Tuesday, April 5, 2016

Sonos Digital Clipping

One of the more important discoveries I've made recently is that the Sonos Connect in the living room can clip it's digital output when the Sonos zone volume control is set to maximum.  The highest volume level that be set without any danger of digital clipping is about -10dB (10dB down from maximum).  When I say clipping I mean the top portion of waveforms is being chopped off, and it can sound very badly distorted.

This is a huge bummer in my opinion, and I believe it is a change from the way Sonos was when I first started using it in 2005.  Back then I recall that when you set the volume control to maximum, you got "0dB" gain, so that recorded music which just barely hits the top digital level, also known as 0dB, came through unaltered.  Back then I also determined that the volume control operated in 24 bit mode, so that you could reduce the volume level as much as 48 dB (8 bits) without losing information.  Now you have to be careful on the high side so that your music doesn't get clipped.

The bummer is even huger because most Sonos interfaces, such as on my Mac and on my iPhone, don't give any repeatable way of setting the level control.  There are no markings and no numbers, it's a fully stupified interface in which you can slide the bar up and down, but with no clear indication of where you are unless your eye is sharp enough to see pixels, and you can't even be sure you can trust pixels.  I really hate stupified interfaces like this.  You know the digital system knows exactly where the level is, but it isn't telling you, just giving you a crude indication like the crudest of crude old potentiometer controls.

Only the original but now sadly discontinued Sonos Controller CR100 had markings on the volume control display.  There were 5 major markings including top and bottom, and between each one there were 6 notches.  I believe this corresponded to 12dB between each major marking and 2dB per notch.

Fortunately I still have a few CR100's.  It appears I can completely eliminate the possibility of digital clipping by setting the volume level 5 notches down (as shown in the picture above) for -10dB.  So it appears that sometime in the past 10 years Sonos has added 10dB of "digital gain" which can produce higher levels than in the recording itself.

Digital gain is often useful, especially when you have ad hoc recordings or use line inputs.  I myself use the line inputs of the Sonos system a lot--and it was the reason I chose the Sonos system over competitors in the first place.  I can have a tuner or two in one room where I get good FM reception, and a turntable in another room, and a tape deck in another room, and still use them all in my whole house audio system.  I love being able to route analog signals to different rooms, sometimes several rooms playing the same source, sometimes different sources.

But digital gain isn't a good idea generally for professionally produced recordings.  Especially on popular music, often the recorded music comes close to or even hits the top level, 0dB.  So if you add digital gain to a signal which is already hitting 0dB, the maximum digital level, you are going to get serious clipping.

And the Sonos digital gain isn't just applied to line inputs, it is applied to the commercial recordings, mostly from CD's and some from downloads, on my hard drive.

When researching this issue I did discover that Sonos now observes the Sound Check option in iTunes.  This could apply digital gain adjustment.  I'd never known about Sound Check and had never used it, I checked to make sure I hadn't set the Sound Check option in iTunes preferences, and I hadn't.

Setting the volume level to "fixed" doesn't help either.  If I set the Living Room system to fixed, it still adds the 10dB of digital gain just as if I had the volume control to maximum, and I will still get serious digital clipping on commercial recordings.

This may be true for all of my 8 Sonos zoneplayers, but I haven't actually tested them yet.  I need to connect a device with digital level readout, such as a Behringer DEQ 2496, and then play test tones at -20dB.  If this plays at -20dB or less, there is no digital gain and therefore no danger of digital clipping.  I was able to do this easily in the Living Room because it already has a DEQ in the signal path.  I see I still have a "spare" (not yet set up for it's intended role) DEQ I can use to run these tests on different zoneplayers.

I'm thinking that it's possible I forced the Living Room zoneplayer into a "gain mode" accidentally by pressing on "volume up" too many times.  I seem to vaguely recall getting some warning that I was setting level too high once.  But there appears to be no option or setting which permits me to undo the unwanted digital gain, and searching online hasn't revealed anything yet either.  I should contact Sonos about this, but that's yet another thing to do and I've been very busy.

The living room system is now so finely adjusted that I can actually play at about 0dB or very close to it on many recordings.

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