[sdiy] Digital modular backplane - update

Rutger Vlek rutgervlek at gmail.com
Sat Mar 22 12:05:18 CET 2014


I've been thinking about the concept of digital modulars too, but I think that until digital technology advances much further (lower latency, higher sampling rate, more CPU power, high bus speeds) the key differences between analog and digital domains will limit the usefulness of a 'digital modular' approach. Some things sound better, and are more cost effective, in the analog domain: such as oscillators and filters with lots of character, a little bit of drifting, etc. And some things sound better, or are more cost effective in digital: noise properties, patch storage, complex connections that would otherwise require lots of messy cables. I see much more benefit from identifying the reason why we actually HAVE analog stuff still around and then searching for an optimal marriage with digital technology. If you look at recent developments of for instance DSI synths, you see that digital is used for the elements it's really good at, and analog is kept for filters and sometimes oscillators. Modeling of analog circuitry becomes more effective and precise every year, meaning that some things that could only be done in analog perhaps can sufficiently approximated digitally very soon, forcing us to constantly think about why we have things in the analog domain and how to merge it with the advantages of digital. Personally, I see the future of modular synths a bit differently:

1) I think the DIY community will search for a new framework to design modules for soon. Probably something that has a nicer digital control surface, keys and controllers built in, but leaves room for (a) custom analog circuitry, such as voice cards or fx cards and (b) room for custom DSP algorithms (reverbs, etc).

2) We the advances in digital technology digital modulars can progress further too. The Clavia G2 was an excellent starting point. If it would be merged with more modern analog modeling algorithms, would run at even higher sampling rates (and more CPU and memory resources), and possibly options for adding your own coded modules, it would be a magnificent step forward. Especially when it would be combined with more modern multi-touch control surfaces, that would bring back a bit of the tactile experience of a modular.

Anyway.... let's see what happens :).

Rutger




On 17 mrt 2014, at 12:24, cheater00 . wrote:

> Hi guys,
> Some time ago we have spoken about what it would take to make a
> digital backplane for a modular synth. One of the issues was that most
> common digital interconnects will introduce large delays in
> transporting the audio which are not acceptable in modular synthesis.
> A requirement was mentioned of ideally having the delay as low as the
> transmission time of 1 bit. This has to take into account the
> situation that an ADC or DAC will operate at low clock speeds, while
> the backplane would operate at very high clock rates, in order to
> accomodate many ADC-DAC links in the switched, TDM fabric.
> 
> I have come across the idea of using a SerDes:
> 
> http://en.wikipedia.org/wiki/SerDes
> 
> They are basically shift registers with additional ISO OSI Layer 1
> processing. in specific, it seems a popular type uses 8b/10b encoding
> which limits the RFI impact, and makes the layout much easier as the
> lower bandwidth limit of the physical links goes further up, while the
> upper bandwidth limit doesn't move.
> 
> In addition this device family addresses the issue of having slow
> links on fast backplanes:
> 
> "Bit interleaved SerDes multiplexes several slower serial data streams
> into faster serial streams, and the receiver demultiplexes the faster
> bit streams back to slower streams."
> 
> It seems in this case there is no bunching or buffering so the latency
> can be kept to a minimum.
> 
> Silicon for SerDes applications exists and is popularly used in loads
> of consumer technologies:
> 
> "Among the areas in which 8b/10b encoding finds application are the following:
> 
> PCI Express at speeds below 8.0 GT/s
> IEEE 1394b
> Serial ATA
> SAS
> Fibre Channel
> SSA
> Gigabit Ethernet (except for the twisted pair based 1000Base-T)
> InfiniBand
> XAUI
> Serial RapidIO
> DVB Asynchronous Serial Interface (ASI)
> DisplayPort Main Link
> DVI and HDMI Video Island (transition-minimized differential signaling)
> HyperTransport
> Common Public Radio Interface (CPRI)
> OBSAI RP3 interface
> USB 3.0
> CoaXPress
> MIPI M-PHY[6]
> ServerNet (From ServerNet2 onward)"
> 
> It might be possible to find switched fabric chips that can route the
> links on an X/Y grid for best bandwidth utilization, and which can do
> so without buffering. At least that is what I would expect of what
> Infiniband, Gb Ethernet, and Fibre Channel are doing.
> 
> Cheers,
> D.
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