Determining if Audio Interface (Steinberg MR816CSX) is distorting signal - Page 1
Forum section: electronics / beginners

author: lescro
author_rank: Newbie
timestamp: August 04, 2021, 10:36:22 am
content: When I use the headphones and listen to the input signal of my audio interface (Steinberg MR816CSX) it sounds slightly distorted.The levels are all set correctly so there is no clipping. No EQ or COMPRESSION is being used.I want to determine if it is actually distorting or not using my test equipment which is :o Brymen BM869 multimetero Rigol DS1104 oscilloscopeo UNI-T UTG962E function generatorhttps://www.uni-trend.com/html/product/NewProducts/UTG900E%20Series/UTG962E.htmlSTEINBERG MR816 CSX SPECSThe MR816 is an analogue to digital, digital to analogue converter.Operation Manual:https://download.steinberg.net/downloads_hardware/MR816/MR816_documentation/MR816_GettingStarted_en.pdfSpecifications:https://www.steinberg.net/fileadmin/redaktion/PDF/Hardware/Technical_Specifications_MR816.pdfANALOGUE INPUT**************MIC/LINE IN ....... XLR connector - balancedNominal Level: -60dBu to 10dBuMaximum Level: +24dBuInput Impedance: 3.5k OhmsANALOGUE OUTPUT***************LINE OUT ...... TRS connector - balancedNominal Level: +4dBuMaximum Level: +18dBuInput Impedance: 600 OhmsHEADPHONE JACKS (2 of)********************Jack 1 & 2 ..... TRS connector - unbalanced - stereoNominal Level: 4mW + 4mWMaximum Level: 25mW + 25mWInput Impedance: 8 OhmsNominal Level: 12mW + 12mWMaximum Level: 75mW + 75mWInput Impedance: 40 OhmsI think the Input Impedance quoted in the specs for the headphone outputs is for the input impedance of the headphones being used.UNI-T UTG962E SPECSThe UTG962E is a Function/Arbitrary Waveform Generator.Amplitude Range: < 10MHz 1mVpp to 10Vpp (50 Ohms)User Manual:https://www.uni-trend.com/uploadfile/cloud/English%20manual/Benchtop%20Instrument/UTG900E%20English%20Manual.pdfMY QUESTIONSI thought I could test the MR816 for distortion by inputting a sine wave and then looking at the output of the LINE OUTS and and also the output of the HEADPHONES with the oscilloscope set to FFT.Connecting the Function Generator******************************1. The MR816 inputs are balanced (3 wires), so should I connect one wire of the Function Generator to HOT and the other wire to COLD and GROUND?2. What amplitude should I use for the sine wave? Say, 0.894Vpp (ie -10dBV signal) or 3.472Vpp (ie +4dBu signal)3. What frequency should I use for the sine wave? 1 kHz?Connecting the Oscilloscope to the LINE OUT***********************************4. I will need to connect a load across the output (balanced - 3 wires). What should the size of the load be? Should it be a high value like 30K Ohms (This would match a bridging input) or should I use a 600 Ohm load?5. The MR816 outputs are balanced (3 wires), so should I connect one wire of the load to HOT and the other wire to COLD and GROUND?Connecting the Oscilloscope to the HEADPHONES***************************************6. I will need to connect a load across the headphone's output (unbalanced - 3 wires - stereo, so will do a test on left side then a test on right side). Would I use a 50 Ohm load for this?Changing the Source Impedance**************************7. My Function Generator has an impedance of 50 Ohms. So my source impedance is 50 Ohms. If I needed the source impedance to be 150 Ohms, how would I do this?CheersLescro

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 04, 2021, 05:32:00 pm
content: aside from your questions is one important point: real distortion measurement require more than a scope and a signal source.below a certain level of distortion, you're unable to detect them just with the scope monitoring the output signal; you'll need a notch filter which eliminates the injected signal and leaves the distortion and intermodulation untouched.easiest possible way to achieve that is an active twin T notch filter with a very low noise opamp.impedance matching is no issue in audio circuits; common output impedance is 600 ohms while input impedance is 10k, so you have voltage matching more or less out of the box; the 50 ohms of the signal source doesn't matterfor a balanced input you should inject a signal with 180° phase offset between hot and cold; most arbitrary generators with 2 signal outputs allow static adjustment of the phase between the 2 channelscaveat: if the interface supports phantom power, disable it before you burn the output drivers of your func-gen by accident.

author: lescro
author_rank: Newbie
timestamp: August 05, 2021, 08:49:13 am
content: Thank you HB9EVI for your reply.I just need to clarify some things.1. To look at the signal from the LINE OUTS that are balanced, would I place a 600 Ohm load across HOT and GROUND and measure across HOT and GROUND?2. I would still use the FFT function on the oscilloscope, but have the oscilloscope connected to the notch filter output. The notch filter input would be from HOT and GROUND of the LINE OUT.3. Are there any notch filter ICs or active notch filter circuits you would recommend?ThanksRetsel

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 05, 2021, 02:14:28 pm
content: hi therelike I said, in audio circuitry you don't work with impedance matching like in rf circuits; very likely your line out driver could drive 600ohms, but not necessarily; if you see need for a termination, use a 10k resistor each between hot-gnd and cold-gnd, but it's not really neededthe FFT function of most DSOs is rather a toy function than a measurement tool; much better results you get with a higher quality sound card and a piece of software; I use a E-mu 0204 which allows quite reliable measurements in a dynamic range of about 85dB; if you want more, it gets very fast expensive.an all-in-one notch filter circuit I don't know about; best way is to carefully match the Cs and Rs from a bigger pile and use a very low distortion opamp; if I remember right, I used a OPA2134 for my twin T notch. I followed this guide here:https://sound-au.com/project52.htmthe trickies part is to determine the precise resonance frequency of the filter setting, but that's a one-time work only

author: lescro
author_rank: Newbie
timestamp: August 05, 2021, 04:40:45 pm
content: Hi HB9EVIThanks for the clarification.I have a RME Fireface UCX Audio Interface. So what you are saying is that I could use it with the appropriate software to determine the THD. I have heard of ARTA software (but never used it.) Is ARTA a good package?CheersRetsel

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 05, 2021, 09:16:17 pm
content: hi therein general these audio interfaces are quite decent interfaces with good technical parameters; I don't know your Steinberg, but I have the mentioned Creative, an older 2ch Tascam and a newer 4ch Behringer; they all do quite fine work; I mostly use the Creative E-mu to measure my home brew PAs; as linux only user I cannot say much about the windows world; i use jack with jaaa and japa

author: mansaxel
author_rank: Super Contributor
timestamp: August 06, 2021, 03:57:45 am
content: Quote from: lescro on August 04, 2021, 10:36:22 amI'd do775mV RMS sine at 1KHz(0dBu). That should produce a -18dBFS signal in the digital domain, at least in pro gear. Start by establishing that, and verify your gain structure. This voltage should come out at the same level on the line outputs. (If you're looking at this on the scope, beware that the waveform will show more than 775mV, since the scope presents Peak on the graticule, not RMS. The formula to convert is Vpeak = Vrms * sqrt(2). Further, you'll see peak-to-peak, meaning you need to multiply with 2 as well. )If the voltage out is the same as in, in unity gain mode, then there might be trouble, if it still sounds bad.Connection-wise I'd start with XLR pins 2 and 3, BNC center going to 2, and playing with connecting 1 and 3 together.Quote from: lescro on August 04, 2021, 10:36:22 amHigh impedance load -- say 10K or so. XLR to BNC as above.Quote from: lescro on August 04, 2021, 10:36:22 am600Ω should be about right.Quote from: lescro on August 04, 2021, 10:36:22 amI'd not bother. As others wrote, this is not at transmission line frequencies; but lower. 50Ω source will do fine.Oh, and what you actually need is an analog distorsion analyser, like a Tek AA501A.