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windguy2
2005-03-28, 16:59
If I understand this, on the Squeezebox2 the volume is controlled
digitally, therefore dropping bits to lower the volume. Because the
DAC is 24 bits, with 16 bit PCM data the loss of resolution would only
come at low levels after 8 bits are dropped. Would this increase the
noise floor since that is an advantage with the 24 bit DAC?




Sean Adams sadams at slimdevices.com wrote:

> 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.
>
> Since the DAC is 24 bits, no resolution is lost (assuming 16-bit PCM
> source) except at the ultra-low volume levels.
>
> If you have a preamp then it's really ideal - set the preamp to the max
> listening level you'd want to use, and then you can use the volume
> control in Squeezebox.
>
> On Mar 28, 2005, at 1:02 PM, windguy2 wrote:
>
> > How is the analogue volume controlled?
> > Is it lossless using shunt resistors in a chip or is it done
> digitally
> > therefore dropping bits?
> >
> > The reason I ask is that I use a passive preamp driving a power amp -
> > if it uses shunt resistors in the analogue domain then I would be
> able
> > to use the remote to control the volume of my system without losing
> > resolution at low volume.
> >
> >

seanadams
2005-03-28, 17:40
At full volume, when you're playing 16 bit PCM into a 24-bit DAC, the
low 8 bits are always '0' so it's still effectively 16-bit precision
(can't squeeze blood from a stone theorem). i.e. you're just
multiplying by 1 and "filling in" the least significant bits with
zeroes.

However at volume levels between 1/256 and full volume, having 24-bit
output is advantageous because effectively you do not lose any
resolution, since the output resolution is sufficiently higher than the
input. Below 1/256 however, you are losing precision because the bits
start "falling off then end".

Here's the sample "5/16" represented in ascii with 4-bit (16 level)
precision:

*****----------- In binary: 0101

If I need to divide by two, I lose precision because I can't draw 2.5
stars:

**-------------- In binary: 0010 (precision is lost - the least
significant bit was lost)

However, if I append a zero to get 5-bit (32 level) precision, I would
represent 5/16 as

**********---------------------- In binary: 01010

Now I can divide by two without losing precision:

*****--------------------------- In binary: 00101


That's the basic idea... extend this concept to 16 bit (range of 65,536
values) versus 24-bit (range of 16,777,216 values), which would be
rather time consuming to draw in ASCII. :)



On Mar 28, 2005, at 3:59 PM, windguy2 wrote:

> If I understand this, on the Squeezebox2 the volume is controlled
> digitally, therefore dropping bits to lower the volume. Because the
> DAC is 24 bits, with 16 bit PCM data the loss of resolution would only
> come at low levels after 8 bits are dropped. Would this increase the
> noise floor since that is an advantage with the 24 bit DAC?
>
>
>
>
> Sean Adams sadams at slimdevices.com wrote:
>
>> 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.
>>
>> Since the DAC is 24 bits, no resolution is lost (assuming 16-bit PCM
>> source) except at the ultra-low volume levels.
>>
>> If you have a preamp then it's really ideal - set the preamp to the
>> max
>> listening level you'd want to use, and then you can use the volume
>> control in Squeezebox.
>>
>> On Mar 28, 2005, at 1:02 PM, windguy2 wrote:
>>
>> > How is the analogue volume controlled?
>> > Is it lossless using shunt resistors in a chip or is it done
>> digitally
>> > therefore dropping bits?
>> >
>> > The reason I ask is that I use a passive preamp driving a power amp
>> -
>> > if it uses shunt resistors in the analogue domain then I would be
>> able
>> > to use the remote to control the volume of my system without losing
>> > resolution at low volume.
>> >
>> >