Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

in the Broadcast/Netcast Facility 39
44.1kHz. This is unfortunate because a 32kHz sample rate optimizes the performance
of HE-AACv2 low-bitrate netcasting.
� The encoded output signal is the ratio of the input audio signal and the PPM
signal.
� The Digital PPM encoder has a five-sample delay at 44.1kHz: 113.38us.
� The Digital PPM Low Level Audio Alarm threshold is –18dBFS using normalized
pink noise or –20dBFS using a 1kHz sinewave.
� In installations where the program line has 18-20dB of headroom (typical of
good engineering practice), the Low Level Audio Alarm may often be in the
Alarm state.
� The Digital PPM Low Level Audio Alarm has somewhat inconsistent behavior:
It takes as long as 30 to 60 seconds to reset from a Low Signal Level condition,
from no audio to normal level audio, and takes as long as 10 minutes
to enter Low Signal Level condition after a transition occurs between normal
audio level and no audio.
� The Digital PPM encoder continues to encode under Low Signal Level alarm
conditions.
� The alarm is independent of PPM encoder/injection functions.
When audio dynamic range compression (not to be confused with bit reduction
compression) is used after the PPM encoder, the low level audio alarm appears to be
unimportant because the compression will maintain both modulation levels and
PPM signal levels.
The encoder PPM signal injection is linearly proportional to the audio level at the
encoder’s input. Therefore, the location of the PPM encoder (before or after the dynamic
range compressor) does not materially affect data throughput. If the compressor
is placed after the PPM encoder (as it is in most facilities), the audio signal
processing systems will amplify the PPM signal and program audio equally, so the
location will not affect the PPM encoded signal except for a slight, inaudible signalto-noise
penalty. However, placing the compressor before the PPM encoder will prevent
the low level audio alarm from tripping because the compression keeps the
level constant at the input of the PPM encoder.
While the PPM system attempts to increase ratings accuracy by replacing the old paper
diary reporting system with an automated one, there are a number of technical
reasons why this is not completely foolproof. High average audio levels are required
to maintain PPM signal integrity. Program material with many quiet periods such as
talk formats offers less opportunity to produce an encoded signal. This is a limitation
of this kind of technology.
It is clear that using considerable amounts of audio dynamic range reduction can
help maximize PPM data throughput and give stations higher ratings.. Orban Opti-