Belar FMHD-1, Руководство оператора

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FMHD-1 Operator’s Manual
Page 34 FMHD-1
before transmission, or the recovered signal combination emerging from a broadband FM demodulator,
before its separate components are processed.
Decibel
Formally, a decibel is the power ratio of two signals based on a logarithmic scale. Decibels, or dB, may
be equivalently expressed as ratios of the corresponding signal voltages or currents.
𝑑𝑑𝑑𝑑 = 𝑑𝑑𝑑𝑑 = 10 𝑙𝑙𝑙𝑙𝑙𝑙
10
�
𝑃𝑃
2
𝑃𝑃
1
� = 20 𝑙𝑙𝑙𝑙𝑙𝑙
10
�
𝑉𝑉
2
𝑉𝑉
1
� = 20 𝑙𝑙𝑙𝑙𝑙𝑙
10
�
𝐼𝐼
2
𝐼𝐼
1
� .
dBFS
This is the abbreviation for “Decibels relative to Full-Scale”. In pulse-code modulation, analog signals
are represented as a series of digitally-encoded numbers, each corresponding to the amplitude of the
analog waveform at successive instants in time. The largest signal amplitude that may be accurately
represented in a pulse-code modulation system is referred to as “full-scale”. This maximum is the
reference voltage used for calculating dBFS:
𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 = 20 𝑙𝑙𝑙𝑙𝑙𝑙
10
�
𝑉𝑉
2
𝑉𝑉
𝐹𝐹𝐹𝐹
� .
Because the coding process breaks down (clipping occurs) at levels above V
FS
, dBFS readings are always
negative.
dBc
This abbreviation may be translated as “Decibels relative to Carrier Power”. (See the definition for
“Decibel”, above.) In this case, the reference power in the expression,
𝑑𝑑𝑑𝑑𝑑𝑑 = 10 𝑙𝑙𝑙𝑙𝑙𝑙
10
�
𝑃𝑃
2
𝑃𝑃
𝑟𝑟𝑟𝑟𝑟𝑟
� = 10 𝑙𝑙𝑙𝑙𝑙𝑙
10
�
𝑃𝑃
2
𝑃𝑃
𝑐𝑐𝑐𝑐𝑟𝑟𝑟𝑟𝑐𝑐𝑟𝑟𝑟𝑟
�
is the power level of the carrier. For FM hybrid IBOC systems P
carruer
is the assigned power of the analog
FM carrier.
De-Emphasis
Because FM transmission is susceptible to high frequency noise, special high-frequency attenuation,
referred to as de-emphasis, is applied to the detected signals at FM receivers. To compensate for this
high-frequency attenuation at the receiver, high frequencies in the program signal are boosted prior to
transmission. This is referred to as “pre-emphasis.” By making the pre-emphasis and de-emphasis
processes fully complementary, high-frequency noise is suppressed at the receiver without compromising
the fidelity of the received signal.
In the FMHD-1, the low-pass filtering (a mathematical integration process) of the monaural or stereo
audio signals precisely compensates for the pre-emphasis of these signals at the transmitter. The
de-emphasis time-constant (i.e. 75 microseconds) refers to the resistor-capacitor-value product that sets
the break frequency of the corresponding R-C low-pass filter.
De-emphasis is not relevant to the program audio carried by the digital transmission portion of the hybrid
IBOC system.