Strengths and shortcomings of a FIFO and Phase noise measurements

Hi all,

This topic – i.e. discussing strengths and (possible) shortcomings of phase noise measurements was initiated in the diyaudio forums, however, one of the other diyaudio members suggested posting the relevant posts here in the TWA forums so as to maybe get a comment from Andrea – which we reckon may have given this some thought. Below I have inserted the three relevant posts from diyaudio in sequence:  … Please feel welcome to comment, anyone!

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Post #437 from The Well synchronized asynchronous FIFO ** thread:

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Sources have always had major impact to me aswell, always.

When I get I my hands on the DAC lite+FIFO one of the most important tests will be:
PC > XMOS USB to toslink > toslink to i2s > FIFo lite
vs
SD player > FIFO lite

 

Andrea thinks his FIFO should be practically* independent from source (*if FIFO lite was perfect there would be no need for the more elaborate ‘top’ version of FIFO he mentioned is in the works).
I was sceptical, still am, but he strengthened his case a whole lot after demonstrating, with PN measurements, some of the issues with Ian’s FIFOs, which would help to explain why people were reporting they could still hear influence from the source with them.
Ian revised his FIFOs and offered some add-ons after that, but without new PN measurements there is no reason to believe any of those original issues were even resolved.

 

At the same time there is no reason believe source influence can be entirely correlated to PN measurements.
PN gives us an even better insight into what’s actually going on but I have to think that they are still another form ‘steady state’ or average-over-time measurement and might not catch some very complex, real time effects and behaviours when jittery signal enters the FIFO, jitter that is likely to be constantly morphing and changing depending on activity in the pc, noise on supply and so on.

 

SD card is an absolute PITA to use in all aspects, if FIFO lite can eliminate the majority of source influence using PC the rest can be forgiven.
I imagine seperating source with a long toslink cable should help make FIFOs job somewhat easier.

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Post #438

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I have suspected even with optical isolation, and galvanically isolated mains (no earthing), that disturbances on mains AC caused by the PC could still make it way through the power transformers in FIFO/DAC and influence it on some level, laptop on battery would obviously address that.
You still have unrelated mains disturbances and for consistent peak performance in FIFO/DAC you probably need batteries there aswell.

I have always noticed major changes in how all systems sound depending on time, day etc.
always hard to tell how much of if it psychological and real but the difference can be pretty extreme sometimes and there is a clear pattern of better sound at very late or very early hours, times when you’d expect activity on national grid to be lowest.
With enough care and filtering in mains supply I would hope the effect could be made insignificant enough not to have deal with hassle of batteries

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Post #439

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@lasercut: Hi   … interesting comments & thoughts you share.

As it is I have actually been wondering why an FIFO would/should have such a positive effect … While I recognise that an FIFO may shift the incoming data stream to another point in time on the output (like e.g. in the TWTMC FIFO, as I understand it) as I see it this may mainly cause the input data to be time-wise uncorrelated with the output data.

However, if there somehow is an “electrical influence/coupling” between the input & the output of the FIFO (or the FIFO for some reason adds its own processing noise structure) then one may – due to this time shift – have a point(s) in time where the input data are noisy (e.g. many data level transitions) and the output data experience less data transitions. I reckon this could be unfeasible due to PCB GND disturbances, increased PSU noise etc. …

On the other hand if there is not an “electrical influence/coupling” between the input & output of the FIFO (or the FIFO for some reason does not add its own processing noise structure) then increasing or decreasing the time distance between the incoming data (the FIFO “buffer”) should not cause any SQ difference. I.e. the different buffer lengths in e.g. the TWTMC FIFO should not lead to different SQ impressions …

Which leads me to my point which is that if the electrical isolation between the incoming clock & data (from the PC/data player) really is efficient (e.g. well-designed optical isolation and re-clocking) then I cannot see that the FIFO would necessarily add isolation ..? It may change the structure of the “input data to output data” electrical coupling but since an FIFO necessarily also in practice contains ICs with a small size (i.e. crosstalk, PCB trace currents influencing eachother, and IC inter-pin crosstalk) then such ICs may also couple signal from input to output – as other ICs do.

With this in mind: My guess would be that what really may matter is to isolate the data input from the data output, phase noise & other noise-wise, – using ICs and PSU structures that make this happen. And in this context an FIFO would be equally vulnerable to such a design not being executed “feasibly” – as is a more “common” isolation and re-clocking design.

PN gives us an even better insight into what’s actually going on but I have to think that they are still another form ‘steady state’ or average-over-time measurement and might not catch some very complex, real time effects and behaviours when jittery signal enters the FIFO, jitter that is likely to be constantly morphing and changing depending on activity in the pc, noise on supply and so on.

I have had these thoughts as well, yet have not remembered them when communicating with Andrea. Would be interesting to have some light shed on this …

Cheers,

Jesper

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Now, it would be interesting to get more insight into the intricacies of phase noise measurement – & maybe also its shortcomings, so again, please feel welcome to comment.

Cheers,

Jesper