[sdiy] uLaw DACs in the modern era?

[Tom Wiltshire]

> On 19 May 2018, at 11:31, Adam Inglis <21pointy at tpg.com.au> wrote:
> 
> 
>> On 19 May 2018, at 8:08 PM, Tom Wiltshire <tom at electricdruid.net <mailto:tom at electricdruid.net>> wrote:
>> 
>> Does anything like this exist any longer? It seems to me that the unusual DAC response is big part of the “thump” of these early machines, since it has considerable distortion, but manages to spread it to areas where it’s perceptually less noticeable (so higher level signals have less detail than lower level signals).
> 
> 
> “Less detail”? Can you elaborate a bit on that, Tom?

Ok, in basic terms it works like this:

The first 16 steps of the DAC output (0-15) are mapped directly to the output 0-15.
The next 16 steps (16-31) are mapped to a 0-31 range instead, so you get output values 16-47.
The next 16 steps (32-47) are mapped to a 0-63 range, so you get output values 48-111.

Etc etc..

(The detail of this isn’t exactly correct, and there are some fairly minor differences between u-Law and A-law, but the principle is clear)
Essentially the step size changes depending on the level of the output - that’s the “Companding” element.

I’m thinking that the simplest way to get a similar effect these days is to use a simple 8-bit linear DAC coupled to a exponential VCA like the 2164. The VCAs input comes from a reference voltage, the DACs output feeds the CV input, and VCAs output goes to the usual I-to-V stage. You finish up with an output voltage controllable over a very wide dynamic range, but with only a limited number of steps.

Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================

-------------- next part --------------
   On 19 May 2018, at 11:31, Adam Inglis <[1]21pointy at tpg.com.au> wrote:

   On 19 May 2018, at 8:08 PM, Tom Wiltshire <[2]tom at electricdruid.net>
   wrote:

   Does anything like this exist any longer? It seems to me that the
   unusual DAC response is big part of the "thump" of these early
   machines, since it has considerable distortion, but manages to spread
   it to areas where it's perceptually less noticeable (so higher level
   signals have less detail than lower level signals).

   "Less detail"? Can you elaborate a bit on that, Tom?

   Ok, in basic terms it works like this:

   The first 16 steps of the DAC output (0-15) are mapped directly to the
   output 0-15.

   The next 16 steps (16-31) are mapped to a 0-31 range instead, so you
   get output values 16-47.

   The next 16 steps (32-47) are mapped to a 0-63 range, so you get output
   values 48-111.

   Etc etc..

   (The detail of this isn't exactly correct, and there are some fairly
   minor differences between u-Law and A-law, but the principle is clear)

   Essentially the step size changes depending on the level of the output
   - that's the "Companding" element.

   I'm thinking that the simplest way to get a similar effect these days
   is to use a simple 8-bit linear DAC coupled to a exponential VCA like
   the 2164. The VCAs input comes from a reference voltage, the DACs
   output feeds the CV input, and VCAs output goes to the usual I-to-V
   stage. You finish up with an output voltage controllable over a very
   wide dynamic range, but with only a limited number of steps.

   Tom

   ==================
          Electric Druid
   Synth & Stompbox DIY
   ==================

References

   1. mailto:21pointy at tpg.com.au
   2. mailto:tom at electricdruid.net