Specifications for audio input?

[bklebke]

Is it line level? Or does it have preamp capabilities? I'm interested in using the RPi as a real-time audio "repeater" towards other devices over the network or internet, and want to be sure I don't fry the soundcard with giving it too hot of a signal.

Also, this may be a silly question, but is there a technical overview of the card anywhere? e.g., I don't actually know if it does onboard audio processing or offloads it to the Pi's CPU.

Thanks!

[flatmax]

Hi there,

Thanks for asking these questions.

For the RCA connectors (line inputs) :
The input signal level of the codec (chip) is 1V RMS. There is a voltage divider at the input which reduces the input voltage by half before it hits the chip.
The input lines fot he chip have a digital gain control in the range of -34.5 dB to +12dB. The digital gain is programmable in steps of 1.5 dB. There is also the Analogue potentiometer to control the signal level, which should suit your application very well.
There are rather large voltage compliant DC blocking caps on the inputs.

For the Microphone input :
Same input signal level of 1 V RMS with a gain boost of up to 34 dB. This gain however is in addition to the gain of the electrical Microphone bias system. The bias voltage is roughly 2.5 V. Each sound card is setup for a microphone, however only those purchased with the microphone are populated with the electret microphone capsule.

For the Headphone Output :
1 V RMS voltage level.
30 mW @ 32 Ohm headphone load = 50 mW @ 16 Ohm headphone load. They are rather loud.

As far as a digital repeater goes, should be very good at that job.

The actual card is the wm8731 audio codec and it directly interfaces the I2S silicon block on the PI's I2S bus. All processing has to happen on the Pi's CPU.

Matt

[bklebke]

Thanks! Super helpful.

Unrelated question but perhaps requiring similar knowledge: in trying to used forked-daapd (a DAAP server) with the sound card, I run into a few problems.

When outputing to Airplay as well as the local output, although it plays stably for some time it soon glitches out. The error it spits out is

laudio: SNDRV_PCM_IOCTL_SYNC_PTR failed (-32)

does that mean anything to you? I'm trying to chase it down but having very little luck in deciphering this one.

Thanks!

EDIT: BTW, just realized omxplayer is sending audio through the HDMI output rather than the 3.5mm jack. Is this normal behaviour?

[flatmax]

Hi again,

Can you please send more context regarding the IOCTL error ? Is it related to an underrun ?

Regarding the omxplayer default output, can you set that by right clicking on the sound icon (top right) ?

thanks
Matt

[bklebke]

Figured out the OMX player issue -- apparently OMX doesn't use ALSA, so the card is completely transparent to it. Only sends out over HDMI or the built in RPi output, since this card uses ALSA.

Found another player that used alsa, proved to myself beyond a doubt that the hardware's working. Haven't done a full sweep test or anything but it looks good.

Regarding the laudio error, unfortunately this is all the daap program's log has to offer. Is there another log I should look at?

Use case was, as I mentioned, outputting audio both to apple TV over network and locally through the sound card. Worked for a time then starting getting intermittent until it quit. This was the only error thrown.

[caco_viejagorda]

Why the input voltage is halved before hitting the chip?

[flatmax]

This is done to stop clipping. You can boost the gain in the mixer to get the 1 bit back !

Matt

[abcdefg]

Hi,

Adding on to this chain because I am having an issue with the signal attenuation on the input side as well. When I put a 1Khz (0 db) tone into the card and have it set to bypass mode, the output is returning a -15 db signal. This is with both pots all of the way up and the input set to Line. I also have all of the digital faders in alsamixer maxed out. Is there another control I'm missing?

-Tom

[flatmax]

I have been thinking about this. Is it possible to try to use the headphone jack ? That has more amplification. Can you report back on your findings ?

I know there is a 6dB drop on input to stop clipping. I assume the rest of the drop comes from other unreported attenuation to the RCA jacks. I am wondering what the headphone output bypass level is like ?

thanks
Matt