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 transmittion 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 bandwith for each channel is NOT sufficient to produce
a robust digital signal that will provide dropout free preformance 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 shortcommings 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
transmittion 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 imediately 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 transmittion
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. 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 preception 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
unnecesary 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®
vraiable 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.
NEW!
Robb Spewak Show
on KCJJ with Samples of AM Stereo.
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.
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.
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.
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.
Magazine Articles
AM Stereo articles from Radio-Electronics Dec77, Popular-Electronics Dec78, and Popular-Electronics Aug80.
~29MB in size. This will take at least 10 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.
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.
