The
AM  Stereo
Tech Zone



This page is for the promotion of AM Stereo for enthusiasts with technical info on tweaking your AM Stereo receiver plus articles and graphs with the pros and cons of various systems.
Introduction

Studying AM stereo has been both a technical and electornic education and information presented here is the product of that and if I seem partial to ISB and linear synchronous detection it's because that I am. I feel that these two issues are the most important for good stereo and mono reception during adverse conditions like nighttime interference, multipath and skywave reception. The current adopted system addresses neither of these issues direcly but instead uses other approaches that are at best second rate. For DX listening nothing beats synchronous detection for extracting a signal out of a high noise enviornment and ISB filters can be used to choose either the upper or lower sidebaed to deal with interference and/or multipath problems.

I first got into AM-Stereo in the mid to late 80's when I bought the book "AM Stereo & TV Stereo New Sound Dimensions" by Stan prentiss and later I purchased a Realistic TM-152 tuner on clearance for ~$10. Not satisfied with the narrow banwidth 4.5KHz sound quality I obtained wider filters, added 10KHz notch filters, added synchronous detection and post detection equilization to boost the higher frequencies. I was able to get high quality sound out of the MC13020 chip especially in synchronous mode. Looking at the newer chips it does not appear that some will easily lend themselves to synchronous conversion and others not at all. Although the MC13020 chip does have its flaws it's the easiest to get synchronous detection out of.

I lost interest a while after my favorite station switched formats and later turned off their C-QuAM® AM Stereo. There was another station that broadcasted in the Kahn ISB system that would trigger the chip's pilot tone circuit. Decoding Kahn ISB with C-QuAM® gave a strange phasing effect for the lack of phase shift filters but the chip actually preformed better during noisy conditions on ISB than it did on a C-QuAM® broadcast. This station later turned off their AM-Stereo also. The only two stations left are Spanish ones listed as Kahn and the other one as Harris in the Stan Prentiss book. They only light the light but I hear no separation. The last station I get that has any separation is ~250 miles away from me that played old time country music and catered to truckers. Their commercials got so bad that you would hear three to four songs per hour and other spots in stereo and after September they switched to an all talk call in format. There seems to be an extreme drought around here of AM Stereo broadcasts and what a waste for all those exciters sitting on shelves collecting dust. I eventually lost interest and found computers and only recently have I found the online AM Stereo community, and I though I was the only one out there.

Most of this information is stuff I've dug out of my archives and only recently have I reassembled it and put it on the web. If some of the information seems dated its because it probably is but I want to share what I have and in time hope to bring it up to date. I have done signal analysis and have constructed some of the circuits. The others are variations of the ones I did build. Some tweaking may be required though depending on the particular application. My technical view may be different on certain issues but is just my prespective. My descriptions may or may not be completely accurate but should provide good ideas. This is presented for information purposes only so enjoy at your risk ;-).


Hardware Modifications

MC13028 Decoder Board with wide ceramic filter and post detection filtering: NRSC Eq, high Q 10KHz tunable notch filter, and 2nd order low pass filter with a ~+11db boost at ~9.25KHz.

Extra Mono - Has your favorite AM Stereo music station been assimilated by the "Evil Empire" and the station managenemt has been seduced by the "Dark Side" into turing off the AM Stereo Exciter with there being no possibility of it ever being turned back on? Well most likely they are still broadcasting out to the 10.2KHz NRSC limit and you can still enjoy that wideband sound. This curcuit will give you that wideband response and provides a 10KHz notch filter to eliminate the adjacent carrier whistle. It is much simplier than an AM Stereo decoder to build and could be assembled and installed in a short period of time.

Synchronous Detectors Technical article about the superiority of synchronous detection over envelope detection. Here is a theoretical circuit for QuAM (Harris) Stereo detection using discrete components.

Independent Sideband Technical article about the virtues of transmittion and reception of an Independent Sideband system.

NEW! Here is some theory on Audio Phase Shift Networks for Single/Independent Sideband Modulation and here is aCircuit to build one. This circuit can be used with a C-QuAM® exciter to generate a C-ISB© (envelope compatible) Stereo Signal. Place it between the last auduo processor and the exciter. Maximum sideband separation is obtained when both of the exciter's channels are phase and amplitude matched. With the phasers properly calibrated it will provide up to a theoretical minimum 53db sideband separation with a maximum ~±¼° phase error from 150Hz-12.8KHz when used with a pure QuAM exciter, and somewhat less with a C-QuAM® exciter. A C-QuAM® decoder with post detection phase shifters similar to these can be used to properly receive the C-ISB© signal distortion free and also can be received with two mono radios tuned to upper and lower sidebands with minor distortion.

75us De-Emphasis Here is a graph of the de-emphasis curve that is needed for the correct equilization when receiving a broadcast from a station that follows the NRSC ANSI/EIA 549-1988 standard metting the AMAX specification. To select the correct resistor capacitor combination here is a table with various combinations along with frequency response of each combination. The formula for this is F(f)=((jf/~2122)+1)/((jf/8700)+1) where: ~2122=105/15π

NEW! Signal Pre-Processing for TX. 600ohm Balanced Input with Gain Adj., 5th Order 50Hz High Pass Filter, 3rd Order 170Hz High Pass L-R Filter, High Frequency Peak Smoother, 5th Order 10.2KHz Low Pass Filter, Choice of Audio Compressor, and NRSC Pre-Emphasis.

NEW! eRIAA Phono PreAmp with 3rd Order 20Hz Butterworth High Pass Rumble Filter. Here is the eRIAA equalization curve for the preamp section. More info available from the Phono Page.

NEW! Realistic TM-152 Schematic

Realistic TM-152 AMAX Upgrade This circuit is to update the Realistic TM-152 to meet the AMAX frequency response using a dual IF filter setup, 10KHz notch filters, NRSC 75us de-emphasis, 6db buffer amp, tweaked up pilot tone circuitry and an optional sychronous adapter. The only thing missing is a noise blanker.

Realistic TM-152 Harris Synchronous Detection Hack This circuit is more complex than the one in the AMAX drawing to hack the Motorola®reg;reg; MC13020 decoder chip for Harris synchronous detection. It provides automatic switching when the PLL is locked and a center tuning indicator.

Harris Synchronous Detection Hack for DTR This is a somewhat less complex circuit than the one for the TM-152 as it does not provide a center tuning indicator because it is not necessary but does provide automatic switching when the PLL is locked.

NEW! A sample of 820AM WBAP AM Stereo from a MC13020P chip in synchronous detection (QuAM) mode.

Toyota 56801 AM Stereo radio.

C-Quam Decoder This is a schematic for a C-QuAM® adapter using the Motorola® MC13020 decoder chip to convert a mono AM receiver to stereo. It has a ±7.5KHz IF ceramic filter, 10KHz notch filter, low pass chebychev filter to boost frequencies above 7.5KHz for a 3db response at 9.KHz that also provides of 6db overall gain, AGC with a PLL style loop filter for a constant carrier level tracking, and a tweaked pilot tone and co-channel circuit to for positive stereo detection even under marginal conditions. This has a flat frequency response and does not have AMAX equilization.

Building A Better C-QuAM® Decoder A more advanced version with PCB and Docs using the Motorola®reg;reg; MC13020 decoder chip.

NEW! AM Stereo Equations System equations for the 5 competing systems.

Graphs displaying Envelope, I*Cosθ, Q*Cosθ, θ & Cosθ for C-QuAM® & C-ISB© The phase in these composite plots are in radians. For direct comparison between C-QuAM® & C-ISB© for Phase & Cosθ look here.

Spectral Analysis This is the spectral analysis of the Proposed systems. The Belar system is not displayed because the formulas use intergration and are more compex. The only difference between FM & PM is that one is the derivative of the other. Most FM systems use pre-emphasis and for signals below the corner frequency the sytem acts as like a pure FM system but above the corner ferquency the pre-emphasis sort of acts as a derivative and causes the FM modulator to act like a phase modulator. This quasi-derivative action causes a ~90° phase shift along with a 6db/oct boost. As a result the signal has sort of a ISB effect when amplitude modulated and sidebands on one side of the carrier are slighty stronger than the other side as compared to regular PM. Cosine Analysis is the spectral content of various signals that have been cosine corrected. C-ISB© refers to an Independent Sideband version of C-QuAM® where L-R is audio phase shifted by +90°. Armstrong refers to a type of phase modulation where the in phase 'I' channel of a QuAM signal contains a carrier with no modulation and the out of phase 'Q' channel is modulated with the signal. Limiting the signal applies cosine modulation. Linear Analysis shows the spectrum of systems based on linear phase modulation. Magna ISB refers to the Magnavox system with L-R audio phase shifted by 90°. Various ISB Analysis shows all the various ISB schemes. Enough information on the Kahn system was not available regarding the 2nd harmonic distortion cancellation term to derive the actual spectrum of this system. The latest version of the Kahn exciter has been made completely compatible with the default decoding technique so the analysis is based upon a signal that is mathematically generated be the reverse process of the decoding process. It is not directly based upon linear phase modulation is derived through the rectangular co-ordinates of I & Q vectors. Magna ISB probably closely represents the Kahn system without 2nd harmonic correction. Given the limitations of Excel I can't guarantee the complete accuracy of these graphs but the spectrum for C-QuAM® and Magnavox AM/PM does appear to closely resemble other published data.

The Original Motorola® C-QuAM® Patent 4,218,586

Magazine Articles AM Stereo articles from Radio-Electronics Dec77, Popular-Electronics Dec78, and Popular-Electronics Aug80.

Audio Filters Here are some audio filters, high pass, low pass, 2nd & 3rd order, bandpass filters, notch filters, phono preamp with rumble filter, and mic preamp.

All these modifications are centered around the Motorola® MC13020 C-QuAM® AM-Stereo decoder chip. In the future I hope to have more information on the newer Motorola® chips and maybe some of the japanese chips if they are still available. I will try to add more technical articles, schematics, and graphs in the future. Information may be revised from time to time to improve and clarify for accuuacy purposes. I want all you hobbyist and tinkerers to take full advantage of this information in the promotion and enjoyment of AM Stereo. Download it and share it with your freinds. If you have any questions or comments about this page and its links contact me.

AM Stereo Forum (Yahoo groups)

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©2001 J. S. Gilstrap All Commercial Rights Reserved.

Use of "The AM Stereo Tech Zone©" is restricted to refering to this site and is copyrighted. If anyone wants to use any of this information in a national publication in printed or electronic form it must be used with my permission so please contact me.