[sdiy] Novation peak NCOs

[Richie Burnett]

Sounds like the delta-sigma trickery required to get good results with just 
one output bit isn't trivial then.  I did some work with Class D audio 
amplifiers about 20 years ago, and remember having problems with different 
switching times of the MOSFETs leading to distortion just like you saw with 
the mismatched rise and fall times of the logic.  If you can't feed the 
analogue output back into the digital part, then it's hard to detect and 
correct for this type of error (distortion.)

I am also not privy to the trade secrets to making a one bit converter work 
well, but I know that MASH converters were all the rage in CD plays several 
decades ago, before today's multi-level delta-sigma converters became 
popular.  I agree with you , I too would be tempted to leave the conversion 
to the pro's like AD and Cirrus Logic, but credit to Novation for what they 
appear to have done here.

-Richie,

-----Original Message----- 
From: James J. Clark
Sent: Monday, April 24, 2017 7:13 PM
To: Olivier Gillet
Cc: Richie Burnett ; *SYNTH DIY
Subject: Re: [sdiy] Novation peak NCOs

I originally tried an on-chip sigma-delta followed by an analog lowpass. I
found the result to be quite noisy, even with a 3rd order loop. There was
also an issue of some distortion caused by differences in rise time and
fall time of the fpga digital outputs. This can be handled by encoding the
bitstream, but lowers the data rate. Also, I didn't do any dithering, so
there was probably some fixed pattern noise.

I could only get 6-8 bits worth using an on-chip sigma-delta. The external
sigma-delta DAC chips are much better. The Cirrus chips we use are
multi-bit sigma-delta, meaning that they combine sigma-delta with multibit
converters in the loop. In this way the sigma-delta doesn't need to
provide all of the bits. Also, the sigma-delta filters are
presumably optimized in a way that I don't have the experience to
duplicate.

I wouldn't recommend doing an on-chip sigma-delta conversion, unless you
are only shooting for 8 bits or so. For higher bit-widths I would rather
leave the conversion to the pros.

Jim


On Sat, 22 Apr 2017, Olivier Gillet wrote:

>> They probably do as I do on the Cyclebox, which is run everything at 
>> 24MHz
>> then decimate (subsample) to 96KHz (or whatever the DAC SR is) to feed 
>> into
>> the oversampled DAC.
>
> I don't think there's a need for a DAC at all. They just run the
> sigma-delta loop(s) in the FPGA, taking the already oversampled signal
> as input, and directly produce a 1-bit stream at 24 MHz straight from
> a FPGA pin - that is externally low-pass filtered.
>
> It would be inefficient (and would introduce some unnecessary latency
> due to filter length) to downsample to 96kHz in the FPGA, then
> re-upsample to MHz in the DAC.
>
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