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-   -   Soundcard clipping when volume control set to full (http://www.diyaudio.com/forums/digital-line-level/175134-soundcard-clipping-when-volume-control-set-full.html)

billybudapest 10th October 2010 08:10 PM

Soundcard clipping when volume control set to full
 
I have a focusrite saffire that I'm finally getting around to examine properly, and there is an issue with clipping when the volume knob is set to full output.

The purpose of this thread is to try to figure out why it distorts, if this means that the performance is reduced significantly, and if there is anything I can do to correct it.

Here are some of the details:

The dac chip is a CS42428 codec:
http://www.cirrus.com/en/pubs/proDat...CS42428_F1.pdf

I'm providing these numbers from memory and they are likely off a little bit, but this should give an idea of when the problem happens.
The volume knob potentiometer is fed (I believe it was) ~3.2V and it attenuates it. After about 2.8V it begins to clip heavily.

The output after the pot goes to the power board and I couldn't follow it, and I tried to measure the change outside the dac but couldn't find it.

If anyone can help me try to figure out what it all means and if there is anything I could do about it I would greatly appreciate it. Thanks.

billybudapest 10th October 2010 10:03 PM

In the software control, there is a way to override the hardware knob and force full volume. This too has the same issue, it must be at like 75-80% of the way up before it clips.

Does this mean that the dac doesn't have enough voltage being supplied to it, and it is simply clipping trying to put out the volume? The analog voltage measures around 4.88. The digital voltage measures around 3.2. The requirements for the chip are:
VA: min 4.75, typ 5, max 5.25.
VD: min 3.13, typ 3.3, max 5.25.

Is any of that indicative of the clipping issues?

Globulator 10th October 2010 10:06 PM

Have a look at the waveform in Audacity or similar.
Most modern pop is clipped and this is very noticeable with some DACs

wakibaki 10th October 2010 10:17 PM

Quote:

Originally Posted by billybudapest (Post 2328381)
...if there is anything I could do about it...

Don't set the volume to full.

w

billybudapest 10th October 2010 10:22 PM

Quote:

Originally Posted by wakibaki (Post 2328472)
Don't set the volume to full.

w

:usd:

Well am I missing anything by having to run the volume down so far? Globulator its not the material, I am outputting a sinewave at -1 and just raising the output knob.

wakibaki 10th October 2010 10:58 PM

Quote:

Originally Posted by billybudapest (Post 2328474)
Well am I missing anything by having to run the volume down so far? Globulator its not the material

No, not unless noise is becoming intrusive or actual volume is inadequate for listening. If the actual volume is inadequate then you can put a preamp in the system, again, as long as noise (hiss) does not become intrusive, or use a more powerful power amp.

w

billybudapest 10th October 2010 11:18 PM

Quote:

Originally Posted by wakibaki (Post 2328500)
No, not unless noise is becoming intrusive or actual volume is inadequate for listening. If the actual volume is inadequate then you can put a preamp in the system, again, as long as noise (hiss) does not become intrusive, or use a more powerful power amp.

w

That isn't the issue, there is perfectly adequate output level at any of the levels. The concern is if the attenuation is either being done digitally and I'm losing steps in the material or if the dac is being fed all the steps but because of the level the dac is no longer operating within its optimal conditions and I'm losing some SNR performance.

If the answer is yes to option 2 above then i can live with it. I would rather not have a device which has to truncate data to operate properly.

wakibaki 11th October 2010 12:00 AM

Quote:

Originally Posted by billybudapest (Post 2328517)
The concern is if the attenuation is either being done digitally and I'm losing steps in the material or if the dac is being fed all the steps but because of the level the dac is no longer operating within its optimal conditions and I'm losing some SNR performance.

Neither of these are true. Losing steps in the material would require software modification of the numbers going into the DAC i.e divide by 2, divide by 4 which would result in low numbers dropping out, not clipping. It would result in loss of resolution and SNR, but this is not what is happening, what is happening is clipping. It's not a question of the DAC no longer operating within its optimal conditions and a loss of SNR performance. If there is a loss of SNR it would mean increased audibility of hiss, that is what is meant by Signal to Noise Ratio. It's not something abstract, if you can't hear it, you can't hear it.

The attenuation is being done digitally insofar as it is under software control, but this will be a digital pot (which may be alternately controlled by a wheel) or other form of digital gain control after the DAC. There must be some gain device before the output if you can drive it into clipping.

w

OK I had a look at the DAC datasheet, the volume control is integral to the DAC, it will not be responsible for the effects you describe, who in their right mind would design a chip that acted that way?

billybudapest 11th October 2010 01:45 AM

wakibaki,
Thanks for taking a look at the pdf. Can you briefly explain what you think is going on? I'm sorry if you have already done so with the edit but I can't figure out if you are saying its really is a strange design or if my concern is ill-founded because nobody would implement it such that my concerns would even be warranted. :spin:

wakibaki 11th October 2010 03:14 AM

OK, it's 3 in the morning here, this is taxing my brain and it's going to take a fair bit of writing. I was a bit over-hasty with the remark about the digital pot or gain control. I need to get some sleep but I will write a comprehensive explanation tomorrow after I have had a chance to do some calculations to verify that my understanding is correct. The datasheet is by no means forthcoming about exactly how the volume control is implemented, but on reflection it looks as though there is a loss of resolution and SNR as the samples being presented to the DAC are no longer full-scale. Let me sleep on it and I will get back to you...

w


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