TAS5705
SLOS549–JUNE 2008...................................................................................................................................................................................................... www.ti.com
Attack
and
Decay
Filters
Energy
Filter
Compression
Control
Audio Input
DRC Coefficient
a, w
T, K, O
aa, wa / ad, wd
0x3B / 0x3C
0x3E / 0x3F
DRC1
DRC2
0x3A
0x3D
0x40, 0x41, 0x42
0x43, 0x44, 0x45
Alpha Filter Structure
S
a
Z–1
w
NOTE:
a = a
w = 1 – a
B0265-01
Figure 44. DRC Structure
AM Tuner Interference Management
Digital amplifiers produce AM interference by radio energy emissions near the digital amplifier switching rate and
the harmonics of that switching rate. The digital amplifier emits an interference spectrum in the AM band that is
centered on the second though sixth harmonics of the digital amplifier switching frequency. Because the digital
amplifier switching rate is a multiple of the input data sample rate, the interference frequencies can be changed
by changing the sample rate.
Anatomy of a Receiver
AM receivers are composed of six sections as shown in Figure
The radio-frequency (RF) section has a variable tuner that preselects the frequencies to be received. The RF
section provides only enough filtering to significantly attenuate signals that are significantly above or below the
desired tuned frequency. The RF-section gain is controlled by the AGC to compensate for variations in the signal
strength.
The mixer, variable-frequency oscillator (VFO), and intermediate frequency (IF) sections perform the bulk of the
receiver tuning. In home receivers, the variable oscillator is adjusted so that it is a constant 455 kHz higher than
the desired tuned frequency. The output of the mixer consists of four frequencies, RF, VFO, RF + VFO and RF –
VFO. These signals are then passed into the IF section, where sharp discriminating filters to accept only the
constant 455-kHz (RF – VFO) signal. The resulting signal is amplified and then passed to the detector section,
where it is transformed into an audio signal.
38
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