Nearly all stereo decoders use a 38 kHz square wave to demodulate the L−R subchannel, which lies between 23 and 53 kHz. An unintended 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 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 typically exhibit flat amplitude and group delay over the 53 kHz stereo-composite passband. Although it rolls off slowly, active filters usually employ a phase-linear Bessel response. For example, the Philips TEA6880H stereo-decoder chip for car radios includes an eight-pole, 80 kHz Bessel filter that provides 19 dB of attenuation at 190 kHz. The TDA1502 yields 17 dB with a four-pole filter. With the help of narrow IF filters and a little road noise, these rather anemic postdetection filters still may render HD Radio self-noise inaudible.
To address the problem for home tuners with wide IF filters in a quiet environment, I wrote a computer program to model the stereo decoding process and optimize a postdetection filter. The program seeks the filter with maximum attenuation at 190 kHz for a specified minimum stereo separation and maximum frequency response error. Instead of using a classical response function, the program directly optimizes the pole locations. It imposes no spectral flatness constraint on the filter itself, only on the decoded output. By including the stereo decoder in the model, the optimizer can take advantage of redundancy in the double-sideband L−R signal. This approach requires just three poles.
The program couples a downhill-simplex local optimizer with a stochastic global optimizer. The resulting optimal filter is down 3 dB somewhere between 30 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 vector sum of the demodulated sidebands is nearly constant. When installing the filter, you compensate for the additional L−R attenuation by readjusting the tuner's stereo separation control. (Often you must change a fixed resistor that limits the control range.)
Freq Sep L|R L+R L-R Fc 34869 Hz 1000 68 0.0 0.0 0.0 Inj -4.0 dB 2000 62 0.0 0.0 0.0 114 -30.2 dB 3000 59 0.0 0.0 0.0 190 -43.7 dB 4000 56 0.0 0.0 0.0 5000 54 -0.1 -0.1 -0.1 R 2400 Ω 6000 52 -0.1 -0.1 -0.1 C1 6243 pF 7000 51 -0.1 -0.1 -0.1 C2 2909 pF 8000 51 -0.2 -0.2 -0.1 C3 361 pF 9000 50 -0.2 -0.2 -0.2 10000 50 -0.2 -0.2 -0.2 11000 50 -0.3 -0.3 -0.3 12000 51 -0.3 -0.3 -0.3 13000 52 -0.4 -0.4 -0.4 14000 52 -0.4 -0.4 -0.4 15000 50 -0.5 -0.5 -0.4
This is the program output for 50 dB minimum stereo separation and 0.5 dB maximum frequency response error. 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. All figures assume perfect performance before adding the filter.
Fc is the filter −3 dB corner frequency. Inj is the L+R stereo matrix injection. 114 and 190 are the attenuations at 114 and 190 kHz where the HD Radio noise lives. R, C1, C2, and C3 are active filter component values. See the next section for the circuit.
The audio level will change by as much as Inj when you install the filter and readjust 1-kHz separation for maximum. Stock L+R injection often is about −0.8 dB so the actual level change usually will be less than shown. The lower output level seldom is a problem, but you can restore it to normal by increasing the gain of the stereo decoder op-amps or the output amplifiers.
The optimizer assumes that the tuner compensates for any excess phase shift between 19 and 38 kHz due to the IF filter. Normally this is done with a capacitor like C1 in the Sanyo LA3450 circuit or C212 in the Hitachi HA11223W circuit shown above. You may need to add a capacitor if none exists. For the AN363, AN7470, HA1156W, HA1196, LA3400, LA3401, LA3410, and µPC1235C it goes on pin 3; for the µPC1223C, pin 18; and for the TCA4500A, pin 2. See the next section for more.
This shows convergence zones in the complex plane for the global optimizer. Blue and red dots are local-optimizer starting points, while yellow and cyan are where it wound up. The black region is beyond the filter specifications and remains unexplored. The global optimum is at the rightmost cusp.
The previous example assumed perfect performance before adding the filter. This is unrealistic. The stereo composite response of analog tuners is never flat. It slopes due to the IF filter, and the detector may contribute roll-off as well. If you can measure the composite response, you can improve optimizer accuracy. Performance will degrade from the ideal case, but the optimizer can make the best possible trade-offs given the true response. Moreover, if you allow the program to optimize pilot phase, which is equivalent to adjusting the stereo decoder phase compensation capacitance with the filter installed, you can actually improve performance.
Freq Sep L|R L+R L-R Fc 38014 Hz 1000 77 0.0 0.0 0.0 Inj -4.3 dB 2000 67 0.0 0.0 0.0 114 -31.3 dB 3000 61 0.0 0.0 0.0 190 -44.9 dB 4000 57 0.0 0.0 0.0 5000 54 -0.1 -0.1 0.0 R 2400 Ω 6000 52 -0.1 -0.1 -0.1 C1 9059 pF 7000 51 -0.1 -0.1 -0.1 C2 3144 pF 8000 50 -0.1 -0.2 -0.1 C3 268 pF 9000 50 -0.2 -0.2 -0.2 10000 51 -0.2 -0.3 -0.2 Pilot phase +2.6° 11000 52 -0.3 -0.3 -0.3 Composite SONY.TXT 12000 56 -0.3 -0.3 -0.3 13000 66 -0.4 -0.4 -0.4 14000 59 -0.4 -0.4 -0.4 15000 50 -0.5 -0.5 -0.5
This is the program output for 50 dB minimum stereo separation and 0.5 dB maximum frequency response error for a typical composite response with optimized pilot phase.
The pilot phase value is for reference only. Simply adjust the phase compensation capacitance and the stereo separation trimpot for maximum separation at 1 kHz. To avoid interaction with the separation trimpot, adjust the capacitance alone for minimum response to a test signal with 1 kHz quadrature L−R modulation. Create the signal with this stereo generator and a 192 kHz sound card. Instead of swapping fixed capacitors, try a larger capacitance with a series trimpot for continuous adjustment. The phase compensation capacitor may affect the low-frequency stereo distortion of some stereo decoders.
The optimizer ZIP file includes SONY.TXT. This composite response should be typical of many tuners that use two wide ceramic filters. If you can't measure your tuner's response, use SONY.TXT instead.
This is the filter circuit. Any wideband, low-distortion op-amp will work. Add a 0.1 µF ceramic between +V and −V if the supplies aren't bypassed to ground nearby. Ground the −V terminal in a single-supply system. Adjust the first resistor value to account for the source impedance. If it's too high or unknown, use a TL072 and configure the second op-amp as a voltage follower to provide a low impedance. I use selected parts within 1% of the values shown, but this is probably overkill. You can specify the filter resistance in the optimizer to use different capacitor values. A high resistance may lower C3 enough that you must account for the op-amp input capacitance. After installing the filter, adjust the stereo separation trimpot and the stereo decoder phase compensation capacitance for maximum separation at 1 kHz.
Freq Sep L|R L+R L-R Fc 39005 Hz 1000 66 0.0 0.0 0.0 Inj -3.5 dB 2000 60 0.0 0.0 0.0 114 -22.5 dB 3000 57 0.0 0.0 0.0 190 -34.9 dB 4000 55 0.0 0.0 0.0 5000 53 0.0 0.0 0.0 R 2400 Ω 6000 52 0.0 0.0 0.0 C1 2926 pF 7000 51 0.0 0.0 0.0 C2 1575 pF 8000 50 -0.1 -0.1 0.0 C3 481 pF 9000 50 -0.1 -0.1 -0.1 10000 50 -0.1 -0.1 -0.1 Composite SONY.TXT 11000 50 -0.1 -0.1 -0.1 12000 51 -0.1 -0.2 -0.1 13000 51 -0.2 -0.2 -0.2 14000 51 -0.2 -0.2 -0.2 15000 50 -0.2 -0.3 -0.2
This result is for the same constraints but without pilot phase optimization. HD Radio self-noise suppression is 8.8 dB worse at 114 kHz and 10 dB worse at 190 kHz.
The blue curve is the filter response with pilot phase optimization and the red curve is without.
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 the postdetection filter of the first example (not optimized for the composite response) 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 in the 125 kHz region to automatically select the IF bandwidth and the 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 then 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 phase-compensation capacitor to the CXA1064 stereo decoder increased 1 kHz stereo separation from the high-40s to the mid-60s in dB.
This shows a postdetection filter built directly on the PCB in a Technics ST-9030 tuner. Parallel capacitors comprise C1 and C2. The 1kΩ resistor is not part of the filter circuit.
Harmonic cancellers, which are implemented as nonlinear postdetection filters, are described here.