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In the digital domain, the threshold of detection lies between -31 dBFS and -16.5 dBFS, depend-
ing on the volume setting, reference level and respective hardware output.
DC affected sources
If you just want to listen to music, you expect (and rightly so) that it does not contain inaudible
signal components that can cause effects ranging from slight distortion to destruction.
Typical DC sources in commercially available music, for example, date back to the early days of
digital technology. First AD converters (Beta, Video, DAT) suffered from the same due to lack of
sophisticated DC filters, typical values are between -60 and -40 dBFS. This is harmless, although
it can produce crackling noises at the beginning/end.
Many early DSD recordings have a relatively high DC content of about -33 dBFS - because the
digital DC filter of the AD chip only works in PCM mode, i.e. it is completely switched off in DSD
mode. But thank logarithm - even -33 dBFS do not represent a dangerous signal after conversion
to linear volts. Nevertheless, this is close to the threshold that DCP uses in the most sensitive
case.
Shocking
Today's DC mostly comes from purely digital sources, in this case probably rather to be described
as faulty. An example posted in the RME forum may even be the worst thing you can find in your
music collection: the track Blue Light by the band Bloc Party, in the Anti Gravity Remix version.
Starting at 0:27, a DC of -13 dBFS (not a typo) takes your shoes off, and at 0:51, an unbelievable
-8 dBFS is unleashed on your pitiful hi-fi system. This mix should never have been released.
Another, albeit less drastic example, is The Turnaround by Gemini. The pumping lower bar in the
analyzer starting at 0:05 unfortunately is not an ingenious low sub-bass, but actually DC (between
-20 dBFS and -14 dBFS). This was probably overlooked during the otherwise very nice produc-
tion.
The effect of such signals can be shown very vividly - if you have the opportunity to look directly
at the cone of a woofer. It is true that almost all power amplifiers have a capacitively coupled
input, i.e. they block DC. But this isolation is usually applied very low. Which means that when
DC occurs, the diaphragm is much displaced - but then slowly moves back. When the DC is
turned off, the same thing happens in the other direction. And with The Turnaround the diaphragm
moves slowly back and forth (nomen est omen), with maximum excursion already at medium
volume. That doesn't have to be, therefore: DCP Filter to the rescue!
Infrasound
While there are movie soundtracks that are professionally cut off at the deep end (around 10 Hz)
to not stress your subs for nothing, there are also completely unlimited productions that cause
amps to shut-off and cones to swing like they want to leave the enclosure. What might look funny
or even impressive is of zero use – there is no sound that low, nor are these subs able to turn
those low frequencies into physical, perceptible motion (that happens above 10 Hz). But those
low frequency swings cause additional distortion and consume valuable power!
Therefore it is recommended to always use the setting DCP Filter. Switching between DCP Filter
and OFF is a quick and easy way to confirm that nothing is lost with the then added high pass,
and while the speaker cones no longer move like crazy the sound itself should not have changed.
Note: For technical reasons the DCP infrasound filter is placed behind level meters and loopback.
Its impact is therefore not visible on the level meters, nor digitally measurable via the loopback
function. It can only be verified at the analog outputs.
User's Guide ADI-2/4 Pro SE – v 1.0
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