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Normalizing
The IBOC And FM/AM-Analog Audio Signal Paths
 Frank
Foti, Omnia Audio
Glynn Walden, iBiquity Corp.
A critical
component to the IBOC transmission system is the relative
audio levels of the IBOC and FM/AM-Analog signals in the
receiver. The key issue here is being able to create audio
levels that are perceived subjectively to be relatively the
same, in average volume, when the signal is blending1
between modes. The listener experience could be adversely
affected if the audio level abruptly changes, in the blending
process. Thus, the question becomes: How can this be
normalized, and what are the operating levels required to
accomplish this? There may be differences between the
"offset" needed for AM and FM, this report covers a
testing program to develop the "offset" for the FM
channel, testing to determine the AM "offset" will
follow.
The analog
signals are limited by channel conditions and receiver
performance to less than 70 dB of dynamic range. In addition
the broadcast stations use audio processing to further reduce
dynamic range to overcome the effects of the analog channel.
The digital signal is not subject to such limitations and may
be delivered with a dynamic range of greater than 92 dB. There
are four key elements that differentiate the IBOC and
FM/AM-Analog signals:
1. RMS
Level: . The function of audio processing in an analog
transmission system is to reduce the dynamic range and
increase the audio RMS level. In analog systems, this
function essentially is used to mask the effects of a noisy
channel. More moderate use of audio processing on the
digital transmission is used to enhance the digital quality
and differentiate digital from analog audio. Without similar
dynamic range restriction on the digital channel the analog
channel, will always have greater apparent loudness.
Imposing analog processing criteria destroys the digital
audio quality advantage.
2. High
Frequency Limiting on the Inverse of the Pre-Emphasis Curve:
. AM and FM transmissions use pre-emphasis to increase the
high frequency energy in analog transmission and de-emphasis
in the receiver to restore flat frequency response and mask
noise in the channel. Pre-emphasis compensation reduces the
overall average audio level by7-8 dB for AM and 15 dB for
FM. Current analog audio processing uses limiting, following
the pre-emphasis compensation to preserve overall audio
level and increase the density of the high frequencies at
the expense of clarity. No such processing is done in the
digital channel, therefore the high frequencies in the
digital transmission are more open and less distorted, a
primary advantage of IBOC.
3. Frequency
Response: . Analog FM is limited at both the high and low
extremes of the frequency range. The lower frequencies are
limited to prevent problems with the phase locked loops of
the analog exciter and the high frequencies are limited to
15 kHz to provide protection to the stereo pilot at 19 kHz.
Analog AM is primarily a 3 - 5 kHz medium, due primarily to
receiver limitations, however, IBOC imposes a 5 kHz
limitation on the analog transmission. The digital frequency
response of the IBOC FM channel is approximately 18 kHz and
10 - 15 kHz in the AM digital channel.
4. Dynamic
Range: . The dynamic range of an analog transmission, under
ideal circumstances, is 70 dB for FM and 50 dB for AM. The
dynamic range of an IBOC digital channel is perceptually 96
dB. Analog audio processing dynamically restricts dynamic
ranges to increase RMS audio levels masking channel
impairments at the expense of quality. There are no such
benefits to dynamic range restriction on the digital
channel. Minimal use of digital audio processing preserves
the dynamics and qualities of the IBOC audio.
Transmission
Path Reference Levels…IBOC and FM/AM-Analog
The
FM/AM-Analog path transmits, over the air, using a maximum
peak deviation of +/-75kHz for FM, and double the RF carrier
level for AM. These levels correlate to 100% modulation.
Assuming that 100% modulation will create a peak level of 0dB
in a receiver, an average level will be derived based upon the
amount of processing employed, which produce a larger RMS
level value when more processing is used.
Most FM and AM
Stations employ sufficient processing, that the RMS level is
usually within a few dB, whether or not the processing is set
for light or aggressive operation. Present generations of
audio processors do an extremely good job of maintaining a
normalized RMS level. Basically the sonic difference to light
and aggressive processing is the perceived density. It sounds
anywhere from packed up and thick, to open and airy. The
concern for this discussion is not the aural texture but the
perceived average level of the analog path.
As stated
earlier in this discussion, one of the advantages to the IBOC
path is the ability to offer wider dynamic range, which allows
less processing to be employed. This would reduce the RMS
average level, and sound perceptibly softer or quieter to the
FM/AM-Analog path. The question, to be determined herein is
what "offset" to set in either the IBOC signal or
analog path so that perceived audio levels are equal.
While the IBOC
path offers a wider dynamic range, one that approaches the CD,
it makes sense to employ some level of dynamic control so that
a consistent and pleasing audio level is presented to the
listener. A perfect example of this is automobile listening,
where it is desirable to provide a consistent level so that
road noise does not drown out what is being heard. Moderate
audio processing will provide this, while maintaining
extremely high audio quality. This is due to the fact that
processing for IBOC is not penalized by high frequency
limiting the inverse of the preemphasis curve, which drives
the presence and treble frequencies into heavy processing.
This will cause a difference in the texture of this frequency
range. The IBOC processor operates on a flat signal, and thus
the presence and treble range appear perceptibly more open and
less dynamically distorted.
The IBOC
processor will create an aural texture that will be different
than it's FM/AM-Analog counterpart. The already mentioned
preemphasis issue is one reason. Additionally, the IBOC system
will operate on a wider frequency response, and the level of
processing will generally be reduced to allow the benefits of
wider dynamic range. Make no mistake, aggressive processing
can be employed in the IBOC system, but it will not generate
the same effective sound as it's counterpart due to the
differences in the transmission channel.
Determining
the FM "Offset"
As a means
towards finding what would be the optimum operating level for
the IBOC processing system, the following subjective test was
derived. Two dedicated processors, one for FM/AM-Analog and
one for IBOC, were set side by side and aurally compared. The
FM/AM-Analog processor was set for a relative reference level
of 0dBu. Think of this as the given 100% modulation level that
would feed a transmitter. Since all preemphasis/deemphasis and
peak control are handled in the processor, the output level
can be set to a known reference. Likewise the IBOC processor
can be setup in the same fashion. Since both units will
provide absolute, precise peak control, the output levels can
be set to maximize the dynamic range of their respective
transmission channels.
The
FM/AM-Analog processor was set for heavy, aggressive
processing, whereas the IBOC system was set for light
processing. Once setup, the processing parameters were not
further adjusted during the testing. Fourteen audio clips
including voice, voice-over and music were recorded with the
digital processor, serving as the reference, held at 0 dB and
the output of the analog processor recorded in 1 dB steps from
0 to - 9 dB. The cuts were then assembled into a matrix on a
computer screen where, through a mouse, instantaneous
comparisons between the digital reference and the 9 levels of
the analog processed audio could be performed. Figures 1 and 2
depict the interface used in the evaluation program. The
information on the test setup of the processors is contained
in Appendix A.
Figure 1
Figure 2
Thirty one
subjects from iBiquity's offices in Maryland, New Jersey, and
Omnia's Office in Ohio participated in the test. The subjects
were asked to match the audio level2
of the reference to the samples A through I and mark their
choice with a checkmark. Figure 3 depicts the results with the
14 sample cuts represented along the X axis and the level
deemed to match is shown on the Y axis. The red line is the
average for the point and the orange line shows the running
average. The result of this evaluation shows that the
perceptual difference between a heavily processed analog and
lightly processed digital transmission is 3.57 dB.
Figure 3
Red (Solid Line)= Average Score
Orange (Dashed Line) = Running Average
Normalized
IBOC FM Levels:
In the
transmission system there are two parameters that can't be
changed; first the FM/AM-Analog is set to 100% peak
modulation, and second the IBOC channel is limited to 0dBfs
peak operation. Therefore to insure smooth blending between
the analog and digital sources, the "offset" to
equalize these levels will have to be incorporated in the
receiver
Based upon the
results of the subjective test, it is recommended that to
normalize the audio levels between the IBOC and FM/AM-Analog
signal paths, a 5.0 dB relational difference in level needs to
be implemented. This number, for FM operation, is derived from
the 3.57 dB determined through subjective evaluation and
approximate 1.5 dB of pad to allow broadcaster flexibility. To
insure proper blending, every IBOC radio, independent of
manufacture, will be required to have the same relative
offset.
Automotive
receiver manufacturers currently match the levels of various
sources including CD players, AM & FM radio, DVD's such
there is minimal level disparities between the devices. For
IBOC the offset can be done in one of two ways either the
receiver manufacturer can choose to increase the level of the
digital by 4.5 dB or decrease the level of the analog by 4.5
dB.
At the
broadcast transmission facility, the station will use a
calibrated reference receiver to match the apparent loudness
between the analog and digital signals.
Footnotes
(1)
The IBOC system the analog and digital paths carry the same
audio. Upon initial tuning to an IBOC station the receiver
produces audio output from tha analog stream and blends to the
digital stream when the signal is acquired. Furthermore, the
analog signal is used as a back-up to the digital stream. In
the event of the loss of the digital signal the receiver
smoothly blends back to the analog audio. (Return
to text)
(2)
It is of interest to note that with some cuts the sonic
differences were clearly evident between the digital and
analog paths. Actually, due to the broader dynamic range, and
lack of emphasis processing in the IBOC unit, the analog
signal sounded noticeably dull and lacking clarity, when
compared to the IBOC path. When quickly switching between the
two signals, the IBOC signal appeared to have more excitement
to the aural presentation. (Return to text)
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