A Medium Wave Audio Processor

by Bruce Carter, Texas Instruments Applications

Introduction

AM radio has long been the domain of talk radio and sports broadcasts. Many people seldom change to "the other band" on their radio unless it is to hear their favorite team play, to hear a talk show, or to get the news. Before FM became widely available, AM was the primary radio band used for music. Throughout much of the third world, this is still the case. Vast distances between cities make it impractical to use FM as a broadcast band for small, isolated groups of listeners.

Many attempts have been made through the years to revive the AM band. Most recently, the FCC extended the band to 1700 kHz in an attempt to alleviate some of the nighttime clutter. In the late 1970's and early 1980's, the FCC attempted to revive interest in the band by approving several standards for stereo broadcasting. AM stereo never caught on with the public, due to the reluctance on the part of the commission to select a standard, combined with the growing popularity of talk radio and a proliferation of FM music stations.

Ironically - the rebirth of the AM band is happening due to a seeming unrelated event, the passage of the Telecommunications Act of 1996. This act has led to wholesale consolidation of radio station ownership. The large radio corporations program a bland mixture of rock, country, and talk stations in every city, and have disenfranchised many niche format fans. Niche formats are forced off the air, to streaming internet audio, or onto the AM band.

As more and more specialized music ends up on the AM band, there is renewed demand for improved AM receivers. Unfortunately, there are some disadvantages to AM:

High frequency interference is a problem at night. AM frequency response is not limited (unlike FM, which is severely rolled off above 15 kHz). Even though AM stations are allocated on 10 kHz channels, the FCC does not require that the audio frequency response be limited to less than 10 kHz. High frequency response can extend to 20 kHz or even beyond. At night, however, distant stations will be present on adjacent frequencies, and audio from those stations will mix with the high frequency audio, producing noise.

The 10 kHz spacing of AM stations causes another problem. The channel spacing is close enough that the carrier frequencies from adjacent channels are audible. These carriers produce audible tones in the audio - 10 kHz in North America, and 9 kHz in Europe and much of the rest of the world. Because AM propagates much farther at night, the problem becomes worse. In almost every case, there will be a carrier below and above the desired station. Not only will both carriers contribute to a 10 kHz whistle, but they will be at slightly different frequencies. The FCC allows AM stations to deviate (20 Hz from their assigned frequency, so the two carriers will mix together, creating an even more annoying warbling 10 kHz whistle tone.

When the carriers from adjacent stations mentioned above mix together, they create sum and difference frequencies. The sum frequencies are above the range of human hearing, and not of concern. The difference frequencies, however, are a concern. Each individual carrier can be (20 Hz from the assigned frequency. This means that their difference frequency can vary from 0 to 40 Hz (If one is 20 Hz low and the other is 20 Hz high). This difference frequency will show up in the received audio, and may be quite annoying through a subwoofer.

The Motorola C-Quam system has become the worldwide standard for AM / Medium Wave stereo transmission. Unfortunately for stations that broadcast music, C-Quam utilizes a 25 Hz pilot tone in the audio from the phase of the carrier that carries the difference (L-R) modulation. This 25 Hz pilot, unlike the pilot used for FM stereo, is not used in the demultiplexing process. It is only there to identify the station as an AM stereo station (to light the stereo light). Because it is low in frequency, it may take longer for the pilot detection circuitry to light the light than it does for the decode circuitry to operate - so the listener is likely to hear stereo separation before the light turns on. The stereo pilot will appear in both channels of the received audio. Unfortunately, this stereo pilot will make low bass unlistenable on AM Stereo stations. The pilot will also beat with any carrier difference frequency, creating even more low-frequency tones.
Whistle Filter

The schematic above is the notch filter section of the AM signal processor. It is extremely difficult to form a high Q notch filter at high audio frequencies. The Twin T notch configuration was chosen because it is unity gain, and relatively easy to tune. All passive components should be 1% except for R11 through R14, which can be 5%. The depth of the notch depends to a large extent on the matching of the components. C3 should be formed from placing two 120 pF in parallel. Care should be taken that these capacitors should be taken from the same batch. Similarly, R1 and R4 should be from the same batch, as should R2 and R5, and R3 and R6.
There are areas of the world where both 10kHz and 9kHz stations are receivable. In these areas, virtually any frequency from 1 kHz to 20 kHz is possible. It would be difficult to design a filter to reject all of these frequencies. In the unlikely event that a particular frequency is objectionable, this design can be modified to reject it: the center frequency of the notch is 1/2pR1C1. R2 = R1, C2 = C1. C3 is twice C1, R3 is one half R1. If there are more than one tone in the audio, two or more notch stages can be cascaded to eliminate them. Sections of the filter can be eliminated. A North American listener can eliminate the 9 kHz notch. The 5 kHz notch can be eliminated for listeners that do not care about shortwave.
Low Pass Filter

Each op-amp is configured as a Sallen-Key low pass filter. Stage 1 has a Bessel characteristic, and stage 2 has a Chebyshev characteristic. The combination of the two form a Butterworth filter with a roll-off of 0.5 dB at the indicated frequencies. The three frequencies selected - 13 kHz, 8 kHz, and 3 kHz are selected to match the settings on a Hammarlund SP-600JX receiver - which the author uses for Medium wave and shortwave listening.
Rumble (High Pass) Filter

The rumble filter above has been designed to reject audio below 50 Hz. It is a 5 pole elliptical filter with no more than a couple of dB ripple. Since the ripple occurs in the low bass end of the response, it should be inaudible.
Response

This audio processor has several switches to enable and disable functions as needed. The response below has the rumble filter enabled, with the 13 kHz, 8 kHz, and 3 kHz low pass filter progressively each separately enabled. The "flat" postion is not shown. The notch filter is disabled in the first response curve, and enabled in the second. 9 kHz and 5 kHz notch response is not shown, but will produce similar results.

The action of the notch makes the 13 and 8 kHz settings of the low pass filter almost identical, except for a small band of high frequencies above 10 kHz. The 10 kHz notch is not needed for the 3 kHz low pass filter. Even without it, 10 kHz is attenuated more than 30 dB.
A Simpler Alternative

There is an alternative to the audio processor above. All of the undesirable audio components can be eliminated in a single filter, if high frequency audio (above 10 kHz) can be sacrificed. The audio processor shown below is a 5 pole 50 Hz high pass elliptical filter with 0.5 dB ripple, followed by a 5 pole 8 kHz elliptical low pass filter with 1 dB of ripple. It has over 40 dB of rejection below 40 Hz and above 10 kHz.

Two of these filters should be constructed for AM stereo. If monaural reception is all that is desired, only one filter is required. Component values are critical. There is no way to convert to 5% resistor values, 1% are absolutely required. If 5% are used, filter characteristics will degrade badly. The response of the filter is:

The high pass filter will remove the pilot difference frequencies as well as the stereo pilot, while allowing bass frequencies of 50 Hz and above to pass unaltered. Because FM radio also rolls off below 50 Hz, the listener should not notice any difference between AM and FM in bass response. High frequency noise and the 10 kHz pilot tones are removed by the low pass filter. This filter also limits the high frequency response of the AM audio to 8 kHz, so the listener may notice a difference compared to FM. Because high frequency noise only occurs on distant stations in the daytime - the listener should be able to listen to local AM stations with the filter section switched out the circuit.
The circuit above is the rest of the circuit, showing the power supply, the creation of Vcc/2 (U1D), and the bypass switch. If monaural reception is all that is desired, only one set of input / output connectors are needed, and SW2 can be a SPDT.
Hosted by www.Geocities.ws

A Medium Wave Audio Processor

Introduction

Whistle Filter

Low Pass Filter

Rumble (High Pass) Filter

Response

A Simpler Alternative