedals, guitars, and amps instead !!! 

[RobA]

You have to get to pretty expensive scopes before you are going to get above 8-bit sampling. But, it's a bit more complicated than just looking at the bit depth.  The signal that's output to the screen is an average of multiple frames and the probes have attenuation on them that you can use to kinda get more resolution on the data. That said, the noise floor is still pretty much set but the bit depth and 8-bit isn't really good for that. I went through pretty much the same analysis that you are doing a few months ago and decided that since one of the primary things I wanted to measure was the noise from power circuits I'm working on, that the low end DSO's weren't really going to cut it for what I wanted to do. Another thing that contributed to that decision was the dead time that happens in the DSO's. That could also be a problem for the type of analysis I wanted to do. So, in the end, I decided to go for an analog scope and bought one of the cheap GW Instek GOS-630FC scopes. So far I've been really happy with it and it's allowed me to measure everything I wanted to.

But, if what you want to do is look at output waveforms and LFO's and that sort of thing, the DSO's are probably going to be good enough for this. They also have the advantage of being able to capture and store waveforms for comparison. So, it depends a lot on what you really want to do. If you are going to get into looking at digital signals, then the analog scope bandwidth limits could be an issue. The general rule is that you need about 10 times the bandwidth as the signal you are looking at and the scope I have is 30MHz. Though, once again, the analog scope is more forgiving in how it degrades as you approach the bandwidth limit. And if you are going to do much digital stuff, I'd guess that you'd end up wanting a logic analyzer anyway.

The question about whether or not your eyes will be able to discern what your ears can hear is a bit complicated to begin with. To a large degree, our ears are better than our eyes to begin with. In one key way or eyes are better than are ears in that we can see the relative phases of the harmonic components and we can't hear them. But that somewhat means that our eyes deceive us when two waveforms can look very different but sound the same. I think that for that kind of analysis, a spectrum analyzer is going to be more helpful anyway. For audio work, there are spectrum analyzer/FFT programs that'll work with high bit depth audio interfaces.

I found it to be a pretty difficult decision to make and it took me a while to make it. I got to the point where I really didn't trust any reviews or blogs or forums or pretty much anything except trying to understand the specs because the world of O-scopes seems to be totally flooded by astroturfed nonsense. I still don't really know if I made the right choice, but I am having blast playing around with the one I did get and it's been a really useful tool.

[BillyBoy]

I'm glad you're happy with your choice.  Sounds like you went through about the same thinking to end up at about the same place I am right now.  I am still considering analog scopes.  There's a lot to think through.  If it wasn't for my bit depth concern, I think the digital scopes are the better match for my needs, particularly the better USB ones.  I have found one 16bit scope, but at over $1000, I'm stretching my budget past the breaking point.  There are also a few 10, 12, and 14bit ones (both real and emulated or "enhanced").  Most of the bench models, even some really expensive ones, are 8bit.  That makes me wonder if I'm missing something in my thinking.

I've also briefly looked into audio analyzers, but they don't seem to be the answer – expensive, confusing requirements for add-on modules and software, don't seem to have the interface/UI I would like, functionality missing, etc.

I may hedge and get a cheap analog scope, or a cheap USB digital scope that could get me by until maybe there are other options available. 

[teknoman2]

After having the same discussion with RobA in another thread I decided
to buy this analog scope (same as RobA) and this DDS Function generator.
http://www.gwinstek.com/en/product/productdetail.aspx?pid=3&mid=6&id=70
http://www.gwinstek.com/en/product/productdetail.aspx?pid=5&mid=72&id=94
I think this combo is pretty alright and covers my needs and it doesn't cost a fortune.

Cheers,
Panos.