[sdiy] Bus for digital patching of synths

cheater00 . cheater00 at gmail.com
Sat Nov 16 11:03:58 CET 2013

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

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:


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:


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