DSD Decoder Project

What people are working on at the moment
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Ray P
No idea why I do this anymore
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Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

#1 DSD Decoder Project

Post by Ray P »

After my previous experiments with decoding DSD with a simple low pass filter arrangement (no DAC chip required!) I am looking at more sophisticated decoding solutions using FIR filters as featured in the Signalyst DSC-1 decoder. The Signalyst design is an open-source hardware project and full schematics and even PCB layouts are available;

http://www.signalyst.com/hardware.html

Signalyst is responsible for the highly regarded HQPlayer software.

I've got two DSD decoder projects on the go currently, for both of which I will be using a Beaglebone Black running the Botic Linux distro, outputting DSD (or PCM) to an isolator/reclocker - this should be good for DSD512 (circa 25MHz)

The first DSD project is using an ElectrArt DSD Principle board, which I picked up used over on DIY Audio. The ElectrArt board uses 8tap FIR filters in differential mode, so four filters for stereo. The filter sections are followed by OpAmp based I/V and balanced to single-ended stages. I'm just tidying up the board I purchased and hope to have it working next weekend. Here's some information about the ElectrArt;

http://fpga.cool.coocan.jp/electrart/FIRTST.pdf

http://www.xtremeplace.com/yabbse/index ... c=190645.0

Despite my nervousness about smd assembly I am also continuing with a 'board-up' build of a differential version of the signalyst decoder based on this, differential 32tap filter variant of the Signalyst decoder;

http://www.ebay.co.uk/itm/272342961175? ... EBIDX%3AIT

The differential approach is intended to interface with output transformers rather than the OpAmp stages of the original, though it could also use a tube-based balanced to SE I/V stage (I'm thinking of using a Broskie Unbalancer).

In the event, after some practice the smd soldering has gone well (if rather tedious, there are 128 smd resistors just for the filters);

Image

Image

I had to locate and fix a solder bridge under a decoupling cap, other than that is looks OK. Just waiting for some parts to arrive before I can see whether it creates smoke too.

Finally a question for those amongst us who understand these things at the circuit level (I mostly don't). I mentioned that the differential Signalyst board has a minimal output stage, output transformers being accommodated on the board but there are a few components between the FIR filters and the transformers that I don't understand the purpose of. Here's the schematic of the output section;

Image

When I was looking at the pictures on the ebay listing I thought R1, R2 an C25 were defining a low-pass filter but the values and the extra R3 appear to discount this. So what is R1, R2, R3 and C25 doing?

Cheers

Ray
Sorry, I couldn't resist!
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