an idea + zipper noise

[J.D. McEachin]

On Thu, 14 Nov 1996, Ian A. Vine wrote:

> 
> 	Suggestions for hardware, there are some neat 8051 derivatives
> 	available these days with built in DAC/ADC and of course
> 	the standard serail i/o and a lot of digital i/o. So adding
> 	the MUX i/o, battery backed RAM, ROM, display and MIDI buffer
> 	to one of these '8051' and you're half way there (yea right).
> 	
> 	Seriously, the 8051 route seems good, cheap component cost, 
> 	cheap (or free) developement tools also using the chips i/p
> 	sorry should be i/o, ports you do away with a lot of the
> 	glue logic you normally need.

Philips makes some kick ass 8051 variants, some of which run up to 
40Mhz!  There's also a C compiler, but that costs money.  Europa is 
developed with a freeware 8051 assembler (w/ some hacks by Neil).


> 	Someone mentioned zipper noise when using ADC/DAC. I thought 
> 	that zipper noise was only caused like this. You have a signal
> 	(audio) going through a VCA the gain of which is controlled by
> 	a DAC. If the DAC output voltage is changed when the signal is
> 	not at the zero crossing point, you get a sudden jump in signal
> 	level giving rise to the unwanted distortion. Maybe the system
> 	needs some zero crossing feedback or the DAC/MUX outputs need toi
> 	be suitable filtered.

I always thought zipper noise referred to audible stairstepping caused by 
too few DAC bits.  It shows up on a lot of synths w/ 8 bit DACs (usually 
only 7 bits are used, because in those days you only had 7 bits of linearity.
It's especially annoying w/ filter cutoff.

JDM