instrument for audio signal analysis - Page 1
Forum section: electronics / testgear

author: Rax
author_rank: Super Contributor
timestamp: October 19, 2015, 06:13:52 pm
content: Hi all,First post here though I seem to be coming here more and more often when researching something having to do with my bench...I'm looking for something suitable for audio signal spectrum visualization (some call them "spectrum analyzers," some "dynamic signal analyzers...") I'd like to be able to cover 10Hz to 100kHz at a minimum, have a decent noise floor. It seems that most digital scopes come with some FFT options, so they may be an option. Especially given my budget for this - I'd keep this well under $1,000, though under $500 would be really awesome if useful - they may be all I got. I am based in the US, if that helps with the suggestions.I've been looking at some Instek models (for instance, GDS-1000A-U), Tektronix (TBS1000, though turned off by the backlash on this forum following their release), Rigol DS1054Z. Agilent may have some affordable models. In the way of specialized spectrum analyzers, I've also been looking at some vintage HP models, such as 3582A etc. I admit to being a fan of these old HP/Tek instruments, of which I have some (a Tek 453A and an HP334A, for instance) and they are built like works of art. Having said that, I will listen to the voice of reason and look for something that's the best for the intended use in the given budget.It may be said that a true solution to what I'm looking for may be an Audio Precision box and such niceties, but those are far out of my hobbyist reach...Thanks a lot for your input! Looking forward to your thoughts.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: October 19, 2015, 06:56:21 pm
content: Why spend so much money, if a good sound card and the programcalled "Audio Tester" for a few bucks might serve your needs?See enclosed example. Bandwith can go up to just under 100 kHzwith the sound card I use (M-Audio - Audiophile 192) .Just noticed. The Audiophile 192 is no longer available.So you might have to look at an ESI Juli@ or something similar.But make sure that the frequency response extends to around 100 kHz.Some high sampling rate sound cards have a fixed built-in filter,cutting in already at 30 kHz regardless.Yours Messtechniker

author: Messtechniker
author_rank: Frequent Contributor
timestamp: October 19, 2015, 07:00:28 pm
content: Addendum: The graph shown is that for a small power amp justbefore clipping (add +20 dB to the levels shown) .The soundcard itself has a much lower distortion.Yours Messtechniker

author: Rax
author_rank: Super Contributor
timestamp: October 19, 2015, 07:30:05 pm
content: Quote from: Messtechniker on October 19, 2015, 06:56:21 pmMesstechniker - thanks for pitching in. Doing this is an interesting idea, though I should have probably make clear I am looking for an instrument to put on the bench/shelf. I built a buffer to place between the computer and DUT a while ago (which I never really ended up using), but I remain very weary of risking the computer assigned for this use with a direct route to the testing end of the probes. This is also because a lot of my work is with tubes.Also, I'm not sure what happens to the calibration when this solution is adopted.Besides, I learned in time to appreciate dedicated instruments for given tasks - they just tend to perform better, take much less time to perform, present much better logistics around them. A touchpad/mouse and a large screen (with a computer attached to it) near or on the bench are pretty inconvenient. If what I need ends up being rendered to be a scope, it may also replace my old 453 for most tasks... That much more an advantage, as the space I have is at a dear premium.I am willing to do this as a temporary solution, though.

author: nctnico
author_rank: Super Contributor
timestamp: October 19, 2015, 07:56:37 pm
content: A Tektronix TDS500/TDS700 series with the FFT option could be a good one for little money. These scopes have high-res acquisition and the FFT also has averaging so it isn't jumping allover the place. The FFT isn't very fast though especially with a lot of averaging. I have used these scopes for some audio analysis work in the past to measure the bandwidth of a system. I'm quite sure the FFT on the low end Chinese scopes are not up to this task.Another option would be a spectrum analyser which goes down to 10Hz. IIRC some radio communication analysers have audio analysis capabilities as well but I don't know if they work up to 100kHz. You'll need to adapt your signal to 50 Ohm inputs though.

author: deadlylover
author_rank: Frequent Contributor
timestamp: October 19, 2015, 09:53:28 pm
content: I have a Rigol DS1054Z and it is bloody hopeless for audio FFT measurements (for measuring distortion anyway).If you're more interested in measuring THD and such, how about taking a sort of half measure and getting an audio analyser? You can then perform FFT on the analyser's post notch monitor outs with a sound card, with no risk of blowing up your PC.IIRC, the Shibasoku 725 with a few simple mods can dance with the Audio Precision flagship at the -120dB THD+N mark for 1kHz@20kHz BW. They are still very much state of the art, and if you look hard enough you can easily get one for <$1000 delivered. And for when you don't want to perform FFT, you still have a nice instrument that can measure levels on individual harmonics up to the 5th separately.Not quite what you were looking for, but it's another wallet destroying option perhaps.

author: amirm
author_rank: Regular Contributor
timestamp: October 19, 2015, 10:43:30 pm
content: Scopes are designed to have very high speed but pretty low resolution. For audio, you want it the other way around. An 8-bit Analog to Digital converter is not going to be at all useful for audio. As said, if you want cheap, just get a sound card and use the free software out there. You would have to be careful with input levels and such (don't try to measure an amp!) but should be good for line level outputs.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 20, 2015, 02:51:08 am
content: You could go "retro",& use a Noise & Distortion meter.(N&D Set),which was the standard way of testing in Broadcasting for many years.OK,in a straightforward Distortion test,they really show youTHD+Noise.Noise is normally much lower in level & has very little effect upon the result,but if the reading appears abnormally high you can then do a Noise Test to see if you have a Noise,rather than a Distortion problem.Youcanchase up various harmonics by nulling them out with the filters,if you really want to know how much each contributes.Another possibility is to approach it from the other direction,& use a Selective Millivoltmeter,which is what they used in testing Hearing Test equipment.With these,you can tune in the various Harmonics & so determine their contribution to THD.You can look for Intermods with a Selective Millivoltmeter,too---that's how we used to check our ISB transmitters back in the day!On eBay,I have often seen HP Audio Spectrum Analyser plugins for the various Spec An mainframes.Not many people want them,so they languish for ages,only to come up again in a new listing.

author: Keicar
author_rank: Regular Contributor
timestamp: October 20, 2015, 03:31:13 am
content: If you're on a budget, there's a lot you can do with the demo version of ARTA - although you've got to deal with the fact that your simultaneously testing your audio interface in addition to the DUT - which can be frustrating. I ended up getting a HP 3562a Dynamic Signal Analyzer - which does a fine job, although the limitations imposed by the 13-bit ADC mean that while the dynamic range is acceptable, you can't necessarily see down to microscopic levels of distortion - but it's certainly more trustworthy than using a PC audio interface that was never intended for test & measurement duty. Another product that has piqued my interest is the QuantAsylum QA-400https://www.quantasylum.com/content/Products/QA400.aspxwhich might be worth a look.

author: Cubdriver
author_rank: Unknown
timestamp: October 20, 2015, 03:43:45 am
content: With some patience and judicious sniping you should be able to snag a functional 3582A, 3561A or 3562A from the 'bay for well under $1k; likely even cheaper if you're willing and able to repair it.-Pat

author: kc8apf
author_rank: Regular Contributor
timestamp: October 20, 2015, 05:02:37 am
content: After playing with a HP331A for a while, I jumped on the 3562a bandwagon. As others have mentioned, it doesn't have the impressive noise floor of an AP but I find it to be a pleasure to use. I got mine for $500 on eBay. They usually run around $1k but occasionally you find them lower.

author: radhaz
author_rank: Regular Contributor
timestamp: October 20, 2015, 05:09:35 am
content: Check this repair:http://amp8.com/vv-amp/onlife/u-22_3.htm

author: singapol
author_rank: Frequent Contributor
timestamp: October 20, 2015, 08:12:18 am
content: AHP8903Bperhaps although this is not a SA? You can find one on ebay at hobbyist prices.http://www.keysight.com/en/pd-1000002339%3Aepsg%3Apro-pn-8903B/20-hz-to-100-khz-audio-analyzer?cc=SG&lc=eng

author: SteveLy
author_rank: Regular Contributor
timestamp: October 20, 2015, 08:24:54 am
content: The way I do audio signal analysis is to use a digital recorder to capture the signal, convert it to floating point and then do the maths on the computer. It's cheap, easy and very effective if you don't need real-time. (I code what I need in C/C++; but you could also use higher level tools such as Matlab, Octave, Scilab, etc - even Audacity will do basic FFT stuff with a few clicks.)You could of course also use a sound card for capture but stand-alone audio recorders tend to give better signal-to-noise per buck than sound cards. Most of them will also give you live digital audio via USB so real-time processing should also be doable, but I have not explored that. If you don't need real time, then the convenience of having all the data in a file and being able to analyse the heck out of it is very nice.

author: Marco
author_rank: Super Contributor
timestamp: October 20, 2015, 09:07:50 am
content: What did you use for a pure oscillator? (Although I guess you could do a pass through first and subtract the oscillator's harmonics in the complex frequency domain.)

author: Marco
author_rank: Super Contributor
timestamp: October 20, 2015, 09:50:42 am
content: How do you cancel the harmonics from the DAC?

author: GK
author_rank: Super Contributor
timestamp: October 20, 2015, 10:04:11 am
content: Quote from: blueskull on October 20, 2015, 08:49:30 amDynamic range (DR) is essentially the same as SNR. Most high-performance 24-bit audio DACs only manage 16-bit performance with the best 32-bit DACs reaching 20 or 21 ENOB. 21 ENOB returns a SNR of 126db - well short of 140dB, which requires 23-bit performance.For instance this one only does -123dB SNR and its THD performance is well short of bettering -126dB.http://www.ti.com/lit/ds/symlink/pcm1795.pdfI'd be interested to know the part # of the DAC you are using.

author: GK
author_rank: Super Contributor
timestamp: October 20, 2015, 10:16:27 am
content: I built my own analyzer. Feeding the distortion residual into a PC sound card for FFT gives the best of both worlds.http://www.users.on.net/~glenk/thd/thd.htm

author: Rax
author_rank: Super Contributor
timestamp: October 21, 2015, 02:00:36 am
content: Wow. Such great feedback - thanks all! Let me itemize a little.Quote from: nctnico on October 19, 2015, 07:56:37 pmThanks a lot for the Tek TDS recommendation. I am closely examining the series. I wonder how this would stack up against a dedicated HP DSA/SA, such as the 3582/3561/3562 types. In other words, if getting a dedicated such instrument is really needed, or assigning two majors tasks (scope & DSA/DA) to one instrument is enough for most intents and purposes.On the (RF) spectrum analyzer option - it just seems too much of a complication and I am uncomfortable with the different standards that would interface. A little like I would replace the compromise of using a sound card with another compromise.

author: Rax
author_rank: Super Contributor
timestamp: October 21, 2015, 02:11:22 am
content: Quote from: deadlylover on October 19, 2015, 09:53:28 pmNot a bad idea, though I'm not sure which ones exactly allow for such pass-through use. If sticking with my current guns, I could do this with my HP334A. I'm not sure if the piece of gear I'm considering right now - HP 8903B - allows for this. In any case, I may be contemplating getting an audio analyzer (such as HP 8903B)anda DSA/SA....Quote from: deadlylover on October 19, 2015, 09:53:28 pmI was completely unaware of the brand, even - sorry. I guess they're not that easy to come by in the US (or I should change my hangouts). I will take a closer look.

author: Rax
author_rank: Super Contributor
timestamp: October 21, 2015, 02:53:08 am
content: Quote from: Cubdriver on October 20, 2015, 03:43:45 amQuote from: kc8apf on October 20, 2015, 05:02:37 amThe 3562a sound like a very interesting piece. My best pick this far for an DSA/SA. Ideally, I think I'd like one of these and an 8903a on my shelf. $500 for a fully working one is a fantastic deal.

author: kc8apf
author_rank: Regular Contributor
timestamp: October 21, 2015, 02:56:44 am
content: Quote from: Rax on October 21, 2015, 02:53:08 amWhat does the 8903 do that the 3562 does not?

author: Rax
author_rank: Super Contributor
timestamp: October 21, 2015, 03:02:11 am
content: Quote from: Keicar on October 20, 2015, 03:31:13 amThanks a lot for the details on the limitations of the 3562A (may you be talking about the 3563A, though?...). I'd also be interested to hear if anybody knows the QA-400 hands-on. I've seen it before and pondered on it for a while.

author: Rax
author_rank: Super Contributor
timestamp: October 21, 2015, 03:18:48 am
content: Quote from: kc8apf on October 21, 2015, 02:56:44 amThey are different animals, to my mind (never had either, so I go by spec sheets etc.). I'd use the 3562A to visualize the content of the audio spectrum (invaluable, surely), while with the 8903A you can obtain similar information, but all numerical: distortion, S/N, RMS AC measurements, even DCV if one needs it, etc. They may look at the same content, but their output I see useful in fundamentally different ways. Also, it's true that the 3562A has a signal source, but if I go for another DSA/SA, it may not have it. Besides, I've learnt to appreciate having multiple instruments with overlapping duties, which is never truly superfluous. But seriously, never mention this to my wife.I'm looking forward to feedback on this though; I am still figuring out my needs, including (instrumentally) through this conversation.

author: kc8apf
author_rank: Regular Contributor
timestamp: October 21, 2015, 03:25:08 am
content: Quote from: Rax on October 21, 2015, 03:18:48 amI highly suggest reading the 3562 manual andFundamentals of signal analysis

author: singapol
author_rank: Frequent Contributor
timestamp: October 21, 2015, 03:50:39 am
content: Quote from: Rax on October 21, 2015, 03:18:48 amYou might be interested in Standford Reasearch SR760/770 SA similar to HP3562a but only 1 channel and-90db dynamic range vs -130db hp3562a but 770 has usb albeit older standard. I think SR760/770 does not have AM/FM modulation but it does have signal source.There is also SR780.http://www.testequipmentdepot.com/usedequipment/hewlettpackard/spectrumanalyzers/3562a.htmhttp://www.thinksrs.com/downloads/PDFs/Catalog/SR760770c.pdf

author: deadlylover
author_rank: Frequent Contributor
timestamp: October 21, 2015, 05:03:47 am
content: Quote from: Rax on October 21, 2015, 02:11:22 amI've been on the lookout for the HP 8903B, but not such luck here down under. I *think* a few simple mods to the oscillator section will make it perform quite decently.Keep an eye out for the Panasonic VP-7722 because it's relatively unknown and you might score a deal, I freaking love mine and it's probably the best $300 I've ever spent on test gear. -112dB(0.00023%) THD+N @ 1kHz/30kHz BW, not too shabby at all eh for hobbyist use.

author: davorin
author_rank: Unknown
timestamp: October 21, 2015, 06:10:09 am
content: Hmm...from the HP 3562A datasheet it looks like it can't do THD and IMD measurements...or do I miss something?Have you had a look at the QA400 from Quantasylum?https://www.quantasylum.com/content/Products/QA400.aspxNot going up to 100kHz due to 192kHz sample rate.I bought one cause I found the combination of USB soundcard and commercial software in the same price range as the QA400.The newer QA401 and QA405 will be more expensive though, but feature isloated inputs/outputs and differential signals...

author: GK
author_rank: Super Contributor
timestamp: October 21, 2015, 07:48:07 am
content: The AWA F242A is a reasonably sophisticated auto-tuning analyzer that pops up on ebay on a semi-regular basis; generally around the $200 - $250 mark. Since I've built my own analyzer with better performance I have no need for one, but have procrastinated over buying one a few times now after stumbling across examples on ebay.Manual here:https://isurplus.com.au/manuals/AWA%20F242A%20User%20Manual.pdfI just nicked this image from a current ebay auction:

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 21, 2015, 07:59:05 am
content: As hardware analysers I use the Amber 3501 "Distortion and Noise Measuring System" and occasionally the HP3563A . The Amber has an excellent low distortion generator with THD about -115dB at midrange (400Hz) and the best SNR about 105dB in 30kHz BW and ~100dB in 80kHz BW. The most useful feature is the distortion only output for the scope, which allows you to see the distortion only waveform. Also it can be used as a good TRMS voltmeter and a selective amplifier.CheersAlex

author: Kjelt
author_rank: Super Contributor
timestamp: October 21, 2015, 08:18:02 am
content: Is it my superstition or is it still bad to use a PC to sample analogue signals? Only the bad powersupply and noise inside would give me shivers.I whould think it is a two stage job, input and signal generation hardware fully standalone analogue build and then the data transfer to a pc that does the math and presents the GUI.Nobody on this forum build his own version of a audio precision system one or are the soundcards and pc's that good you can reach the same specs ?

author: fivefish
author_rank: Frequent Contributor
timestamp: October 21, 2015, 08:24:53 am
content: I have an 8903B, useful for getting numbers... not a spectrum analyzer though, so for frequency response visualization, I use FuzzMeasure and my MOTU 828mk2 audio interface.

author: Rax
author_rank: Super Contributor
timestamp: October 21, 2015, 06:15:55 pm
content: Quote from: fivefish on October 21, 2015, 08:24:53 amWhoaaAaaw!... It seems I just snatched one exactly like this. Somewhat unfortunate - though it probably helped with the price - with rear hookups like you have it ("option 001" I guess it is). How did you put together that installation there to bring them to the front? Looks pretty neat. It's briefly mentioned here:http://www.neurochrome.com/test-equipment/hp-8903/,but I will have to get the list of parts together in an order.Thanks!

author: Rax
author_rank: Super Contributor
timestamp: October 22, 2015, 02:12:01 am
content: Quote from: Alex Nikitin on October 21, 2015, 07:59:05 amHi Alex,I just discovered this (before reading your post), and I am quite impressed. It's sort of like an HP339a but better... One of the good things about it is that, even if in a tighter spec'd category, it retains the needle meter, which is useful at finding the minimum output when adjusting a filter, for instance, etc. There are some things easier done with an old school meter.It also puts little damage in one's pocket, relative to what it does.

author: fivefish
author_rank: Frequent Contributor
timestamp: October 22, 2015, 02:30:19 am
content: >How did you put together that installation there to bring them to the front?It's just a DIY 1u front panel with BNC pass-through connectors, and some short BNC patch cables connected to the rear of 8903B then to the 1u front panel. It adds a few decimal points in the readings (but almost negligible), but the convenience it brings outweighs that.

author: kc8apf
author_rank: Regular Contributor
timestamp: October 22, 2015, 04:12:58 am
content: Quote from: davorin on October 21, 2015, 06:10:09 amIt can definitely do THD. See pages 19 and 8-18 ofHP 3562A Operating Manual. The signal source isn't capable of two-tone generation so they don't include an IMD feature. If you use an external two-tone generator, you'll definitely see the IMD visually and can use the markers to measure it.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 22, 2015, 07:03:47 am
content: Quote from: GK on October 21, 2015, 07:48:07 amBack in the day,somebody thought it would be a nice idea to replace our manual AWA N&Ds with HP auto units.Well,the HPs were a major disappointment!Probably good in a Lab,but when you have limited time to do multiple tests,as is common in Broadcasting,their slo-o-o-www nulling made it a hassle.The only way to use them was to manually null,then let the auto do the last few "tweaks".We thought all our birthdays had come at once when we got the AWAs---fast auto null,equal or better specs.The HPs got shut away in the equipment cupboard!

author: Rax
author_rank: Super Contributor
timestamp: November 06, 2015, 12:12:18 pm
content: OK, time for some updates.Since my last post on this tread, I received my 8903B and got a chance to test it and all that. I must have ran into the nicest seller on the entire bidding site, because they not only replaced my purchased unit (they have a large stock at hand) with one with front hookups (no option 001), but also with one having all filters I wanted (400Hz, A-weighing, 30kHz, and 80kHz - all available for this unit that are most useful in my opinion), in pretty much perfect shape, and - best of all - withcurrent calibration until spring of next year!... They even sent me the long form calibration certificate. This is in fact much better than I expected. The price was far from excessive also.Anyway, my decision to get this is in a juncture with getting myself geared up for doing the audio spectrum analysis on my 17" laptop (I gave in on this one...). I am thinking that maybe later I will be able to afford an HP 3562a or similar to have this analysis done in a more calibrated manner, so to speak. For now, my task is to get myself a suitable USB sound card and maybe also build Pete Millett's interface kit to work with it, and I think I have this nailed down for now.Thanks all for your input - extremely useful guiding for me in this endeavor.

author: fivefish
author_rank: Frequent Contributor
timestamp: November 06, 2015, 01:29:06 pm
content: Congrats. If yours didn't come with the hardbound operation and calibration manual, I'd suggest also buying one.

author: Cubdriver
author_rank: Unknown
timestamp: November 06, 2015, 04:36:14 pm
content: Excellent - glad to hear that you found something!And I'll second the suggestion to get the manual.-Pat

author: Rax
author_rank: Super Contributor
timestamp: November 06, 2015, 06:41:55 pm
content: Thanks a lot, fivefish and Cubdriver!I am confused on your recommendation, though, as the Keysight website has all of the manuals for this instrument (and pretty much all historical HP instruments) available for free as PDFs. I have the one I need the most (Op. & Calib.) printed out.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 06, 2015, 07:06:53 pm
content: I found interesting nobody mentioned the Keithley 2015. Just curious, as I am not an expert in the audio analysis field: how different is the Keithley from the animal you bought?

author: Cubdriver
author_rank: Unknown
timestamp: November 06, 2015, 10:14:27 pm
content: Quote from: Rax on November 06, 2015, 06:41:55 pmAs long as it's legible you're good to go, but I've found that a lot of those scans are less than optimal as far as quality goes - especially for things like the schematics, which in the factory manuals are often multi-page foldouts and color coded, and display screen reproductions (which don't really apply here upon further reflection, since the 8903B doesn't have a display screen, just digital readouts and an analog meter). Don't get me wrong - I think it's wonderful and a great thing that Agilent/Keysight have the scans for so many instrument manuals posted and easily available, and have downloaded plenty of them myself for my own instrumentation, but I find it (for me at least) best to try to get a real hardcopy of an original manual if possible. I find doing cut-and-pastes with the multi-page schematic printouts to be a royal PITAYMMV.-Pat

author: Rax
author_rank: Super Contributor
timestamp: November 07, 2015, 02:33:03 am
content: Quote from: rsjsouza on November 06, 2015, 07:06:53 pmNot a specialist either, but the first observation would be that the Keithley seems to kind of hit a different price bracket. Not by much, and on some sites where you can find both used they may really not differ much at all, but enough to be substantive for a decision (at least for me). Additionally, I think it's easier today to find a good deal on an HP 8903B than you typically can on a Keithley 2015.From the specs, I think the HP offers a bit more consistent performance across the board and is a bit better in some critical points (the Keithley is better in some other points). It may also be geared more specifically towards audio measurements.Then again, certainly, the Keithley could replace a bunch of instruments on my - chronically lacking room... - bench.Maybe guys having used both may pitch in here with more hands on information than what I have.

author: RNA
author_rank: Regular Contributor
timestamp: December 14, 2015, 01:59:17 am
content: I have been using the transfer function a lot on my hp 3582a. In this video I use it to examine the hippos filter mod for the GSSL bus compressor. Very useful.