Nearly all stereo decoders use a 38 kHz square wave to demodulate the L−R signal, which occupies 23 to 53 kHz in the stereo composite. An unintended consequence is that the waveform's fifth harmonic demodulates power near 190 kHz. HD Radio digital sidebands, which appear at 129 to 198 kHz, can cause an annoying background noise when demodulated by the fifth harmonic. Extended hybrid HD Radio signals, whose spectrum may go as low as 102 kHz, can cause additional noise when demodulated by the third harmonic at 114 kHz. A lowpass filter between the detector and stereo decoder can eliminate this HD Radio self-noise.
To avoid degrading frequency response and stereo separation, conventional postdetection filters have flat amplitude and group delay over the 53 kHz stereo composite passband. But high attenuation above 100 kHz requires a complex filter. I wrote a Windows program to optimize a simple postdetection filter that takes a different approach. The optimizer models the stereo decoder and seeks the filter with maximum attenuation above 100 kHz for a specified minimum stereo separation and maximum frequency response error. Instead of using a standard filter response, it directly optimizes the pole locations. It imposes no spectral flatness constraint on the filter itself, only on the audio output.
The program uses a Differential Evolution optimizer. The resulting optimal filter is down 3 dB somewhere between 35 and 50 kHz. Roll-off this low attenuates the L−R signal, particularly the upper sideband. But like the vestigial sideband system of NTSC television, the sum of the demodulated sidebands is nearly constant. This approach requires four poles for contemporary digital sideband levels.
Sep is stereo separation in dB. L|R, L+R, and L−R are the demodulated levels for single-channel, in-phase, and antiphase signals. Fc is the filter −3 dB corner frequency. Lev is the audio level change. 122 and 187 are the filter responses at 122 and 187 kHz, centers of the MP3 extended hybrid and MP1 hybrid mode sidebands that demodulate to 15 kHz or less. C1–C4 are the active filter capacitor values.
The filter example uses the composite response of a Sony ST-S555ES tuner with two SFE10.7MP3-A 250 kHz Murata filters. This response, down 1.5 dB at 53 kHz, should be typical of tuners that use two wide ceramic filters. To improve accuracy, measure your tuner's composite response. The program can use both amplitude and phase data. Phase is optional. It makes little difference when group delay variation is small.
This is the filter circuit. Any wideband, low-distortion op-amp will work. I've used a TLE2027A. Add a 0.1 µF ceramic across the power pins if the supplies aren't bypassed to ground nearby. Adjust the first resistor value to account for the source impedance. If it's too high or unknown, use another op-amp configured as a voltage follower to provide a low impedance. Change the filter resistance in PDO.EXE to use different capacitor values.
Select and parallel parts to come close to the target values. Then enter measured values in CHECK.EXE to check the filter response as implemented.
The stereo decoder pilot lag capacitor compensates for a phase lead in the PLL. PDO.EXE assumes that you increase its value to compensate for the group delay difference between 19 and 38 kHz in the postdetection filter. The capacitor, which some tuners omit, is C1 in the circuit on the left and C2 on the right. It goes on this pin:
Pin Stereo Decoder 1 CXA1064S LA3450 2 LM1800 LM4500A TCA4500A µA758 3 AN363 AN7470 HA1156W HA1196 HA11223W HA12016 KA2261 LA3400 LA3401 LA3410 MC1310 µPC1173C µPC1235C 12 KB4437 PA1001A 18 µPC1223C 20 LA3390
Adjust the stereo separation trimpot and pilot lag capacitance for maximum separation at 1 kHz. You may need to parallel a resistor with one that limits the separation trimpot range. For continuous lag adjustment, try a 10kΩ trimpot in series with 470 pF across the original lag capacitor.
The filter will drop the audio level by 4.4 dB. To restore the original level, increase the gain of the stereo decoder op-amps. In this circuit, parallel the 10kΩ resistors on pins 4 and 5 with 15kΩ. The −3 dB low-frequency corner will then increase from 16 to 27 Hz. If this is an issue, parallel the grounded 10 µF capacitors with 6.8 µF.
Eight years after initial authorization at −20 dBc, the FCC permitted an increase in digital sideband level to −14 dBc for most stations and −10 dBc with special permission. Today the level for 86% of stations is > −20 dBc and for 71% it is ≥ −14 dBc. The four-pole filter should fully quiet any HD Radio signal, with the possible exception of those at −10 dBc (5%) using MP3 extended hybrid mode (?%). At −20 dBc a simpler three-pole filter renders HD Radio self-noise inaudible for signals using MP1 hybrid mode. A trace of noise may remain for MP3 mode. I installed three-pole filters before digital sideband levels increased.
A postdetection filter can reduce noise for any stereo signal, not just one with HD Radio sidebands. Detected FM noise increases 6 dB per octave, the same rate that squarewave harmonic amplitudes decrease. Thus each 38 kHz harmonic can potentially contribute as much noise as that in the L−R region. The IF filter will attenuate some of this harmonic noise. A postdetection filter can eliminate the rest.
For the wide IF filter (two 250 kHz Murata MXs), 50 dB stereo quieting sensitivity for a Yamaha T-1020 was 42.4 dBf. Adding a three-pole postdetection filter increased sensitivity 2.4 dB to 40.0 dBf. For the narrow IF filter (two 110 kHz Murata MHYs cascaded with the 250s), sensitivity increased 1.5 dB from 40.6 dBf to 39.1 dBf.
This is the detected spectrum to 200 kHz for an HD Radio signal in a Yamaha T-1020 (wide IF filter).
This is the spectrum after installing the postdetection filter.
This shows the filter installed on a perfboard in the tuner. The T-1020 uses a noise-detection bandpass filter near 125 kHz to automatically select IF bandwidth and stereo/mono mode. After installing the postdetection filter, the tuner still thought most clean signals were noisy. I had to route the postdetection filter output to the noise filter and boost its gain somewhat to restore normal operation. This is typical of the complications you may encounter when adding a postdetection filter.
Here a postdetection filter is installed in a Sony ST-S444ESX. This filter uses 2.7kΩ resistors. I selected three that measured within a few ohms of 2811Ω and then used this value in the filter optimizer to determine the capacitor values. Adding a 750 pF pilot lag capacitor to the CXA1064S stereo decoder increased 1 kHz stereo separation from the high-40s to the mid-60s in dB.
This postdetection filter is installed in a Carver TX-11b tuner. I added a 1300 pF pilot lag capacitor.
This shows a postdetection filter built directly on the PCB in a Technics ST-9030 tuner.