Emphasis - RME Audio ADI-2 Pro SE User Manual

34.5 Emphasis

In the early times of digital audio, with AD and DA converters of only 14 bit resolution, a technique
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 distortion 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-2/4
Pro SE 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-2/4 Pro SE as audio interface to record SPDIF into
an audio file, the emphasis state is lost. Similarly there exists no mechanism to let the audio
playback software control the emphasis state of the ADI-2/4 Pro SE's DAC during playback of
that recorded file. An option De-emphasis On in the channel's I/O menu allows for a manual
activation in such a case.
The ADI-2/4 Pro SE can also perform both pre- and de-em-
phasis outside the DAC with just a single band of its Para-
metric EQ. The emphasis filter is based on a simple first order
RC filter with time constants of 50 μs and 15 μs. The fre-
quency 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 ac-
tive, set Q to 0.5, Frequency to 5.2 kHz and Gain to -9.5 dB.
Similarly, a pre-emphasis is done with the same settings but
Gain to +9.5 dB.
The measurement below shows these settings against the DAC's de-emphasis.
User's Guide ADI-2/4 Pro SE - v 1.0
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