Featured Articles
Announcing Pi
Technical Specifications
Raspberry Pi FAQs
Win a Pi
Raspberry Pi Wishlist
Actions
Forum Thread Details
  • State Not Answered
  • Replies 5 replies
  • Subscribers 675 subscribers
  • Views 625 views
  • Users 0 members are here
Related

Input Impedance

Former Member

Hello,

I bought the Cirrus Audio Card for my Raspberry Pi B+. I am using it as follows: Audiomixer --> Line In on the Cirrus --> Icecast Server --> Webstream output. I successfully set up with the provided Image. I put the file "Record_from_LineIn.sh" in rc.local and start my Icecast Client as well. But I have a little issue: My Audiomixer has an integrated VU-Meter. It works quite well connected to a normal Amp. But as soon as I route the Audio Path with the provided Shellscript,all LEDs light up. It doesnt affect the functionality of the system, but still it bugs me. I guess it is related with the input impedance. So I built a little circuit with an OP- Amp (Voltage Amplifier with Factor 1 (Impedanzwandler)). This resolves my LED issue, but produces noisy Output... So i guess there must be some setting in the software, since the Wolfson Chip is quite powerful. Does anyone has a clue where to dig in?

The only information I have from my audiomixer is: output: 775mV (max. 1.5V).

Thanks, Simon

  • 0

    Hello,

     

    I think the micbias voltage is always on on line in of the Cirrus Audio. I measured it on scope. When I start recording the voltage at the input jumps to about 2.5V (only the oscilloscope was on the input). This fools the VU-meter of my tape recorder too.

     

    I have Rpi3 and Arch linux (I don't like the debian line).

     

    Does anybody knows how to switch off the micbias?

  • 0 in reply to griphand

    Could you test if this helps?

    amixer cset name='Line Input Switch' off

     

    Just checked the mixer settings and noticed this seems to be on by default...

    # amixer cget name='Line Input Switch'
numid=341,iface=MIXER,name='Line Input Switch'
  ; type=BOOLEAN,access=rw------,values=1
  : values=on

     

    so long,

     

    Hias

  • 0 in reply to hiassoft

    Hello!

     

    I tried

    $  amixer -Dhw:sndrpiwsp cset name='Line Input Switch' off

    the result:

    numid=345,iface=MIXER,name='Line Input Switch'

      ; type=BOOLEAN,access=rw------,values=1

      : values=off

     

    $ arecord -Dhw:sndrpiwsp -c 2 -f S16_LE -r 192000 -d 10  float1.wav

     

      arecord: pcm_read:2096: read error: Input/output error

     

    I think this switch off the line in path entirely.

     

     

    $ cat /sys/devices/platform/sound/WM5102/dapm_widget

     

    In idle state: all switches are off.

     

    During recording: (listed only the swithes are on)

    AIF1TX2 Mixer: On

    AIF1TX1 Mixer: On

    LHPF2 Mixer: On

    LHPF1 Mixer: On

    LHPF2: On

    LHPF1: On

    MICBIAS3: On

    IN3R PGA: On

    IN3L PGA: On

    MICVDD: On

    SYSCLK: On

    PM State: On

     

    Can I influence on the behavior of alsa dapm widgets? I'd like to switch off MICBIAS3 and maybe MICVDD.

  • 0 in reply to griphand

    Thanks for testing. It looks like micbias has always been enabled on linein in the wolfson/cirrus driver and the "Line Input Switch" amixer setting in Record_from_lineIn_Micbias.sh didn't have any effect at all.

     

    To change the behavior we'd have to modify the dapm routes (search for dapm_routes in rpi-cirrus-sound-pi.c) and add a separate "Line Micbias" switch, probably with a few lines of extra code. I'm not 100% sure yet how a proper setup should look like, DAPM can be tricky.

     

    so long,

     

    Hias

  • 0 in reply to hiassoft

    Hello!

     

    I removed the two resistors (R33, R26) that connect the DC to the input from the board. I think this is a design fault. There must be hw jumpers to switch off the DC and the resistors from the input when not used with electret microphones.

     

    If these resistors stay in the curcuit reduce the input impedance or cause crosstalk depending of the state of the MICBIAS pin of the chip.

     

    The input impedance of the chip is 16Kohm said the manual. So the input impedance of the board without bias resistors is: 10Kohm + (12Kohm || 16Kohm) = 16.8Kohm.

    With bias resistors this drops to 1.95Kohm. (+ some annoying DC at present)