Republic of the Philippines
Iloilo Science and Technology University
La Paz Iloilo City
College of Engineering and architecture
www.isatu.edu.ph
ECE 406
(PRINCIPLES OF COMMUNICATION)
Experiment No. 6
FM STEREO TRANSMISSION
Artillo, Joshua Benedict
Deveza, Raimel Angelo
Declarador, Luelson Jay
Roldan Precious Joy
BS ECE 4-A2
I. OBJECTIVES:
1. Analyze and evaluate FM stereo transmission.
2. Plot the output waveform of the FM stereo Transmission and its frequency
spectrum.
II. Basic Theory:
In the late 1950,s several systems to add stereo FM radio were considered by
the FCC. Included were systems from 14 proponents including Crosby, Halstead,
Electrical and Musical Industries, Ltd(EMI), Zenith and general Electric. The
individual systems were evaluated for their strengths and weaknesses during field
test in union town, Pennsylvania using KDKA-FM in Pittsburgh as the originating
station. The Crosby System was rejected by the FCC because it was incompatible
with existing subsidiary communications authorization (SCA) services which used
various subcarrier frequencies including 41 and 67 kHz. Many revenue-starved FM
stations used SCAs “storecasting” and other non-broadcast purpose. The Halstead
system was rejected due to lack of high frequency stereo separation and
reduction in the main channel signal-to-noise ratio. The GE and Zenith system, so
similar that they were considered theoretically identical, were formally approved
by the FCC in April 1961 as the standard stereo FM broadcasting method in the
United States and later adopted by the most other countries. It is important that
stereo broadcast be compatible with mono receivers. For this reason, the left (L)
and the (R) channels are algebraically encoded into sum (L+R) and difference (L-R)
signals. A mono receiver will use just the L+R signals so the listener will hear both
channels through the single loud speaker. A stereo receiver will add the
difference signal to the sum signal to recover the left channel, and subtract the
difference signal from the sum to recover the right channel.
A stereo signals consist of two channels which can be labeled L and R (left
and right), once channel for each speaker. The ordinary mono signal consist of
summation of the two channels, I,e. L + R, and this can be transmitted in the
normal way. If a signal containing the difference between left and right channels,
L and R is transmitted then its possible to reconstitute the left and right only
signals. By adding the stun and difference signals I.e. (L+R) + (L-R), gives 2L, i.e. the
left signals. And subtracting the two signals, I.e. (L+R) – (L-R) gives 2R, the right
signal. This can be achieved relatively simply by adding and subtracting signals
electronically. It only remains to find a method of transmitting the stereo
difference signal in a way that does not affect any mono receivers.
To generate the 38 kHz subcarrier, a 19 kHz pilot tone is transmitted.
The frequency of this is doubled in the receiver to give the required 38 KHz signal
to demodulate the double sideband stereo difference signal. The presence of the
pilot tone is also used to detect whether a stereo signal is being transmitted. If it
is not present the stereo reconstituting circuitry is turned off. However when it is
present the stereo signal can be reconstituted.
The (L+R) Main channel signal is transmitted as baseband audio limited to the
range of 30 Hz to 15 kHz. The (L-R) signal is amplitude modulated onto 38 kHz
double sideband suppressed-carrier (DSB-SC) signal occupying the baseband
range of 23 to 53 kHz. A 19 kHz pilot tone, at exactly half the 38 kHz sub carrier
frequency and with a precise phase relationship to it, as defined by the formula is
generated. This is transmitted at 8-10% of overall modulation level and use by the
receiver to regenerate the 38kHz sub carrier with the correct phase. The final
multiplex signal from stereo generator contains the main Channel (L+R), the pilot
tone, and the sub-channel (L-R) This composite signal, along with any other sub-
carrier, modulates the FM transmitter Another way to look at the resulting signal
is that it alternates between left and right at 38 kHz, with the phase determined
by the 19 kHz pilot signal. Converting the multiplex signal back into left and right
audio signals is performed by a decoder, built into radio receiver.
In addition, for a given RF level at the receiver, the signal-to-noise ratio and
multipath distortion for the stereo signal will be worse than for the mono
receiver. For this reason, many stereo FM receivers include a stereo/mono switch
to allow listening in mono when reception condition are less than ideal, and most
car radios are arranged to reduces the separation as the signal-to-noise ratio
worsens, eventually going to mono while still indicating a stereo signal is being
received. As with monaural transmission, it is normal practice to apply pre
emphasis at the receiver after decoding.
Antenna
III. DIAGRAM:
Fm
R ADDER DELAY
. RF
SUBTRACTOR
L SCA
FM carrier
Oscillator
Frequency Pilot
doubler subcarrier
IV. Equipment/material/components:
Electronic Communication Systems, 5th edition by Wayne Tomasi
V. Procedure:
By plotting the diagram of the FM transmission and understanding of its principle using
the book we were able to derive some conclusion.
VI. Result and discussion:
FM stereo Transmitter
FM=2kHz
2kHZ 19kHz 36kHz 38kHz 40kHz 67kHz
A Pilot tone of 19 kHz is transmitted in which it was doubled at the receiver
to attain the required 38kHz signal to demodulate the doubled-sideband stereo
difference signal
The left and right signals enter the encoder where they are passed through
a circuit to add the required pre-emphasis. Then Land R audio channels are
combined in a matrix network to produce the L+R and L-R signals. Then the L+R
signal is passed straight to the final summation circuit to be transmitted as an
ordinary mono audio. Meanwhile, the difference L-R signal is passed Through the
balance modulator to provide a double sideband suppressed carrier signal
centered on 38kHz.The other signal entering the balance modulator is 38kHz
pilot. After that it will now passed the final summation circuit as a stereo
difference signal.
Raimel Angelo Deveza S.
Observation:
The left and right signals enter the encoder where they are passed
through a circuit to add the required pre-emphasis. Then they are
passed into a matrix circuit. This adds and subtracts the two signals to
provide L+R and L-R signals. The L+R signal will enter the summation
circuit to be transmitted as the ordinary mono audio. The L-R signals
will enter a balance modulator to give the double sideband suppressed
carrier signal centered on 38 kHz. And this will be the stereo difference
signal. The other signal entering the balance modulator is a 38 kHz
signal which has been obtained by doubling the frequency of the 19 kHz
pilot tone. The final modulating signal consisting of the L+R mono
signal, 19 kHz pilot tone, and the L-R difference signal based around 38
kHz is then used to frequency modulated the radio frequency carrier
before being transmitted.
Reception of a stereo signal is very much the reverse of the
transmission. A mono radio receiving stereo transmission will only
respond to the L+R signal. The other components being above 15 kHz
are above the audio range, and in any case they will be suppressed by
the de-emphasis circuitry. For stereo receiver the baseband consisting
of the stereo sum signal (L+R) and the difference signal (L-R) centered
around 38 kHz and the pilot 19 KHz tone are obtained directly from the
FM demodulator. The decoder then extracts the left only and the right
only signals. The block diagram of one type decoder is shown although
this is not the only method which can be used it shows the basic
processes that are required. The signal is first separated into its three
constintuents. The L+R mono signal between 0and 15 KHz, the pilot
tone at 19 kHz, and the stereo difference signal situated between 23
and 53 kHz. First the pilot tone at 19 kHz is doubled in frequencies.
Once the L+R and L-R signals are available they enter a matrix where
they are added and subtracted to regenerate the L and R signals. Both
signals are amplified before being converted into sound by loud
speaker or headphones.
Conclusion:
A stereo signals consist of two channels which can be
labeled L and R (left and right), once channel for each speaker. The
ordinary mono signal consist of summation of the two channels, I,e. L +
R, and this can be transmitted in the normal way. If a signal containing
the difference between left and right channels, L and R is transmitted
then its possible to reconstitute the left and right only signals. By
adding the stun and difference signals I.e. (L+R) + (L-R), gives 2L, i.e. the
left signals. And subtracting the two signals, I.e. (L+R) – (L-R) gives 2R,
the right signal. This can be achieved relatively simply by adding and
subtracting signals electronically. It only remains to find a method of
transmitting the stereo difference signal in a way that does not affect
any mono receivers.
Generating frequency modulated signal happens in FM
transmission. It work on the principle of frequency modulation
standard FM broadcast is 88—108 MHz, otherwise it is known as RF or
radio frequency ranged. Nut as long as a receiver has been turned to
demodulate they can be in any range. RF carrier wave and the input
signal can’t do much by themselves, they must be modulated. There
are also major setbacks in FM transmitter, its noise and frequency
control.
Noise can interfere with an FM signal and particularly with the
high frequency components of the modulating signal, but FM are better
at rejecting noise, A simple high-pass filter can serve as a transmitter
pre-emphasis circuit; this technique used to overcome high frequency.
Precious Joy Roldan
Observation:
From the left and right channel to antenna, the signal will pass
first through the pre-emphasis. The pre-emphasis helps amplify high
frequency signal components such that they will have magnitude higher
than noise components. After that, the signals will pass through the
adder and subtractor. The adder (L+R) with frequency of 50Hz to 15 kHz
will directly goes to multiplexer while subtractor (L-R) with frequency of
50 Hz to 15 kHz will pass through the balance modulator that mixes the
audio signal and the radio frequency carrier, but suppresses the carrier,
leaving only the sidebands. That’s why there is a DSBSC in the system.
Then after passing through balance modulator there is a pilot sub
carrier frequency of 19 kHz going to multiplexer then it will pass
through the FM modulator
From antenna to the channels, the signal will pass first through
the FM receiver. The signals will spread from different ways, it will pass
through the band pass filter with frequency of 23-25 kHz, band pass
filter with 19 kHz, and (L+R). then the both band pass filter will pass
through the balance detector that will let the signals pass if the
frequency is 50Hz – 15 kHz only. Then it will go through the adder and
subtractor going to the de-emphasis which designed to decrease,
(within a band frequencies), the magnitude of some( usually higher)
frequencies with respect to the magnitude of the other (usually lower)
frequencies in order to improve the overall signal to noise ratio by
minimizing the adverse effects of such phenomena as attenuation
distortion or saturation of recording media in subsequent part of the
system. Then it will pass through the left and right audio frequency
going to the left and right speaker.
Conclusion:
Frequency modulation (FM) was originally developed to handle undesirable
noise which is one of the main concerns when amplitude modulation (AM) is
used. FM radio broadcasting station are operating in the very high frequency
(VHF) range. The FM signals requires a considerably greater bandwidth as
compared with the maximum frequency.
A stereo signal consist of two channels that can be labeled L and R. (left
and Right), providing one channel for each of the two speaker that are needed.
An ordinary mono signal consist of the summation of the two channels. L+R,
and this can be transmitted in the normal way. If a signal containing the
difference between the left and right channels. L-R is transmitted then it is
possible to reconstitute the left only and right only signals. Adding the sum
and difference signals. (L+R) +(L-R) gives 2l the left signal. And subtracting
the two signal. (L+R) –(L-R) gives 2R. the right signal this can be achieved
relatively simply by adding and subtracting the two signal electronically. It
only remains to find a method of transmitting the stereo difference signal in a
way that does not affect any mono receiver.
Luelson Jay C. Declarador
Observation
I observed that the range of FM transmission is related to the transmitter's RF power,
the antenna gain, and antenna height. Interference from other stations is also a factor in
some places. It is important that stereo broadcasts be compatible with mono receivers. For
this reason, the left (L) and right (R) channels are algebraically encoded into sum (L+R) and
difference (L−R) signals. A mono receiver will use just the L+R signal so the listener will
hear both channels through the single loudspeaker. A stereo receiver will add the difference
signal to the sum signal to recover the left channel, and subtract the difference signal from
the sum to recover the right channel. The (L+R) Main channel signal is transmitted as
baseband audio limited to the range of 30 Hz to 15 kHz. The (L−R) signal is amplitude
modulated onto a 38 kHz double-sideband suppressed-carrier (DSB-SC) signal occupying
the baseband range of 23 to 53 kHz. A 19 kHz pilot tone, at exactly half the 38 kHz sub-
carrier frequency and with a precise phase relationship to it, as defined by the formula
below, is also generated. This is transmitted at 8–10% of overall modulation level and used
by the receiver to regenerate the 38 kHz sub-carrier with the correct phase. The final
multiplex signal from the stereo generator contains the Main Channel (L+R), the pilot tone,
and the sub-channel (L−R). This composite signal, along with any other sub-carriers,
modulates the FM transmitter.
Conclusion
Therefore i conclude that Random noise has a triangular spectral distribution in an FM
system, with the effect that noise occurs predominantly at the highest audio frequencies
within the baseband. This can be offset, to a limited extent, by boosting the high frequencies
before transmission and reducing them by a corresponding amount in the receiver.
Reducing the high audio frequencies in the receiver also reduces the high-frequency noise.
These processes of boosting and then reducing certain frequencies are known as pre-
emphasis and de-emphasis, respectively.The amount of pre-emphasis and de-emphasis
used is defined by the time constant of a simple RC filter circuit.In addition, for a given RF
level at the receiver, the signal-to-noise ratio and multipath distortion for the stereo signal
will be worse than for the mono receiver.For this reason many stereo FM receivers include a
stereo/mono switch to allow listening in mono when reception conditions are less than ideal,
and most car radios are arranged to reduce the separation as the signal-to-noise ratio
worsens, eventually going to mono while still indicating a stereo signal is being received. As
with monaural transmission, it is normal practice to apply pre-emphasis to the left and right
channels before encoding and to apply de-emphasis at the receiver after decoding.
