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hey, do any of you know audio hardware stuff? i have a keyboard with a 6.35 mm (1⁄4 in) audio output. i believe it's stereo (trs). i want to record it without horrible usb interference noise, so i want something with uhhh some kind of isolation? (what is that called). what do i buy. i would prefer to spend less than 100 €

i probably want an “audio interface”, that's my going assumption so far, but something that is driving me up the wall is not even the good manufacturers seem to specify whether they can record in stereo. the fact the output is stereo means nothing, what about the input!!!

oh okay, the fact the input of the “scarlett solo” is mono is actually on their site at least (buried deep in the manual, not explicitly stated in the specification list)

oh my god why is every connector standard a compatibility nightmare. the way mono (ts) and stereo (trs) connectors interact is scary enough by itself, but then there's mono with differential pairs (“balanced audio”, also trs)…

from now on i am going to release my music recordings as 2-channel flac files, but instead of the second channel being intended for stereo, it's just the inverse of the first one. i'm calling it “balanced flac”

it won't be mid-side encoded because that would destroy the balanced property

the right audio channel isn't silent, it's just the mid channel of a mid-side-encoded differential pair,

huh, the “zoom ams-22” is a usb audio interface with stereo line-in. true stereo, not mixed down to mono. i think it's the first one like that i've found! it's also like, tiny. weird, fascinating product

the things we lose in the transition to laptops and smartphones, huh? with a laptop, you're lucky to have even a single audio input channel, whereas your average PC has three!

many pc motherboards have eight audio output channels too (7.1 surround). i think mine does as well, but it's one of those more compact models with just 3 ports rather than 6, so i would need to do some driver shenanigans to switch the functions of the ports (yes that's a thing)

haha omg it even has 192kHz, 24-bit recording

there's no way the noise floor is low enough on this thing that 24-bit is worth it, but hey this is still way better than your average usb audio device can do :3

anyway here's a song from the yamaha psr-350 recorded via my pc's stereo line-in. https://hikari.noyu.me/etc/2023-07-30-psr-350-song-009-Sometime.flac

(does the fact i didn't try to filter out the noise mean you can steal my encryption keys now? lol)

what did we learn from this? pc users stay winning and usb delenda est, but we knew that already

omg… since i have line-in monitoring turned on, and my keyboard is turned on and connected to my pc via both line-in and my usb midi interface, i can just use it as my normal midi synth for everything. like, i can hear its output and normal sound from my computer. this is the 1990's pc midi dream

ah goddamnit that introduces noise from the usb midi interface

ok so, having my usb midi interface connected seems to unavoidably create an audible (circa -32dB) hum when recording my keyboard's audio output. is this the kind of thing that's avoided if i get an audio interface with a built-in midi interface? the midi interface i have might just be bad though

i think it is telling that that all the usb audio interfaces out there only seem to require special drivers on windows, not on apple platforms. i think the devices are usb audio class compliant and the drivers mainly exist to work around the windows audio stack sucking lmao

ok so i ended up buying a used “Steinberg UR22mkII”. it's much like any other two-channel usb audio interface, but i wanted this particular one because:

• both input channels have xlr/6.35mm combo jacks, so i can use a so-called “insert cable” to connect it to my keyboard's unbalanced stereo jack
• both inputs are designed to allow line-level input, and the hi-z and phantom power are switchable, so no amplifier explosion
• it's a class-compliant usb device, so i don't need to install drivers and it works on any operating system
• it's also a midi interface (in and out), so i can eliminate two potential ground loop sources with this box, not just one, and i only need a single usb cable :3
• steinberg/yamaha are pretty trustworthy
• it has a very sturdy and attractive metal construction

it arrived today and i've just started using it. it feels just as solid as i expected, it's really pleasant to physically interact with. it was plug-and-play, as expected. the audio quality is excellent and free of noise, as expected.

one drawback: it seems to be downmixing to mono for the earphone (zero-latency monitoring) output. i think that's configurable in the custom drivers, so i may end up installing those anyway. but the recording received by my laptop is definitely stereo! so i'm still very happy with it.

ah, no, it's always mono monitoring i think; i should have read the block diagram. well, that's fine with me :)

i am now just filling up my ssd with flac files recorded from my keyboard playing back various midis <3

@hikari is it 1kHz or 50/60Hz or something else

@rcombs here's a spectrogram if that helps

@rcombs left is before connecting the midi interface, right is after

@hikari oh, so it was always there, but connecting the interface made it louder? hmmmmmmm

okay maybe this is something something rectified european AC (thus 100 instead of 50)? maybe off a crappy power supply?

and maybe connecting the external device is creating a ground loop and exacerbating the existing problem?

@rcombs yeah probably. i suspect the usb buss power from my pc is quite noisy and i doubt this cheap usb midi interface is properly isolated. also, the keyboard has its own psu which might not be good either. i generally am very suspicious of the quality of my ac power lol

@hikari huh, so somewhere around 100Hz, with a whole lot of harmonics? that's… unexpected, not sure what the source would be :/

@rcombs well, considering there's at least two power supplies involved here, there's plenty of possible sources of noise

@hikari do you have things plugged into separate outlets? how much stuff here has ground connections? is there a MIDI cable involved that had continuity from shield to shield?

@rcombs uhh, the pc is connected to the same power strip as the keyboard's power brick, but only the pc is earthed. the midi interface is a unit with three fixed/integrated cables: one usb (power and i/o) and two midi

@rcombs @hikari to my completely untrained eyes, it looks like the 100hz (+harmonics) noise is still present on the left, but just quieter. Could it just be an amplifier being turned on?

@hikari do you have a spare PCI slot for a Sound Blaster 16?

@hikari it took me until this post to realize you weren't reverse engineering some weird computer keyboard with an inexplicable audio output 😅

@PaddyCo new side-channel just dropped

@hikari is this one of the songs that come premade on the yamaha psr-950?

@mikoedit i can't find a keyboard by that name, but uh, if that was something yamaha made circa 2000, quite possibly

@hikari if you downsample from 192 kHz you get to lower your noise floor too!

@whitequark oh huh, how does that work?

@hikari @whitequark When downsampling, multiple samples become one, noise is different in the different samples and (mostly) cancels out when averaged, while signal is mostly the same and so it stays

That's my guess anyways, I'm not an audio engineer 🤷‍♀️

@quirk @hikari yes

@whitequark @quirk oh, right, just like with downsampling images

@hikari you could use this to control a bunch of galvanometers and make a really cool laser show!!

@whitequark poor catgirl's glasgow