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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.

Old Intro

Analog AM is still the best transmission mode for commercial broadcast bands below 30MHz. Attempts at going to a digital system have not proven to be viable. Receiver technology required is more complex and expensive. The power required to run the digital processing is many times greater than a simple analog AM radio. On regular inexpensive batteries a simple analog radio will last quite some time. The available bandwidth for each channel is NOT sufficient to produce a robust digital signal that will provide dropout free performance and a high level of sound quality of 50Hz-12.5KHz (200Hz-10KHz minimum in Stereo) at the same time. There is a trade off for one or the other. A robust encoding scheme that will provide the same range as an analog signal will produce a much lower fidelity signal for all receivers regardless of range where analog AM can provide near FM quality during good signal conditions with a wide bandwidth receiver and narrow bandwidth modes can be used on marginal signals. Analog AM, with all its shortcomings of weather interference, skywave issues, etc... has proven itself over decades of use to be a good all around choice for MW and SW bands and can be received on simplest to the most advanced receivers.

Choosing an envelope compatible analog AM STEREO transmission method as an overlay onto the existing broadcast system is the least disruptive and least expensive for both TX & RX. Older radios will still be usable and in developing countries where the simplest and cheap radios are the most widely used replacement of them is unnecessary, where most could not afford to do so. Even in the U.S. complete replacement of all analog radios is more than an inconvenience it is costly and many will decide not to purchase one immediately if at all. Just look at the issue of migration to digital TV and the need for vouchers to purchase converter boxes for analog sets. Through attrition these older radios could eventually be replaced with ones providing AM STEREO reception just as it did with the introduction of FM STEREO.

The term 'envelope compatibility' has been thrown around as criteria to be met for existing 'diode' type detectors. The truth be told any method that forces the envelope of a phase modulated carrier to carry 1+L+R is considered non-linear in the sense that the sidebands contain harmonic content that did not exist in either 1+L+R or L-R modulating signals. This is true for C-QuAM®/C-ISB™©, Magnavox® PMX®, Belar® AM/FM and Kahn® ISB transmittion methods where Harris®(QAM) and linear ISB(QAM with audio phasing) are considered linear and produce sidebands with the same harmonic profile as both the L+R & L-R modulating signals combined. During the transmission and reception process if any antenna and/or IF filtering that introduces any abnormal phase/delay shift to these non-linear signals the envelope will no longer carry 1+L+R properly but will also contain a quadrature type of distortion and older 'diode/envelope type detector' radios tend to have less than perfect linear phase/delay effects on the signal.

This begs the question if the benefit of using a non-linear type of transmission method for compatibility with older radios is realized when their design shortcomings will tend to make them perform less than optimal on these type of signals. For the linear versions of QAM & ISB both the mathematical theory and using test tones during field testing shows that distortion for an envelope detector will be present and may be objectionable to the human ear during two channel modulation, but distortion on regular program material rarely if ever is noticed by the human ear as being objectionable and if it is noticed it will usually appear as a treble boost which can also be a benefit to the older narrow band radios, kind of a natural Spectral Band replication for envelope detectors that sounds better in most cases than SBR in some digital schemes proposed for the MW band. Under extreme conditions where program material might produce objectionable distortion this would be better addressed with a matrix processor that would reduce separation to a point where the perceived distortion would be considered acceptable. The ability to apply any separation reduction would be better tolerated in the lower frequencies where separation perception to the human ear is less critical and distortion caused by the lower frequencies in the L-R channel would have the potential to produce a greater degradation of signal listenability. In the higher frequencies where separation has the biggest spatial effect in the listening environment separation reduction will be mostly unnecessary and any quadrature related harmonic distortion generated will fall outside the hearing range or reduced enough with low pass filtering in the receiver. This approach would produce minimal audible impact to the listener while reaping the benefits of using synchronous detection for these linear type of signals, a feature that has long been desired or available on modern higher-end radios. For the argument of using a linear (less compatible) vs. non-linear (questionably more compatible) system and its effects on existing envelope detector radios it can be observed that envelope detection during nighttime interference conditions can produce more objectionable distortion where a synchronous detector would protect against it than what would ever be produced from a full quadrature stereo type of signal being envelope detected.

The downside to linear QAM modulation is the need for reduction of mono signal loudness to make room for the 14.4% increase in envelope level during +125% single channel modulation, a power decrease for the mono only signal of 23.6%, something that Motorola® addressed nicely with C-QuAM®. However, this issue could be decreased with matrix processing much the same way that the Harris® V-CPM® vriable angle QAM method was used to improve envelope compatibility. For linear ISB the loudness issue is less of a problem thanks to the audio phasing necessary for ISB and the natural effects on the overall peak envelope level. Less level correction with matrix processing will be necessary for maintaining power levels and envelope compatibility providing a more natural sounding signal. The only downside to ISB is the need for the accurate audio phasing filters in the receiver, a trade off that would be well worth using linear ISB for AM STEREO broadcasting. A linear ISB signal using synchronous detection is the most robust analog RX/TX method available during less than optional conditions like DX/Skywave and poor weather conditions.

If a linear ISB system was chosen back in the early 1980's where the low frequency portion of the L-R signal was reduced in level to provide acceptable compatibility with envelope detection and all AM radios made from thereon were required to have synchronous detection then today enough of the older envelope based radios would have been replaced to allow the use of a greatly reduced carrier level by as much as ⅛ the power level leaving more power available for the actual sideband information. At this time the low frequency L-R information could be unattenuated and returned to its normal level. The carrier power needed would only have to be enough to allow the PLL to lock onto even in the noisiest conditions. Reducing the carrier level would also help eliminate the need for the 10kHz adjacent carrier whistle except under the most extreme levels of interference. If this migration path was followed we would have had one of the best sounding AM Stereo systems today.

A linear ISB system would also function as 2 individual audio channels transmitting different programs. Since the channels would be separated by frequency and not phase there would be no crosstalk with well engineered transmitters and receivers even under some of the poorest reception conditions. At night if a station was having trouble covering its area because of interference to only one of its sidebands the receiver operator could just selectively choose the cleaner sideband and the station could also choose not to transmit the sideband that would suffer the most interference. This could also help to reduce some of the the skywave interference problems. All a radio would need for user control is a switch for Mono, LSB, USB, & Stereo. 30 years ago technology existed to do this effectively and if this was adopted then then today we would have doubled the number of channels per carrier that a broadcaster could utilize. 2 mono channels that would more than rival the sound quality of a 192kBPS mono stream during high quality signal conditions especially during daytime hours.

Here is a forum message that was too long so I posted a link in the shorter forum message.

Excerpt from the 1988 ARRL Handbook about Amplitude Compandored Single Sideband
ARRL Synchronous Detection of AM Signals

Harris Synchronous V-CPM Demo Record

A sample of 820AM WBAP AM Stereo from a MC13020P chip in synchronous detection (QuAM) mode demonstrating the robustness in a high noise environment. This was received from a 50kW station at 230 miles (370km).

What's all that buzzing about? No analog AM now, DX or local! Welocme to digital
hash thank's to Din of iNiquity's iBloc HD Radio = All Buzz, All the Time!

Courtesy of Google Translate:
(If there are mistakes you know what's to blame ;-)

Was ist all das geschwirr herum? Keine analogen MW/AM jetzt, DX oder lokale! Willkommen auf digitalis
Rauschen dank Lärm/Höhle von die iNiquity iBlockieren HD Radio = Alle Schwirren, die Ganze Zeit!

Qu'est-ce que tout ce qui bourdonnaient autour? Pas analogique MW/AM maintenant, DX ou locale! Beinvenue au bruit
numéique merci à Bourdonnement/Lieu de l'iNiquité iBloquer HD Radio = Tous les Bourdonnement, en Tout Temps!

O que é tudo o que zumbindo em volta de? Não analógico MW/AM agora, DX ou local! Bem-vindo ao ruído
digital graças a Ruído/Caverna de la iNiquidad iBloquear HD Rádio = Todos Zumbido, Todo o Tempo!

Atención México:
¿Qué es todo ese zumbido alrededor de? ¡No analógica MW/AM ahora, DX o local! Beinvinedo a ruidos
digitalis gracias a Estruendo/Cueva de iNiquidad iBloquear HD Radio = ¡Todos Zumbido, Todo el Tiempo!

Che è tutto quel ronzio intorno? No analogico MW/AM ora! DX o urbano. Beinventui al rumore
digitale grazie a Rumore/Tana di iNiquità del iBlocco HD Radio = Tutto Ronzio, Tutto il Tempo!

NEW! NRSC & IBOC Transmission Mask
A message from the AD Council.

Robb Spewak Show on KCJJ with Samples of AM Stereo received on the Meduci Pro1K.

NEW! Meduci has AM Stereo radios available for sale if you are looking to purchase one.

Hardware Modifications

TAPR Open Hardware License PDF ODT TXT

NEW! - MC13028 Pocket Radio
Schematic for an AMAX spec. pocket sized radio with 11kHz audio bandwidth,
10kHz whistle filters, and post detection eq. for proper de-emphasis.
Here is MC13028 Pocket Radio (Version 2)
Here is MC13028 Pocket Radio (Version 3) using a quad lower power op-amp in the post filter section. Audio Response and the Spreadsheet to calculate it.

NEW! - A Minimal Front End with RF wAGC / LO / Mixer / Ceramic IF Circuit and boards PCB & SMD.

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 +12.2db boost at ~11.3KHz. Here is an SMD version the size of a business card that just contains the decoder and post detection filtering but not the 8V regulator, IF input amps or IF filter. The post detection filtering is tuned for an IF using a ±10KHz ceramic filter.

C-QuAM AM Stereo in Japan

NEW! Stand Alone Post Detection Filtering. This filtering will provide NRSC De-Emphasis and 10kHz whistle rejection.

NEW! 3 Stage IF Filter and a Closup This setup provides a smooth wide bandwidth response with a very flat passband, better than some ceramic filters. Used with a RF bandpass of ±7.5kHz, a 78.5µs de-emphasis with a pole at 5.565kHz, and a Chebyshev Low Pass Filter at 15.33kHz with a Q of 1.75 providing a boost of 4.85dB at 'f' will produce a -3dB response at 12.5kHz for transmitted signal with 75µs pre-emphasis with a 8.7kHz pole.

Extra Mono - Has your favorite AM Stereo music station been assimilated by the "Evil Empire" and the station management 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.
Here is a schematic of a updated Hi-Fi version without pre-emphasis and >15khz detected audio bandwidth where bandwidth is only limited by RF & IF filtering. As signal input decreases the AGC amp will increase gain by narrowing the bandwidth and increasing the input tank Q."

NEW! A A Discrete QuAM Synchronous Detector with Post Detection Eq.

NEW! Discrete QuAM & ISB Document with circuits.

NEW! C-QuAM & C-ISB Exciter Document with circuits.

NEW! C-ISB Decoder using a MC13028 and the 3 stage phase shift network attenuation graph.

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

NEW! Spreadsheet - Transistor Base-Emitter Vbe & r'e vs. Current & Differential Pair Linearity

NEW! Quadrature Modulatoion, 4 Different Ways. A Block Diagram of 4 different AM STEREO modulating schemes using quadrature modulation.
NEW! Here is a 4-Way Quad Mod Decoder using the MC13020 chip this will do C-QuAM, C-ISB, and Synchronous QuAM & ISB.

NEW! Compatible Vestigal SideBand If you don't have stereo audio available for your C-QuAM exciter it can still be used to generate a mono vestigal sideband signal offering spectrum and power saving benefits as well as better modulation levels. This is essentially a C-QuAM version of Kahn PowerSide, which used one channel of a Kahn ISB exciter, and may offer better sideband suppression.

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 audio 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. Here is post detection filtering for a 187Hz to 10.8kHz phase shifter with a ~±1° accuracy along with NRSC Eq. for a MC13028 decoder.

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® 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.

External Antenna Torroid replacement for TM-152

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.

NEW! MC13020 Pilot Tweak PCB

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

Block diagram of a forward acting C-QuAM decoder.

Miscellaneous Schematics

NEW! Shapes for Dia drawing program (Win/OSX/Linux), Schematics & Circuit Boards. ZIP TAR (RPM Older version. Use Zip or Tar instead.)

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 complex. 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.

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

NEW! FCC Broadcast Standards for AM Stereo.

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

NEW! Motorola® C-QuAM® Decoding Chips & Associated Datasheets
MC13020 Motorola C-QuAM® AM Stereo decoder
Application Note AN-HK-07 for the MC13021 & MC13041 support chips for the MC13020
MC13022 C-QuAM® Advanced Medium Voltage AM Stereo Decoder
MC13024 Low Voltage Motorola C-QuAM® Am Stereo Receiver
MC13025 ETR® Front End for C-QuAM® AM Stereo
MC13027 & MC13122 Motorola C-QuAM® AMAX Stereo Chipset
MC13028 Advanced Wide Voltage IF and C-QuAm® AM Stereo Decoder
MC13029 Advanced Medium Voltage IF and C-QUAM® AM Stereo Decoder with FM Amplifier and AM/FM Internal Switch
MC13030 Dual Conversion AM Receiver
MC1496 Gilbert Cell Double Balanced Modulator, Mixer, and Detector
Low voltage single supply Op-Amps. MC34074 High Performance & MC33174 Low Power.
Magazine Articles AM Stereo articles from Radio-Electronics Dec77, Popular-Electronics Dec78, and Popular-Electronics Aug80.
~29MB in size. This will take a few minutes to download.
To save bandwidth: instead of viewing this imbedded in the browser please 'right mouse click --> save link/target as' to disk and view locally to avoid multiple downloads when viewing later on.

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.

Amidon Jan 2001 Inductive Component Catalog with extensive graphs and information for iron powder and ferrite cores.

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.

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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.