Question

Connect no longer bit-perfect?

Forum|Forum|10 years ago
August 23, 2016
450 replies
29769 views

It looks like the Connect is no longer bit-perfect. Here's my evidence: let's discuss this.

First, I constructed a wav file of pink noise with amplitude ramping up from zero to digital max and back to zero.
I play this through my Connect and record the SPDIF output from the coax output into my PC.
The recording uses a Scarlett 8i6 audio interface set to use the Connect as master clock.
I record into a DAW (Sonar) multiple times - all instances are identical.
However, this recorded signal is not quite the same as the original wav file - it can be up to -21dB different.
See https://www.dropbox.com/s/t8od479xo9hi5el/connect_diff.PNG?dl=1
Note the expanded scale on the difference (third) track.

It looks like the difference gets larger when the signal is larger. To confirm this, I import the
original and difference files into Matlab and plot the raw data (difference vs original). There is clearly audio compression
happening here. See https://www.dropbox.com/s/p1yq6wcqafvnhaj/diff_vs_orig.png?dl=1
The scale is such that digital maximum is 1.

There also appears to be a slight bias when the waveform is negative and the signal is below the
compression threshold. See an expanded version of the previous plot
https://www.dropbox.com/s/9001tl9mkle4wly/diff_vs_orig_zoom.png?dl=1

Happy to answer questions about the method and conclusions.

Cheers, Peter.

p.s. Volume is set to fixed - I haven't tried variable.
In a loopback test (8i6 out from DAW to 8i6 in, no Sonos gear involved), I get bit-perfect cancellation.

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ratty

I think you'd better post your CONNECT's serial number (or if you feel cagey about that, submit a diagnostic and post the number), so Sonos can verify whether yours is one of the later model CONNECTs affected by the issue discussed elsewhere, concerning non-defeatable DSP treatment. (EDIT: In fact Ryan suggested that it was CONNECTs manufactured from 2011 on, with hardware version 1.16.5 or higher.)

As a matter of interest is your machine able to report peak jitter?

Peter McAuthor

Hardware version is 1.16.5.5-2. It was bought in Australia in early 2014.

No, I don't think my gear will measure jitter. I've only recently got my brain around the concept.
I'll do a little digging to see if it's possible with my fairly simple setup.
The Scarlett 8i6 itself has jitter less than 500 picoseconds. In the current tests, however, it is
slaved to the Connect.

"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett

ratty

Okay, so yours is a potentially affected CONNECT.

Your graphs are interesting, and do seem to support the suggestion that there's some kind of peak limiting going on.

I suspect jitter is a red herring here. It was mainly for interest's sake. For reference the original ZP80 measurements by Stereophile put the jitter at 388ps peak-to-peak, which they classed as 'low'.

Peter McAuthor

500 ps is 1/1000 of a cycle of a 20kHz signal, less at lower frequencies. It's hard to imagine we could hear that level of error.

"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett

T

The LHC

So does this mean we could see pre 2011 models start commanding a premium on second-hand sales?

Majik

So does this mean we could see pre 2011 models start commanding a premium on second-hand sales?

Unless Sonos make a firmware change that allows this to be turned off in the future. As I understand it, this is a combination of later Connect models plus firmware changes (v6.0 onwards) so it may be possible to defeat it in firmware (e.g. with a user option).

Cheers,

Keith

Peter McAuthor

Just to clarify a number in my original post - the original and recorded signals can differ by up to about 1dB. It is the difference signal that can have an amplitude of -21dB (compared to full scale).

Also, the offset for the negative waveform (before compression kicks in) in my third plot is just 1 bit. This means every time the signal dips below zero, the amplitude is in error by 1 bit. That's got to be a bug. I can't imagine what that does to the sound, or if it's even audible.

"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett

Stuart_W

World-Class Superstar

Just to clarify a number in my original post - the original and recorded signals can differ by up to about 1dB. It is the difference signal that can have an amplitude of -21dB (compared to full scale).

Also, the offset for the negative waveform (before compression kicks in) in my third plot is just 1 bit. This means every time the signal dips below zero, the amplitude is in error by 1 bit. That's got to be a bug. I can't imagine what that does to the sound, or if it's even audible.

If you can discriminate 1 dB you need dissecting - for scientific purposes only you understand!

Arc + Sub (Gen 1) + 2 x Play 1/ Stereo Play 3s/ Era 100 Stereo in Kitchen, One Stereo in Bedroom, Play 1 stereo pair in Study, Play 1in Bathroom, Sonos Move 2

ratty

As has been remarked elsewhere it's been proven in the past that Dolby or DTS content could be piped bit-perfect through the system. Any bit errors there would likely have corrupted the stream. Something seems to have changed, and at present the finger of suspicion is wavering in the direction of the DSP rework.

K

Kumar

It seems that there IS something going on under the hood, and for those of us that can't understand much of it:
1. What is the audible effect, if any, of this behaviour?
2. What is the way to deal with it where music listening is concerned?
And the same questions for where the Connect is used in Variable mode.

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