Postdetection Filter 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. Adding a lowpass filter between the detector and stereo decoder can eliminate this HD Radio self-noise.

To avoid degrading frequency response and stereo separation, postdetection filters typically are designed for flat amplitude and group delay over the 53-kHz stereo-composite passband. Although it rolls off slowly, active filters usually employ a 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 audio-spectrum flatness. It imposes no flatness criteria on the filter itself, and it directly optimizes the pole locations instead of using a classical response function. By including the stereo decoding process 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 filters are 3 dB down somewhere between 35 and 50 kHz. Roll-off this low attenuates the L-R signal, particularly the upper sideband, but the response is such that the vector sum of the demodulated sidebands is nearly constant. This is similar to the vestigial sideband system used for NTSC television transmission. 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.)

This is the output of the filter optimizer for 50-dB minimum stereo separation and 0.5-dB audio-spectrum flatness:

           --------- A m p l i t u d e --------  --------- P h a s e ---------
 Freq Sep      L      R   L+R   L-R   LSB   USB      L   L+R   L-R   LSB   USB
 1000  67   0.00 -67.20  0.00  0.00  0.39 -0.26    0.0   0.0  -0.0   7.0  -7.7
 2000  61  -0.01 -61.17 -0.01 -0.01  0.69 -0.62   -3.5  -3.4  -3.5   3.1 -11.3
 3000  58  -0.02 -57.65 -0.02 -0.02  0.97 -0.99   -7.0  -6.9  -7.1  -0.8 -14.9
 4000  55  -0.04 -55.19 -0.04 -0.04  1.23 -1.38  -10.5 -10.4 -10.6  -4.8 -18.4
 5000  53  -0.06 -53.37 -0.07 -0.06  1.47 -1.78  -13.9 -13.8 -14.1  -8.8 -21.8
 6000  52  -0.09 -52.01 -0.10 -0.09  1.69 -2.19  -17.4 -17.3 -17.6 -12.8 -25.0
 7000  51  -0.12 -51.05 -0.13 -0.12  1.89 -2.62  -20.9 -20.7 -21.0 -16.8 -28.1
 8000  50  -0.16 -50.44 -0.17 -0.15  2.07 -3.05  -24.3 -24.1 -24.5 -20.8 -31.1
 9000  50  -0.20 -50.20 -0.21 -0.19  2.23 -3.48  -27.7 -27.6 -27.9 -24.7 -34.0
10000  50  -0.25 -50.34 -0.26 -0.23  2.38 -3.93  -31.1 -31.0 -31.3 -28.6 -36.8
11000  51  -0.29 -50.89 -0.31 -0.28  2.51 -4.37  -34.5 -34.4 -34.7 -32.5 -39.4
12000  51  -0.34 -51.81 -0.36 -0.33  2.63 -4.82  -37.9 -37.8 -38.0 -36.4 -42.0
13000  52  -0.39 -52.76 -0.41 -0.38  2.74 -5.27  -41.3 -41.2 -41.3 -40.1 -44.4
14000  52  -0.45 -52.63 -0.47 -0.42  2.84 -5.71  -44.6 -44.6 -44.6 -43.9 -46.7
15000  50  -0.50 -50.50 -0.53 -0.47  2.93 -6.16  -48.0 -48.0 -47.9 -47.6 -48.9

Fc = 34809 Hz   Inj = -4.0 dB   3rd = -30.3 dB   5th = -43.8 dB
S/N = -0.0 dB to 5 kHz, +0.1 dB to 10 kHz, +0.2 dB to 15 kHz
R = 2400 ohms   C1 = 6294 pF   C2 = 2943 pF   C3 = 358 pF

Fc is the filter -3 dB corner frequency. 3rd and 5th are the attenuations at the third and fifth harmonics of the 38-kHz stereo-decoder oscillator where the HD Radio noise lives. S/N is explained later.

Inj is the L+R stereo matrix injection required for the separations shown. Audio output will drop by this amount after the filter is installed and separation is adjusted for maximum. Usually the lower output level isn't a problem, but you can restore it to normal by changing the gain of the stereo-decoder op-amps, output op-amps, or detector.

These are the filter amplitude, phase, and group-delay responses from a circuit simulation program.

This is the filter circuit. If the detector output impedance isn't low, use a TL072 and configure the second op-amp as a voltage follower. Any wideband, low-distortion op-amp will work. Add a 0.1-uF ceramic between +V and -V if the supplies aren't bypassed to ground nearby. Ground the -V terminal in single-supply systems. I use selected parts within 1% of the values shown, but 5% parts should be good enough. You can specify the filter resistance in the optimizer.

When calculating stereo separation, the optimizer assumes that your tuner compensates for any phase shift between 19 and 38 kHz due to the IF filter and detector. This may not be the case unless your stereo decoder has a capacitor like C1 in the Sanyo LA3450 circuit or C212 in the Hitachi HA11223W circuit shown above. For best separation you may need to add such a capacitor. For the LA3400, LA3401, LA3410, HA1156W, HA1196, and Mitsubishi AN363, it goes on pin 3. For the NEC µPC1223C, pin 18. This capacitor may affect stereo distortion.

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.

Examples

I installed a postdetection filter in a Yamaha T-1020 tuner. This is the detected spectrum to 200 kHz of an HD Radio signal prior to the filter (wide IF filter selected).

This is the spectrum after 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.

This shows a postdetection filter built directly on the PCB in a Technics ST-9030 tuner. C1 and C2 are composed of parallel capacitors. The 1kΩ resistor is not part of the filter circuit. 1-kHz stereo separation was 43 dB. Instead of a fixed capacitor, the ST-9030 uses an LC circuit to set pilot phase. Adjusting the inductor increased separation, but it also increased second-harmonic distortion in the AN363 stereo decoder. I reset the inductor for minimum distortion per the service manual.

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.

S/N Enhancement

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.

There is one more noise effect. Because detected FM noise increases with frequency, the upper L-R sideband is noisier than the lower sideband. A postdetection filter may increase S/N by reducing the USB noise contribution. It may also decrease S/N by lowering the signal level even more, since signal components combine coherently while sideband noise combines incoherently. The net S/N effect depends on the particular filter response. The S/N figures in the filter-optimizer output show the small gain or loss over three frequency bands.

With the wide IF filter (two 250-kHz Murata MXs), 50-dB stereo quieting sensitivity for the Yamaha T-1020 was 42.4 dBf. Adding the postdetection filter increased sensitivity 2.4 dB to 40.0 dBf. For the narrow IF filter, which cascades a pair of 110-kHz Muratas with the 250s, sensitivity increased 1.5 dB from 40.6 dBf to 39.1 dBf with the postdetection filter.

Nonlinear Filters

Harmonic cancellers, which are implemented as nonlinear postdetection filters, are described here.

More is here.

Updated May 11, 2008