Harmonic Cancellers for HD Radio Signals
Nearly all stereo decoders use a 38-kHz square wave for demodulating the L-R subchannel, which lies between 23 and 53 kHz. An undesired consequence is that the waveform's fifth harmonic demodulates power near 190 kHz. HD Radio digital sidebands, which occupy spectrum from 129 to 198 kHz after FM detection, can cause an annoying audio background noise when demodulated by the fifth harmonic. Extended-hybrid HD Radio signals, whose detected spectrum may extend as low as 102 kHz, can cause additional noise when demodulated by the third harmonic at 114 kHz. Demodulation waveforms without third or fifth harmonics eliminate this HD Radio self-noise.
Prior Art
In U.S. patent 4,264,784 (1981), Lorea describes a four-level waveform with no third or fifth harmonic. Sansui used this waveform, known as a Walsh function, in a discrete stereo decoder. The Sony ST-S800ES uses a Walsh function, as does the Denon TU-680NAB. Richards and Field describe a Walsh-function stereo decoder in U.S. patent 5,027,402 (1991), implemented by Allegro Microsystems as the 20-pin A3828EA IC. All of these stereo decoders avoid HD Radio self-noise for both ordinary and extended-hybrid signals.
The Siemens TCA4500A stereo decoder, second-sourced as the Motorola TCA4500A and the National Semiconductor LM4500A, uses a three-level demodulation waveform with no third harmonic. The Sanyo LA3450, produced by Sony as the CXA1064, uses a four-level waveform with the same result. These decoders do not suppress noise for ordinary HD Radio signals, which have no power near 114 kHz.
Fifth-Harmonic Canceller
If the duty cycle of the TCA4500A waveform is increased to 40%, the fifth harmonic disappears. A stereo decoder using this waveform would eliminate self-noise for ordinary HD Radio signals. The waveform can be synthesized for a squarewave stereo decoder by synchronously blanking 20% of the input signal each 38-kHz half-cycle. A circuit that phase-locks a 380-kHz oscillator to the stereo pilot and drives an analog switch with a decoded decade divider would accomplish this. But for stereo decoders such as the Sanyo LA3380, Hitachi HA11223W, or NEC µPC1223C that generate a 76-kHz sawtooth VCO waveform, a simplified circuit using analog timing is possible.
A 4066 analog switch level-detects the sawtooth waveform. An RC network delays the trailing edge about 1.3 µS, two additional switches in the quad package square up the pulse, and a final switch blanks the detected FM signal. Set the pot for 20% blanking. Although not critical, you can adjust the RC network to center the blanking interval about the sawtooth transition. The circuit drops the L-R signal 0.4 dB, yielding a slightly lower audio output level. L+R drops 1.5 dB more so you must readjust the separation control. Timing may vary somewhat as the input threshold changes with temperature. For a bipolar signal, connect Vss and circuit grounds to the negative supply. Vdd - Vss is 12 volts maximum for a 74HC4066. For 15 volts maximum, use a CD4066B.
Third- and Fifth-Harmonic Canceller
This circuit uses a quad op-amp and FET to implement the four-level Walsh
function with analog timing. The unconnected noninverting op-amp pin goes to
the detector midpoint voltage in a single-supply system or to ground for dual
supplies. Set the threshold pots so that the FET is off during the middle 50%
of each 38-kHz half-cycle. Set the gain pot so that the half-cycle end levels
are about 40% of midlevel. Make final adjustments for minimum 1-kHz audio
output when injecting 115- and 191-kHz sine waves into the SCA input of a
stereo generator.
If a 10kΩ load impedance is too low for the FM detector, use the spare
op-amp as a voltage follower. The circuit compensates for the 1.6-dB
Walsh-function loss at 38 kHz so the audio output level doesn't change. L+R
drops 1.4 dB so you must readjust the separation control. Any high-speed
op-amp will work. Any FET that turns off with the input op-amps saturated low
and whose on-resistance is small compared with the 8kΩ pot setting is suitable.
If FET resistance is too high, harmonic distortion may result as the
resistance varies nonlinearly with drain-source voltage. Find a better FET, or
replace the FET, resistor, and diode with a low-resistance analog switch.
A three-level waveform eliminates gain adjustment, while a digital timebase
eliminates timing adjustment. A waveform with no third or fifth harmonic
requires two complementary fifteen-bit sequences. The sequence 010111111110100
maximizes the fundamental. The bit length is one-thirtieth of the period of a
38-kHz sine wave. Implementation requires a phase-locked 1.14-MHz oscillator,
a counter with decoding logic, and an analog switch. Although it increases
only 0.1 dB, a half-bit delay maximizes the in-phase fundamental. This can be
done digitally by starting at 2.28 MHz. The resulting waveform drops L-R 1.6
dB and L+R 3.5 dB.
The circuits implement the cancellers as nonlinear postdetection filters. A
linear postdetection filter is described here.
More Updated May 11, 2008
Digital Implementation
Linear Filter
is here.
