Emphasis - RME Audio ADI-2 Pro User Manual

The Advanced Balanced mode of the ADI-2 Pro is as unique as brilliant. Balanced mode never
made as much sense as when implemented like done here!
In Advanced Balanced mode the ADI-2 Pro's maximum output level rises to +13 dBu for Hi-
Power Off and +28 dBu for Hi-Power On. The signal to noise ratio rises from 117 dB / 120 dBA
to 120 dB / 123 dBA. Or in other words: while the output level rises by 6 dB, the noise rises only
by 3. And therefore stays inaudible as in normal operation.
While the higher output voltage might be useful for some older exotic headphones, a 4 times
higher output power (around 5 Watts, per channel) does not make sense. Fortunately the cur-
rent limit circuit in the ADI-2 Pro takes care of that and prevents the output power to rise higher
than 3 Watts, and below 2 Watts at impedances below 24 Ohms. See chapter 34.13 for details
about the available output power at the phones outputs.
The Advanced Balanced mode design does have one drawback though: it will work in DSD
mode, but not in Direct DSD mode. With the ADI-2 Pro that is no real issue, as in Direct DSD
mode phones are turned off anyway due to the missing volume control.

34.4 Emphasis

In the early times of digital audio, with AD and DA converters of only 14 bit resolution, a tech-
nique was used that is also known from radio transmission: pre- and de-emphasis. The audio
signal is equalized to have treble boosted before the conversion. When played back an analog
treble filter (the term high cut seems a bit strong) is required. Overall the audible noise and dis-
tortion caused by the AD and DA conversion was hoped to be reduced this way.
Some older CDs were recorded with Emphasis, and indeed Emphasis is part of the Red Book
standard. Listening to them requires a filter on the playback side or their sound will seem too
bright. The playback of older digital recordings from tape might also require de-emphasis, and
even one of the first DAT recorders used Emphasis constantly.
Fortunately digital to analog converter chips have support for de-emphasis included. The ADI-2
Pro activates the DAC's de-emphasis automatically when the current source is AES or SPDIF
and the Emphasis bit is set in the incoming Channel Status. The State Overview screen can be
used to track this state, a WARNING SPDIF EMPHASIS message will be shown.
Why warning? Because when using the ADI-2 Pro as audio interface to record SPDIF into an
audio file, the emphasis state is lost. Similarly there exists no mechanism to let the audio play-
back software control the emphasis state of the ADI-2 Pro's DAC during playback of that re-
corded file. An option De-emphasis On in the channel's I/O menu allows for a manual activation
in such a case.
The ADI-2 Pro can also perform both pre- and de-emphasis
outside the DAC with just a single band of its Parametric
EQ. The emphasis filter is based on a simple first order RC
filter with time constants of 50 μs and 15 μs. The frequency
response curve looks like a low-Q treble boost with its +3
dB point at 3183 Hz, and the upper shelving point at 10610
Hz. At 20 kHz gain hits +9.49 dB.
For an inverted filter curve select band 5 with type shelf
active, set Q to 0.5, Frequency to 4.7 kHz and Gain to -10
dB. As the measurement below shows these filter settings
compensate the pre-emphasis curve with 0.1 dB accuracy.
Similarly, a pre-emphasis is done with the same settings but Gain to +10 dB.
User's Guide ADI-2 Pro © RME
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