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VP-8IP / Sep 2013
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P R O C E S S M O D E S
FM Mode Summary
In the FM mode standard 50 or 75 microsecond pre-emphasis and peak limiting are
performed and the audio is tightly band limited in order to protect the 19kHz stereo
reference signal (pilot). This operating mode is also accompanied by a reference grade
stereo encoder providing two independent composite multiplex outputs and a pair of
subcarrier (SCA) insertion inputs which are digitized at a 192kHz sample rate. While
the composite signal is in the digital domain aggressive filtering is applied to constrain
the output bandwidth to 53kHz to protect RDS or SCA subcarrier signals that may
occupy the spectrum above 53kHz. Baseband192 technology is a standard feature.
AM Mode Summary
In the AM mode NRSC AM pre-emphasis is available within the eight band limiter
section and may be applied pre or post multiband limiters. The most commonly used
AM and medium wave broadcasting low pass filter frequencies are provided, and
all filters are overshoot-free to assure accurate modulation control. Low Frequency
Tilt Correction is also independently available for both transmitter outputs to offset
inadequacies in the transmitter’s low frequency response that can compromise
modulation accuracy. Specialized limiting and clipping algorithms permit up to 150%
positive modulation for those applications where it is warranted.
FM-HD Mode Summary
In the FM-HD mode special look ahead algorithms developed by Vorsis condition
the audio in a manner that makes it most suitable to be coded by the iBiquity FM HD
perceptual codec. The audio source for the HD mode look ahead limiter is the output
of the four band AGC/Compressor.
AM-HD Mode Summary
This mode has been optimized to operate favorably with the HDC (Hybrid Digital
Coding) codec, a proprietary and lossy audio compression scheme developed by
iBiquity for specific use by US AM-HD stations. HDC is known to utilize SBR
( Spectral Band Replication), and in 2003 it replaced the earlier PAC codec which had
been proposed for AM-HD. Very little is known (outside iBiquity) about the codec’s
inner workings, although some codec experts believe that it may be a customized
version of MPEG-4 HE-AAC.
The input to the AM-HD mode look ahead limiter comes from the output of
the four band AGC/Compressor. While there are others involved, two important
pre-processing steps involve (1) carefully managing the stereo image to ensure that
non-stereo information is never presented to the codec, and (2) managing the transient
high frequency spectrum to ensure that certain kinds of high frequency energy is also
never presented to the codec.
MP3/AAC >48k & MP3/AAC <48k Mode Summary
In the previous “codec” modes the end user knows what codec is being used
to perform the audio data compression and therefore has some insight as to
its probable behavior with his program material. The two MP3/AAC>48k and
MP3/AAC<48k modes assume that the user may not always know what codec(s)
are being used during transmission of his audio; however he may at least know what
bitrate the codec is utilizing.
