Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

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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-
40 Maintaining Audio Quality mods can provide high average audio levels for PPM encoders without introducing side effects that drive audiences away due to listening fatigue.A PPM explanation and performance plots are available here: http://www.indexcom.com/ppm Part 3: The Production Studio The role of the production studio varies widely from station to station. If used only for creation of spots, promos, IDs, etc., production studio quality is considerably less critical than it is where programming is “sweetened” before being transferred to a playout system. Our discussion focuses on the latter case. Choosing Monitor Loudspeakers The loudspeakers are the single most important influence on studio quality. The production studio monitor system is the quality reference for all production work, and thus for the final sound to be broadcast/netcast. Achieving accurate monitor sound requires considerable care in the choice of equipment and in its adjustment. Loudspeakers should be chosen to complement room acoustics. The space limitations in production studios usually dictate the use of bookshelf-sized speakers. You should assess the effect of equalization or other sweetening on small speakers to make sure that excessive bass or high-frequency boost has not been introduced. While such equalization errors can sound spectacular on big, wide-range speakers, it can make small speakers with limited frequency response and power-handling capacity sound terrible. The Auratone Model 5C Super Sound Cube has frequently been used as a small speaker reference. Although these speakers are no longer manufactured, they are often available on the used market. We recommend that every production studio be equipped with a pair of these speakers or something similar and that they be regularly used to assure the production operator that his or her work will sound good on small table and car radios. The primary monitor loudspeakers should be chosen for: � high power-handling capacity � low distortion � high reliability and long-term stability � controlled dispersion (omnidirectional speakers are not recommended) � good tone burst response at all frequencies � lack of cabinet diffraction � relatively flat axial and omnidirectional frequency response from 40-15,000Hz � physical alignment of drivers (when all drivers are excited simultaneously, the
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Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

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