HD Radio Self-Noise

You arrive home from a long day at work, flip on the classic stereo system you've spent nights and weekends restoring to perfection, and tune to an FM broadcast of a live local concert. As you settle back to enjoy it, you notice something peculiar. The normally pristine audio from a station that prides itself on a high-quality signal has an annoying background noise. You check your directional rooftop antenna, but it's aimed right at the station not more than ten miles away. Your tuner's signal-strength meter is pegged to the right, as usual. What gives? Then you vaguely recall that the station recently announced it was converting soon to HD Radio.

HD Radio Signals

In its present hybrid implementation, HD Radio adds digital sidebands to an analog FM signal. The two sidebands, each consisting of hundreds of digital subcarriers, occupy spectrum from 129 to 198 kHz away from the channel center frequency. Each sideband is 23 dB below the level of the analog carrier. In some modes, additional subcarriers may extend the sidebands down to 102 kHz from channel center. These extended-hybrid signals are rare.

This shows the RF spectrum of an HD Radio signal. The horizontal scale is 50 kHz per division and the vertical scale is 10 dB per division.

Although the digital sidebands occupy spectrum beyond the normal bandwidth of an analog signal, they remain outside the receive passband of other channels that may be locally allocated. But they can easily interfere with reception of a distant station on an adjacent channel 200 kHz away since the signal spectra overlap. FCC rules do not protect long-distance reception from interference.

An HD Radio station's digital sidebands may interfere with reception of its own analog signal. A stereo decoder may mistake the sidebands as part of the analog stereo subcarrier. Because the digital subcarriers are numerous and their data randomized, analog detection yields noise. This is HD Radio self-noise.

Stereo Decoding

A stereo encoder lowpass-filters the left and right audio channels to 15 kHz and adds them to form the L+R signal. It subtracts them to get L-R and amplitude-modulates this signal onto a 38-kHz subcarrier. The subcarrier is suppressed and replaced by a phase-coherent 19-kHz pilot. These signals are combined to form a composite signal consisting of L+R to 15 kHz, pilot at 19 kHz, and double-sided L-R from 23 to 53 kHz. The composite signal frequency-modulates the RF carrier.

To recover the left and right channels, a receiver detects the FM signal and passes the composite waveform to the stereo decoder. The decoder phase-locks a 38-kHz oscillator to the 19-kHz pilot, demodulates the L-R signal to baseband with the oscillator, and then adds and subtracts the baseband L+R and L-R signals to get L and R. Lowpass filters eliminate everything above 15 kHz.

An ideal way to demodulate the L-R signal is with a multiplier. Multiplying the composite signal by a 38-kHz sine wave phase-locked to the 19-kHz pilot yields L-R at baseband plus unwanted mixing products at ultrasonic frequencies. Lowpass filtering at 15 kHz leaves L-R. This demodulator has no inherent spurious responses. Practical issues with four-quadrant analog multipliers include linearity, temperature stability, complexity, and cost. A few high-end tuners have used this approach in discrete stereo decoders, but it seems never to have been implemented as a standard integrated circuit (a custom Delco chip is described here).

IC stereo decoders instead use synchronous signal inversion. The composite signal is either passed unaltered or is inverted, with the changeovers at a 76-kHz rate synchronized to the 19-kHz pilot. The signal never undergoes variable attenuation as with an analog multiplier, so linearity depends almost entirely on the fidelity of the inversion transitions. This is easily controlled in integrated circuitry. This approach effectively multiplies the composite signal by a 38-kHz square wave with amplitude levels of +1 and -1. A square wave contains odd harmonics of the fundamental frequency, in particular the third at 114 kHz and the fifth at 190 kHz. But since the composite signal has no components near these frequencies, it decodes as if a sinewave multiplier had been used.

The problem with squarewave decoding is that detected HD Radio signals have power near 190 kHz, and extended-hybrid signals have additional power near 114 kHz. The squarewave harmonics demodulate this power to baseband. This is the source of HD Radio self-noise.

Special Stereo Decoders

The four-quadrant analog multiplier is a special stereo decoder. So is a harmonic canceller. This type of decoder uses a multilevel waveform to eliminate one or more squarewave harmonics without resorting to an analog multiplier.

This four-level Walsh-function waveform, generated by a discrete Sansui stereo decoder, has no third or fifth harmonic. The first nonzero harmonic at 266 kHz is beyond the spectrum of a detected HD Radio signal.

Pioneer developed a novel stereo decoder IC that couples to a pulse-count FM detector. Reversing roles, the binary composite signal passes or inverts a 38-kHz sine wave locked to the 19-kHz pilot. Since the pulse rate is above 1 MHz, spurious responses lie well beyond the spectrum of a detected HD Radio signal. (The multiplication circuits are simple analog switches, not four-quadrant multipliers.)

Finally, the composite signal can be decoded with digital signal processing. The 100-dB stopband rejection typical of A/D anti-alias filters eliminates HD Radio signals, while L-R demodulation by numerical multiplication incurs no linearity penalty.

Noise Level

The HD Radio self-noise level depends on receiver IF bandwidth, postdetection filtering, and stereo decoding method. Since the late 1970s, a standard receiver architecture has become common. It uses two wideband ceramic IF filters, no postdetection filter, and a squarewave stereo decoder IC. For such systems HD Radio self-noise typically is about 50 dB below the level of a full-deviation 1-kHz sine wave. The derivation of this figure is here. The great majority of home tuners and receivers made in the past thirty years should exhibit an HD Radio self-noise level within a few dB of this figure. It can be up to 10 dB higher for an extended-hybrid HD Radio signal. It will be 10 dB higher if the FCC approves the NAB proposal to increase HD Radio digital sideband levels 10 dB.

I've measured the RMS level of program audio as anywhere from 5 to 25 dB below that of a full-deviation 1-kHz sine wave. The upper figure is for highly compressed pop or rock, while the lower figure is for quiet passages in classical music. HD Radio self-noise on standard-architecture receivers therefore is 25 to 45 dB below the program level. Not surprisingly, I've found HD Radio signals to be unlistenable on every home tuner or receiver I've tried that lacked a special stereo decoder, narrow IF filter, or postdetection filter.

IF Filters

Although some equipment is more elaborate, home tuners and receivers commonly have used a cascaded pair of 280-kHz ceramic IF filters to set the receive passband.

This shows the detected spectrum in one such tuner when receiving an HD Radio signal. The horizontal axis is 0 to 200 kHz and the vertical scale is 10 dB per division. The signals to the left of midscreen are L+R, pilot, residual L-R, RDS, and two SCAs. To the right are the HD Radio sidebands.

The sideband spectrum, flat at RF, rises 6 dB per octave after FM detection. The roll-off of cascaded 280-kHz IF filters matches the rise around 150 kHz, and the filters take over above. But the sidebands are not down much between 175 and 198 kHz, the region a squarewave stereo decoder will demodulate to baseband with its 190-kHz fifth harmonic.

Narrower IF filters make quite a difference. This shows overlapped spectra for three different sets of IF filters in the same tuner. The traces are for pairs of 280-, 180-, and 150-kHz filters. The differences in HD Radio sideband level are pronounced, as are the differences in audible self-noise.

Narrow IF filters increase harmonic distortion and susceptibility to multipath distortion, primarily due to greater variation in group delay across the IF passband. Elevated multipath distortion can be quite noticeable, while the increase in harmonic distortion is measurable but not necessarily audible. Manufacturers almost universally use wide filters in home equipment that has a single IF bandwidth. This makes HD Radio self-noise louder than it otherwise might be.

I've never heard HD Radio self-noise in a car radio. This may be due to the narrower IF filters commonly employed or to more complex signal processing. Because reception is more difficult in a moving vehicle and because more people listen to FM there than at home, advanced techniques such as digital signal processing or postdetection filtering are more common in car radio designs.

Postdetection Filter

Few home tuners or receivers provide a dedicated lowpass filter between the FM detector and stereo decoder. Only a small capacitor to eliminate IF ripple is usually present. Audio bandwidth and stereo separation are sensitive to amplitude and phase irregularities in the 53-kHz composite passband. A postdetection filter that makes a significant dent below 200 kHz needs careful design to maintain flat composite response. Nevertheless, some high-end tuners do use such filters, often multipole/multizero designs with response nulls that target the 38-kHz squarewave harmonics at 114 and 190 kHz.

This passive postdetection filter in a Kenwood KT-880D completely eliminates HD Radio self-noise.

Postdetection filters sometimes are called birdie or anti-birdie filters when intended to prevent the demodulation of extraneous composite signals as tones. Such filters may or may not be effective against HD Radio self-noise. While some are complex lowpass filters, others are just LC traps resonant at the 67-kHz SCA frequency.

Eliminating HD Radio Self-Noise

The simplest way to get rid of HD Radio self-noise is to hit the mono button. The noise corrupts only the stereo subchannel. If you make no use of the L-R signal, the noise won't appear. I don't like listening to stereo music monophonically, but I often listen to monophonic spoken programs on public radio stations. Some stations kill the stereo pilot for these programs, causing your receiver to revert to mono and any HD Radio self-noise to disappear. For stations that retain the pilot, press the mono button to eliminate HD Radio self-noise.

If a signal has stereo content and you'd like to preserve some of it, engage the multiplex high-blend filter. This rolls off the highs of the L-R signal, or subjects it to flat attenuation, before combining it with L+R. The result is 3-6 dB of noise reduction with 10-18 dB of midband stereo separation. This provides a noticeably quieter signal with a surprisingly good stereo effect. However, HD Radio self-noise still will be audible unless the listening level is low.

Obtaining a wider stereo image without HD Radio self-noise is difficult. If your tuner has a selectable narrow IF bandwidth, try it. Even if this merely cascades additional wideband filters to improve alternate-channel selectivity, HD Radio self-noise should noticeably drop. Filter bandwidths of 180 kHz or less will drop the noise substantially. Using such a filter may not eliminate the last traces of HD Radio self-noise, but I think the result will satisfy most listeners.

Although narrow filters measurably degrade harmonic distortion and stereo separation, I've never been able to hear a difference in sound quality between wide and narrow IF filters in any properly aligned tuner on speech or music, except when receiving multipath-laden signals. (Distortion products and channel crosstalk are easy to hear on a sinewave test signal.) You may object to the sound of a narrow IF filter on some signals, especially when you must contend with multipath, but I think you'll find the reduction in HD Radio self-noise worthwhile in most cases.

You can retrofit your own equipment with narrower ceramic filters. Digi-Key and Mouser stock a variety of bandwidths, and filters also are available on eBay. Filter matching and tuner realignment are necessary for lowest distortion and best performance. But if you obtain a dozen filters and a breakaway in-line socket strip with which to make three-pin filter sockets, you can swap the filters around until you're satisfied with the sound.

A solution with negligible sonic penalty is to add a postdetection filter. You can make one with an op-amp and a few resistors and capacitors.

Here is one such design. This particular circuit degrades channel flatness by 0.5 dB and stereo separation to 50 dB. The filter is down 44 dB at 190 kHz, rendering hybrid HD Radio self-noise inaudible. It is down 30 dB at 114 kHz, leaving just a trace of residual noise on an MP3 extended-hybrid signal. You can design a filter to your own standards with software you can download here, where the filter design is more fully explained.

This shows a postdetection filter installed in a Carver TX-11b tuner.

Tuners

The final solution is simply to buy another tuner. I don't mean a new tuner, since home tuners are out of fashion these days and few new ones are marketed. I mean a high-quality used tuner, one made after 1980 or so with essentially mature analog technology.

Any tuner with a selectable narrow IF bandwidth ought to reduce HD Radio self-noise, perhaps to below audibility. You'll have to put up with any degradation in harmonic distortion, multipath susceptibility, or stereo separation that results. Tuners that cascade more than two wideband IF filters will reduce HD Radio self-noise, but probably not to inaudibility. The effect of IF filters is more fully explored here.

To receive an HD Radio signal without self-noise at full IF bandwidth, a postdetection filter or special stereo decoder is essential. Here is a list of tuners and receivers that have one or the other.

Postdetection Filter
Braun T2
Carver TX-11
Denon TU-800
Harmon Kardon TU920
Hitachi FT-5500MKII
Kenwood KT-880D, KT-917, KT-5020, L-07TII
Marantz 2130, 2265B
Onkyo T-4500, T-4700, T-4711, T-9060, T-G10
Phase Linear T 5200
Pioneer F-9
Sansui TU-717, TU-X1
Sony ST-J75, ST-S555ES
Sumo Charlie
Teac TX-500
Technics ST-G7, ST-G70, ST-S8

Analog-Multiplier Decoder
Kenwood KT-990D, KT-1100SD, KT-3300D, KT-7020, L-1000T
Rotel RHT-10, RT-990BX
Yamaha T-85, TX-1000, TX-2000

Walsh-Function Decoder
Denon TU-680NAB
Sansui TU-S77X, TU-S77AMX, TU-D99X, TU-D99AMX
Sony ST-S800ES

Switched-Sinewave Decoder
Pioneer F-90, F-91, F-93, F-99X

DSP Decoder
Accuphase T-1000

There must be more home equipment that will suppress HD Radio self-noise at wide IF bandwidth without modification, but I've not come across it. Technical information on hundreds of FM tuners is available here.

More 88-108 MHz is here.

Updated February 21, 2008