Converting outputs to inputs for mic'ing band

[suberimakuri]

Hi there,

Looking at using the balanced breakout octo card with a collection of Shure SM57 mic's to record our garage band.
Could use an analogue mixer that still allows multi tracking (multiple outputs on mixer), but prefer not if possible.

Built a lot of car EFI computers, fairly decent with soldering iron, linux, etc.

Haven't had a real good look at the code or (GPIO?) limitations of rpi or this board, but is it possible to convert the 8 outputs to inputs?
Bit of fun perhaps.

Thanks!

[flatmax]

Hi,

Dynamic microhpones should work well. You may want to look at the balanced breakouts. You may need a little extra preamp on the mics, however you could test directly - preferably using balanced microphone cables.

thanks
Matt

[suberimakuri]

Hi Flatmax,

Thank you!
Sorry for lots of questions.

1. Balanced breakouts - yes this was what I was looking at and this post based on using this board:
http://www.ebay.com.au/itm/Audio-Inject ... Swcj5ZSKwH

2. Input options: 1/4" or RCA. The 1/4 looks like 3 way (tip/ring/sleeve), so I believe I can go XLR to 1/4" (TRS). Easy. Yes?

3. Additional Preamp for mics - DC15V
Can I connect suitable power supply into the 15V 2.1mm connector on the blue board?
From other threads it looks like that's what it is intended for?

4. Balanced breakouts convert output -> input.
I just looked up the Cirrus chip from your link in the kickstarter and I see this chip is limited to 6 balanced input/8 bal output.
So 6 mics is the limit.
Does this sound correct to you? Or do you think it's possible to repurpose inputs as outputs with a modified driver or otherwise?

5. Run 2x Octo boards at once? - I would like to hook up ~10 microphones.
I would think Alsa would end up seeing two sound cards, which should be fine. Might need to massage asound.rc perhaps.
Any other perceived issues?

[flatmax]

Hi Flatmax,

Thank you!
Sorry for lots of questions.

1. Balanced breakouts - yes this was what I was looking at and this post based on using this board:
http://www.ebay.com.au/itm/Audio-Inject ... Swcj5ZSKwH

Yes - that is the one, however you may want to run them without the preamp - I'll explain why below ...

2. Input options: 1/4" or RCA. The 1/4 looks like 3 way (tip/ring/sleeve), so I believe I can go XLR to 1/4" (TRS). Easy. Yes?

Yes sounds right.

3. Additional Preamp for mics - DC15V
Can I connect suitable power supply into the 15V 2.1mm connector on the blue board?
From other threads it looks like that's what it is intended for?

OK - the preamp ... it takes the roughly 3v p-p differential output from the Octo and steps it up to 6v p-p. It doubles the voltage on the output. The input divides by two - which probably isn't what you want to do. I would imagine that the dynamic microphone signal levels are much less then 3 v p-p ... for that reason, probably would be best to not use the preamp - however some experimentation would help you decide. If you aren't going to use the preamp - then your signal goes in 1:1 rather then being divided by 2.
You can see the wiring diagram here : http://forum.audioinjector.net/viewtopi ... 2987#p4792

4. Balanced breakouts convert output -> input.
I just looked up the Cirrus chip from your link in the kickstarter and I see this chip is limited to 6 balanced input/8 bal output.
So 6 mics is the limit.
Does this sound correct to you? Or do you think it's possible to repurpose inputs as outputs with a modified driver or otherwise?

Yes 6 input and 8 output. I don't quite understand your re-purpose concept yet.

5. Run 2x Octo boards at once? - I would like to hook up ~10 microphones.
I would think Alsa would end up seeing two sound cards, which should be fine. Might need to massage asound.rc perhaps.
Any other perceived issues?

OK - 2 Pis and 2 boards (can't do two on one as only 1 I2S bus). In that case, when you record, you will need to manually align the captured wavforms in your mutitrack editor. This alignment is required because the Pis will be out of sync slightly.

When I record a new instrument, I typically have 4 to 8 bars of lead in music and I cup the headphones on a microhpone. When you edit you can manually align the waveforms based on the initial loopback. If you haven't seen this, you will probably be surprised at how out of alignment they are ... and recording track after track forces this latency to make more slop - and your tracks no longer sound tight !

Matt

[suberimakuri]

Interesting. I'm new to mics so thanks for explaining.

I was hoping to repurpose the Cirrus chips outputs as inputs, but it doesn't seem possible looking at the datasheet. I don't need the outputs. This would have given me 6+8? inputs.

Hmm. 2pi's and 2 octo's... Hadn't thought about bus limits. Two cards would talk over each other.
Thanks for the extra info on syncing tracks, makes sense to manually sync based on the waveform and using lead in music to get timing right.

I've read in other forums people have tried to get ADAT audio in support with rpi's, but can't find any definitive products or details.
I'll ponder this all for a bit, might be stuck with commercial offerings only if go above the 6 inputs.