[sdiy] Digital modular backplane - update

Rutger Vlek rutgervlek at gmail.com
Sun Mar 30 20:06:47 CEST 2014

Hi Damian,

Thanks for collecting and sending all the info! Looks great. I am a list member for quite some time already, but have become more "active" in the last months. Here's a brief introduction for anyone interested:

My name is Rutger, 31 y/o, living in Nijmegen (Netherlands). Have been playing keys for various (prog) rock bands and doing synth DIY since age of 21. Have a masters degree in artificial intelligence and just finished a PhD in Brain-Computer Interfacing (with EEG). Looking for job opportunities to move away from academia. I love science, but I hate present academic climate (pressure, ego's, competition, etc). Thinking about starting a small company to share some of my recent DIY efforts with a wider audience. Have recently been working on prototyping a new idea for a discrete OTA and obtained pretty good results! Also very much interested in the "bigger picture" of synths and other new instruments, large innovations, a new standard to replace MIDI, digital modulars, etc :). 



ps. And did already mention that I'm looking for job opportunities in synth design (sorry, shameless pluggin' going on here).

On 30 mrt 2014, at 15:27, cheater00 . wrote:

> Hi John and Rutger,
> thanks for your emails. First of all, welcome to the list - I have not
> spoken with you before, and I guess this is one of the first posts for
> each of you.
> I am guessing you don't recollect, or haven't been involved with, the
> original conversation from November 2013. I have had to ask Paul for
> permission, but I have now forwarded the whole original thread to the
> list. The thread is not all in the synth-diy archive, which is
> troubling.
> On Thu, Mar 20, 2014 at 4:00 AM, john slee <indigoid at oldcorollas.org> wrote:
>> Hi,
>> On 17 March 2014 22:24, cheater00 . <cheater00 at gmail.com> wrote:
>>> Some time ago we have spoken about what it would take to make a
>>> digital backplane for a modular synth.
>> What problem is this attempting to solve? Being able to save signal
>> routing and control state in a patch would be nice, I suppose, but
>> - physical settings != patch settings
>> => replace controls with "soft equivalents" => encoders/momentaries
>> => current state no longer obvious => more indicator lights etc
>> => may require more front panel real estate than "real" controls
>> => more $$$
>> What other benefits of going all modern do you see?
>> John
> The basic idea (which you will find more details on in the original
> thread I have just forwarded) is that you use analog modules. Those
> modules are connected via digital patch cables. Much like Rutger
> suggests below...
> On Sat, Mar 22, 2014 at 12:05 PM, Rutger Vlek <rutgervlek at gmail.com> wrote:
>> 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.
> ...the idea is to take the best of analog, and the best of digital.
> Analog is obviously better at generating and processing sound. Digital
> is better at moving it around, meaning recording and playback. Michael
> Jackson's album Thriller was multitracked in digital.
> My latest update, which I again include below (as it's been 10 days
> and many readers have likely deleted the original post), talks about a
> possibility of realizing a modular bus for hauling a lot of audio
> around in digital with as little latency as possible. Ideally the
> latency should be as much as it takes to digitize a single bit of
> signal, send that single bit, and then reproduce it with a DAC. This
> should make even feedback topologies useful.
> Naturally, for situations where feedback isn't good in digital,
> front-panel patch points should still be available.
> Cheers,
> Damian
> On Mon, Mar 17, 2014 at 12:24 PM, cheater00 . <cheater00 at gmail.com> 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
>> 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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