Topic: Edirol UA5 distortion *before* 0dB???

[poorlyconditioned]

You may have read my hackery with the UA5, changing opamps and such.

I decided to verify my results before taking it out into the field.  So, I used a "sweep", a sinewave that goes from 20 to 20K over about twenty seconds.  Well, I noticed some harmonic distortion (multiples of the base frequency) with anything above -6dB or so. 

Demonstration: To isolate the problem I focussed on one frequency.  I put a *single 1600Hz sine wave* onto a Creative NJB3 and put line out into *line in on the UA5*.  I used a laptop USB to record from the Edirol UA5 for approx ten seconds.  Then I did an FFT of the recorded wave using the Adobe audition program.

Here is the recorded wave when the input to the UA5 is at approx. -3dB.  Notice the secondary peaks at multiples of 1.6kHz.


Here is the same wave input at -6dB.  The peaks are mostly gone:


The problems seemed to go away below -12dB or so.  FWIW I noticed no such probems running a MD line input near 0dB.

OK, here are the files used:
Input (sinewave): www.cs.uwaterloo.ca/~mannr/UA5/tone1600.flac
Output of UA5 (-3dB): www.cs.uwaterloo.ca/~mannr/UA5/ua5-tone1600-3dB.flac
Output of UA5 (-6dB): www.cs.uwaterloo.ca/~mannr/UA5/ua5-tone1600-6dB.flac
Output of UA5 (-12dB): www.cs.uwaterloo.ca/~mannr/UA5/ua5-tone1600-12dB.flac
Output of UA5 (-18dB): www.cs.uwaterloo.ca/~mannr/UA5/ua5-tone1600-18dB.flac

Any guidance welcome!  Note that I used line input, so this should have nothing to do with my mods.  (And I tried it with and without mods too, because that is what I thought originally.)

  Richard

[keith]

Richard,

You refer to various dB levels, i.e. you say that the plot you show is with an input of "-3dB".  What is that with reference to, are you talking about dBu, dBV, or something else?  Since a decibel is a relative measurement it is always with reference to some level.

It definitely looks like the Edirol is beginning to clip - I wonder if the front end is designed with a different max level than your MD recorder?  What do the specs on the 2 devices claim for max input levels?

-Keith

[poorlyconditioned]

Richard,

You refer to various dB levels, i.e. you say that the plot you show is with an input of "-3dB".  What is that with reference to, are you talking about dBu, dBV, or something else?  Since a decibel is a relative measurement it is always with reference to some level.

It definitely looks like the Edirol is beginning to clip - I wonder if the front end is designed with a different max level than your MD recorder?  What do the specs on the 2 devices claim for max input levels?

-Keith

When I say 0dB and so on, it is with respect to the UA5 *digital output*.  So, to get -3dB, I just a moderate signal into line in (from a NJB3 line out BTW), and turn the trim pot on the back of the UA5 until the signal coming out is at -3dB.  That is all.  So, I'm not saying anything about actual analog levels here.

Note: I get problems with both mic and line in, so it must be something at, or near the ADC...

  Richard

[poorlyconditioned]

Richard,

You refer to various dB levels, i.e. you say that the plot you show is with an input of "-3dB".  What is that with reference to, are you talking about dBu, dBV, or something else?  Since a decibel is a relative measurement it is always with reference to some level.

It definitely looks like the Edirol is beginning to clip - I wonder if the front end is designed with a different max level than your MD recorder?  What do the specs on the 2 devices claim for max input levels?

-Keith

When I say 0dB and so on, it is with respect to the UA5 *digital output*.  So, to get -3dB, I just a moderate signal into line in (from a NJB3 line out BTW), and turn the trim pot on the back of the UA5 until the signal coming out is at -3dB.  That is all.  So, I'm not saying anything about actual analog levels here.

Note: I get problems with both mic and line in, so it must be something at, or near the ADC...

  Richard

OK, I'm going to reply to my own post here...

After talking with some people who know more than me, the conclusion is, that for cheap ADC at least, you shoud *not* run near 0dB.  Common sense would say get more signal and less noise, but it appears this ADC works best at say -6dB.  I'm not sure how to do this at shows though.  The CLIP light comes on at -3dB.  I guess you should get the clip light and then back off.

Just to reiterate, I observed secondary peaks at -40dB when the input signal was at full scale.  This is 1% distortion!  So, even if you run at -6 or even -12 and boost in post, you're going to do better than that.  Now that is distortion at the peaks only, but distortion none the less.

Now I don't know how other ADC's compare, but I do know that my MD has no such distortion at full scale!  The amps might not be as good, but the ADC seems alright.  I'm going to try my NJB3 line input as well.

I guess the moto is: trust your ears, but also run some tests!

  Richard

[Brian Skalinder]

Just to reiterate, I observed secondary peaks at -40dB when the input signal was at full scale.  This is 1% distortion!  So, even if you run at -6 or even -12 and boost in post, you're going to do better than that.  Now that is distortion at the peaks only, but distortion none the less.

I'm not very technical, so I'll ask:  might this be related to your associated hackery and not necessarily the stock design and ADC?

[poorlyconditioned]

Just to reiterate, I observed secondary peaks at -40dB when the input signal was at full scale.  This is 1% distortion!  So, even if you run at -6 or even -12 and boost in post, you're going to do better than that.  Now that is distortion at the peaks only, but distortion none the less.

I'm not very technical, so I'll ask:  might this be related to your associated hackery and not necessarily the stock design and ADC?

I tested with a stock unit too.  Same problem.  Same problem reported with "cheap" soundcards too.  Don't drive near 0dB is the message I guess.  I don't know whether pro gear (eg., Grace V3, etc) is subject to this or not.

  Richard

[SparkE!]

Still, this almost has to be a problem with the analog feed to the A/D.  The A/D itself should be very linear.  Otherwise they would not be able to claim the THD specs that they quote for a full scale signal.  I've been wondering about this myself.  I've noticed that the gain settings on the UA-5 tend to be pretty forgiving, like there is some soft clipping going on when you get close to the rails.  Soft clipping would give you the type of distortion you see in the 0 dB spectrum.

[poorlyconditioned]

Still, this almost has to be a problem with the analog feed to the A/D.  The A/D itself should be very linear.  Otherwise they would not be able to claim the THD specs that they quote for a full scale signal.  I've been wondering about this myself.  I've noticed that the gain settings on the UA-5 tend to be pretty forgiving, like there is some soft clipping going on when you get close to the rails.  Soft clipping would give you the type of distortion you see in the 0 dB spectrum.

I think it is "soft clipping" of some kind too.  I don't know why, bit it seems OK up to about -3dB.  So, my advice is to *never* let the clip light come on even though it looks like you're 3dB down in the recording.

It seems to me like some kind of bug in the design.  An ideal ADC would go right up to 0dB.  And in fact, I've got some docs on the chip (the "evaluation board" for the chip) that show a plot at -0.2dB!  But I think it is a bug perhaps in the Edirol implementation, either the setup of the ADC or the Cirrus resampling chip after it.

  Richard

[aberg]

Richard, that's great info to know, since I think most people were under the assumption that seeing occasional clip indications was the ideal setting.

[it-goes-to-eleven]

I'll try and do some tests of this with gear I have.  Of course I often use usb>ua5 for doing these tests.

Baudline is a Great free tool for doing noise analysis but it is linux only.

[SparkE!]

Richard, what type of op amps did you substitute into your box?

[poorlyconditioned]

I'll try and do some tests of this with gear I have.  Of course I often use usb>ua5 for doing these tests.

Baudline is a Great free tool for doing noise analysis but it is linux only.

Thanks, please keep us posted!

Can you give a brief description and URL to "baudline"?  Or is that "bodeline"?  Anyway, I do run Linux on my workstation computers.  I only use Windows for audio stuff.

  Richard

[it-goes-to-eleven]

Absolutely love it.

http://www.baudline.com/

[poorlyconditioned]

Richard, what type of op amps did you substitute into your box?

I finally decided on a BurrBrown OPA2228 for the first two (mic pres), and then OPA2134 for the next two.  I forget the numbers on the circuit board but the first two are right beside the XLR connectors.  My guess is that those are the most significant.  I chose OPA2228 because they are optimized for gain of >=5, which is good.

I'm kind of bummed out on the ADC though.  While the OPA's may reduce noise a bit, I really think the distortion is a much more significant issue.

  Richard

[SparkE!]

Richard, I think that the problem you are having is that neither of those op amps have rail-to-rail outputs.  The OP2134's output will swing closer to the negative supply than it will to the positive supply, so when you run too hot, you'll see more even harmonic distortion than odd harmonic distortion in the lower order harmonics.  I see evidence of that in your plots.  You may want to find an op amp that has rail-to-rail output capability and low noise.  I think that the op amps used in the stock UA-5 design also have this issue.

FYI, the 2134 will get within 2.2 V of the negative supply and to within 2.5 V of the positive supply when driving a 600 ohm load.  It will get to within .5 V of the negative supply and to within 1.2 V of the positive supply when driving a 10k ohm load.  I don't know what impedance is seen looking into the A/D circuit, but I suspect that it's probably closer to the 10k ohm scenario.