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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.
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
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.
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
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 linearISB for AM STEREO
broadcasting. A linearISB 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 linearISB
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.
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.
A sample of
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!
¿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!
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
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.
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."
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.
Technical article about the virtues of transmittion and reception of an
Independent Sideband system.
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)
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.
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.
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
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
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.
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.
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.
AM Stereo articles from Radio-Electronics Dec77, Popular-Electronics Dec78, and Popular-Electronics Aug80.
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Here are some audio filters, high pass, low pass, 2nd & 3rd order,
bandpass filters, notch filters, phono preamp with rumble filter, and mic
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