Inovonics JUSTIN 808, Инструкция по установке и эксплуатации

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ing-in on a final correction. Once the timing error is corrected,
the indicator should remain within a sample or two of zero at
all times.
An
FM-HD1 Alignment
meter also
heads the top of each Web interface
page, duplicating the action of the
front-panel meter, save for ‘strobing’
effect while delay is being corrected.
DELAY ERROR CORRECTION
Autocorrelation
Defined
The term ‘autocorrelation’ refers to a very complex process that
is the heart of the JUSTIN 808. There is no timing signal
broadcast as part of the ‘hybrid’ HD Radio FM/HD1 broadcast.
The HD Radio encoding/decoding process takes in excess of 8
seconds, requiring the FM broadcast channel to be ‘diversity-
delayed’ by this same time period.
Diversity delay may be set with some degree of accuracy by lis-
tening to both the FM and the HD1 programs in the ‘split mode’
offered by many HD Radio receivers, including the JUSTIN 808.
The human ear/brain interface is sensitive to a time and a
phase difference between audio sources, and the proper delay
was historically dialed-in by this method.
To perform this task automatically, the JUSTIN 808 must actu-
ally listen to the program audio and perform a cross-correlation
in much the same manner as a human operator. This requires a
very sophisticated mathematical process implemented with dig-
ital signal processing (DSP) algorithms.
The time required for autocorrelation is dictated in large part
by the 8+ seconds of delay inherent in the HD Radio system,
and further aggravated by the degree of time misalignment be-
tween the FM and HD1 programs. A second factor is the com-
position of the actual program material, which takes account of
the spectral and amplitude distribution of the signal waveforms
and even the ‘rhythm’ of the audio.
Once autocorrelation has been achieved, the JUSTIN 808 will
immediately begin to correct for FM/HD1 timing errors.
Delay
Plot
Delay alignment is shown with best clarity through the Web in-
terface. On the
Now Playing
screen, both the measured and the
applied delays are shown graphically as one-minute history
plots, as well as numerical values of the current number of
program audio samples at 44.1kHz.
The following example shows the JUSTIN 808 correcting a tim-
ing error that was purposely introduced to illustrate this action.
The diversity delay in the FM airchain was increased by 500
samples, which was quickly correlated and displayed on the au-
to-ranging
Measured Delay
screen. The
Applied Delay
readout