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How about going all digital

Theo Verelst

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All that talk about DACs and mixers makes you wonder, doesn't it, if if we couldn't do everything digital.


Software synthesis and effects don't ever go analog, except at the end, as well as digital workstations and simulation synthesizers, digital organs, everything except (partially) analog synthesizers and microphones, works all as samples in a computer or Digital Signal Processor right till pretty much the last stage, where the circuit board connects a Digital to Analog Converter to the computer circuits, that turns numbers into small voltages.


So wouldn't it be a great idea to have all software modules, digital synthesizers, workstations and digital effect units give out digital signals, and connect all those signals up to a digital mixer ? Sort of the converse of the early digital mixer Yamaha brought out in the late 80s, which would convert a number of analog inputs into digital signals, just for the purpose of mixing them. Digital mixing of analog signals seems a bit strange (pure mixing, i.e. addition of analog signals) isn't that electronically difficult or particularly expensive, but mixing all digital sound sources digitally together is actually a great idea, you put a big-aus quality DA converter at the end with many bits and good specs, maybe a ground separation circuit (preventing a lot of hum and rattle), a good electronic buffer at the output and setting up becomes easy, and with great quality!


Sure, that would work, but there's a problem: those digital signals have to be output by all the equipment, and should preferably be synchronized, i.e. linked to the exact same (preferably very stable) digital (word and bit-) clock. Some might think, no way, all you need is a re-sampling unit, so that all different clock sources can be made to synchronize even though the sources are slightly different in frequency.


Well, that's not a very good idea, especially for live use, so we prefer another solution. Why? Because technically/scientifically re-sampling isn't a very nice filter (and has negative properties as a side effect, like most filters, like phase shift, group delay issues and amplitude inaccuracies) and for it to do it's work properly, it needs to be relatively long, and therefor show an amount of delay, for more accuracy, more delay.


There can be a solution, known very much in pro audio: use a master clock that makes all signals synchronized, i.e. the bits run over the cables or glass fibers simultaneously, or the words run over ether net cables at exactly the same pace.


That makes it possible to create a digital mixer that works pretty perfect. Some equipment has this option already, I know from practice the PC3 (Kurzweil) and Lexicon Omega and MX400 make this idea possible. Of course it remains a question if the setup won't go wrong because of inaccuracies, or needs some sort of solution for solving drop outs or signal errors that doesn't cause problems with the audience.


Maybe when 10G Ethernet will be out, some equipment will start to allow connecting it's digital ins and outs up to some sort of broadcast with Tcp-check protocol that lets you just plug in, maybe set up some "easy" re-sampling filters to allow 49/96/192 or even higher digital audio clock rate equipment to work together. Until that time, there nicely ground separated (like MIDI is, too) glass fiber cables that can transmit digital audio over quite a distance reliably (when it is kept in decent shape) even at 192kHz, without having to cost much. Equipment that can take a master clock in and a simple digital mixer should be enough to need only one good DA converter at the end of that.



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