[PD] headroom in Pd

Martin Peach martin.peach at sympatico.ca
Wed Jan 1 20:03:41 CET 2014

I can see how a filter circuit following a DAC can swing more than the 
DAC for example if two successive samples are full-scale, but there's no 
way a DAC can output beyond its own full scale except momentarily while 
it's settling to a value inside its range.
The scaling has to be done before the DAC.

You just can't reconstruct a clipped signal unless the clipping is very 
mild or the signal is very simple, like a sine wave. What if the signal 
is +12dBFS white noise?

I meant that if you take 16 bits to be full-scale but you have a 24-bit 
DAC you _could_ use the 16 LSBs of the DAC as full scale, then you have 
a lot of headroom but your signal to noise ratio is not as good, and 
maybe something like this is happening in the default MacOS headphone 


On 2014-01-01 13:50, Chris Clepper wrote:
> Nope, the DAC can freely construct intersample peaks as it sees fit and
> those can easily exceed 0 dBFS.  It has been common practice in the
> industry for more than a decade to reconstruct clipped samples well
> above 0 dBFS - partially to make up for shitty mixing and mastering
> prevalent in music, and also because it's the right way to do it.
> 16 bits full scale and 24 bits full scale are the same 0dBFS signal.
>   The bits are added at the bottom not the top.
> On Wed, Jan 1, 2014 at 1:34 PM, Martin Peach <martin.peach at sympatico.ca
> <mailto:martin.peach at sympatico.ca>> wrote:
>     On 2013-12-31 19:32, Chris Clepper wrote:
>         It's very, very easy to avoid any sort of clipping processing by
>         using
>         hardware with drivers that don't have any!  Avid, Apogee, MOTU,
>         RME, and
>         many others have bit transparent OSX CoreAudio drivers.
>         Also, any DAC worth it's using can reconstruct far beyond 0dBFS
>         without
>         distortion, so hearing volume increase past -1..1 in software is not
>         surprising.  I recall the ADI 1955 and equivalent TI part
>         putting out
>         +12dBFS or something ridiculous, but those ain't Wolfson low power
>         headphone codecs neither!
>     A DAC can only go to 0dBFS by definition. If it appears to go beyond
>     that then something is scaling the input to be less than full scale
>     at "full scale".
>     For instance a 24-bit DAC could be sent 16 16-bit full-scale streams
>     and not clip. Only if 16-bits is considered "full scale" does that
>     make it +12dBFS.
>     Martin
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