elskardio

PCM2906C usb codec - high shelf in background noise
« on: March 03, 2021, 09:01:46 AM »
Hi guys,

I'm working on a small channel strip with a built-in USB output.
I'm using the PCM2906C for the codec and after a few revisions and tweaks, I'm pretty happy with the result.
The noise floor sits at -79dBFS in DAWs.

But there's one thing I would like to improve... if it's possible. I know this 16bit chip has its limitations.
When I look at the noise floor with a frequency analyzer (please see the attached picture), there's clearly a high shelf peaking at 18.5KHz.



The codec is powered with an external regulator and I'm using more than enough coupling capacitors.

What could cause this high shelf?
Any hint would be appreciated.

Cheers



abbey road d enfer

Re: PCM2906C usb codec - high shelf in background noise
« Reply #1 on: March 03, 2021, 09:45:16 AM »
Basic questions: is it the same with the ADC input shorted?
Are you using the recommanded regulator for Vccci?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Rochey

Re: PCM2906C usb codec - high shelf in background noise
« Reply #2 on: March 03, 2021, 10:09:29 AM »
check your layout and grounding, ensuring that your digital traces aren't coming close to your analog inputs. Also ensure that you have adequate decoupling on your power rails and especially your VRef pins too. noise on VRef pins couples directly to the outputs.

But also - real world, at those high frequencies, I don't think you'd notice the noise at -80dBFS. We have enough difficulty hearing full scale signals up there as it is.

Cheers

/R
Expat Audio Home: http://www.expataudio.com

elskardio

Re: PCM2906C usb codec - high shelf in background noise
« Reply #3 on: March 03, 2021, 12:14:53 PM »
Basic questions: is it the same with the ADC input shorted?
Yes

Are you using the recommanded regulator for Vccci?
Yes I'm using a REG103A feeding 3.7V to the VCCCI (bypassing the internal regulator)

------------------------------------------------

It look like it has something to do with the IC's internal filters... switching the clock to 48 KHz removes the peak at 18.5KHz.



So i guess the codec offers better performances at 48KHz. But by default the clock is loaded at 44.1 KHz. That means the user has to change it manually  :(

Re: PCM2906C usb codec - high shelf in background noise
« Reply #4 on: March 04, 2021, 08:45:10 AM »
I'm not sure the value of being limited to 16 bit audio at this stage of the game ,
Of course the PCM2906c does offer a simple one chip solution which is neat and tidy .
I found a small cheap A/D board capable of 192khz , it doesnt have USB but it can connect via i2s to other Xmos based boards with USB connectivity.

https://www.audiophonics.fr/en/devices-hifi-audio-adc/stereo-adc-board-wm8782-i2s-24bit-192khz-p-14897.html

Heres an Xmos board with USB which does 2 channels of  i2s in and out and comes with an Asio compatible driver from 'Thesyscon' .

https://www.diyinhk.com/shop/audio-kits/170-xmos-32bit-384khz-dxd-dsd256-usb-tofrom-i2sdsdreclock-spdif-pcb.html

elskardio

Re: PCM2906C usb codec - high shelf in background noise
« Reply #5 on: March 04, 2021, 11:20:39 AM »
I found a small cheap A/D board capable of 192khz , it doesnt have USB but it can connect via i2s to other Xmos based boards with USB connectivity.
Heres an Xmos board with USB which does 2 channels of  i2s in and out and comes with an Asio compatible driver from 'Thesyscon' .

Yes I already tested the USB to I2S solution... but that means you need the USB to I2S converter + I2S DAC + I2S ADC and that brings you close to 100$ USD minimum.
For a diy, one unit only project, it could work. But the moment you need to build multiple units and produce a PCB for it, its close to impossible.

Yes 16 bit is kind of outdated in 2021... but the easy integration of the PCM2906 IC + no drivers needed on PC and MAC is hard to beat.

If only TI could release a new chip to support 24bit/96Khz  :D

Re: PCM2906C usb codec - high shelf in background noise
« Reply #6 on: March 04, 2021, 05:45:26 PM »
On one hand no drivers is hard to beat for convienience and fool proof instalation ,
it also means no asio support which would be a prerequisite for low latency from your daw.
There are a number of third party asio drivers which might help you get the best out of what you have  ,Asio4all being the more popular , worth a try anyway as most other options have bad latency , of course latency might be of no consequence in your particular application . REW measurement software also appears to work best under asio drivers .


JohnRoberts

Re: PCM2906C usb codec - high shelf in background noise
« Reply #7 on: March 13, 2021, 09:43:49 AM »
Oversampling convertors can have non-flat noise floors. Is the noise within spec for the convertor or high?

JR
Cancel the "cancel culture", do not participate in mob hatred.


 

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