DCO claimed as digital

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DCO claimed as digital

Post by gridsleep »

Having just reread the old (2002) retread of the Akai AX80 keyboard, I could not ignore that the author declared its DCOs to be digital oscillators. Without having to look through all the back issues, I would like to know if SOS still allows this foible to be printed without correction. A DCO being an analog oscillator with auto-tuning always enabled, as it were, is not the kind of quality that should be overlooked or hammered into a digital sobriquet á là square peg and round hole.
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Re: DCO claimed as digital

Post by Eddy Deegan »

There seems to be a bit of debate out there on the subject, but a post I found on vintagesynth.com asserts:

"Some things *called* DCO's have analog oscillators at the 'root'. EG> Siel DK600 which uses the SSM2031 to generate a clock for the divide down and TOS circuitry in the TMS3631 chips I think it is. HOWEVER in the AX80 I repeat there are NOT analogue oscillators. It's a digital set of chips purely. CPU, and bus control chips and 8253 programmable counters. There are the 'oscillations'. Purely digital anyway you look at it.

*THEN* AN ANALOG SAW CONVERTER is used which is functionally a hybrid circuit since the digital clock resets the process. It's not an analog oscillator by definition but rather a saw converter. It takes an analog voltage converted from the key value usually and uses it to control the ramp rate of an op amp circuit then restarts the ramp with the digital clock. So it's a nice smooth sawtooth lacking the analog artifacts that accompany an actual analog sawtooth wave generated by a real vco perhaps. There are no analog oscillators in these machines except for the clock crystal which technically is an analog component I suppose one could say.

Roland JX series, The Kawai SX-2xx series, Korg Poly61, and probably elka EK-22 oh and then Crumar 'Bit' series all use the programmable timers for tone generators as I recall and the saw converstion technique is used in all but the Bit machines plus it's used in the Siel DK machines as well.

...

A rep at Akai did an errant write up claiming vco's in the AX80 when really it's the only Ax synth that used non-vco's."


The above could be wrong but I found it interesting enough to inspire a bit more research on my part. I don't know enough about hardware at that level to be sure myself but being curious I hunted down the AX80 service manual (a detailed parts list for the voice boards is on pages 22-23).

There are a number of CEM3372 chips in the AX but these do not contain oscillators, rather a voltage controlled mixer, a 4-pole LP filter and a VCA. The parts associated with the voice board listed in the service manual appear to me to closely match the description above but maybe someone better versed than I in interpreting this kind of documentation (not to mention knowledge of electronics) can clarify further.

I'm not saying you're definitely wrong by the way, more than it might not be as straightforward as you think.

Edit: I also found this schematic of the voice board in a later section of the manual which is quite revealing:
AX80VoiceBlock.JPG
IC17 is on the CPU board and is a Texas Instruments SN74LS00N Quad 2-input NAND gate. To my eye this does look a lot like a high frequency digital clock being divided down to create DCO 1 and DCO 2, these signals being then fed into the analogue sawtooth generators.

Also note that Akai have labelled those digital sources as DCO 1 and DCO 2. Again, I'm not that knowledgeable about electronics but I do like a bit of sleuthing!

I too would be interested in a qualified opinion. Hugh, Will, Dave ... ? ;-)
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Re: DCO claimed as digital

Post by gridsleep »

Interesting. Thank you for all the detail. It's rather off-putting that the SX-240 may well be a hybrid and not analog.
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Re: DCO claimed as digital

Post by Hugh Robjohns »

Yes, it looks like the master oscillator is a digital squarewave generator which is divided down and then shaped with analogue integrators to make saw waves.
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Re: DCO claimed as digital

Post by Eddy Deegan »

gridsleep wrote: Sat Jul 24, 2021 11:59 pm Interesting. Thank you for all the detail. It's rather off-putting that the SX-240 may well be a hybrid and not analog.

You're welcome :) ... but as to your latter statement: why?!

Hugh Robjohns wrote: Sun Jul 25, 2021 12:26 am Yes, it looks like the master oscillator is a digital squarewave generator which is divided down and then shaped with analogue integrators to make saw waves.

Thanks Hugh, I'm relieved I didn't overlook something glaringly obvious to the contrary.
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Re: DCO claimed as digital

Post by gridsleep »

My SX-240 sounds splendid but I would have dickered the price down more.
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Re: DCO claimed as digital

Post by Eddy Deegan »

gridsleep wrote: Sun Jul 25, 2021 1:07 am My SX-240 sounds splendid but I would have dickered the price down more.

:lol: fair enough, kinda (although there is no technical justification for it and it sounds splendid so I'd say that's money well spent!) but I hope we can all agree that when it comes to the AX-80, the Sound On Sound review by David Hughes in 2002 was correct.

On a random note, a year or three ago I posted a plea to the forum to identify a synth I'd played in a shop 30-something years ago which had little fluorescent bar-graphs for the parameters. I can't remember what it sounded like but I loved the interface. I have a feeling that at least one responder identified it as the AX-80, though I can't find the post in question any more :crazy:
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Re: DCO claimed as digital

Post by resistorman »

It seems like the synth world is dividing into the “give me analog or give me death!” and “hey, this sounds great, I don’t care how it’s made!” camps.
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Re: DCO claimed as digital

Post by Eddy Deegan »

resistorman wrote: Sun Jul 25, 2021 2:51 am It seems like the synth world is dividing into the “give me analog or give me death!” and “hey, this sounds great, I don’t care how it’s made!” camps.

Hehe, they've been doing it for decades. In recent years it has become increasingly apparent that 'I don't care' is the way to go but there are a lot of people yet to get that message ;-)

Ditto for software vs. hardware! We all have our preferences in terms of the hands on experience but academically there's not much, if anything, in the sound and even where there is you'll find 'betters' in the product offerings of both camps.

Digital vs. analogue synths is (should be!) a dead argument. Both have their strengths and weaknesses but both rock. Additionally, some software sounds better than some hardware, and vice versa.
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Re: DCO claimed as digital

Post by BillB »

gridsleep wrote: Sun Jul 25, 2021 1:07 am My SX-240 sounds splendid but I would have dickered the price down more.

Interesting outlook. You bought a vintage machine with a splendid sound. How it generates that sound isn’t really relevant to its price. Be happy!

Eddy wrote: On a random note, a year or three ago I posted a plea to the forum to identify a synth I'd played in a shop 30-something years ago which had little fluorescent bar-graphs for the parameters. I can't remember what it sounded like but I loved the interface. I have a feeling that at least one responder identified it as the AX-80, though I can't find the post in question any more :crazy:

Eddy, AFAIK the AX-80 was the only synth to employ this method of parameter display, which is a shame. It’s a great idea to be able to ‘see’ the patch. It’s conceptual successor is the Nord Lead/Rack 3, which I own. It is brilliant for flipping to a new sound and thinking ‘how was that programmed?’ Then taking a look at the ring LEDs to show values, the switch LEDs to show routeing and the LFOs etc to show rate. And of course, a doddle to then edit further with no wild parameter jumps. It’s a real shame that more synths don’t use this kind of visual interface.

I suspect I would very much enjoy using an AX-80. Shame about tape-based backup/restore though!
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Re: DCO claimed as digital

Post by Eddy Deegan »

BillB wrote: Sun Jul 25, 2021 8:45 am I suspect I would very much enjoy using an AX-80. Shame about tape-based backup/restore though!

On my old DX9 I used to save to a sampler instead of a tape! It worked just fine and I have a feeling I might still have a floppy with my own patches on it in Replay 16 format.

It might or might not be readable now (assuming I can find it at all) but it would be interesting to try and recover those sounds at some point, partly to archive the data safely but also as one of them was a fantastic lead sound which I've failed to reproduce on anything else since :D
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Re: DCO claimed as digital

Post by gridsleep »

I'm watching an AX-80 on eBay right now. It started at $231 and, hope against hope, I kept an eye on it, but so did a couple of dozen others. Now, it's creeping up toward $1000. There will probably be a bidding war at the end. I guess it's less discouraging than the current $3500 price range for a Juno. Not that I actually want a Juno. Not really. The price wars are for a different topic.
I guess the original question is answered. SOS does not, indeed, disseminate the fallacy that a DCO is digital.
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