[thomase]

I'm trying to understand how line-out on soundcards works. From what I understand the output signal has a voltage range with a lower and upper bound. The DAC takes a 16-bit (or 24-bit input). Does volume adjustment on the line-out happen in the analog or digital domain? If it happens in the digitial domain, do you have to increase the line-out volume to max in order to get the full 16-bit (or 24-bit) dynamic range? If so, is THIS the reason for headphone amps? For example, set the line-out to maximum (max dynamic-range) and let the amp scale this up or down in the analog domain such that you preserve the maximum dynamic range

Any links?

 

[viator122]

I'm a bit confused by your post, but I want to point out that you don't use a DAC with a line out. The DAC provides a line out. If you want to use a DAC with your computer, you'll need to get a soundcard that has digital out (toslink, coax, etc.). Your computer will send the digital signal to your DAC which will convert it to a line-level analogue signal. Then you'd connect the line out from your DAC to your headphone amp.

 

[thomase]

Quote:

Originally Posted by viator122 I'm a bit confused by your post, but I want to point out that you don't use a DAC with a line out. The DAC provides a line out. If you want to use a DAC with your computer, you'll need to get a soundcard that has digital out (toslink, coax, etc.). Your computer will send the digital signal to your DAC which will convert it to a line-level analogue signal. Then you'd connect the line out from your DAC to your headphone amp.

I was referring to the DAC on the soundcard that provides the line-out. I guess what I'm getting at is, assuming your source material uses the full 16/24 bit dynamic range (ie. in the case of 16-bit, samples are as low as 0 and as high as 65535), if you "lower" the volume on the line-out, does this effectively result in discarding information (least-significant bits for example) from the sample data? Perhaps I should just ask staight out, how is volume control on the line-out actually implemented? I'm an EE/CS btw, so you can err on the technical side.

 

[Glassman]

digitally of course.. only gain switching on professional soundcards is implemented in analog (not all the times) via relays.. with today's 24bit DACs with dynamic range up to 120dB, you can somewhat safely attenuate 16bit source by as much as 24dB without loosing performance (well, theoretically)..

 

[Cerebral_Mamba]

You will have to set you software volume levels (on the computer) to max inorder to get the full dynamic range. The amplifier attenuates the volume in its analog form. There may be DACs with volume control, but It most likely will be controlling the volume after the Digital-to-Analog conversion has occured.

Again, reaffirming what viator122 has said, you cannot connect the line out or simply an analog output as input to the Digital-to-Analog converter.

 

[thomase]

Quote:

Originally Posted by Glassman digitally of course.. only gain switching on professional soundcards is implemented in analog (not all the times) via relays.. with today's 24bit DACs with dynamic range up to 120dB, you can somewhat safely attenuate 16bit source by as much as 24dB without loosing performance (well, theoretically)..

The last time I did any analog stuff was freshman year, which was 9 years ago. I don't really remember much of it. For example, I don't know what you mean by "gain switching".

Does volume control change the way the DAC maps discrete sample values to voltage levels? In other words, does the minimum difference between 2 discrete voltage levels change depending on the volume level (bigger with higher volume, smaller with lower volume), or is this minimum difference fixed, forcing you to effectively discard information if you use anything less than the full range of allowed voltage. If it is variable, perhaps this is what you mean by "gain switching"? Actually, I'd imagine it HAS to work this way. The complication is in desiging a DAC that maps samples to voltage levels linearly for a good range of volumes.

I'm still failing to see how a separate analog amp effects sound quality other than potentially making things louder. Maybe the problem is typical soundcard DACs don't have a nice linear output at volume levels that we want? I'm trying to understand the interaction between line-out volume level and amp volume. How can you know what is the best combination of the two? How does a separate application (game, media player, etc.) volume control factor into this?

 

[thomase]

Quote:

Originally Posted by Cerebral_Mamba You will have to set you software volume levels (on the computer) to max inorder to get the full dynamic range. The amplifier attenuates the volume in its analog form. There may be DACs with volume control, but It most likely will be controlling the volume after the Digital-to-Analog conversion has occured. Again, reaffirming what viator122 has said, you cannot connect the line out or simply an analog output as input to the Digital-to-Analog converter.

So, if i connect my headphones directly to the line-out of my soundcard, and the volume at full is too loud, am I better off lowering the line-out volume, or putting some kind of in-line volume control on the output to lower the current going through the headphone drivers?

 

[sonance]

Just a warning, if you plan to use digital out exclusively, and need line in support (I needed it for my TV card) you will have a problem with the Chaintech AV-710. It just flat out does not work in digital mode, and I've seen other posts about this on the net.
If anyone knows how to get this working let me know, but as far as I know this is a limitation of this card.
I had to fall back to my Extigy to get this working.

 

[thomase]

Quote:

Originally Posted by sonance Just a warning, if you plan to use digital out exclusively, and need line in support (I needed it for my TV card) you will have a problem with the Chaintech AV-710. It just flat out does not work in digital mode, and I've seen other posts about this on the net. If anyone knows how to get this working let me know, but as far as I know this is a limitation of this card. I had to fall back to my Extigy to get this working.

Jeez, I guess it was a mistake to use the word DAC in the subject line. Let me repeat that I am not talking about a standalone DAC. I want to understand how the soundcard's DAC works with regards to line-out volume level.

 

[Glassman]

Quote:

Originally Posted by Cerebral_Mamba There may be DACs with volume control, but It most likely will be controlling the volume after the Digital-to-Analog conversion has occured.

false.. it's digital volume control, just like what you do in software..

 

[Glassman]

Quote:

Originally Posted by thomase The last time I did any analog stuff was freshman year, which was 9 years ago. I don't really remember much of it. For example, I don't know what you mean by "gain switching". Does volume control change the way the DAC maps discrete sample values to voltage levels? In other words, does the minimum difference between 2 discrete voltage levels change depending on the volume level (bigger with higher volume, smaller with lower volume), or is this minimum difference fixed, forcing you to effectively discard information if you use anything less than the full range of allowed voltage. If it is variable, perhaps this is what you mean by "gain switching"? Actually, I'd imagine it HAS to work this way. The complication is in desiging a DAC that maps samples to voltage levels linearly for a good range of volumes. I'm still failing to see how a separate analog amp effects sound quality other than potentially making things louder. Maybe the problem is typical soundcard DACs don't have a nice linear output at volume levels that we want? I'm trying to understand the interaction between line-out volume level and amp volume. How can you know what is the best combination of the two? How does a separate application (game, media player, etc.) volume control factor into this?

first of all you should get rid of the idea that DAC works in a way you think it does, it's all different these days and you would be surprised, but the step is in fact only no voltage and full scale voltage, but this switching happening at frequencies of units of MHz.. this is then filtered by analog low pass filters, which yields you the audio signal.. so as you see the theory with discrete steps can't be applied in real life..

by gain switching I mean switching of the actual feedback resistor of the opamp circuit.. this is used on professional gear to accomodate both consumer and pro line levels without sacrificing performance, because otherwise you would have to attenuate in digital to lower the output level from pro to consumer.. E-MU cards for example does that in digital, while CardDeluxe switches the closed loop gain of the output opamps by relay, so in analog..

amplifier does not neccesarilly need to amplify the volume, but it's task is to present low impedance on the output with the ability to supply sheer amount of current when the load asks for.. something that opamps can't really do..

 

[Cerebral_Mamba]

Quote:

Originally Posted by thomase So, if i connect my headphones directly to the line-out of my soundcard, and the volume at full is too loud, am I better off lowering the line-out volume, or putting some kind of in-line volume control on the output to lower the current going through the headphone drivers?

Ideally, it would be beter if you performed the volume control externally and that is what Echo Audio says for all their computer interfacing audio products. But I have not actually been able to make a difference either way. Its just not an audible difference therefor don't bother getting any external volume control.

Quote:

Originally Posted by Glassman false.. it's digital volume control, just like what you do in software..

Well, then the really expensive ones may employ complicated DAP to control volume without sacrificing quality. Anyway, have you experienced any improvement in audio when listening with

< Full Volume on DAC + controlled volume on AMP >
vs
< controlled volume on DAC + Full volume on AMP >

 

[Glassman]

the sound signature is not going to change, but the noisefloor gets rised and low level signals will get below it, thus degrading performance..

 

[thomase]

Ok, I read a little bit on delta sigma converters and I get the general idea. Convert each sample to a sequence of pulses such that the duty cycle corresponds to the desired discrete output level. Filter the high-freqeuncy inaudible noise.

My intuition is still failing me regarding volume control. I looked at the specs for the CS4382 DAC used on the Audigy 2 and X-Fi soundcards. The DAC itself has per-channel volume control. In the block diagram, this volume control seems to be occuring before the delta sigma converter which suggests that the actual sample value is modified in some way first. Ultimately, what I'm getting at is whether or not you effectively throw away information (bits) by attenuating the signal. If so, what does this mean qualitatively? For every -6dB attenuation, does this correspond to one less bit, such that if the attenatuation is -48dB on a 16-bit signal, its as if I was using only 8 bits? If so, that seems bad, and it seems you would be better off loading with a POT if you want to attenuate rather than effectively reduce the bit precision in half, although, people have said they can't tell the difference. Is this related to how a good analog-domain amp can "bring out details" even if you are not necessarily increasing the overall volume or dynamic range?

When you adjust the volume level on your computer with the control panel, does it directly effect the volume control on the CS4382 DAC? What is happening when you adjust volume control in an application such as a game (ie. music and FX settings)? Is this setting combined with the windows setting and mapped to a final volume control setting for the DAC? Or do the samples actually get attenuated (ie. throwing away bits) in software before even sending them to the soundcard?

 

[thomase]

Maybe throwing away bits is "ok" because although the SNR will be lowered, you don't perceive it as being noisier because the dynamic range is similarly lowered. Jeez, I remember back in the day we played around with Matlab in my Intro to EE lab. I wish I remembered more of it...