Potential DirectStream firmware improvements


#1

Hi @tedsmith

Where do you see potential firmware improvements in the future for the DirectStream?
I can remember you writing something on keeping the signal out of the noisy FPGA memory. Are there other improvement areas worthwhile to investigate?

Thanks


#2

One path is to have the sigma delta modulator look further into the future so that it can generate bit streams that more closely track the input signal. Similarly doing multiple SDM’s in parallel and picking the “best” bit stream (with the stream we have so far.) These can both be done at the same time, but that’s a lot of resources.

I can do approx. 5 x as much SDM work with no additional resources and the last few releases have freed up more resources in the FPGA that could be used for the above.

There’s another kind of improvement where we generate a bitstream and then try to incrementally make it better: a very simple example is to flip random bits and see if that happens to work better (similar to dither), a more complicated example is to look at errors in the output and to modify the input a little to lower those errors (this can be similar to the above look ahead, but it is more targeted.)

As always, I’m revisiting code to redistribute the resources in space and time to lower the FPGA’s self generated noise and jitter.

I have a few ideas that from the above that I’ve started to develop that I need to finish and either integrate or abandon.

One class of things that can improve the sound quality are refining previous releases again, perhaps pushing things a little farther (more bits, faster clock, moving filter parameters a little further out…) or perhaps using results of new releases to revisit older releases, say with more resources or better tuning of any parameters.

Snowmass was the result of doing work towards the TSS software, as the TSS software evolves I’ll back port improvements that I implement for the TSS into the DS and DS Jr where their hardware permits.

As always, without infinite time, I have to choose a few of these that I think will get the biggest improvement in sound quality.


#3

Nice job as usual Sir


#4

Ted,What is TSS,please?


#5

Has any DAC manufacturer done the input signal tracking part?
This ‘thinking out of the box’ seems quite extraordinary and promising :star_struck:


#6

TSS = Click


#7

Ted Smith Signature (DAC). A working name, or at least what it’s become know in these forums.


#8

There are research papers about things like some of the ideas above. (If you are an AES member searching for “sigma delta” will net a lot of papers: it would be cheaper to join the AES than to pay for very many :slight_smile: ) It’s been my experience that many of these things don’t perform well in practice with real signals, but there’s a lot to research. Few other DACs do sigma delta modulation in software so I doubt that many are doing these things. There’s no reason that software on a PC can’t do them, but most implementations simply use a black box/toolkit approach instead of writing things from first principles.


#9

One of my favorite hack software based SDMs is to “simply” search thru all possible bit streams for the one which (when low pass filtered) most closely matches the input. As stated this is clearly infeasible, but by using path pruning techniques and other software optimizations, it does work. It has all of the foibles that the standard SDM implementations have but it was a good learning experience.


#10

Ted, there is something different about your picture? Or have I not paid attention. Are you holding a gnome, sweet. While others have elephants on their speakers, I have a Christmas elf. Care to share a story?

Chas


#11

The gnome was a Christmas gift from my grandkids. I like that they have a sense of humor:


#12

I don’t get it.

:wink:


#13

The point being, he is a wonderful grandfather.


#14

I guess I need to hunt around for a “polite sarcasm” emoji or symbol or something…

:slight_smile:


#15

No emoji needed, your quip was clear.


#16

MY apologies Scotte1, I misunderstood in my haste to praise Ted. The picture he sent, touched me and cracked me up, at the same time.

I could use a numb emoji, or maybe even one, telling me to pay attention.

My Best to you always,

Chas


#17

No apology necessary, but I appreciate it. As my wife often says, “you think your pretty funny.”

Cheers.