Samplerates of multiple audio inputs of the Audio Expander

giuliomoro

What's AoA? You can mix and match analog and audio channels but be mindful of their phase difference. The audio ADC has approx 20 frames group delay due to its internal filter. For the analog ADC that's more probably below 2 all included, however analog inputs are using a single ADC with multiplexes inputs , so its channels will all have a different phase. If your application is heavily phase-dependent, you'll have to compensate for these differences

Bethanychm

giuliomoro I am sorry for writing wrong analog channels in https://forum.bela.io/d/3201-samplerates-of-multiple-audio-inputs-of-the-audio-expander/37, now I have modified it to analog IN2-3. I want to use 4-mic array to measure the arrival of angle (AoA) for the reflected signals. It is indeed heavily phase-dependent. I understand that analog multiplexes inputs introduce different phases between IN0-3. How about the interval when swtiching from audio IN0/1 to analog IN2-3 (actually relates to IN0 I think?)? Can you give me some methods to compensate for these differences?

giuliomoro

Bethanychm

Bethanychm . How about the interval when swtiching from audio IN0/1 to analog IN2-3 (actually relates to IN0 I think?)?

The audio inputs are sampled synchronously, so they will not have a phase difference between them. They will be about 18 samples later than the analogs. For sub-sample phase adjustment, things are complicated ... you'd need a fractional delay line with decent interpolation, or a resampler where you can set the group delay arbitratrily. Could start with a four-point interpolator like the one in Pd's [tabread4~], or look at what they are using for [vd~]. For scientific accuracy, howeverm you may need a FIR linear-phase filter. Libresample can probably do what you need, but it may be overkill. You'd need to find a sufficiently short set of FIR coefficients that do the phase shift you require and then can apply them with the Convolver class to apply them to the signal.

Bethanychm

giuliomoro Sincerely thank you for your answer! It seems to be more straightforward to solve the contact problem between the cape and the capelet. How can I compensate the phase for analog In0-3 when they are used to measure the arrival of angle (AoA)?

giuliomoro

Ultimately, if you wanted to fix this in hardware, a better option is to only use audio inputs that are all of the same type and synced. We have the Bela Mini Multichannel Audio Expander which provides 6 audio inputs. That has microphone inputs and it's cheap, however, that's designed for the BelaMini, which is now discontinued so, unless you already have one of those, you'd need to make a PCB adapter to connect it to the BeagleBone Black you already have.

In the BBB form factor, there's the CTAG FACE, which you could even use in combination with your Bela cape's analog inputs and outputs (not the audio ones). The inputs of the CTAG are not designed for microphones though and they don't have a preamp (unlike the ones you've been using so far) and they have full-scale 0.9Vrms (I think) input, so the signal may be a bit quiet, but it may work.

If you can wait a few months, have a look at the Bela Gem Multi (with 10 mic inputs): https://bela.io/gem

Bethanychm

giuliomoro Thank you for your valuable suggestions. I need to use them urgently recently, so I cannot wait for a long time.

Bethanychm

giuliomoro I want to know if I purchase a new product, how long does it usually take to ship to Asia?

giuliomoro

you'd need a fractional delay line or resampling filter where you can specify a fractional delay as I mentioned above

giuliomoro

Bethanychm we ship on Wednesdays and if you pick DHL as an option, it will take 4 or 5 days I think. If the DHL option doesn't show up for your country, email orders at bela.io to add it.

Bethanychm

giuliomoro So the latest ship date is next Wednesday?

giuliomoro

the earliest shipping date is next Wednesday. How much phase sensitive is your application? If you can withstand up to 1 sample of phase offset, then it's not a problem.

Bethanychm

giuliomoro Can I program the multi-channel audio processing of the CTAG FACE with C++?

giuliomoro

Yes

jakedorado32

Is it possible to program multi-channel audio processing on the CTAG FACE using C++?

I'm exploring audio development with the CTAG FACE platform and wondering if C++ can be used to handle multi-channel audio processing efficiently. Has anyone here worked on similar projects or have insights into the development workflow with C++ on this hardware?

max

jakedorado32 start a new thread with a new topic?

giuliomoro

Nah, @jakedorado32 is probably a spam bot anyhow

ZEJIAN_SONG

Dear giuliomoro @giuliomoro ,
I'm currently working on a project that involves acquiring six input channels and processing them into two output channels. Ideally, I would like to maintain a sampling rate of 44.1kHz throughout the system. I am using the Bela Starter Kit (REV C) along with the Audio Expander.

From the discussion before, I understand that when using more than four channels on the expander, the sampling rate is reduced to 22.05kHz. I was wondering: would it be possible to instead use 2 audio input channels from the audio channel on the Bela cape and 4 analog input channels from the expander, all at 44.1kHz, to achieve the required six input channels?

Do you know if this configuration is supported, or if there are any limitations I should be aware of?

Thank you very much for your time and assistance.

giuliomoro

Yes that configuration can work.

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