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View Full Version : Does Squeezebox 1/II/III VOLUME adjust DECIMATE?



milosz
2009-08-08, 10:02
Does the SB VOLUME adjustment work by decimation of the stream being played, or does it actually adjust gain in the analog domain?

Obviously decimation (i.e. "digital volume control") would reduce the audio quality as you lowered the volume.

I can't find any thread that discusses volume control and decimation.

Thanks

qball
2009-08-08, 10:49
volume control is in software if I understand correctly.
PRE-AMP setting in squeezebox uses the volume control on the DAC (I thought I read somewhere). This often also is in the digital domain.

Mnyb
2009-08-08, 10:51
It is digital but it is working with 24 bit resolution, whether this is good or bad depends on the situation.
witch correct gain staging in the systems ie amps with a suiting sensitivity or with analog atentuators with a fixed damping you really are operating at the upper end of the bits.
And btw the analog parts of pedestrian SB is quite "normal" not so higendish so with this in mind within it's price bracket a digital volume control will have the best performance.

have the SB3's modest SN ratio in mind 24 bit is 144dB 16 bit is 96dB i *think* the SB3 has slightly worse than 96dB sn ratio ( i can not find spec rigth now ).

I'm also convinced that digital volume controls can be made *better* than any analog counter part .

dsdreamer
2009-08-08, 11:47
Does the SB VOLUME adjustment work by decimation of the stream being played, or does it actually adjust gain in the analog domain?

Obviously decimation (i.e. "digital volume control") would reduce the audio quality as you lowered the volume.

I can't find any thread that discusses volume control and decimation.

Thanks

I think you are somewhat misusing the word "decimation" at least in the usual signal processing sense of the word. http://en.wikipedia.org/wiki/Decimation_(signal_processing) However, we are understanding your post to mean loss of bit precision rather than loss of sample rate.

There is no alternative to loss of bit precision when doing volume control in the digital domain with fixed precision DACs.

I would argue that it doesn't matter as long as don't turn the digital gain way down, and then compensate by turning the following analog gain up high! My rule is that the volume of the entire chain should only be a shade higher than is comfortable to listen to when the digital volume is set to 100%. Then it is safe, to pull the volume down to an ideal listening level with the digital volume control and leave the analog volume control well alone.

In slightly more technical language, I would argue that the quantization noise floor of the digital part of the signal chain will remain well below the thermal noise floor of your playback system as long as you don't crank the following amplifier gain to silly levels. This is, in part, what Mnyb is saying, if I understand his post well.

Cheers.

milosz
2009-08-08, 13:09
I didn't know it was 24 bit at that point, that makes some difference certainly. Using a fixed attenuator one could set up the amp so that max vol on the SB is max listening level, sure.

My idea is a minimal system just an SB and a power amp. Well, speakers too! ;-)

And, right, it's not decimation; is the correct term LSB truncation?

dsdreamer
2009-08-08, 14:55
I didn't know it was 24 bit at that point, that makes some difference certainly. Using a fixed attenuator one could set up the amp so that max vol on the SB is max listening level, sure.

My idea is a minimal system just an SB and a power amp. Well, speakers too! ;-)

And, right, it's not decimation; is the correct term LSB truncation?

Truncation of LSBs would be a perfectly fine way to express it.

For safety you want to be sure that an accidental reversion to 100% digital volume will not damage your speakers. A passive TVC (transformer volume control) might be worth considering, depending on your definition of minimalist.

A resistive divider can be good if you need a lot of attenuation, because you can then get a good ratio of input impedance to output impedance. You would like your passive volume control to be easy to drive from the Squeezebox side, but still have low enough source impedance to drive the power amplifier without impairing the frequency response. Much depends on the input impedance and sensitivity of your power amplifiers.

Mnyb
2009-08-08, 16:01
Yea you understood my post right (English is not my first language I love Firefox spell check), btw rothwell does some nice inline rca attenuators, or you could do your own for pennies.


Our nice wiki have some articles of interest:

http://wiki.slimdevices.com/index.php/Connect_To_PowerAmp

http://wiki.slimdevices.com/index.php/DIYPassiveAttenuation

If the power amp has enough room the resistors could easily fits inside saving you the trouble to build a box