Author Topic: USB Impedance  (Read 625 times)

Offline emil_murillo

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USB Impedance
« on: December 02, 2018, 08:20:28 PM »
Hi Guys,

I need your thought guys! (Tech Guy, Audio Tech, Sound Engineer) or anybody knowledgeable.. Regarding the issue of desktop/laptop USB ports having input impedance at 90Ω differential for both inverting (DM) & non-inverting (DP) data signal path and USB guitar link having an output impedance of 50Ω.. Considering the usual "10 times higher rule" that most of us are used to in handling impedance matching..
thanks in advance! God Bless

  USB (desktop, laptop, etc) Input Impedance 90Ω:

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  USB Guitar Link Ouput Impedance 50Ω:

« Last Edit: December 03, 2018, 08:41:05 AM by emil_murillo »

Offline emil_murillo

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Re: USB Impedance
« Reply #1 on: December 03, 2018, 08:51:36 AM »
sorry i overlooked.. 50Ω was the headphone out impedance lol

i wonder what was the output impedance on its USB (D+ & D-)..

thanks again!

Offline emil_murillo

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Re: USB Impedance
« Reply #2 on: December 03, 2018, 09:19:19 AM »
Logically when we apply the "10 times higher rule" as most of us were taught.. Therefore, desktop/laptop USB ports having 90Ω input differential impedance would mean a and lower USB guitar link output differential impedance → hypothetically speaking..

That's too close to be a speaker impedance level.. You can drive an speaker with that kind of impedance out from that of Guitar Link USB output if that would be the case or logic.. "Considering most guitar link manuals do not say or include the output impedance out of the type A USB plug"

Information that was so important in solving the puzzles regarding noise issues occurring on most usb guitar links.. Again, hypothetically speaking..

Thanks again! and don't hesitate to include your inputs

Offline emil_murillo

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Re: USB Impedance
« Reply #3 on: December 15, 2018, 10:53:57 AM »
ok guys i hate doing hypophoric line of questions that may spoil the topic's element of interest.. but here it is:

Analog Audio Vs Digital Audio:

though usb guitar link do not use analog audio signal in transfering data to usb ports.. they are actually digital data.. usb guitar link converts the analog audio signal to digital data via analog-digital converter system (ADC).. the same method used on our smartphone's camera for capturing image and audio.. ► but recording an audio primarily as digital, even though if you set it at the best quality (e.g. 64bit or 192KHz) but when it was smothered with harmonic distortions, (THD%) it was not the same as you record it in analog.. where as to analog recordings that was converted to digital → even though you compressed it to let's say 16bit or 44.1KHz and from 6 channels to 2 channels.. the analog sound is still there.. so what was it? → High Fidelity or HiFi sound..

Noise Issues:

In analog recording the analog signal is recorded in 8mm or 16mm or 1 inch or 2 inch open film reel.. In my case i experienced recording studio recording it in vhs tapes.. the recording is somewhat hybrid.. sound engineer uses digital preamp rack with digital fx and 12ax7.. for that we sounded very different from our demo tape (analog recordings).. analog recordings in reel, vhs, vhs-c, betamax, hi-8, mini-dv tapes → any impertfections on any said storage devices will result into physical noise such as cracklings, popping noise and static-like scratching noise.. storage discs (dvd, cd) will have it too..

USB Ports 90Ω Impedance Vs Analog Signal's Line Impedance:

though usb do not transfer analog signal but digital ones only.. the differential pair data signal (D+ & D-) characterized by polarity or voltage and impedance which is given (Z) and measured by ohms (Ω).. as i mentioned earlier 90Ω is about as high as (professional) headphone impedance level.. impedance at this level contains higher currents than those higher impedances.. high currents flowing in this ports may introduce magnetic fields and may be an origin of 60 cycle hums, RFIs, statics.. line impedance level of analog signal can contain high voltages (48V) from a phantom power and may contain lower currents.. an opposite characteristics with headphone & speaker impedance levels..

My Conclusion:

If you're not in a hurry.. if you have the money and luxury.. If you wanna hear yourself and your band sounds like you were actually in a gig or up on stage or live like metallica sounded like on live and on recordings..

→ record and mix your tracks in analog (multitrack the better).. keep atleast 3 to 5 master record safe and clean.. convert it to digital but in higher bit rate and bandwidth e.g. 64 bit 192KHz.. each tracks in each channels e.g. 6 tracks = 6 channels.. that way the digital master copy is in best quality..
→ or record each tracks in analog then convert it to digital (best quality) then mix in with your chosen daw.. do not use sound fonts

this way you will not be given any impressions that your demo sounds like "you stole it from another band" compare when you play it live.. sorry folks

now let me here about your take & Enjoy music!

God bless
« Last Edit: December 16, 2018, 05:09:46 PM by emil_murillo »