Digital Audio Measurements; Analog Audio Measurements - Cisco D9887B Installation And Configuration Manual

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 D9887B receiver, for a -20 dBFS test tone, the D9887B
receiver 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 D9887B receiver, 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 D9887B receiver are derived from the decoded digital audio in the D9887B
receiver.
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Digital Audio Measurements
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