Multiple audio channel broadcast system

A microwave system is provided for the broadcast of multiple channels of audio programming to a wide listener base, in which noise-free transmission of multiple audio channels is accomplished through microwave transmission followed by down converting the received signal to television band frequencies, accomplished in one embodiment through the use of a single MDS channel.

FIELD OF INVENTION 
This invention relates to the transmission and distribution of multiple 
channels of audio programming such as music and more particularly to 
microwave transmission of the audio channels followed by down conversion 
to TV channel frequencies. 
BACKGROUND 
In the past either telephone lines or subsidiary communication 
authorization (SCA) systems which involve subcarriers on FM broadcast 
stations were utilized to transmit background music and the like. Land 
line systems are expensive, while SCA systems which broadcast on 
subcarriers of FM radio stations in the 88-108 MHz band, are prone to 
noise and only facilitate two good quality subcarriers. Moreover the 
bandwidth of both systems is only 5 KHz which eliminates most high 
frequency audio components. 
On the other hand, multi-point distribution system (MDS) channels operating 
at 2 GHz have in the past been utilized for dissemination of video to a 
limited number of locations. Originally, the MDS common carrier system was 
authorized for only point to point video applications. Because MDS systems 
were used exclusively for video programs, this particular service was 
underutilized and the Federal Communications Commission has now provided 
licenses for audio programs to be transmitted via microwave as a 
replacement for sub-carrier authorization service or the use of telephone 
lines. 
In order to adapt the video MDS system to the provision of multiple audio 
programs, in the past it has been suggested that one transmit audio 
program material on the microwave TV audio channel, with sub-carriers 
multiplexed to provide for multiple program channels. This permits a 200 
KHz audio frequency response range so as to accommodate and surpass the 
requirements of high fidelity material. The problem with this system is 
that since there is no video transmitted there is an exorbitant amount of 
wasted energy transmitted. This is because video related signalling such 
as the video carrier and color bursts are transmitted even if there is no 
video. This means that the effective power of the audio channel is reduced 
dramatically. For instance, assuming 100 total watts power, the entire 
audio transmission can only utilize approximately 25 percent of the 
allocated power. This dramatically reduces the possible coverage to a 
quarter of what it could have been had all of the energy been concentrated 
in audio programming. Note, with respect to MDS systems the transmission 
is from a single transmitter location to multiple points which gives rise 
to the designation of multiple-point distribution system. 
SUMMARY OF THE INVENTION 
As the solution to the problem of power and range, the subject system 
utilizes individual audio sub-carriers throughout what was originally the 
video band width. To this end FM subcarriers are generated, one each 
corresponding to an audio channel, with the FM subcarriers being combined 
and transmitted at microwave frequencies to remote locations where they 
are down-converted to TV channels and detected by FM detectors, each tuned 
to a different subcarrier frequency. Here in one embodiment numbers of 
subcarriers, each tuned to a different frequency corresponding to a 
different audio channel, are combined and used to modulate a 2 GHz AM 
transmitter. The output of the transmitter is filtered to remove the AM 
carrier, with the resultant signal amplified and coupled to an 
omnidirectional microwave antenna. By the utilization of this type of 
system the filter normally utilized after the AM modulation of the video 
signal in the above multiplexed MDS service can now be retuned to 
eliminate the carrier, thereby providing nearly double the power for the 
audio programs. The result is the transmission of individual FM 
subcarriers, one each attributable to a different audio channel or 
program, with the AM carrier and unwanted sidebands removed. The result is 
that the entire transmission power is dedicated to these subcarriers. It 
will be appreciated that these FM subcarriers are in essence the same as 
FM radio stations found on the FM broadcast band. The difference is that 
the FM subcarriers of the subject system appear within one of the channels 
designated for MDS service, for instance the 2150 to 2156 MHz band. What 
this means is that the audio signals are transmitted in the microwave 
region to various locations. 
At each recipient location the microwave signal is heterodyned to TV 
channel 5 or 6, where through the utilization of conventional FM receiver 
technology the signals are individually detected and reproduced on 
different audio channels corresponding one each to the individual 
programs. 
It is therefore possible to provide 5 or more programs on a single MDS 
channel. The resultant power for multi-channel audio programming is for 
instance 20 watts per channel for a 5 channel system, whereas only a few 
watts per channel is available with the prior multiplexed MDS video 
system. 
Moreover, the receiver section for the subject system is greatly simplified 
because only two basic components are required for reception; namely the 
integrated antenna feed, low-noise amplifier and down converter package at 
the antenna; and a basic FM receiver. The FM receiver is easily tuned to 
the appropriate subcarrier frequency corresponding to the particular 
program channel to be received. This is in contradistinction to the MDS 
multiplexed method of providing audio channel de-multiplexing in that in 
the multiplexed system the video carrier has to be mixed with the audio 
carrier in an amplitude modulation detector to obtain an inter-carrier 
sound signal. This has to be limited and further demodulated in an FM 
detector. Subsequently the individual sub-carriers have to be demodulated 
to extract their individual informational content. Obviously, such process 
is both complex and inefficient. The basic problem with such a system is 
that it uses inter-carrier sound which requires all of the complexity of 
TV reception. Moreover, such a system is subject to interference present 
on the video carrier. The result is also that the entire multiplexed 
system has extremely poor sensitivity because of the wider bandwidth 
involved in obtaining all of the information including the carriers and 
the subcarriers. While the subject system requires a stable oscillator, it 
is not a difficult requirement that the local oscillator associated with 
the down converter have a frequency stability of 0.001%. 
The subject system utilizing a single audio channel per subcarrier as 
opposed to a multiplexed channel provides greater power per channel of 
program material, less interaction between program sources, greater 
simplicity in transmitter and receiver design and ultimately less noise 
and better range. 
More particularly, a microwave common carrier broadcast system is provided 
for the transmission of multiple audio channels to large numbers of 
receivers in a coverage area, in which noise-free transmission is 
accomplished through microwave transmission followed by down converting 
the received signal to television band frequencies. 
In one embodiment, multiple audio frequency sources are applied to a 
corresponding number of audio frequency subcarrier generators, the outputs 
of which are combined at a combiner, with the output of the combiner 
driving a 2 GHz AM transmitter, the output of which is filtered and 
linearly amplified prior to the coupling of the linearly amplified output 
to a suitable antenna. Here the AM transmitter is in essence a heterodyne 
mixer, and the in-line filter is tuned to the carrier frequency and one 
set of side bands. When the audio frequency carrier generator outputs are 
mixed in the AM transmitter with the carrier frequency, sum and difference 
components are generated corresponding to the carrier and the individual 
audio generator subcarrier frequencies. The in-line filter is set up to 
eliminate the carrier frequency of the AM transmitter and the undesired 
side bands produced in the mixing process. 
The output of the system is therefore a carrier-removed transmission such 
as a single side band transmission with the exception that the output 
signal to the antenna is a number of independent frequency modulated 
carriers. While it would be possible to downconvert the 2 GHz transmission 
to the low end of the FM broadcast band, this approach was rejected 
because of interference from local FM broadcast stations and particularly 
low power small college stations that are located in the immediate 
vicinity of the receiver. Rather, a local oscillator frequency was chosen 
so that with downconversion the received signal would be in the bands 
corresponding to TV channels 5 and 6. This eliminates the interference 
Problems having to do with feedthrough associated with the aforementioned 
local stations. The choice of microwave transmission coupled with 
downconverting to the TV band provides an interference free system in 
which there are a number of readily available FM/TV band receivers, as 
opposed to the utilization of the FM radio band which while interference 
makes such a system unusable. 
As to the receiver section, a microwave antenna system having a specialized 
feed, a low noise amplifier and a down converter, all at the antenna, down 
converts the received signal to television channels 5 or 6 depending on 
the availability in the given area. The output of the down converter is 
supplied to an FM receiver tuned to the subcarrier frequency corresponding 
to the desired audio source. It will be appreciated that each of the 
individual audio subcarrier generators is tuned to a different frequency 
within the chosen MDS channel. In one embodiment the MDS microwave channel 
is between 2150 MHz and 2156 MHz, with each of the audio frequency carrier 
generators being tuned to a frequency between those two limiting 
frequencies. It will be appreciated that at the receiving site the down 
converter heterodynes the microwave signal with an intermediate frequency 
signal thereby to provide a resultant signal in either the channel 5 or 
channel 6 band. Channel 5 and channel 6 operate between 76 and 88 MHz 
making the required down conversion from 2 GHz to approximately 80 MHz. 
As to the transmitting section of the system, in an alternative embodiment 
each audio frequency source has associated with it a separate 2 GHz FM 
transmitter, the outputs of which are combined in a combiner and then 
linearly amplified, with the amplified signal being provided to the 
transmitting antenna.

DETAILED DESCRIPTION 
Referring now to FIG. 1, in one embodiment the transmitter section 10 of 
the subject system includes a plurality of audio frequency sources 12 
coupled to a like plurality of audio frequency sub-carrier generators 14 
which are in turn coupled to a combiner network 16 which involves a 
network of resistors to prevent interaction between the signals. Each 
individual audio frequency sub-carrier generator can be the conventional 
4.5 MHz generator normally used for the generation of the audio portion of 
a video signal. However these generators are modified to operate at 
frequencies from a few hundred KHz to the band width of the MDS channel, 
e.g. 4 MHz or 6 MHz. Typically, however, the audio frequency carrier 
generator produces a carrier having a frequency for instance of 1 MHz, 2 
MHz, 3 MHz, 4 MHz and 5 MHz corresponding to 5 audio channels. The exact 
frequencies are selected according to the desired system parameters. Each 
of the audio frequency sub-carrier generators is a frequency modulated 
carrier generator modulated with a deviation corresponding to a maximum 
band width of a couple hundred KHz in the embodiment presently described. 
Each of the audio frequency sub-carrier generators includes its own 
pre-emphasis network which can typically be set at 75 micro-seconds and is 
commercially available from Comwave Inc. of Mountain Top Pa. The output of 
each of these generators is therefore an FM modulated signal having its 
own unique carrier frequency, with the center frequency being that 
associated with a particular channel of audio frequency programming to be 
demodulated at the receiver section of the subject system. 
The output of the audio frequency sub-carrier generators is applied, as 
mentioned before, to a resistor network which forms combiner 16, with the 
resistor network forming summing junctions, with the resistors in each of 
the legs of the summing junction providing a termination isolation for 
each of the generators, and with resistor values being such that the 
individual nodes match to the impedance of an AM microwave transmitter 18 
here illustrated to be a 2 GHz transmitter. In this case each output of 
the audio frequency carrier generators is loaded with a resistor 20 to 
ground, with the output of each individual carrier generator passing 
through a resistor 22 to a summing node 24 having a resistor 26 to ground. 
The purpose of the provision of the resistive combining network is to 
match the output impedance of each individual carrier generator to the 
input impedance of transmitter 18 and to provide isolation between the 
generators. It will be appreciated that the AM transmitter, in one 
embodiment is a one watt 2 GHz transmitter modulated with the signal 
available at output node 24. In one embodiment transmitter 18 is tuned to 
2150 MHz with the output being supplied to a filter 30 the purpose of 
which is to remove the 2 GHz carrier, or in the above example the 2150 MHz 
carrier. The filter also is designed to eliminate undesired side bands 
generated by the mixing process of the carrier and the signal from node 
24. These are commercially available as vestigual side band filters 
retuned to the carrier frequency which provides the desired result. A 
vestigual side band filter typically leaves the carrier and part of the 
undesired side band. Such filters are available from Comwave Inc. of 
Mountain Top, Pa., which are easily retuned cavity filters. 
The resultant signal from the output of filter 30 is a plurality of FM 
modulated carriers each centered about 2151 MHz, 2152 MHz, 2153 MHz, 2154 
MHz and 2155 MHz based upon the prior example of setting the audio 
frequency carrier generators to 1 MHz, 2 MHz, 3 MHz, 4 MHz, and 5 MHz. 
The output of the filter is applied to a linear RF amplifier 32, typically 
a 50 or 100 watt unit, the output of which is coupled to a conventional 
omni-directional microwave antenna (not shown). 
Referring now to FIG. 2 for the receive section here illustrated at 40 an 
MDS antenna 42 typically either a YAGI or a parabolic dish is coupled 
through a feed 44 to a low-noise amplifier 46 all of which are located at 
the feed to the antenna. The resultant signal is down converted at the 
antenna by a down converter 48 of conventional design tuned such that its 
local oscillator is tuned to a frequency of 2330 MHz. Thus when the 2251 
MHz signal is heterodyned therewith, the resultant signal is a signal at 
79 MHz which is within the channel 5 TV band. The FM receiver, here 
illustrated at 50 is a conventional FM receiver used for demodulating the 
audio components of the 79 MHz FM modulated carrier which is applied 
thereto. This FM receiver is standard in all aspects with the exception 
that it is not variable tuned but rather has its frequency controlled by 
stable frequency controlling elements which are fixed. This includes 
crystals, phase lock loops, or other conventional means of automatic 
frequency control. It is however important to note that the receiver is 
specially configured so as to respond to one of the multiple audio 
frequency program channels and, should program selectivity be appropriate, 
receiver 50 may be provided with a front panel switch to change the 
frequency of the receiver to correspond to one of the program channels. 
Note that the bandwidth of the mixer of the receiver is augmented to 
preclude the necessity of retuning for each program channel. Moreover, the 
receiver is provided with a 75 microsecond de-emphasis. 
In operation, various audio frequency sources corresponding to 
predetermined channels of programming are generated and supplied at the 
transmitting station to transmitter 18. The programs are transmitted 
omni-directionally, with the intent that the signals be picked up by 
directional antennas having a low noise characteristic at which point the 
signals are down converted from the original microwave frequencies to 
frequencies compatible with the channel 5 and 6 frequency bands. The 
result is that with hundred watt transmitters, coverage is typically line 
of sight, although because of refraction and reflection of the signal, 
adequate reception can be achieved beyond the nominal line of sight 
distance. Moreover, the signals are relatively noise-free, thereby 
eliminating the problem of complicated filter circuitry to eliminate cross 
talk that would be present if the FM broadcast band was utilized. Because 
of the utilization of the MDS system utilizing microwave frequencies and 
omni-directional transmission, it is possible to increase the range of 
such a system over that associated with FM broadcasting due to the 
availability in this frequency range of extremely directional high-gain 
antennas, and very low atmospheric noise. Also electromagnetic radiation 
interference is considerably less of a problem at microwave frequencies 
providing an exceptionally quiet system. In a preferred embodiment, the 
bandwidth for each of the audio frequency sub-carrier generators is on the 
order of 200 KHz due to the ready availability of inexpensive FM receiver 
band pass filters which can easily handle the proposed 200 KHz maximum 
band width for each of the audio channels. 
Referring now to FIG. 3 in an alternative embodiment each audio frequency 
source 12 is coupled instead to a 2 GHz FM transmitter 60 tuned in a 
preferred embodiment for instance to 2151, 2152, 2153, 2154 and 2155 MHz 
respectively to correspond to the above-mentioned example. The outputs of 
these transmitters which are typically one watt, are applied to a 
microwave combiner circuit 62. This type of combiner can include a 
resistor network or typically includes cavity mixers or circulators. The 
output of combiner 62 is coupled to linear amplifier 32 which can be 
identical to the linear amplifier of FIG. 1. 
Having above indicated a preferred embodiment of the present invention, it 
will occur to those skilled in the art that modifications and alternatives 
can be practiced within the spirit of the invention. It is accordingly 
intended to define the scope of the invention only as indicated in the 
following claims: