[sdiy] Function Generator ICs

Edward King edwardcking2001 at yahoo.co.uk
Sat Sep 22 17:03:23 CEST 2007


My original designs and work towards the S3 were purely DSP based (quite 
early DSPs initially). Eventually I found that creating audio engines based 
soley on DSP's left me with huge control problems because using a dsp as an 
audio engine board host processor  was wasteful and the manufacturers had 
(have) a nasty habit of pulling a chip out of production.  pics just werent 
up to the job either.

So I started learning to work with FPGAs thanks to Scott and the intention 
was to use FPGAs for the host processors (custom control, custom interfacing 
and high speed thoughput) AND the audio engines themselves. A bit more 
research revealed - at least in my case and for my needs - that FPGAs alone 
werent the way to go, but to combine them and DSPs on the same boards. That 
way offering an FPGA host processor for all the custom interaction, FPGA 
based hardware for certain types of processing and DSPs for the flexibility. 
Also, by having FPGAs providing the "glue" between the various busses 
(system, event and audio), when a dsp retires, there is at least a fighting 
chance of getting whatever is new at the time to fit.

In hindsight, when I reached the point where I was trying to compensate for 
any lack of flexibility through over-use of softcore processor it should 
have been clear that it wasnt going to be a "one size fits all". This may or 
may not be a relevant take on things for you depending on what types of 
synthesis you want to provide and how much flexibility you need. In my case, 
I would have had to reconfigure the fpga everytime a different type of 
synthesis was used or suffer dire resource deficits through redundancy in 
the design.


----- Original Message ----- 
From: "Tom Wiltshire" <tom at electricdruid.net>
To: "Paul Perry" <pfperry at melbpc.org.au>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, September 19, 2007 3:23 PM
Subject: Re: [sdiy] Function Generator ICs

> On 19 Sep 2007, at 09:59, Paul Perry wrote:
>> Maybe a micro would be the go today? Anyone tried tio make a  general 
>> purpose
>> voltage controlled audio osc from a PIC?
> I have. I tried at first with a 8-bit processor (PIC 16F series chip) 
> which is fairly limited, but is still able to copy the 16-bit divider  DCO 
> of the Roland Juno synths. I wrote firmware for a chip which  takes a 
> 7-bit digital MIDI note input and generates a square wave of  the 
> appropriate frequency. The Juno followed this with a waveshaper  stage to 
> generate ramp and PWM waves.  You could use the PICs 0-5V  analogue inputs 
> for frequency modulation, although I didn't bother.  The analogue inputs 
> are only 10-bit, so you have to watch the range  of the modulation to 
> avoid zippering. You get true analogue  waveshapes though, not stepped 
> waves from a DAC.
> I'm currently working on a dsPIC (16-bit processor) audio osc,  although 
> this won't be directly voltage controlled. Instead, it will  have its 
> basic pitch set digitally (either via an SPI connection or  directly from 
> a MIDI serial stream) and use analogue inputs for  frequency and waveshape 
> modulation. This gives you digital stability  and the analogue ability to 
> feed anything you like into it. Output is  16-bit via a cheap 
> 24-bit/192KHz Cirrus DAC. The DAC is stereo, so I  hope to be able to have 
> two independent oscillator outputs.
> Bruce Duncan/Eric Brombaugh of Modcan Electronics have a commercial 
> example of something similar with the VCDO 58B, which also does two 
> operator FM synthesis.
> There are definitely problems with this approach. I'm vaguely keeping  in 
> mind that I'd like to build a polysynth, but the current  consumption of 
> the processor (70-100mA at the speed I'm using) means  I'm going to be 
> using a big power supply if I put 16 voices in it -  300+W just for the 
> oscillators! (How does this compare to synths of  old?)
> If anyone has any great ideas about a better way, I'd love to hear  it. 
> I'm starting to think that Scott Gravenhorst is going in the  right 
> direction with the almost-custom-hardware and massive  parallelism of 
> FPGAs.
> T.
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