Forum Rule: Always post complete source code & details to reproduce any issue!
Results 1 to 13 of 13

Thread: Hash noise from display getting into audio when screen updates ..

  1. #1
    Member
    Join Date
    Apr 2020
    Location
    Guelph Ontario Canada
    Posts
    66

    Hash noise from display getting into audio when screen updates ..

    okay teensy 4.0 , ra8875 display , audio shield ..currently running off usb power the ra8875 is powered off vcc and gnd looks good ..also running metro at 250ms as the display was way to frantic .. the problem ..I have has noise during the sweep ...So I figured I had best isolate the power for the display on a seperate power supply ..fine no problem .So I though okay ..I can run the teensy 4.0 off usb power and power the ra8875 from a seperate power supply ...my external supply checks good .no issues ..however with the teensy on USB ..and the ra8875 on external power the display will not come uo ..if I re-connect the display power back to the teensy it runs ...a little stumped as to the behaviour ..Am I missing something with respect to power ? I though that perhaps the power supply took more time to stabilize so I dropped in a 5 second delay in setup no luck ....thoughts ?

  2. #2
    Senior Member+ defragster's Avatar
    Join Date
    Feb 2015
    Posts
    12,341
    Thread title suggests 'Noise' - but post suggests trying to solve the noise with external power resulted in no display function? Are the display and Teensy safely/well grounded together?

  3. #3
    Member
    Join Date
    Apr 2020
    Location
    Guelph Ontario Canada
    Posts
    66
    yes .. I re-read my post my head is going in too many directions ...sorry!

    1. ra8875 is connected to power on the teensy 4.0/audio shield combination ..
    2. teensy 4.0 produces a 1024fft which is sent to the screen.
    3. audio on audio shield has hash noise while the display is updating.
    4. rationalized that I needed to separate display/tft power to an external power supply.
    5. ran a separate power supply to power the ra8875
    6. power is at the display .
    7.Connected USB to teensy board to power t4.0
    8.Reloaded software screen does not display !?????
    9. re-attached ra8875 to power on teensy ... display works!!
    10. something has me really puzzled ..I expected the screen to function with power applied from an external source..( didn't happen)
    11. I've checked with a voltmeter and proper power is at the Display..
    .
    .. open to suggestions .. I know they should have a common ground ..

    I've pulled the project apart on the bench to see what's going on .. but I can't see a reason the display will .... well not display LOL..

  4. #4
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Posts
    22,651
    A ground loop isolator might help.

    https://www.pjrc.com/store/audio_ground_isolator.html

  5. #5
    Member
    Join Date
    Apr 2020
    Location
    Guelph Ontario Canada
    Posts
    66
    I'm running ground isolation now ..the device uses 2 x 600 ohm isolation transformers ......still working my way through it
    I tried putting the ra8875 display on a separate regulated 5 vold supply but the display does not come up when I power up the reensy 4.0 ..initially I was powering everything off USB in development ..Then the hash noise started when The screen was rewriting an fft .... to the screen fft1024 ..I though by placing the 8875 on a separate power supply zit would bleed it off ..however , when I connect the external power supply vcc and ground up to the display , and run a common ground from the teensy to the supply ..The screen does not display anything ..I re-attach the screen to the teensy 5.0v running off the USB port and It runs .......if I leave tge screen powered up on the external supply and restart the teensy ..no display ..So a bit puzzled at the moment ...the external5v supply can easily supply 1 amp (7805 regulator on a heat sink.).

  6. #6
    Member
    Join Date
    Apr 2020
    Location
    Guelph Ontario Canada
    Posts
    66
    maybe I need more bypass caps on the external supply .. will add a few and see what happens

  7. #7
    Member
    Join Date
    Apr 2020
    Location
    Guelph Ontario Canada
    Posts
    66
    Okay ..1 problem identified ..7805 regulator is ..well not regulating ..checked voltage at regulator and it has dropped to 3.65 volts from 5v !!!..so the display would just blank out..!!..

  8. #8
    Senior Member
    Join Date
    Jul 2020
    Posts
    385
    I think the problem is you are trying to regulate 5V down to 5V ? Sounds like it. Doesn't work like that, a linear
    regulator drops about 2V in operation, so you'd need an external supply >= 7V to be able to use the 7805 regulator.

    Or am I getting the wrong end of the stick? It helps to post the circuit up front to spot things like this quickly.

  9. #9
    Member
    Join Date
    Apr 2020
    Location
    Guelph Ontario Canada
    Posts
    66
    I'm running 12v into the 7805 with a heatsink ..but there is a problem with the regulator ..I have a second identical circuit I built and it's running fine ..checked it out pin for pin and end to end ..one board is good but the second bad ...

  10. #10
    Member
    Join Date
    Apr 2020
    Location
    Guelph Ontario Canada
    Posts
    66
    Anyhow ..Will swap the regulator out ...have a 10 pack here lol

  11. #11
    Member
    Join Date
    Apr 2020
    Location
    Guelph Ontario Canada
    Posts
    66
    Was not expecting that ...!!!..

  12. #12
    Senior Member
    Join Date
    Jul 2020
    Posts
    385
    Not from eBay are they?

  13. #13
    Member
    Join Date
    Apr 2020
    Location
    Guelph Ontario Canada
    Posts
    66
    Regrettably yes ....from EBay ..I was going to swap one out of the bag but didn't trust them ,,got in my van and hit the electronic store for the real deal .....lol ...NOW I KNOW!

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •