Preamp Volume Control - how does it work?

[sbn]

Hi, I'm just wondering about how the "Preamp Volume Control" attenuation works.

I can see that it only impacts the analog outputs, so my first guess was that it was using some kind of analog volume control, controlled by the slimserver...

But then i read about all these guys noticing a diffent sound via SPDIF, using the -63dB tweak... And that means that the attenuation is being done on the digital side of the DAC, right?

If so, what is the impact on sound quality? I could fear that the dynamic range was decreasing, due to a less amount of usable bits, but maybe that's all wrong?

In short: What's the impacts of the volume control and the preamp attenuation, and how do they work?

[pfarrell]

sbn wrote:
> I can see that it only impacts the analog outputs, so my first guess
> was that it was using some kind of analog volume control, controlled by
> the slimserver...

There is nearly nothing analog in the SqueezeBox.
Its a digital box. Sean and others can correct me, but
I would expect that the SlimServer sends a command to the computer in
the SqueezeBox to turn down the volume. And it either adjusts the
digital signal sent to the DAC, or adjusts a VCA.

There sure isn't anything that looks like a motorized potentiometer.


> But then i read about all these guys noticing a diffent sound via
> SPDIF, using the -63dB tweak...

Not clear if the world famous 63 dB tweak is real or just an urban folklore.

> In short: What's the impacts of the volume control and the preamp
> attenuation, and how do they work?

Would be interesting to know. Or it might be a trade secret of SD.
The firmware and design are not open source.

--
Pat
http://www.pfarrell.com/music/slimse...msoftware.html

[andy_c]

Hi, I'm just wondering about how the "Preamp Volume Control" attenuation works.

I can see that it only impacts the analog outputs, so my first guess was that it was using some kind of analog volume control, controlled by the slimserver...

But then i read about all these guys noticing a diffent sound via SPDIF, using the -63dB tweak... And that means that the attenuation is being done on the digital side of the DAC, right?

If so, what is the impact on sound quality? I could fear that the dynamic range was decreasing, due to a less amount of usable bits, but maybe that's all wrong?

In short: What's the impacts of the volume control and the preamp attenuation, and how do they work?

The Squeezebox uses the Burr-Brown/TI PCM1748 DAC chip, according to Stereophile. The datasheet of the chip can be found here: http://www-s.ti.com/sc/ds/pcm1748.pdf. If you look at the first page of this datasheet, you'll see that the chip internally implements a variable attenuation scheme, 0 to 63 dB in 0.5 dB steps. It doesn't give details of the implementation though. To set this attenuation, the appropriate digital word is written to the serial control port of the chip.

It's not at all obvious if or how this would affect the S/PDIF output, which is presumably tapped off upstream. I'll refrain from further editorial comment :-).

[sbn]

If you look at the first page of this datasheet, you'll see that the chip internally implements a variable attenuation scheme, 0 to 63 dB in 0.5 dB steps. It doesn't give details of the implementation though. To set this attenuation, the appropriate digital word is written to the serial control port of the chip.

Okay, so the attenuation might be a feature of the DAC, adjusting some kind of master voltage in the DAC, affecting the output?

[andy_c]

Okay, so the attenuation might be a feature of the DAC, adjusting some kind of master voltage in the DAC, affecting the output?

Yes, it's internal to the DAC chip itself. There are at least two ways it could be done. One way might be to scale the digital data prior to conversion to analog inside the chip. Another possible way is in the analog domain. Digitally-controlled analog volume controls, such as the Burr-Brown/TI PGA2310 chip, have a switched resistor network used as the feedback network of an op-amp. By switching different resistor networks in and out, using FETs as switches, the analog gain of the op-amp after D/A conversion could be varied. The choice between these techniques is probably done on the basis of cost.

The DAC chip is almost the last thing before the analog output as far as signal processing is concerned. There may be some additional analog filters after the DAC, but that's about it. The digital signal from which S/PDIF is obtained is picked off before it ever gets to the DAC.

[occam]

http://forums.slimdevices.com/showpo...84&postcount=3
per Sean Adams -

Right now we are doing the volume in the CPU, for both s/pdif and DAC outputs. The DAC also has volume control capability but we're not using it at the moment...

[sbn]

http://forums.slimdevices.com/showpo...84&postcount=3
per Sean Adams -

Thanks but i already knew that (according to the link I posted above)

The thing I'm asking about is the Preamp volume attenuation feature, and I hope that some of the slimdevices guys will come by for a look at this thread...

[andy_c]

http://forums.slimdevices.com/showpo...84&postcount=3
per Sean Adams -

That post is from last year before they added the capability to use the DAC's volume control feature. The post you referenced relates to the software option:

Player Settings, Audio, Digital Volume Control

The subject of this and some other threads here is the software option:

Player Settings, Audio, Preamp Volume Control.

[occam]

Andy - Thanks for the clarification.

[seanadams]

Thanks but i already knew that (according to the link I posted above)

The thing I'm asking about is the Preamp volume attenuation feature, and I hope that some of the slimdevices guys will come by for a look at this thread...

the "preamp control" uses the digital volume control feature in the DAC. It is separate from the main volume control, which we do in software. See several recent threads for reasons why you should not use large levels of digital attenuation to compensate for mismatched analog levels.