Fixing analog audio on the $2.58 HDMI-to-VGA adapter

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• Hacker News
• A picture of the adapter would have been nice.
  by Mistletoe
• I love it when people go so far into the weeds to solve a problem on their own and then document the process for everyone.

  I learned a lot from this writeup, and really enjoyed the read even though I know I would have made it about 10% of the way myself and chickened out and just bought a different product in hopes it would solve the problem.

  by BizarroLand
• It seems like this person is incredibly intelligent and resourceful. Why not just design a new PCB for this instead of modding one? Surely the component cost of 2 connectors and a little PCB could still be in the < $20 range and actually output good quality analog signals? $20 is the same as $3 if you’re also buying CRTs just to play Switch games. This is such a hobby level activity I’m sure people would love a kit + a guidebook, could even provide a 3d printed case.
  by lukevp
• Maybe the chips aren't available, or maybe they just wanted to tinker.
  by actionfromafar
• To reduce e-waste? Not everything has to be optimised on time or money spend.
  by waffleiron
• I think you misunderstand the obsessive, blissful, motivations of a nerd. :)

  I was like this when I was younger, where a goal like this would entirely consume me, regardless of practicality, and more of a "see if I can". Some of the best days of my life, where I was most alive, were those times of complete, detrimental, obsession. As I grew older, and patterned by employment, it slowly faded, and sometimes I miss it so much.

  https://xkcd.com/356/

  (funny enough, I was once nerds sniped about a grids of resistors, to understand PCB trace resistance better, for high currents, because an EE said my intuition was wrong. I made a paint program where you could paint/paste an image, with red being + voltage sources, blue being negative, and greyscale being resistance, and it would show you the current flow through whatever you drew. I was able to prove that my intuition was actually correct!).

  by nomel
• Sometimes it's fun to mod something that other people can also easily buy and follow your mod recipe. It's more of a network effect than publishing Gerbers and a BOM.

  For example, there are (or maybe were) lots of cheap LED controllers that use proprietary phone apps, but if you crack them open, you find that they're ESP-8266-based, and if you can find the right locations on the PCB, you can reflash them with WLED.

  by sowbug
• What kind of mixer do you use?

  If it has balanced xlr inputs a DI will completely eliminate the earth hum. That the signal is dropped to mic level doesn’t matter much as the impedance is correct, the connection balanced and the mixer preamp designed for the purpose. // I am somewhat professional sound tech :)

  by olelele
• 3.5mm, I now know why conventional consumer audio devices don't use mixers, they're a ground loop party

  It's also that I don't know how to build a differential input not limited to power supply range, I do think they are sold and I have a mixer at home but it's somewhat bulky for my computer desks.

  by nyanpasu64
• These chips appear to be 8051s with multi hundred MHz DACs.

  Be fun to program them but I can’t find any programmable devboards with the Capstone CS5213.

  by andrewstuart
• Theres been some work on the firmware for the Realtek RTD2660H LCD driver that's also 8051 based.

  I believe pine/the community worked on that. They may have some how tos / pointers.

  by benj111
• Extron made great audio extractors and they can be found for cheap. Also nice is it provides balanced stereo output. I have a bunch lf them for getting audio to my mixer from whatever with hdmi.
  by blightful
• This post just makes me realize that one day I need to buy myself an oscilloscope.
  by nticompass
• The weird edge cases that live around connectors never cease to amaze. I recall one, I think it was also HDMI (maybe displayport or VGA), that showed there's a very low resolution digital graphic or text signal and some connectors even have a screen embedded in them.
  by 0xWTF
• > Annoyingly the input pins had a constant DC offset of over 2 volts, which went away when shorted to ground but reappeared moments after I removed the short.

  Mic inputs on audio codecs always have DC bias, because they are DC biasing the microphone inputs so that signals can pass. By default, you would expect ~2-2.7V of DC bias on each microphone channel. This is typically connected through a high impedance (~1kOhm) source impedance. The actual impedance of the input should be extremely high resistance, though, typically in excess of 50kOhm.

  The proper way to connect this would have been to a line-in jack on your motherboard. There should be no DC bias on those jacks. Or use an audio input that is 'retaskable' to select the line-in function instead of the mic-in function.

  > This large of capacitance is not easily available as C0G. The usual X7R MLCC capacitors are piezoelectric and will change voltage when bumped, which can add interference to audio. Electrolytic caps are an option, but will not fit well on pads meant for 0402 ceramics.

  C0Gs are not needed for passing something like this. X5R/X7Rs are commonly what is used for DC blocking capacitors in ADC/DAC solutions and you can still keep extremely good audio performance even with the theoretical microphonic problems for Type 2 dielectric capacitors.

  > If we wanted to improve audio filtering, we could design a replacement PCB (four-layer for impedance matching and signal integrity?) with footprints and traces to install a second-order Sallen-Key filter. This would be about the same difficulty as the HDMI2SCART, but likely reusing the existing case and screws unlike the HDMI2SCART's 3D-printed case.

  Why not just add your filtering circuit externally via the 3.5mm jack? No limits to what you could do. Cascade as much as you want at that point. Might be a bit ugly, but it wouldn't require re-designing the board.

  by Kirby64
• Both of your points were the very first things I thought reading this.

  A Sallen-Key filter is trivially easy to design if you work within certain constraints - a 2-pole Butterworth filter has the feedback capacitor exactly twice the value of the "second" capacitor to ground, if both resistors are the same value. If you pick 10kΩ for both resistors, 1nF for the feedback cap, and 470pF for the cap to ground, then you'll get pretty damn near a Butterworth response (Q of 0.707, maximally flat in the passband and then as fast a transition as possible to the stopband) at around 23kHz.

  This is perfect.

  And guess what? If you want to scale the cutoff, just scale the component values! If you use 15kΩ resistors you get 15kHz, if you use 22kΩ you get 10kHz, and so on. The minor error in Q will not be audible.

  by ErroneousBosh
• >> A photo revealed the chip was a NX3303X; I could not find any datasheets

  Not much information out there but these are similar chips, possibly it’s a clone?

  Capstone CS5213 datasheet:

  https://file.elecfans.com/web2/M00/2B/BD/pYYBAGHVUbyARd_-AAU...

  And also the AlgolTek AG6200 / AG6201 Series, datasheet:

  https://assets.yoreparo.com/attachments/28SGHqLW2Z2MfSzEBJB4...

  by andrewstuart
• The CS5213 appears pinout-compatible with the MX929x, interesting. The AG620x is a chip I've had the misfortune of encountering; it was common in Amazon DACs, dropped signal output every 15 minutes or so unless you disconnected the EDID pins from the (monitor?), and interprets input signal in a cursed fashion where 16 or below produces black but only 255 produces full white. If your computer outputs limited range HDMI the analog signal can't reach white, but if your computer outputs full range HDMI the shadows are clipped. This is one of the chips that gave HDMI DACs a reputation for black crush and caused CRT communities to recommend DP.

  Interestingly the MiSTer game system community actually patched their FPGAs to output digital signals from 16-255, and have some other way of avoiding the signal dropouts (never talk to EDID?), so this chipset is almost seeked out over incompatible models.

  by nyanpasu64