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Signalyst DSC1
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Old 6th April 2015, 08:18 AM   #71
gentlevoice is offline gentlevoice  Denmark
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Signalyst DSC1
Hi abraxalito - thanks for your input ... Without knowing I had the impression that it performed some kind of filter function. Interesting also that Bruno uses this in his latest DAC - IMHO very often his approaches are indeed both unusual and inspiring (not least with Grimm audio).

BTW Jussi made some measurements of the DSC1 that he posted an computeraudiophile:

Preliminary measurement results for the DSC1 DAC - Blogs - Computer Audiophile

It's interesting to see that the DAC has some HF noise - wonder if this may be related to inter-output timing differences in the '595 shift registers? I've noticed that soekris R2R DAC (also?) has some HF spikes (although at lower frequencies) that is suggested could be such timing differences ... Or maybe it could be inductive/(capacitive) crosstalk between the tracks/components which look as if they are quite closely placed ...

Quote:
I suspect that Jussi's circuit will suffer in terms of dynamics - getting a clean enough power supply is going to be a challenge as a shift register DAC will on average have about 6dB PSRR
I am not familiar with this but Jussi's DSC doesn't load the shift registers very much (15k) so maybe it is of less influence here ..??

Cheers,

Jesper
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Old 6th April 2015, 09:02 AM   #72
abraxalito is offline abraxalito  United Kingdom
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Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Very difficult to estimate the audio band noise floor given only an FFT with unknown number of bins. Typically though an FFT has 64k bins so is subject to an FFT gain of at least 30dB. Since the 'grass' in those plots is above the -120dB mark this tends to indicate a lower than 90dB SNR - not at all impressive.
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Old 8th April 2015, 08:57 AM   #73
Signalyst is offline Signalyst  Finland
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Quote:
Originally Posted by abraxalito View Post
Very difficult to estimate the audio band noise floor given only an FFT with unknown number of bins. Typically though an FFT has 64k bins so is subject to an FFT gain of at least 30dB. Since the 'grass' in those plots is above the -120dB mark this tends to indicate a lower than 90dB SNR - not at all impressive.
My analyzer is not very good for estimating SNR due to limited 16-bit resolution of the high speed ADC...
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Old 8th April 2015, 09:04 AM   #74
Signalyst is offline Signalyst  Finland
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Quote:
Originally Posted by gentlevoice View Post
It's interesting to see that the DAC has some HF noise - wonder if this may be related to inter-output timing differences in the '595 shift registers? I've noticed that soekris R2R DAC (also?) has some HF spikes (although at lower frequencies) that is suggested could be such timing differences ... Or maybe it could be inductive/(capacitive) crosstalk between the tracks/components which look as if they are quite closely placed ...
Yes, it is some capacitive switching glitch leakage, but luckily I know how to improve on this front in future versions. You need to take into account that this is first version of the design...

This is problem with lot of commercial DACs too, for example if you look at iFi iDSD Micro:
iFi iDSD Micro measurements - Blogs - Computer Audiophile

One reason is that many tend to use just analyzers like AP that have limited bandwidth and thus don't see how things look like outside the 100 or 200 kHz analysis bandwidth, which is essential for designing a DAC.
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Old 8th April 2015, 09:08 AM   #75
Signalyst is offline Signalyst  Finland
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Quote:
Originally Posted by gentlevoice View Post
I somehow wonder why Jussi didn't just use a flip-flop (e.g. potatosemi) and a suitable filter after the flip-flop ...
The shift register I used has a separate output latch, so I saved bunch of components for not duplicating the latch. I tried to keep the circuit as simple as possible while at the same time having sensible demonstration on technical level how I think a DSD DAC should be designed.
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Old 8th April 2015, 09:29 AM   #76
Signalyst is offline Signalyst  Finland
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Quote:
Originally Posted by gentlevoice View Post
Any of you can just briefly explain what happens? It seems to me that there's some kind of averaging done (?) when then 1 bit signal passes through the registers ... Or may it be something else?
Yes...

Shift register forms a "scrambled thermometer code" that is then directly converted to analog through a latch. It averages 32 samples of data, so the output gets value between 0 and 32 and thus there are 16 possible negative output levels, a zero level and 16 possible positive output levels. (converted output value is log2(33)=5.044 ENOB for Nyquist sampling)

It is also an analog FIR filter with benefit of having optimal step (transient) response without any ringing and also reduced jitter sensitivity. This allows both cutting down the the noise shaping noise at higher frequencies and relaxes requirements for the following analog filter stages.

With careful design of the following analog filter stage, the transient response is possible to get very good:
Squarewaves from DACs - Blogs - Computer Audiophile
While noise leakage at higher DSD rates becomes very small.
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Old 25th April 2015, 12:39 AM   #77
bibo01 is offline bibo01  Italy
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Please, would anyone give me an idea of the cost of making a DSC1 board (pcb print out) and of the parts.
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Old 26th April 2015, 10:24 AM   #78
leon77 is offline leon77  Italy
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Hi bibo, you can looking for here

https://oshpark.com/shared_projects/Dcm5mq1P

https://oshpark.com/shared_projects/Zx4dezQf
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Old 28th April 2015, 02:26 PM   #79
zaum is offline zaum
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If somebody is interested i have 4 PCB's (DSC + PSU) for sale. Send me a mail if interested.
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Old 1st May 2015, 03:20 AM   #80
mcluxun is offline mcluxun  United States
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I tried to repeat Jussi's squarewave test with files generated by audicity
@2822400Hz
7k:
http://i.imgur.com/pFKuSoK.jpg
10k:
http://i.imgur.com/jtaYzid.jpg
bonus pic: 7k square wave at 44100Hz
http://i.imgur.com/HZptHGV.jpg
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