MRD 3187B - User Manual
Page 240 (245)
automatically down mix an AC-3 Dolby stream to two channels. “Raw” simply passes
thru the Dolby AC-3 data stream to be decoded by an external decoder such as the
Dolby 569. The analog channels can be assigned to any of the digital services. The
analog gain may be adjusted for the desired level. The gain setting does not affect
the level of either the embedded audio or the digital services. A setting of 7 provides
an output of approximately 0 dBu for a digital signal level of -20 dBFS. To check the
audio output level, set “User / Custom1 / L/R” to disabled. This will remove any signal
processing in the down mix. Set the digital service to be measured to “PCM.” A test
stream of -20 dBFS will output from the digital services as -20 dBFS. If the analog
channels are set to a gain setting of 7, the output should be -0.7 dBu plus or minus
0.5 dBu. Each gain number increments or decrements approximately 2.5 dBu per
step. When you are setting the output levels it is suggested that an AC-3 stereo tone
(2.0) be used as the source. Do not use a Dolby AC-3 5.1 tone source as your test
signal. The five channels of equal tones will down mix such that one channel will be
more than twenty dB greater than the other.
Digital Audio Measurements
Digital measurements are made in dBFS, decibels related to Full Scale. The normal
operating level of a digital audio system is 20 db below the full digital signal. This full
digital signal is the point where there are no more bits to describe a higher level. This
would be analogous to clipping in the analog world. Since test tones are set to -20
dBFS, there is 20 dB of headroom in the digital system before the system runs out of
bits. To test for maximum level, a 0 dBFS test tone is used.
The dynamic range is read directly by reading how many dBFS below 0 dBFS is the
noise. If the signal plus noise is -85 dBFS, then the dynamic range is 85 dB. The
same method is utilized for THD measurements. Again, the THD reading in dBFS is
read directly on the analyzer. If the amount of distortion is wanted in per cent, it can
be calculated from the value in dBFS. For example, -84 dBFS would be equivalent to
.0063% distortion.
If there are analog outputs that are delivering the same signal as the digital outputs
for a digital source, the normal digital level must be compared to a resulting analog
level out of the system. In the MRD, for a -20 dBFS test tone, the MRD analog audio
outputs deliver a -0.78 dBu level. If a 0 dBFS test tone is used, the analog outputs
deliver a +19.98 dBu level. The analog level is adjustable and the levels above result
if the analog gain control is set to “7”. This is the setting Sencore uses for testing.
The signal to noise in a digital system is measured by stimulating the system with a
test tone at -60 dBFS, notching out the tone, and measuring the noise that is left. This
measurement is made with the bandpass set by a low pass filter that is slightly below
½ the sampling frequency. In the case of the MRD, that is ½ of 48 kHz or 24 kHz.
Practically, the filter in the analyzer cuts off about 22 kHz. The tone is necessary
because the system mutes if it is not receiving a digital signal.
Analog Audio Measurements
The analog audio measurements utilize the same test signals as the digital test
with the exception of the THD measurement. The longer setting time of the
analog THD analyzer requires a slower frequency sweep test signal. The analog
audio signals in the MRD are derived from the decoded digital audio in the MRD.
General Audio Setup for Measurements
The MRD has three primary operation modes that affect the quantity of
compression with decoded Dolby AC3 signals. All tests are made with the “user”
mode of operation. This eliminates any signal compression on AC3 Dolby signals
downmixed in the MRD. This mode has no effect on PCM encoded audio (AES).
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