Possible to use difference math of DSO as distortion analyzer?

[Jacques Merde]

Originally Posted by benb

These DSO's only use 8 bit A/D's (at least one of these, this one shows the spec here Instek GDS-1062A Oscilloscope Digital Storage 60MHz Instek GDS-1062A GDS1062A GDS 1062A), so with the FFT you're not really going to see anything (such as a distortion harmonic) below circa 48dB of the fullscale input. A modern 24-bit computer interface with free software will beat that bigtime.

+1

The shallow bit depth of a typical DSO makes one pretty much useless for this.

[perfknee]

Yes, I've found the FFT function on the instek 1062a is more of a toy than a useful tool, for the reasons given above.

As to why so many people chose the rigol 1052e instead, I think it's just because the Instek wasn't around when most of the 1052e posts were made. Kind of like how people still buy the products with hundreds of amazon reviews even when there's newer better available.

[cliffyk]

The FFT function on any 8-bit DSO is no more than an interesting toy for nearly any audio frequency work. You need a minimum of 12-bit DACs to get even close, and even at that the very best dynamic range will be only 72dB, with 66db more likely.

FWIW A number of the older Lecroy 'scopes (93xx and 94xx series) offer oversampling "resolution enhancement" modes that can create solid 11-bit equivalent resolution (66dB dynamic range) at audio frequencies...

[perfknee]

That does bring me to ask another newbie question: What piece of equipment would actually show me the spectrum of an audio (20-20k) signal? I had a craigslist notification on "spectrum analyzer", and most of the ones I came across were focussed on higher frequency radio bands. Other than the 1/3 octave displays usually found attached to equalizers, is there such a thing as a standalone audio spectrum analyzer? What would I google to find it? I'm thinking ideally something that would clearly show harmonics and other frequency domain processing artifacts would be cool. This is really just for my education, not trying to solve any particular problem.

[benb]

Firstly, it depends on what you mean by "audio" whether an 8-bit DSO/FFT is useful. I used a Tek TDS-whatever-it-was (crt display) scope circa 1997 when designing and testing a modem for use over the POTS phone line. The 1 GSPS sampling was way overkill, but the 48dB was just enough to see what was going on with the modem and phone line. On the other hand, this was surely one of the last POTS-connected modem designs ever.

To post #14, yes, there is such equipment, I recall an HP FFT audio spectrum analyzer, I forget the model number, that did something like at least 70dB S/N and up to maybe 100kHz bandwidth. There are also more specific devices made by Audio Precision and perhaps another such company, specifically for testing audio devices.

But furthermore, there are good quality USB-connected computer A/D and D/A audio interface boxes in the $100 range that go to 24 bit/192k sampling rate, and PC (and I presume Macintosh and Unix/Linux) software to do FFT's and other tests. I recall there's some thread with a list of such software here on DIYAudio somewhere.

Googling this brings up some good hits:
audio test software

[audiomik]

Re: "To post #14, yes, there is such equipment, I recall an HP FFT audio spectrum analyzer, I forget the model number, that did something like at least 70dB S/N and up to maybe 100kHz bandwidth."

HP Agilent 3562A perhaps?

A more basic Analog one might be the HP3580A.

I use a Schlumberger SI 1220, 4 inputs, tracking generator and Power Spectrum noise floor of -120dB pk/hz between 0.05 and 50kHz, but these sort of instruments are very rare to find at an affordable price

Mind you the R&S®UPV Audio Analyzer looks like an instrument to have for all things Audio:
http://www.rohde-schwarz.us/en/products/test_and_measurement/audio/UPV-|-Overview-|-100-|-6077.html
Would need to win the Lottery to ever afford one though

Mik

[gootee]

Originally Posted by perfknee

That does bring me to ask another newbie question: What piece of equipment would actually show me the spectrum of an audio (20-20k) signal? I had a craigslist notification on "spectrum analyzer", and most of the ones I came across were focussed on higher frequency radio bands. Other than the 1/3 octave displays usually found attached to equalizers, is there such a thing as a standalone audio spectrum analyzer? What would I google to find it? I'm thinking ideally something that would clearly show harmonics and other frequency domain processing artifacts would be cool. This is really just for my education, not trying to solve any particular problem.

HP 3580A Spectrum Analyzer - eBay (item 170621729256 end time Apr-03-11 08:50:17 PDT)

The HP 3580A does 5 Hz to 50 kHz. The linked auction closes in less than 21 hours. The current price is $56.

[benb]

Originally Posted by audiomik

Re: "To post #14, yes, there is such equipment, I recall an HP FFT audio spectrum analyzer, I forget the model number, that did something like at least 70dB S/N and up to maybe 100kHz bandwidth."

HP Agilent 3562A perhaps?

I had to google to see the front panel, but yes that's it. It was overkill for what I was doing (again, phone line stuff).