[sdiy] Bus for digital patching of synths

charlie wallace charlie at finitemonkeys.com
Mon Nov 18 23:53:45 CET 2013


i have a cyclone gen2 pci expansion box, if memory serves it was about
3K  USD, the board was around 1K on its own.

stability could be better, but i am beating on it bandwidth wise.



On Sat, Nov 16, 2013 at 2:03 AM, cheater00 . <cheater00 at gmail.com> wrote:
> Hi guys,
> one of the list members has pointed me to the VME Bus backplane
> format, which is apparently a popular format in industrial computers,
> and traces back to the 68000 memory interface. The bus goes up to 320
> MByte/sec which is 2.5 gigabits per sec. This is 39% faster than the
> calculated example I cite above: "on a single collision domain, there
> could be 10 monophonic modules which each have 6 inputs or outputs at
> 100 kHz at 20 bits". However, instead of monophonic, the VME 320
> caters for 16-voice polyphony, so it's obviously very powerful. Much
> older versions can be used for monophonic bus.
>
> The 16 voice polyphony example above calculates at 1831 Mb/s or 228 MB/s.
>
> VME 320 is supposed to be very expensive. VME 64 (which is 64 MB/s) is
> supposed to be much cheaper.
>
> This got me thinking. Perhaps it would be a good idea to use PCI
> express? A single "backplane" would be a computer with PCIe slots.
> Even nowadays, an older motherboard with 4 slots can be bought
> inexpensively. A slow, cheap CPU can be cooled fanlessly with a huge
> heatsink, so no issue there. Flex cable risers can be used to route to
> the modular interface, so that you can use any motherboard really. The
> CPU could do additional processing, such as automation or signal
> processing or even recording. The cost for 5 slots is:
>
> - motherboard + cpu + a niminum of ram = 100 €
> - heatsink = 80 €
> - silent PSU = 100 €
>
> The best thing is that it uses cheap consumer products.
>
> My worry is though whether 1. this can be used easily from a digital
> board on a synth (is there inexpensive silicon?) and 2. whether it can
> achieve latency as low as one would achieve with a per-sample TDM
> setup.
>
> If one wishes to go in that direction, though, PCIe seems to be a
> flourishing ecosystem of relatively inexpensive hardware. The
> programming model becomes very simple, and there are numerous good
> real-time operating systems.
>
> Because PCIe can be switched, there are numerous backplane products.
> Every typical PC motherboard has at least one PCIe x16 slot. This can
> be used to drive a card which then connects to a backplane with
> multiple new PCIe slots that either communicate with each other, or
> with the host board. Even a 16-voice modular needs one PCIe x1 lane
> per 10 modules. A PCIe x16 slot obviously provides 16 lanes (as the
> name suggests). So with use of some challenging electrical signalling
> we're looking at being able to use up to 160 modules with a single
> inexpensive PC "brain", or 3x more with PCIe 3.0, plus we can multiply
> by the amount of 16x slots the host motherboard provides. That is
> obviously more than enough.
>
> There are ready PCIe backplane products. One example is from Cyclone.
> It has a physical backplane with PCIe slots on it, which connects via
> a cable to a PCIe card that plugs into your motherboard:
>
> http://www.cyclone.com/images/block_diagrams/600_2703blk.gif
>
> No idea how much such backplanes cost. Given all the bitcoin rage
> nowadays, I would expect them to be used at least in some capacity.
> The thing is that a bitcoin miner setup has to have a lot of graphics
> cards crunching away at numbers, and those cards don't need any
> powerful uplinks since there's no texture and geometry data coming in,
> just a few initial algorithm parameters the first time the card is
> engaged. So they're perfectly fine with x1 slots, but want a lot of
> them. Since hardware cost is heavily at play here, the only acceptable
> solution is for this hardware to be cheap. So I reckon there must be
> someone doing something like this inexpensively.
>
> Physically, the backplanes might be pitched too tightly. They are
> built for normal PCIe cards, so right next to each other. Perhaps flex
> cable risers can be used, such as here:
> http://www.adexelec.com/pciexp.htm
>
> Cheers,
> D.
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