The "Guitar Workstation"
By Jacob Lister
Posted on July 16, 2003
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All Set Up

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...And what's that cup of coffee doing on the floor?!? Probably has some connection to my discovery that the Digitech processor does not like having Red Bull in it's internals! Still seems to have survived alright (durable beast). Perhaps another reason to switch back to V, which I was pleased not to be deprived of when I was in the UK, but I digress.

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The large white cable in the center of the setup is a USB connection between the Mini-ITX system and the RPx400. This transmits full duplex audio and MIDI control information. Running audio over USB means the A/D and D/A conversion can be done inside RPx400, which is much higher quality than the onboard audio or an internal soundcard in the Mini-ITX system. And the RPx400 has blanced XLR outputs to run straight to a mixing desk. Again, lovely product Digitech.

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USB audio does have it's downsides however. The software I'm developing is real time signal processing, and exploits the ability to achieve < 1ms latency scheduling possible under Linux. Such low latenty is a real must for musical applications, as any perceiveable delay between playing the instrument and hearing the output will have a negative impact on the performance of a musician. Using audio over USB with the RPx400 under Linux I've been able to achieve 128 sample, or ~5.6ms latency. Due to buffering in the computer, the effective working latency is double this, or 11.2ms. Respectable, but far from ideal.

For this reason the RPx400 is also connected to the Mini-ITX system by standard analog audio and in internal Sound Blaster Audigy sound card in the Mini-ITX system. The audio quaitly is lower, but does give the option for very low latency real time processing.


Here's a recent screenshot of the software this beast is running. I don't yet have a general release version, but intend to release the software under GPL when it's ready. The sequencer functions (play/stop/record) etc can be controlled by MIDI over USB from the RPx400s footswitches when it's placed in a special mode.

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All Packed Up

All zipped up in the floorboards' provided carry bag. Just unpack, connect power, guitar input and output ready to rock!

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And for a real master setup, check this out...

Jacob Lister

Jacob's Control and Audio Processing Software, Carbon - is here