[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
driver.
Martin
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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