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Pixel USB-C Audio out measurements (Stepped sine tests added 1/24))

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By bjrmd, Senior Member on 20th October 2017, 02:39 AM
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Important update 11/9/18:
Android Pie seems to have fixed the digital gain defaulting to max. This means that the stock implementation is now optimal and using the gain control app is not needed (unless you want more voltage from a Dragonfly Red).
This was not announced anywhere to my knowledge, but is very pleasant surprise. I found it accidentally while testing the new dongle (above link).
Updated: Edit--I did some further tests and unfortunately the gain optimization only seems to work with line in mode, not with a "forced" headset scenario. So with a headset inserted, the distortion bands are still present.
As of the Nov patch, Pie on the Pixel has fixed both the line out and headset mode distortion banding when using the Google DAC.
When looking at the tracings, assume that stock will look as good as the optimized.





Made a simple spreadsheet with data so far:


The above numbers are using my gain control app, android volume at 25/25 to produce the best possible output signal

Note, the voltage at distortion threshold is the point where harmonic banding becomes >5 dB.
The THD is the harmonic distortion at 250 mV, this would be about 90 dB using full size cans like the Senn 650/800 that have a sensitivity of 103 dB/V.

Will work on uploading the exact images if anyone is interested.

Stepped sine tests at higher output (750mV) - An attempt to see how high impedance, lower sensitivity full size cans will effect distortion.

Edit: I updated the measurements and comments

Edit: Tested Apple USB C Dac

Edit: Tested the HTC dongle

Edit: Tested the Razer Dac

Edit: Tested the Realtec Dac

Edit: Tested the Geekria Dac

Edit: Tested the Audiolab USB device

Edit: Brief test of Dragonfly red. I would consider this to be the gold standard, very low impedance (<.1), minimal crosstalk, high volt out (not on stock), very low distortion.

Edit: There appears to be 3 (at least) "modes" that the Google dongle will do. Depending on the impedance of the headset/line out device attached, the output voltage varies quite a bit.

Low impedance - .36 Volt (high efficiency iem's)
High impedance - .9 Volt (full size headphones)
Very high impedance- 1.8 Volt (line out)

What gets interesting is the effects on testing. The "reference" sites like GSMarena, are testing with a Y cable setup (as I was) and will see high voltage out even with headphones attached since the use case was taken from the line out initially. What made me aware of this was that my app was not reading the Spl correctly, as if the max voltage was about 7 dB down (which it was). Also of note, the reduction in output is not due to a change in the Dac digital gain (the one that adjust from 1 to 175), so the distortion bands are just as bad since they kept that gain at 175.

RMAA:
Used both a Senn 800S and Shure 846 as load, as well as no load.
Note the flat FR on the SE 846 which verifies the low impedance above.





So superb impedance, great voltage out, pretty good crosstalk (with low impedance iems).

But, significant distortion using stock gain settings:





And it has 25 volume steps (each tick is 2.3 dB).
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20th October 2017, 04:10 AM |#2  
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going to subscribe for those that have invested in 3.5mm portable audiophile amps/iems.
20th October 2017, 04:24 AM |#3  
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Nice work. Subscribed!

By the way, very nice headphones. I'm looking forward to testing out LDAC on my MDR-1000Xs when my Pixel arrives.
20th October 2017, 08:10 AM |#4  
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Hi bjmrd,

So I was looking at the Phone arena review in regards to the audio quality through the dongle.

It showed this Click image for larger version

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Not sure if this means the 2XL is weaker than the 2?

What does your test conclude? Would this device provide a nice audio experience for the user ?

This is important to me as I use 3.5mm headphones.

Thanks in advance,

Previous devices are the V20 (superb) and the non DAC G6.
20th October 2017, 01:17 PM |#5  
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I double checked my voltage measurements and they seem correct (compared to a dragonfly red).
The dongle may contain a DAC (like apple lightening). It is possible that with root, ALSA, some of the distortion can be reduced if so. Certainly the voltage is good, impedance great.

Oh and each volume tick is about 2.5 dB.

Addendum - The dongle definitely has a DAC

I have ordered a USB audio pass through cable to see if onboard analog is enabled
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20th October 2017, 08:34 PM |#6  
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Quote:
Originally Posted by bjrmd

I double checked my voltage measurements and they seem correct (compared to a dragonfly red).
The dongle may contain a DAC (like apple lightening). It is possible that with root, ALSA, some of the distortion can be reduced if so. Certainly the voltage is good, impedance great.

Oh and each volume tick is about 2.5 dB.

Addendum - The dongle definitely has a DAC

I have ordered a USB audio pass through cable to see if onboard analog is enabled

bjrmd when you have a minute can you explain some of the numbers and what they might mean to the layperson. Why is high voltage good and low impedance good.
20th October 2017, 09:55 PM |#7  
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Quote:
Originally Posted by jawmail1

bjrmd when you have a minute can you explain some of the numbers and what they might mean to the layperson. Why is high voltage good and low impedance good.

The max voltage is what limits the overall volume, so if you are driving over the ear cans and need power, this is helpful. Not important for sensitive iem's.

Crosstalk--a mixing of the R and L channels- not intentional. Better crosstalk--more stereo separation.

Impedance- internal resistance of the amplifier--if this is too high, causes frequency response changes depending on the type of iem and the iem impedance.
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20th October 2017, 09:57 PM |#8  
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Thanks for the definitions bjrmd!
20th October 2017, 10:06 PM |#9  
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A couple of key points:

The dongle is a USB DAC, so basically audio out as supplied by Google is determined by that cheap little item. You could get a better USB DAC like a dragonfly, but it is bigger.

The distortion is based on some sort of driver parameter issue. I think the reason will end up being that the dongle DAC is set for max digital gain which is not optimal (unless needed). The following test shows why:

I used the Dongle on my Pixel orig and Pixel 2 with the Neutron player usb driver and the signal was clean.
This is a tracing, the same on both devices.



This is a dragonfly red, virtually the same:



So, I think the Google designers did not properly design the software, but the hardware is solid. In the meantime, use the Neutron player USB driver and you will have a very clean signal.
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20th October 2017, 11:32 PM |#10  
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Quote:
Originally Posted by bjrmd

The max voltage is what limits the overall volume, so if you are driving over the ear cans and need power, this is helpful. Not important for sensitive iem's.

Crosstalk--a mixing of the R and L channels- not intentional. Better crosstalk--more stereo separation.

Impedance- internal resistance of the amplifier--if this is too high, causes frequency response changes depending on the type of iem and the iem impedance.

Thanks
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21st October 2017, 02:05 AM |#11  
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Dongle optimization on the old Pixel:

Alsa installed (need root, so can't do on the new Pixel yet).
Dongle card output:
numid=2,iface=MIXER,name='Headphone Playback Switch'
; type=BOOLEAN,access=rw------,values=1
: values=on
numid=3,iface=MIXER,name='Headphone Playback Volume'
; type=INTEGER,access=rw---R--,values=2,min=0,max=175,step=0
: values=18,18
| dBminmax-min=-175.00dB,max=0.00dB
numid=4,iface=MIXER,name='Extension Unit Switch'
; type=BOOLEAN,access=rw------,values=1
: values=off
numid=1,iface=PCM,name='Playback Channel Map'
; type=INTEGER,access=r----R--,values=2,min=0,max=36,step=0
: values=0,0



It seems that the android software defaults the dongle digital gain at max:



This leads to the distortion bands previously seen.

But, if we cut the dongle DAC gain down, and raise the android Volume to net the same(or even a bit higher) voltage:



The bands are gone.

In addition we can get 1 volt out without too much distortion as well:



This is not that difficult to do, but root is needed.
I will try to do some more detailed testing over the weekend, but I wanted to get this up so if someone was on the fence due to audio issues, if you are willing to wait for root and a few mods, this is a reasonable solution.

BTW for 9 bucks the dongle is pretty good with the correct gain settings
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