squarewave

Boss DSD-2 Digital Sampler Delay
« on: September 13, 2018, 06:27:11 PM »
Regarding this Boss DSD-2 Digital Delay pedal schematic:



It seems the DAC (RA1) out goes through a buffer (6), through some analog gates (4abcd) and out. But it also goes down to the comparator (5) which is being used as an ADC? Why is the output mixing with the input?

I have another piece of gear I'm trying to fix (see other thead in The Lab) that uses the same chip (RDD63H101) so I'm just trying to understand how this works.
« Last Edit: September 13, 2018, 08:18:55 PM by squarewave »


abbey road d enfer

Re: Boss DSD-2 Digital Sampler Delay
« Reply #1 on: December 17, 2018, 02:55:47 AM »
Regarding this Boss DSD-2 Digital Delay pedal schematic:



It seems the DAC (RA1) out goes through a buffer (6), through some analog gates (4abcd) and out. But it also goes down to the comparator (5) which is being used as an ADC? Why is the output mixing with the input?

I have another piece of gear I'm trying to fix (see other thead in The Lab) that uses the same chip (RDD63H101) so I'm just trying to understand how this works.
My understanding is that the comparator is just a part of the A/D and D/A conversion. I surmise the output of the DAC is compared to the input signal and provides a count stop to the SAR (successive approximation register). That was the technique prevalent before Sigma/Delta.
« Last Edit: December 17, 2018, 03:22:24 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

JohnRoberts

Re: Boss DSD-2 Digital Sampler Delay
« Reply #2 on: December 17, 2018, 02:28:35 PM »
My understanding is that the comparator is just a part of the A/D and D/A conversion. I surmise the output of the DAC is compared to the input signal and provides a count stop to the SAR (successive approximation register). That was the technique prevalent before Sigma/Delta.
+1 crude A/D with resistor array to make a cheap D/A, google "successive approximation".  In addition to that looks like 2;1 companding (ne570) to improve S/N, and HF pre-de/emphasis to further improve perceived S/N.

Mixing output to input is likely part of effect (recirculation).

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

squarewave

Re: Boss DSD-2 Digital Sampler Delay
« Reply #3 on: December 17, 2018, 03:58:34 PM »
I thought I found out what that was doing but this post is so old I don't remember.

It's not feedback. Feedback is in the upper right.

My best guess at the moment is that it makes the single NJM311D comparator ADC more accurate or less susceptible to clock noise or something. The comparator is comparing the input with the delayed output.

Weird. But that's why I asked. Some of these old pedal schematics have clever bits.

abbey road d enfer

Re: Boss DSD-2 Digital Sampler Delay
« Reply #4 on: December 17, 2018, 05:10:23 PM »
My best guess at the moment is that it makes the single NJM311D comparator ADC more accurate or less susceptible to clock noise or something. The comparator is comparing the input with the delayed output.
  No. I already gave you the explanation. During the A/D conversion, the SAR register scans all the binary values and when the output of the DAC exceeds the actual sampled input signal, the comparator stops the SAR counter. It's not comparing with the delayed signal. That was the standard when ADC chips were expensive.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

squarewave

Re: Boss DSD-2 Digital Sampler Delay
« Reply #5 on: December 17, 2018, 07:46:41 PM »
  No. I already gave you the explanation. During the A/D conversion, the SAR register scans all the binary values and when the output of the DAC exceeds the actual sampled input signal, the comparator stops the SAR counter. It's not comparing with the delayed signal. That was the standard when ADC chips were expensive.
I see. So the the SAH pin turns off the analog switches so that the DAC can be used to sample the input by stepping through and testing each bit. Then it flips the gates open and uses the DAC to output the delayed signal and prime the input for reading the next input sample. So the DAC is doing double duty alternating between outputting the signal and outputting bits to compare with the input.

JohnRoberts

Re: Boss DSD-2 Digital Sampler Delay
« Reply #6 on: December 17, 2018, 08:29:50 PM »
I see. So the the SAH pin turns off the analog switches so that the DAC can be used to sample the input by stepping through and testing each bit. Then it flips the gates open and uses the DAC to output the delayed signal and prime the input for reading the next input sample. So the DAC is doing double duty alternating between outputting the signal and outputting bits to compare with the input.
This (SAR) is pretty old (digital) technology. You drive a D/A starting with the most significant bit, and compare the output of the D/A to input for above/below after each step... The trouble is it takes as many comparison steps as bits, but was the "best way" back when.

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

abbey road d enfer

Re: Boss DSD-2 Digital Sampler Delay
« Reply #7 on: December 18, 2018, 01:00:48 AM »
I see. So the the SAH pin turns off the analog switches so that the DAC can be used to sample the input by stepping through and testing each bit. Then it flips the gates open and uses the DAC to output the delayed signal and prime the input for reading the next input sample. So the DAC is doing double duty alternating between outputting the signal and outputting bits to compare with the input.
Right.  :)
« Last Edit: December 18, 2018, 01:04:29 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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