looking for low-noise USB-audio input, for not too much $ - Page 1
Forum section: electronics / projects

author: JBeale
author_rank: Frequent Contributor
timestamp: September 05, 2017, 05:12:01 pm
content: I have a low-level signal between 50 Hz and 4 kHz that I want to get into a computer.Maximum amplitude is about 3 mV (rms). This is similar to a microphone, so a mic-level audio interface should work.The trick is I want 70 dB of dynamic range, meaning the input-referred noise of the audio input device should not exceed 1 microvolt rms over a 50 Hz - 4 kHz bandwidth. So the total noise density of the amp should average < 16 nV/rt(Hz) over that frequency band.There are plenty of low-noise opamps around 2 nV/rt(Hz) and I'm sure professional audio equipment will meet the spec, but I'm looking for the least expensive USB audio adaptor that can do this, hopefully that works on Linux. I have a no-name usb-audio dongle from ebay, super cheap at $5 but no surprise, it has a noisy and distorted input.Has anyone measured the DNR (dynamic range) of USB audio adaptors with a microphone-level input?Only one channel needed, not stereo. I do not need any kind of audio output.I only need a low sample rate, for example 8 kHz or 16 kHz is fine.I expect it's possible without high-priced hardware because the very cheap PS3 Eye camera has good quality audio (and 4 channels!) from its built-in mics, but it's a little bulky and I'd have to hack into it to connect my external mic-level signal. Thanks for any ideas!

author: DaJMasta
author_rank: Super Contributor
timestamp: September 05, 2017, 05:53:56 pm
content: 70dB of dynamic range is about what you get from a record player - you get much better by default with modern equipment. I believe a 16 bit converter can still get you more than 70dB of dynamic range, and most integrated computer audio solutions should do around 90dB or a bit better - high end consumer stuff tends to be 95+dB, the very best professional stuff is around 110-115dB.So I think it's more a matter of getting something and seeing if it meets your requirements than looking for an appropriate spec.... very few budget solutions will give you a real measured specification, and the specifications of the audio chipset they're using are probably way beyond your performance requirement, but are also dependent on the PCB layout and whatnot.I think the only issue you'll run into is the amplitude, you'll need a preamp to get the dynamic range you're looking for. Computer mic inputs will have this, but will also probably introduce a DC bias to the line (needed for powering electret mics that are normally used with computers), and while a low enough noise amp to do what you want is inexpensive, there's also no guarantee that they've used a good one, laid it out well, or kept it isolated from the digital signals all around.My preferred solution would probably to be to look for a decent preamp that meets your performance needs, then just using it before any old sound card in a computer. It lets you configure the gain to put your output at normal line level and then unless you need the repeatability of having the digitizer included in the test apparatus, any old integrated sound card from the last decade should do the job.Alternatively, you could buy an external sound card and get it all built together - decent consumer grade ones will start close to $100, but should be more than sufficient and would come with a preamp.

author: JBeale
author_rank: Frequent Contributor
timestamp: September 05, 2017, 06:12:23 pm
content: Thanks for the reply. I agree you could do this with a good mic preamp and any old soundcard. True, consumer hardware won't have published specs, but I wanted to check if anyone happened to know of a better-than-average consumer device that had the preamp already.Some years ago, I measured audio performance of several consumer audio devices, as posted here:http://www.bealecorner.org/best/measure/audio/For example, the small and fairly cheap iRiver iFP895 (MP3 recorder) was decent. I measured around 81 dB DNR (A-wt.) with a 50 mV input signal. I still have one of those, but unfortunately it is abandon-ware; the proprietary and locked-down USB driver was never updated so it is not compatible with any current operating system.The Sony MZ-RH1 minidisc recorder had an excellent built-in mic preamp, DNR = 58 dB(A) with a -65 dBu signal, meaning its noise level was -125 dBV or 0.56 microvolts. I'd love to see that preamp available separately at a reasonable price, but AFAIK you have to spend about as much as the entire MD recorder cost, to get such a thing.Pro gear like a Sound Devices USBPre 2 with EIN of -127 dBu would be great, apart from the $895 price tag.

author: JPortici
author_rank: Super Contributor
timestamp: September 05, 2017, 06:43:25 pm
content: consumer hardware WILL have spec.https://www.steinberg.net/en/products/audio_interfaces/ci_series/models/ci1.htmlthis is a very old audio interface i've had for years, more or less the same quality as every yamaha mixer with usb stereo (codec's the same, preamp more or less the same)https://www.steinberg.net/fileadmin/redaktion/PDF/CI1/CI1_-_Technical_Specifications_01.pdfi paid 100 euros for it when it was new, circa 2009, i wouldn't pay more than 50 now.why stop at 16 bits? 24 bit is actually cheaper nowadays!preamp is flat up to about 15 kHzthe same can be said for whatever edirol, behringer, focusrite, tascam in this price class

author: ruairi
author_rank: Frequent Contributor
timestamp: September 05, 2017, 07:10:58 pm
content: It's important to remember that audio mic pre amps are optimized for a 150 ohm source impedance and a differential source.What does your signal source look like?

author: DaJMasta
author_rank: Super Contributor
timestamp: September 05, 2017, 08:08:35 pm
content: I didn't mean to suggest 16 bit converters, just saying that the lower fidelity digital audio standards of the 90s were sufficient to meet the performance requirement. And yes, a lot of consumer dedicated audio gear gives you decent specification information, but specifically, motherboard integrated or laptop/tablet integrated solutions generally aren't specified, so you get the datasheet for the chip they're based on and hope the layout designer was paying attention. Even so, 70dB of range would be audible noise, so as long as you're not hearing the noise floor in an integrated solution much, it's probably plenty sufficient.I could have been clearer, but if you're going for absolute minimum cost, maybe a $100 audio interface is actually too much (though it will be more uniform and lower noise, so I'd probably spring for it if I had such a project anyways).

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 05, 2017, 08:17:02 pm
content: What is the impedance of your source? Is it balanced or single-ended? As @ruairi observed, modern audio microphone preamps are optimized for source-impedance of a few 100 ohms, balanced. And most decent ones are capable of superimposing +48V of "phantom power" onto the input connection.Practically any of the "jelly-bean" modern USB audio chips are easily capable of meeting your requirements. I just bought a couple of copies of a USB audio interface board that uses TI PCM2906 (http://www.ebay.com/itm/111309002429)Of course no ADC (or DAC) operate at those signal levels. They all expect to be working with a reasonable "line-level" approaching 1V. So some kind of pre-amp is essential.You should be able without much difficulty to find/make a preamp for whatever source impedance you have. There are many out there, but impossible to start recommending any without knowing the nature of the source you must interface to.

author: JBeale
author_rank: Frequent Contributor
timestamp: September 05, 2017, 08:22:51 pm
content: Good point about the impedance of the source, which I believe is probably under 1k ohm, but I do not know. I will check that. Signal is single-ended, although maybe I could float the whole thing to make it appear differential. Yes, 70 dB isn't a huge DNR overall but the < 1 uV RMS noise at the input is what has me thinking.The Art Dual Pre is spec'd well enough to do it.https://www.bhphotovideo.com/c/product/615405-REG/ART_USBDUALPREPS_USB_Dual_Pre.htmlso now I'm checking if there's anything below that.In fact my first try at this was building my own preamp, using an OPA209 opamp spec'd for 2.2 nV/rt(Hz) at 1 kHz and it does work, but there's some mysterious low-level broadband noise around 400 Hz, with power from either LT3042 low noise regulator, or even on battery power. So I wanted to get a "known-good" amp to narrow down the problem to the source, or my amp circuit. The noise doesn't happen with the input shorted through a capacitor, but I suppose I should try a resistor matched to my source impedance for a better test.

author: BrianHG
author_rank: Super Contributor
timestamp: September 05, 2017, 09:02:03 pm
content: Quote from: JBeale on September 05, 2017, 06:12:23 pmI saw you are using cool-edit 2K in your software list. If you know how, properly used, the it's statistical FFT noise removal tool can eliminate all background noise without any audible artifacts if your background noise in in the -70db range.

author: ruairi
author_rank: Frequent Contributor
timestamp: September 05, 2017, 09:04:03 pm
content: The specs on almost all prosumer grade products are meaningless. Very often the quoted noise, distortion and dynamic range specs are lifted straight from the data sheet of the A/D or D/A chipset used and have no bearing on the reality of the whole circuit.You need to tell us more about your source signal and project.- Is is single ended or balanced? (differential)- The source impedance- Is the signal steady state, periodic, random? (Averaging could be used to drive noise floor down if it's the former).- Is this a one time breadboard build or something to be commercialized?There are some very good all in one mic pre chips available these days but the context will tell us whether they are the right fit.

author: JBeale
author_rank: Frequent Contributor
timestamp: September 05, 2017, 09:29:05 pm
content: Not a product, this is just a one-off personal hobby project. I'm curious how much information I can extract from these cheap sensors, for example HB100 and CDM324:https://www.eevblog.com/forum/projects/playing-with-cheap-hb100-doppler-radar-module-for-car-speed/msg1274864/#msg1274864In both cases the IF output is single-ended, about 3 mV rms peak amplitude, impedance unknown but I'll check. The circuit has only two or three active devices (well, one or two may actually be just a diode).I don't want to do too much signal averaging because I'm trying to detect random single events. My hope was to be able distinguish (for example) between a person walking by, and a cat or a dog walking by.Attached is a spectrogram of the CDM324 sensor output as a person walks away, and then comes back. This is using my own preamp circuit going into my laptop line-level audio input and plotted with Audacity. You can see a 2-Hz "pulsing" which matches the rate of the arms and legs swinging. A constant band of background noise around 400 Hz is also visible.If you just wanted to look at cars on the street around 30 mph => 2 kHz (see 2nd image), the larger velocity and less complex doppler return separates them more clearly from the 400 Hz noise band. The vertical stripes mark the time where a car passed the sensor, and the large signal return momentarily overloaded the receiver.

author: ruairi
author_rank: Frequent Contributor
timestamp: September 05, 2017, 11:04:13 pm
content: Fascinating project and it immediately moves outside my area of "expertise" which is audio. I'm going to check out the data sheets for those modules.

author: JBeale
author_rank: Frequent Contributor
timestamp: September 06, 2017, 12:19:52 am
content: AFAIK the cheap devices sold on ebay with the HB100 and CDM324 part numbers do not have any published datasheets. They are apparently (partial?) copies of devices that may have a datasheet, but it doesn't necessarily apply. For example the CDM324 looks a lot like this parthttp://www.innosent.de/fileadmin/media/dokumente/DATASHEETS_2016/Datenblatt_IPM-165_V8.5.pdfbut that datasheet claims a signal level around 1 or 2 Vpp. I assume that must be from some preamp that the cheap part I got doesn't have.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 06, 2017, 02:59:55 am
content: Why not just build up a little circuit to do it? Audio band stuff will be fine on a breadboard, but it would be fine breadboarded up too. Since you've got all the HF stuff removed from the signal, you can take one of these good audio opamps (<$5 on digikey), spend a few bucks on a PCB, and drop some resistors in to suit your gain requirement. The hardest part will be powering it... but if you find something that will except +5V, you could just filter the USB power a bit and probably do an OK job getting fairly low noise on it. If you went with an external brick you could stick a ferrite on the input and use a local linear regulator and be in good shape too. The circuit would be quite simple... you could even use an active low pass filter to keep any extra HF noise out, giving you an even lower overall noise floor.

author: JBeale
author_rank: Frequent Contributor
timestamp: September 06, 2017, 03:53:31 am
content: Indeed I did build a circuit, which wasn't working well until now, so I was seeking a commercial device to compare it to. But just now I seem to have fixed it. I was misusing a TI TLE2426 "rail splitter" to generate an artificial ground so I could run my opamp at +/- 2.5 V instead of 0-5V, but it seems this part is not stable with too much load capacitance and that caused the ~ 400 Hz noise. Taking that part out and simply using 2 groups of 2 NiMH AA for +2.5 V and -2.5 V gives me a much cleaner result (with some 120 Hz powerline pickup, since I didn't have the lid on the case for this test.) Thanks for the assistance and suggestions folks! I'm going to call this one as problem solved.

author: ruairi
author_rank: Frequent Contributor
timestamp: September 06, 2017, 05:14:41 am
content: I can't tell if those are tears of joy on the board or hot glue :-)You might consider bringing your output signal from your preamp circuit into your audio interface differentially, using impedance balancing. It's as easy as adding a resistor and could clean up ground and power hum.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 06, 2017, 05:34:51 am
content: Could also use a single supply amp and just your regular resistor pair for a midway virtual ground - the key is to get something that can operate pretty close to the rails so you have enough space for you amplified signal. Would prevent any kind of periodic noise from the rail splitter. You could also get an inexpensive audio transformer to entirely isolate the output - eliminating any potential ground loop issues. Not going to have perfectly flat response, but that may not be required for this kind of project.It's certainly a neat one though, hope you can get a suitable preamp/digitizer solution. Looks like the data you've got so far is pretty good though - I'm a fan of Spectrum Lab (free) for this sort of PC based FFT visualization.

author: JBeale
author_rank: Frequent Contributor
timestamp: September 06, 2017, 06:20:23 am
content: I tried adding a 2 ohm resistor in series with the virtual ground output of the rail splitter and that also seems to solve the issue. I've also got a huge 3300 uF cap from V- to (virt-gnd) so it won't be moving around too much.So it's working pretty well now. It can detect me walking (faintly) even through an indoor wall. Also, what I thought was 120 Hz electrical or magnetic pickup, may not be. After that, when I pointed the sensor in different directions, suddenly all trace of 120 Hz is gone and it's just a smooth noise floor. I think that's a real motion signal, maybe the refrigerator?It was on the kitchen table looking past the fridge, which is an LG model with a linear compressor, no doubt running at 60 Hz. The sensor sees motion towards and away as the same signal, so for small vibrations, it acts as a frequency doubler. It's interesting to play with this stuff and find things out.PS. Thanks for the pointer to "Spectrum Lab", that program is amazing.

author: cvanc
author_rank: Frequent Contributor
timestamp: September 06, 2017, 01:43:24 pm
content: Unique, interesting project. Have fun!