Spectrum analyser or a scope with good FFT? - Page 1
Forum section: electronics / testgear

author: JSE009
author_rank: Contributor
timestamp: January 30, 2016, 10:06:56 am
content: I can surely need some help/adviseI am working a lot on audio stuff and especially pre- and power amplifiers (tube and transistor).And for measuring distortion etc. I use two Keithley 2015 and they do the job perfectlyBut what I miss is a closer look on the spectrum of a (test)signal, yes I can use my Rigol scope (DS2072A) but as Dave showed nicely in his review episode #845,the Rigol is not really suitable for this part of the job.I also use a pico scope (2205A) but the noicefloor of this unit is to hight to measure audio signals so I use this unitI only when I am in the field so to say and want to performsome quick measurements.I can buy a standard spectrum analyser but they almost all started at 9kHz and I need less than that.I did like the Oscilloscope FFT Comparison in Dave’s reviewand now I am wondering, is it smart to look for a scope with a “good” FFT function or do I have to look fora spectrum analyser that start at a low frequency? Sometimes they can be found on eBay.I do not want to spent more then $2.000,= for a unit.Thank in advance for feedback.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: January 30, 2016, 12:35:43 pm
content: Quote from: JSE009 on January 30, 2016, 10:06:56 amAs you already found out, spectrum analyzers start at comparably high frequencies (usually 9kHz, some even higher i.e. 10MHz) and are not much use for audio. There are a few audio SAs but they are somewhat rare and pretty pricey (even for some pretty old ones with mono CRT).A scope with good FFT is in my opinion a better alternative. However, FFT on pretty much all Rigol scopes (up to the ridiculously priced DS6000) is nothing more than pathetic as it only uses a few thousand (4k, in some cases even just 2k) points for FFT. That means Rigol is is out.You also want a scope with a somewhat decent processing subsystem so that FFT calculations are done reasonably fast, and that pretty much rules out most B-brand scopes.With a $2k budget, you could get a new Keysight DSOX2000A Series scope which offers decent FFT capabilities. However, I'd consider the 2nd hand market, which has several scope alternatives that give you fast FFT, plus you get a much more capable scope on top (so you could sell your DS2000A and recoupe some of the money).Having said that, the cheapest alternative is a good(!) PC sound card and a software SA for audio.

author: nowlan
author_rank: Frequent Contributor
timestamp: January 30, 2016, 01:14:25 pm
content: Would the quantasylum gear be better than your 2015?https://www.quantasylum.com/content/Products/QA400.aspx

author: ADT123
author_rank: Regular Contributor
timestamp: January 30, 2016, 01:23:44 pm
content: The more points you can have in the FFT the lower the noise floor - going from 1000 points to 1,000,000 spreads the noise floor over 1000 times as many bins so will drop the noise floor by 30dB.With your PicoScope 2205 make sure you have the max no of points enabled in the spectrum (probably 8000 as it has 16K memory) then enable averaging to drop the noise floor more. If thats not enough you need more resolution and more memory - I did a test with 1 million points and 16 bits resolution on a PicoScope 5444B and got the noise floor 120dB down. See reply 25 and 26 herehttps://www.eevblog.com/forum/blog/eevblog-845-oscilloscope-fft-comparison/25/Noise floor is of course not the only issue - a high res scope with a low distortion front end is also important for audio.

author: markone
author_rank: Frequent Contributor
timestamp: January 30, 2016, 01:35:43 pm
content: Quote from: Wuerstchenhund on January 30, 2016, 12:35:43 pm8 bitDSO for High End audio analysis ?

author: _Wim_
author_rank: Super Contributor
timestamp: January 30, 2016, 01:40:34 pm
content: Scopes never have a good enough AD to make usefull audio measurements (well, maybe for valve amplifiers they can be of some use due to their typical high distorsion figures).You have scopes with 16 bit AD (accomplisched though oversamping), but even those get only a THD+N of around -80db (-96db is the theoretical max for 16 bit). The quantasylum is much better then this (THD+N > -98 db, but also most good pc audio cards with free software (for example ARTA, Holmimpulse, Rightmark Audio Analyzer, Room EQ Wizard...) can reach level up to -100dBPersonnaly I use ARTA with an Echo Audiofire 2 sound card (balanced in and out) and this has been very usefull to me. I also have a rigol ds1054z for general electronisc work.

author: markone
author_rank: Frequent Contributor
timestamp: January 30, 2016, 01:45:46 pm
content: Quote from: nowlan on January 30, 2016, 01:14:25 pmI was going to suggest the E-MU 1212M PCIe :http://www.creative.com/emu/products/product.aspx?pid=19169but the QA400, that honestly i did not know, even if with lower specs, @200 USD seems a good deal due to the fact that has all the metrics ready out of the box.Have you direct experience with that ?

author: AF6LJ
author_rank: Unknown
timestamp: January 30, 2016, 01:49:04 pm
content: I am thinking that maybe going old school would give better results...Get a selective voltmeter. They are made for this kind of stuff.

author: _Wim_
author_rank: Super Contributor
timestamp: January 30, 2016, 01:54:55 pm
content: Quote from: markone on January 30, 2016, 01:45:46 pmI have the E-MU 1212M PCIe also, but the drivers are really unstable and cause BSOD very often. It is a very good card (see spectrum plot), but the instability makes it very annoying to use...

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: January 30, 2016, 01:55:58 pm
content: Quote from: markone on January 30, 2016, 01:35:43 pmMost better scopes offer high resolution modes (i.e. 11bits on older LeCroy scopes, 12bit on older Agilents like the DSO5k, 16bits on the R&S RTM) which work pretty well, and the reduction in analog bandwidth is not a problem for audio applications.It does require a decent processing backend but on a good scope FFT performance will still be good even with high res/ERES modes enabled.

author: G0HZU
author_rank: Super Contributor
timestamp: January 30, 2016, 02:04:06 pm
content: QuoteIf you avoid the mid range models you will find that a decent RF spectrum analyser will usually work down to 100Hz or less and it will maintain a decent noise floor down to audio frequencies but it isn't really the right tool for HiFi audio analysis. Too slow and not enough SFDR for audiophoolsThis class of analyser also tends to be very big and heavy. I've got several decent RF spectrum analysers here and they all work very well down to 100Hz or less.I normally use a soundcard for stuff like this but care is needed not to overload or damage the soundcard. The same would apply to a decent RF spectrum analyser in terms of ease of damage. An RF spectrum analyser will usually have a 50R input impedance which often isn't suitable either.I would have thought that a Picoscope would work well here although I haven't used a Picoscope for many years. The last one I used was over 12 years ago.Like you I have a Keithley 2015 THD and this can be configured over GPIB to show the levels of harmonics with a reasonably fast update rate on a PC display. So it looks like a spectrum analyser even though it is really just measuring and displaying the individual harmonics. But it can do this very accurately in my experience. See the image below.The Keithley is also very rugged in terms of damage level. But it isn't going to deliver the ultimate THD (or THD and noise) performance that some die hard audiophools crave.

author: markone
author_rank: Frequent Contributor
timestamp: January 30, 2016, 02:04:13 pm
content: Quote from: Wuerstchenhund on January 30, 2016, 01:55:58 pmEverything below a real100dBdynamic range and proper noise floor is useless for the purpose, your suggestion was for Keysight DSOX2000A, that is utterly out of specs for audio meas.

author: _Wim_
author_rank: Super Contributor
timestamp: January 30, 2016, 02:07:57 pm
content: Quote from: Wuerstchenhund on January 30, 2016, 01:55:58 pm11.5 bits wont cut is, and the R&S RTM have a native 8-bit dac, and only reaches 16 bit through oversampling. They do not state the THD+N figure in their datasheet, but the picoscope 5000 series (https://www.picotech.com/oscilloscope/5000/picoscope-5000-specifications) do something simular but only reach a THD of -70db which is insufficient for audio work. If you really want to use a scope for something like this, you should buy a scope the a native 16 bit ad like a picoscope 4262 (https://www.picotech.com/oscilloscope/4262/picoscope-4262-specifications). But due to their use of a 16-bit AD, these sort of scopes have a very low bandwith, making them less usefull as a general purpose scope. So in my opinion (and as other have posted), you much better off with a general purpose scope, and a dedicated audio measurement system. Due to the power of modern pcs, a pc based solution clearly offers the best bang for buck

author: markone
author_rank: Frequent Contributor
timestamp: January 30, 2016, 02:15:05 pm
content: Quote from: _Wim_ on January 30, 2016, 01:54:55 pmI have the old PCI version, i do not have stability problem but i have to say that its mixer control panel is a huge PITA at first glance, performance wise i think its the best bang for the bucks, much better than even pricer USB versions.

author: _Wim_
author_rank: Super Contributor
timestamp: January 30, 2016, 02:24:21 pm
content: Quote from: G0HZU on January 30, 2016, 02:04:06 pmI can agree the Keitley is also a nice solution. I can also agree that the final device does not need to have THD levels of -100dB or more (audiophool territory), but around -80dB is necesarry for high qualitiy listening, especially so if you not playing at the optimal sound level for the lowest distorsion. Also, when devellong audio gear, it very handy that you measurement system has at least a 10db lower distionsion and noise floor, so you know for sure that what you are seeing is the DUT and not the measurent system itself.This makes using a sound card sometimes difficult, as ground loops cause distorsion in the measument system that can sent you on the wrong track. To avoid this I have been thinking of buying a AD-card en DA-card (evalution boards) and powering these with an isolated power supply individually. Both cards would then be connected to a pc audio card using an optical toslink connection. With some additional input protection this creates a high-end audio measurement system that is very robust...

author: _Wim_
author_rank: Super Contributor
timestamp: January 30, 2016, 02:27:10 pm
content: Quote from: markone on January 30, 2016, 02:15:05 pmYes, the mixer control panel is a pain, but once you have your usual setups configured and saved however, you are in the clear. It was (it is not sold any longer) indeed the best bang for buck performance wise.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: January 30, 2016, 02:27:19 pm
content: Quote from: _Wim_ on January 30, 2016, 02:07:57 pmYou're probably right that the THD+N figures of most scopes will be insufficient for high end audio work, but then OP did only broadly specify what he's looking for so a scope with high res mode (which is what a friend of mine who's dabbling with HiFi gear uses, amongst other gear like an audio analyzer) might (or not) have satisfied what he wants. I never worked much with audio stuff so I can't speak of experience.Which is exactly why I also wrote that a PC sound card is probably a better alternative.

author: deadlylover
author_rank: Frequent Contributor
timestamp: January 30, 2016, 02:28:06 pm
content: I'll echo the suggestions for a sound card + PC for FFT, that QuantAsylum 400 looks pretty good.I have a spectrum analyser (Advantest TR4171) that starts from 10Hz, but the problem is the spectrum analyser's distortion is in the range of -70dBc or so, way too high for audio work, it is basically useless to have a look at individual harmonics.You can use a notch filter to reject the fundamental, but now the problem becomes the veeeery long sweep times, expect anywhere between 3-15 minutes just for a 1kHz or 10kHz span at 3Hz or 10Hz RBW. Not very practical for most uses in audio.Another half measure would be to get a dedicated audio analyser with a monitor output, they're pretty standard so you should find them on basically everything. You can then perform FFT on the monitor output without any risk of blowing up your PC or sound card (they can usually take up to 150V input too, good for power amps), and depending on the audio analyser you can get very decent performance.Sometimes all I really need to look at are the first 5 harmonics, and any good audio analyser can display them individually, no need to set up math functions on a scope or fire up a PC, 1 button and you're golden.Attached is an FFT on the monitor out of a ShibaSoku AM51B audio analyser loopback using the crappy built in sound card from a cheap laptop, that second harmonic is at -139dB from the fundamental.ShibaSoku make great audio analysers, one to look out for is the old 725B or 725C models, with a few simple mods (upgrading 25 year old op amps) they will bring any entry level audio analyser today to their knees in terms of residual noise+distortion anyway. They'll measure down to -120dB THD+N @ 1kHz, 30kHz BW no problem, and for reference the new flagship Audio Precision APx555 goes to like -124ish I think, but they start at 27 grand.

author: markone
author_rank: Frequent Contributor
timestamp: January 30, 2016, 03:08:29 pm
content: Quote from: deadlylover on January 30, 2016, 02:28:06 pm

author: rosbuitre
author_rank: Regular Contributor
timestamp: January 30, 2016, 03:11:21 pm
content: HiI am considering purchasing this audio interface, MOTU Ultralite MK3 that comes with software to measure FFT, someone use?http://www.motu.com/products/motuaudio/ultralite-mk3http://www.motu.com/products/cuemix-fx/tuner-analysis.htmlRegards

author: _Wim_
author_rank: Super Contributor
timestamp: January 30, 2016, 03:36:29 pm
content: Quote from: markone on January 30, 2016, 03:08:29 pmThat's "only" the price of a pair of "good" speaker cables:http://enjoythemusic.com/superioraudio/equipment/0113/siltech_royal_signature_series.htmEuhm correction: one the price for the short version of 1m. The 2.5m version sets you back 62000$. Bargain if you are a real audiophool...

author: Muxr
author_rank: Super Contributor
timestamp: January 30, 2016, 04:56:03 pm
content: There is also the Signal Hound, it goes down to the audio frequencies and it's a pretty capable PC based SA. For my audio work I just use a keithley 2015 for THD measurements and a scope FFT if I need to look at the frequency domain.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 30, 2016, 05:25:00 pm
content: Years ago when I was working with audio, the company I worked for owned an HP3582A. At the time, there was no Labview and I remember using a Polaroid camera to get data off of it, besides collecting it by hand. We also had some ADC board that we used with the VAX at that time. Once the PCs started picking up steam, we had some software / hardware package that could plot waterfall curves. It was pretty impressive as I was doing this by hand up till then.I would bet the worlds worse modern PC sound hardware is better than what we were doing.

author: Muxr
author_rank: Super Contributor
timestamp: January 30, 2016, 05:36:30 pm
content: I think the problem with your standard audio PC hardware is jitter. Nwaguy who kind of set the PC audio community on fire (interesting read, many on here will enjoy his story) has an article on it. dScope is what audio industry uses nowadays but it's out of reach of hobbyists.http://nwavguy.blogspot.com/2011/02/testing-methods.html(it was updated on 4/15)Anyways those who haven't heard of this guy. He's some anonymous EE, who popped up out of nowhere and started challenging these common myths in the Audiophool circles we all like to make fun of. Like a $2 op-amp being superior to these nude virgin op-amps, people were after. But he backed it all up with measurements and facts. He claimed he could make a headphone amplifier that was more objectively accurate than these multi thousand dollar alternatives, and then he did just that and open sourced it. It's called O2. It's kind of the ultimate, I told you so story.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 30, 2016, 06:58:11 pm
content: Quote from: Muxr on January 30, 2016, 05:36:30 pmCare to expand on this?

author: _Wim_
author_rank: Super Contributor
timestamp: January 30, 2016, 07:09:48 pm
content: dScope and Audio precision are indeed commonly used by profesionals. Apart from very good specs, these are mainly used because of features like automated testing and reporting, which is not so important for hobbyists. Also, as a professional, 10 of 20k for something you use on a daily basis is much more acceptable price wise then to a hobbiest who uses this max half a day a week and who gains no profit of it.As to jitter, I do agree it is not so easy to measure accurately, but I do not agree this is very important for audability. I read of a controlled listening test were they introduced jitter in the audiochain, and asked people to indicate the level of jitter. In essense, people only started to recongnise the jitter because the DA was creating skipping sound when the jitter was so worse. I have searched for the reference in my archive, but could not find it. As far as I remember, the study was performed with members of a dutch audio forum.Anyway, somehow in audio people tent to believe that our ears/brains are a much more sensitive instrument that high end measurement gear, and that stuff that can barely be measured accurately, make a "huge" difference in sound. My opinion is that the oposite is true. When it measures okay with reasonable quality gear, it is also ok. This means that is you built something with moden electronics (pre-amp, amp...) normally most well designed gear will "sound" exactly the same.Things change however when you measure the combination of loudspeakers the acoustic environment they are in. When you start measuring reflections, power responces, decay times... you will see HUGE deviations from the "ideal" (currently there is no concensus about the "ideal" however, as to little is known about the phsycho-acoutics of the brain).The point of all of this is that with quite low cost gear you will be able to measure the actual REAL problems in audio as a hobbyist, solving them is a lot more difficult however... Don't spent to much money on gear, but buy some good books on the subject instead.Some good reading on the subject that is freely available:http://www.davidgriesinger.com/http://gedlee.azurewebsites.net/Books/AudioTransducers.aspx

author: Muxr
author_rank: Super Contributor
timestamp: January 30, 2016, 07:23:56 pm
content: Quote from: joeqsmith on January 30, 2016, 06:58:11 pmI think it would be difficult to measure, and my understanding is that the commodity DACs and ADCs in computer hardware don't have terribly stable clocks, so your measurements might be off due to jitter/phase noise. There are some DACs for instance that do this well, but then again we're talking thousands in gear again.I do agree though, with you and _Wim_, try the gear you have (there are some nifty open source tools as well), and invest in books. Also read Nwaguy's blog, it's full of good information regarding audio testing and measurements.

author: _Wim_
author_rank: Super Contributor
timestamp: January 30, 2016, 07:24:06 pm
content: Another factastic book on the subject (and easily readable):http://www.amazon.co.uk/Auditory-Neuroscience-Making-Sense-Sound/dp/0262518023/ref=sr_1_1?ie=UTF8&qid=1352658057&sr=8-1Also check out the wonderfull website:https://auditoryneuroscience.com/I do not want to turn this into an audioforum, but I only want to point out that there are real problems involved in audio, that are dictated by real laws in physics, and when you really make the effort to study the subject, you will understand it is not easy to solve (especially so if you want a commercial viable solution). The amount of BS that is floating around on the internet concerning this topic (in most cases thanks to marketing departments of audio companier) is enormous. So whenever you read something, please check actual physics and electronics books, and also do the calculations. Many marketing departments take real effects (for example skin effect in cables), but fail to tells you that the effect is soooooooooo small (at audio frequencies), that even with the best gear in the world you cannot measure it. And if you say it is unmeasureable, they will tell you that your ears are way more sensitive...

author: amirm
author_rank: Regular Contributor
timestamp: January 30, 2016, 07:36:23 pm
content: Just to provide a reference for what proper audio test equipment does as you look at Scope FFT, here are a couple of measurements I did measuring jitter in DACs/AVRs using Prism Sound Dscope:Notice the noise floor (they do have processing gain).And here are some with my Audio Precision Analyzer:THe last measurement shows the effect of inserting a USB hub in the path of the USB connection to the DAC!You can use sound cards but then you have to build your own analog front-end when testing amplifiers and such as you are not going to want to pump +- 50 volts into their inputs!

author: _Wim_
author_rank: Super Contributor
timestamp: January 30, 2016, 09:36:15 pm
content: Quote from: amirm on January 30, 2016, 07:36:23 pmA simple resistor dividor in parallell with your dummyload is al that is required...In my opinion the problem is really the other way around, how to measure the distorsion when the amp is running at low power (<100mW) Distorsion in that range (crossover distorsion) can be quite audable, so you need a pre-amp after the DUT before feeding it to your sound card...But, if you really want to built yourself a good audio system, I would invest the sparse hobby time into building constant direcitivity speakers and optimizing room acoustics. Off course, then we have to move this topic to a different forum...

author: amirm
author_rank: Regular Contributor
timestamp: January 31, 2016, 03:21:03 am
content: Quote from: _Wim_ on January 30, 2016, 09:36:15 pmNot if you are measuring THD+N relative to power. You need auto-scaling or sit there and screw around with segmented measurements.

author: markone
author_rank: Frequent Contributor
timestamp: January 31, 2016, 12:01:05 pm
content: Quote from: amirm on January 30, 2016, 07:36:23 pmHow can a jitter problem produce 60dBc frequency tonesdistant 8Khz from an 11KHz carrier?What i can see in this picture is AM modulated carrier.Jitter produces phase noise components placednearbycentral freq.

author: FrankD
author_rank: Contributor
timestamp: January 31, 2016, 12:33:27 pm
content: A 2015THD can measure distortion down to 0.002 percent but the build in signal generator has about 0.02% distortion. You can use a state variable filter at 1Khz on the siggen output to get a lower distortion sine. If your 2015THD has a rs232 input you can control it with an Arduino or other uController to automate the sweeps to your liking and tune the state variable filter to the outputs frequency.To view a spectrum you can use a 9KHz spectrum analyzer when your test signal is 10KHz instead of 1kHz. (At one 1kHz the distortion will be lower. )By using a square wave you can use a scope alone simple by observing the resulting square wave from the dut.

author: Loboscope
author_rank: Regular Contributor
timestamp: January 31, 2016, 01:09:17 pm
content: I could recommend the analysis-software "HpW-Works" (http://www.hpw-works.com/).It is used used normally with a soundcard and for reliable measuring it is obvious that a really good one is necessary. Therefore I would recommend the interfaces from RME (http://www.rme-audio.de/), especially the "PCI-Card HDSP Hammerfall 9632" or the new "Babyface Pro". These interfaces have not only dgital IN/OUT (SPDIF / ADAT) but extremely high performance analog IN-/OUT-Connections.The software can connect to the interfaces up to 32 bit and max. samplerate. The software limit of samplerate will be 1 GHz, but there is no soundcard available capable of more than round about 200 KHz. So this will be the real limitation in frequency range, normally ca. 100 KHz.Because the software is able to manage multiclient ASIO, it can handle separate interfaces for Input/Output. This means, you can measure a DUT, for example a Digital Mixer-Desk by sending the stimulus signal via PC-Interface to the analog-In of the Desk and use the digital interface-connection of the desk, for example USB to receive the signal from the Desk. PC-interface and Desk are connected via its own ASIO-driver parallel to the software and so there will be no additional AD/DA-converter in the signal chain.HpW-Works has its own signal source witch can produce absolutely clean test signals who are a must for doing any reliable testing. The test signal can be adjusted in 0,5 dB steps.In my opinion HpW-Works is fantastic analyser tool what produces solid and professional results - at a reasonable price.Looking at the main user list (http://www.hpw-works.com/index.php/2015-08-09-08-41-49/users) you will find quite a lot of the "who-is-who" in the professional audio world.Greetings, Jürgen

author: _Wim_
author_rank: Super Contributor
timestamp: January 31, 2016, 02:00:46 pm
content: Quote from: amirm on January 31, 2016, 03:21:03 amAs explained before, measurement automation is indeed a big advantange of the profesional gear. As a hobbyist, measurements take indeed a little longer. My sparse free time is very valuable to me, but not enough so to spend >10k$ on a single piece of measurement gear...

author: _Wim_
author_rank: Super Contributor
timestamp: January 31, 2016, 02:02:34 pm
content: Quote from: Loboscope on January 31, 2016, 01:09:17 pmThanks for this, I will give this a try also.

author: amirm
author_rank: Regular Contributor
timestamp: January 31, 2016, 05:43:39 pm
content: Quote from: markone on January 31, 2016, 12:01:05 pmAt low amounts that we see in audio products, jitter can be approximated to be AM modulation with just a pair of modulated sidebands (modulation index is so low that the rest of the FM sidebands disappear in the sidebands).That said, further analysis needs to be made to make sure that is not Vref modulation by changing the main tone to see if the amplitude of the sidebands remains the same or not.QuoteDeterministic jitter can have whatever frequency/spectrum it wants. Such was the case in the one DAC measurement you quoted. All you need is your clock polluted by that frequency, say USB packet timing, and it will pop up in the analog output of the DAC. Its frequency will depend on the source of pollution, only its amplitude will depend on the frequency of the main tone.I did show however another case where we get narrow-band but random jitter such as this other example:Here you see two measurements, one of a $1,000 Audio/Video Receiver from Pioneer and the other my dedicated Mark Levinson DAC. In both cases, *identical* digital bit stream is sent to them. What you see is sharply different outputs with the AVR clearly suffering from spreading of the J-test tone due to narrow-band random noise and deterministic jitter. The Mark Levinson DAC despite being nearly 15 years older, is free of both.Thankfully from psychoacoustics point of view, random narrow-band jitter is not very audible so the measured difference doesn't translate into audible difference. The measurements do dismiss the myth that "bits are bits" in audio. Clearly the identical set of bits generated two distinct analog outputs.

author: amirm
author_rank: Regular Contributor
timestamp: January 31, 2016, 05:49:46 pm
content: To expand on the last point, here are two measurements on the exact Audio Video Receiver, one using its HDMI input and the other, its S/PDIF digital audio input. In both cases the source device is my computer and the bit streams identical:So with the identical DAC inside the same device we get two different analog outputs. The reason of course is that with HDMI audio is slaved to video so to receive it, you have to light up the whole video subsystem. And that subsystem is bleeding noise and deterministic jitter into the DAC clock, resulting in far degraded performance versus much simpler S/PDIF that doesn't need the video subsystem to do its thing.Going back to OP, if you are going to do such analysis, then you need a dedicated audio measurement system that is trusted and people can replicate your results.

author: amirm
author_rank: Regular Contributor
timestamp: January 31, 2016, 05:51:25 pm
content: Quote from: _Wim_ on January 31, 2016, 02:00:46 pmAgreed. Peace.

author: _Wim_
author_rank: Super Contributor
timestamp: January 31, 2016, 07:16:19 pm
content: Quote from: amirm on January 31, 2016, 05:51:25 pmPeace off course! If I came over I as being on a war, for sure that was not my intention... Just wanted to explain to people here that you can do an awful lot with low cost gear when it comes to audio.This makes this in my opinion a fantastic hobby, the only real problem is the amount of misinformation and audio foolery around on the net. But when reading the right books, making good measurements (don’t be an audiophile who only “trust” his ears) and maybe do some software simulation (I can recommend ABEC3 for this, free licence available for hobby users), it is possible to build a fantastic sound system for a reasonable amount of money…

author: markone
author_rank: Frequent Contributor
timestamp: January 31, 2016, 08:59:18 pm
content: Quote from: amirm on January 31, 2016, 05:43:39 pmI was expecting more spurious spreaded with lower amplitude level, those side band are quite strong, are you sure that you do not have any sampling rate conversion in you PCM stream chain ?

author: amirm
author_rank: Regular Contributor
timestamp: February 01, 2016, 05:13:18 am
content: Quote from: markone on January 31, 2016, 08:59:18 pmNo sample rate conversion. I don't own a $20,000 instrument without knowing how to use it.That is the measured performance of a $400 DAC that clearly has design flaws. The same sideband does not show up when another digital input is used.Here is its USB input:The sideband is gone. But see what happens when I change the media player used to send out the exact bit stream:Yup, the DAC is sensitive to activities in the PC! WMP in the second example pre-buffers the file and that extra activity bleeds into the DAC due to lack of isolation. Media Player classic reads the bits as it goes along so doesn't generate the same noise profile.Again, this demonstrates that "bits are not bits." Just because there is a digital stream being played, it doesn't mean the output will be the same regardless of how you get it there.My apology to OP for continued off-topic posts.

author: hibone
author_rank: Regular Contributor
timestamp: February 01, 2016, 10:22:59 am
content: What about analog discovery?It has 14-bit adc and 30 Mhz bandwidth, I don't know if FFT runs on board or on the PC though

author: markone
author_rank: Frequent Contributor
timestamp: February 01, 2016, 12:53:00 pm
content: Quote from: amirm on February 01, 2016, 05:13:18 amSurely it was not my intention to say that, anyway i'm still convinced that thoseclean cuthigh level (for the context) lateral side bands are not due to a jitter problem, at least in the sense in which it is normally meant.All the other spectrum graphs that you showed are of course compatible with jitter effect.Quote from: amirm on February 01, 2016, 05:13:18 amS/PDIF perfomance wise is quite a rip-off for a $400 device, but i bet that lot of its proud owners did not even noticed that in listening sessions ;-)