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3-12
Operation
With the FM->HD CONTROL COUPLING set to INDEPENDENT, there are several approaches to minimizing brightness
and conditioning the signal to work well at low bitrates.
•
Use little or no high frequency boost in the HD equalization and band mix sections.
•
Set the HD BAND 4>5 COUPLING to 100%.
•
Set the HD B5 THRESH to match the codec and its bitrate. Adjust the threshold until you find a good compromise
between presence and high frequency codec artifacts. We find the range from -6.0 to +6.0 dB to be useful.
•
Use a moderate Band 5 attack time. 25 ms works well.
•
If necessary, lower the HD B4 THRESH.
BS.1770 Compliance for Digital Radio
The 5750 includes an ITU-R BS.1770 Loudness Meter and Safety Limiter in the digital radio processing path. The
Safety Limiter should only be activated if the regulatory authority in your country requires constraining BS.1770
Integrated Loudness to a specified threshold.
Input/Output Delay
The sophisticated look-ahead algorithms in the 5750 have one significant cost—the input/output time delay is longer
than that of an analog processor and can cause problems if an off-air pickup is used to feed talent headphones.
To make intelligent decisions about how to process, the 5750 needs to look ahead at the next part of the program
waveform.
(Slowly changing bass waveforms require particularly long look-ahead delays.) As digital on-air processing advances
further and further from its analog roots, this is the inevitable price of progress.
The amount of delay depends on several things. Because of their unprecedentedly sophisticated processing, "MX"
presets introduce delays of approximately 265 ms, which makes real-time monitoring through headphones
impossible for talent. The 5750 offers a low-delay headphone monitoring output to solve this problem, which also
occurs in HD Radio installation where the diversity delay is typically 8-10 seconds.
If a non-MX preset is active, the delay is usually low enough to allow talent to monitor live through headphone
although a learning period may be required.
For a thorough discussion of delay issues and solutions, see Monitoring on Loudspeakers and Headphones on page
1-12.
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