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  • Bela capabilities : Guitar FX ? Active Speakers crossovers ?

Greetings everyone.

I wonder what real capabilities could be expected from Bela's hardware considering two projects :

  • As a stand alone guitar pedal (for the bela/bela mini hardware), would the device still operate within a few miliseconds latency, in the case of several (dozen ?) digital chained effects (exemple : ADC --> Split signal // noise gate // overdrive // tremolo // auto wah // EQ --> DAC ) --> time effects obviously excluded.

  • As a audio DAC and DSP device (Bela + analog--> adio extention or CTAG hardware) : would bela be suitable to handle FIR convolution for Room correction and active speakers (latency due to FIR requierd sample excluded) ?

Thanks.

The Bela infrastructure gives you the ability of very low in->processing->out latency.

The processing part is defined by the beaglebone though, for a dozen effects you may well be pushing it depending on the algorithms. You could easily reach the limits with a single high quality overdrive algorithm where you could also have the whole chain you mention, it's all down to how much grunt you want to use.

For the room correction exactly what do you have in mind?

    13 days later

    Based on my experience with processors of similar grunt you could do convolution with a 50ms IR if efficiently coded.

    Obviously on the cheap end of the spectrum you can get some degree of room correction with a well-chosen set of moderate-order IIR filters and delays + speaker placement. It all depends how perfect you want the result.

    As for guitar effects, I have been able to chain the following simultaneously with about 56% CPU loading (this wasn't a practical combination, but just to see if I could handle all my guitar FX at once):
    Kokkinizita Rev1 Reverb (this is >25% CPU loading on its own)
    Flange
    Chorus
    Delay (echo)
    Phaser
    Tremolo
    Dynamic range compressor
    Wah
    8-band graphic EQ
    Envelope filter

    As @AndyCap pointed out it would depend what kind of overdrive algorithm you want.

    An overdrive that sounds ok uses a low-order polynomial (like 3rd order) a few first-order IIR filters, which is all a drop in the bucket for BBB CPU. It's probably lower CPU than the dynamic range compressor.

    If you really want to do serious modeling to the degree where you can't discern the difference between the software overdrive and a real tube pre or stompbox then you will probably use as much CPU as the BBB has just to do that one effect.

    On the other hand, all I listed except the reverb are not very large adders.

    Back in the day I used a Digitech RP20 Valve, which accomplished everything on far less substantial CPU power than the BBB. As for overdrive, these were real analog circuits, sent out a DAC and received on an ADC. I actually really liked that multi-FX unit. I wished Digitech had stayed on the track of analog and digital hybrid hardware rather than jumping on the DSP modeling bandwagon. Now 25 years later the DSP modeling is getting good enough to look at again.

    On the other hand, I have a lot more fun messing with Bela. My plan is to eventually use Bela to take the Digitech RP20 concept where I wished it had gone: I'm planning to chain channel 1 into an analog FX chain, then post-processing in channel 2 and out...

    I seem to recall the latency chugging all my FX in series is <4ms when channel 1 is chained into channel 2 (so 2x the nominal latency).

    Channel 2 uses time-delay effects like echo and reverb, so the added latency when that channel is active is not such a big deal.

    5 months later

    AndyCap Talking abt speaker correction, I wonder what could be the "taps" limit ?

    Considering a three-way stereo speakers setup for which I am looking for a device that allow me to set up the crossovers, high pass protection, amplitude and phase correction of the signal (considering the audio only application, then the FIR own latency isn't much of an issue). For all the audible range down to 20 hz or less (about woofer protection).

    I already own a the core parts of a device (yet to build) made of the adau1701+1452. It may reach 10 000 taps to share btw 6 outputs. From what i saw Bela cape + audio expander or the CTAG both allow 8 audio out, thus i ask about the FIR effectiveness of this two boards.

    Would the Ctag works fine alone (with a BBB but without a bela cape) ?

      I would not use the Bela's "re-conditioned " analog outs as audio outputs for hi-fi applications (see here for details). The CTAG does not need a Bela cape to work.

      So your question is whether a 10000 taps FIR filter can be implemented? Yes it but I am not sure you would be able to do it all in the time domain. You may have to use partitioned convolution and implement the larger convolution stages in the frequency domain. If you don't care about latency, you could just do a single big FFT, without the need for a partitioned algorithm, which would be easier to implement than a partitioned convolution.

      I will check on this partitioned convolution theory. It s true that the filter requirements aren't the same in an home theater setup than for an audio one, considering the acceptable latency.

      What then would be the ctag computation limit ? None but the mathematical convolution delay due to the FIR lengths ?

      The computation limit will be the CPU capabilities. Realistically speaking, people have used the same CPU to achieve convolutions with impulse responses that were several seconds long. In terms of latency, the CTAG can go sub-millisecond, but it seems that for your application you will be limited by the algorithm of choice.