WO1998057455A1

WO1998057455A1 - System and method for transmission of audio signals to remote locations using a power line carrier

Info

Publication number: WO1998057455A1
Application number: PCT/US1997/023213
Authority: WO
Inventors: Scott R. Bullock; Daniel Seeley
Original assignee: Phonex Corporation
Priority date: 1997-06-09
Filing date: 1997-12-11
Publication date: 1998-12-17
Also published as: AU5528898A

Abstract

A system and method for transmission of both stereophonic and monophonic audio signals using a power line are disclosed. The stereophonic system (102) and method include hardware and method steps that compress and modulate a 2-channel input (104a, 104b), filter the signal, and use a transmitter and line interface (116) to place the multiplexed FM signal (115) onto a power line carrier, and then decode and demultiplexed the signal and utilize an audio amplifier (129) to provide high quality audio output at a remote site without the use of dedicated wiring or airwave transmission. Similar steps and hardware are utilized by the monophonic version.

Description

SYSTEM AND METHOD FOR TRANSMISSION OF

AUDIO SIGNALS TO REMOTE LOCATIONS

USING A POWER LINE CARRIER Priority is hereby claimed to United States Provisional Patent Application Serial

No. 60/019,588 which was filed on June 11, 1996.

I. BACKGROUND OF THE INVENTION

A. Field of the Invention

The invention relates to the use of a power line carrier to transmit audio signals to remote locations. More particularly, the invention provides an economical and high

quality system and method for transmitting both stereo and mono audio signals to one or more remote locations by utilizing already existing power lines as the transmission

medium.

B . The B ackground Art In the background art it has generally been known that audio signals could be

transmitted to remote locations by a variety of mechanisms without the use of dedicated wire connections. For example, United States Patent No. 3,590,382 issued in the name of

Kenney discloses a portable wireless stereo sound transmitter that transmits signals over the airwaves. United States Patent No. 3,927,316 issued in the name of Citta discloses a

wireless speaker system which communicates audio signals to remote speakers via an

infrared link. United Kingdom Patent Application No. 2, 159,023 A in the name of

applicant British Telecommunications pic discloses a system for transmitting stereo audio

signals over optical fibers in a cable television system.

More relevant to the field of the invention, United States Patent No. 4,829,570

issued in the name of Schotz discloses a system that converts audio signals into F.M. signals, transmits the F.M. signals over the power lines, reconverts the F.M. signals into

audio signals, and then provides the audio signals to a speaker device. United States Patent No. 4,847,903 also issued in the name of Schotz discloses a system similar in general principle to that of the Schotz '570 patent. Each of the above references is hereby

incorporated by reference.

In comparison with the prior art, the invention provide an economical and high quality system and method for transmitting audio signals to remote locations by utilizing already existing power lines as the transmission medium. π. SUMMARY OF THE INVENTION

It is an object of the invention to provide and economical and high quality system and method for transmitting stereo and mono audio signals to remote locations using a power line carrier. The invention features two preferred hardware embodiments and

related signal processing and transmission methods to transmit signals to a number of intended remote locations. It is an advantage of the invention that transmission of audio signals to remote locations can be performed without the use of dedicated wiring while maintaining high quality sound reproduction at the remote locations. Other objects,

features and advantages of the invention will become apparent to persons of ordinary skill in the art upon reading the specification and appended drawings. m. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a block diagram for a preferred wireless speaker system of the

invention.

Figure 2 depicts a circuit diagram of a preferred audio input stage of the system of

Figure 1. Figure 3 depicts a preferred stereo modulator circuit of the system of Figure 1. Figure 4 depicts a circuit diagram of the preferred stereo modulator filter circuit of

the system of Figure 1.

Figure 5 depicts a preferred transmitter amplifier, AC line interface, power supply

and VCO of the system of Figure 1.

Figure 6 depicts a preferred AC line interface of the system of Figure 1.

Figure 7 depicts a preferred power supply of the system of Figure 1. Figure 8 depicts a preferred receiver of the system of Figure 1. Figure 9 depicts a preferred stereo decoder circuit and left-right select of the system of Figure 1.

Figure 10 depicts a preferred low pass filter, mute, and audio presence detector of the system of Figure 1.

Figure 11 depicts a preferred audio amplifier of the system of Figure 1.

Figure 12 depicts a block diagram of a monaural wireless speaker system of the invention.

Figure 13 depicts a preferred audio input stage of Figure 12.

Figure 14 depicts a preferred AC line interface, power supply, VCO and

transmitter/amplifier of Figure 12.

Figures 15 and 16 depict a preferred power supply of Figure 12.

Figure 17 depicts a preferred filter and receiver of Figure 12.

Figure 18 depicts a preferred filter, mute, and audio presence detector of Figure

12.

Figure 19 depicts a preferred audio amplifier of Figure 12. IV. DESCRIPTION OF THE PREFERRED EMBODIMENTS A. Transmission of Stereophonic Audio Signals

Referring to Figure 1, a preferred wireless speaker system of the invention 101 is depicted. The system 101 includes a transmitter component 102 and a receiver

component 103. Focusing on the transmitter component 102, it can be seen that a left input 104a and a right input 104b audio signals are provided to an audio input stage 105.

The audio input stage is intended to be connected to a four (4) wire (two (2) channel) audio source. Preferably, the inputs will contain protection circuitry that will prevent them from shorting speaker wires together, as that could result in damage to the amplifier

that is driving the inputs. In the preferred embodiment, the audio source is assumed to be either line-level (smaller amplitude) or speaker-level (larger amplitude) audio. A switch

is used to select between these two types of audio level or these two drive levels.

In the "speaker-level" mode, the transmitter's audio inputs are expecting to be driven by a power amplifier that would normally drive 8-ohm speakers. The system is

designed to produce the same levels across the wireless receiver's audio output, thus making the system transparent to the power amplifier source. Further output level

adjustment is obtained by the use of the center-detent volume controls (normally in the

detent position to produce the 1-1 relationship) on each of the wireless receivers.

In the "line-level" mode, the transmitter's audio inputs are expecting signal levels

at +4dBu (nominal). These are the typical signal levels that are used to connect between

audio components in a home stereo system. Line level mode is used in conjunction with

the center-detent volume controls on the wireless receivers to obtain desired listening

levels. Following the audio input stage 105, two outputs are provided to twin

compressors 106a and 106b. Preferably, these compressors are hi-fi compressors with

pre-emphasis such as ¹/₂NE571 commercially available from Philips Electronics. As the V₂NE571 also provides an expander function, it is seen in the receiver 103 as well. The compressor/expander combination is preferred because it provides noise reduction. It also

uses pre-emphasis and de-emphasis to reduce unwanted FM hiss. For greater detail on the ¹/₂NE571, the reader is directed to Philips application note AN 174 from Applications for Compandors NE570/571/SA571, which is hereby incorporated by reference.

Each of 106a and 106b provide an output, both of which are routed to stereo

modulator circuit 107. The preferred stereo modulator circuit is an NJM2035 stereo modulator integrated circuit (IC). It is used to produce multiplexed audio that contains both left and right channel audio information in order to support the intended

stereophonic audio transmission. The preferred circuit illustrated produces multiplexed audio at a 38kHz sampling frequency (50% duty cycle) and a 19kHz square wave (Pilot 108a) and has a direct phase relationship with the multiplexed audio (MUX Out 108b).

The multiplexed audio 108b is filtered by a 57 kHz low-pass filter 109b. The pilot signal 108a is passed through a 19kHz band-pass filter 109a and superimposed via a summing

amplifier 110 with the filtered multiplexed audio. The resulting composite signal 111

contains the multiplexed audio and the pilot tone.

The composite multiplexed audio 111 is then fed into a voltage controlled

oscillator (VCO) 112 and thus frequency-modulates the oscillator. The oscillator has a

nominal frequency of 4.9MHz (carrier frequency). The VCO is bufferred via a

transmitter amplifier 113 and is filtered through a 4.9MHz band-pass filter 114. The carrier 115 is then injected into the power line interface 116 using a filter/safety capacitor and a balun transformer. The carrier is now present on the power lines and can be

received by both receiver/amplifiers A power supply 117 is depicted. With this system, there is no limit the number of receivers that can be employed.

Referring to Figure 2, a circuit diagram for a preferred audio input stage 105

discussed above is depicted that functions as described above. Referring to Figure 3, a circuit diagram for a preferred stereo modulator circuit 107 that produces MUX OUT 108b and PILOT OUT 108a is shown. Referring to Figure 4, a diagram of the band-pass

filter 109a and the low pass filter 109b with the summing amplifier 110 to produce composite signal 111 is provided. Referring to Figure 5, a circuit diagram depicting a preferred arrangement of the VCO 112, transmitter amplifier 113, band-pass filter 114, AC line interface 116 and power supply 117 as discussed above is depicted.

Focusing on the receiver component 103, reference is again made to Figure 1. It can be seen that at a remote location, the signals 118 provided by transmitter component

102 are received by receiver 103. An AC line interface 119 is provided as depicted in greater detail in Figure 6. A power supply 121 is depicted and is detailed in Figure 7.

The receiver 103 is connected to the AC power line via a filter/safety capacitor and a

balun transformer. A 4.9 MHZ band pass filter 120 is used to filter the signal from the

AC power line. The band pass filter 120 and receiver 121 are depicted in Figure 8. The

receiver 122 is preferably a TDA 7021 FM receiver commercially available from Philips

Electronics. The receiver 122 demodulates the FM signal and produces a composite

multiplexed audio signal which is passed on to the stereo decoder 123. A preferred stereo

decoder is PLL Stereo Decoder TDA 7040 commercially available from Philips Electronics. A circuit diagram for a preferred decoder 123 is provided in Figure 9. The PLL stereo decoder circuit is used to de-multiplex the left and right channels of audio

from the transmitter's inputs. The user can select which channel is to be amplified via a left/right channel selector switch 124. The stereo decoder 123 provides left and right channel signals to the left/right select 124.

The single audio channel which is selected above by the left/right select 124 is sent to a 16kHz low-pass filter 125 (see also Figure 10). This filter 125 is used to suppress local oscillator components from the FM receiver circuit and to supply gain

which restores the compressed audio's amplitude corresponding to the level produced by the compressor in the transmitter. This allows for proper matching with the expander circuit.

A mute function is preferred to eliminate any noise that may result from a weak or

absent transmitted carrier. The mute switch 130 is depicted in Figures 1 and 10. The mute function preferably operates from the field-strength indicator (FSI) pin of the TDA 7021 receiver chip. The FSI presents a dc level that corresponds to the strength of the

received carrier and thus can reach a threshold where an audio pass transistor is turned

off.

The audio presence detector 127 (Figures 1 and 10) operates by charging a capacitor when audio is present. The voltage across this capacitor decreases slowly when

no audio is

present. If no audio is present for about 90 seconds, the voltage reaches a threshold

where the standby pin of the audio amplifier is set high. This will place the audio

amplifier in "standby" mode which allows no audio to reach the speaker and draws little power so that the heat sink of the amplifier will cool down.

An expander 128 is provided that is a hi-fi compressor/expander combination, preferably Vι NE571 available from Philips Electronics and is used primarily for noise

reduction. The combination also uses pre-emphasis and de-emphasis to reduce unwanted FM hiss.

An audio amplifier circuit 129 (Figures 1 and 11) is used to drive preferably a

low-impedance speaker (such as 8-ohm). The preferred amplifier circuit 129 utilizes a 30W audio power amplifier LM4700 which is commercially available from National Semiconductor. The amplifier circuit 129 preferably has additional filtering to eliminate high frequency oscillations. A center-detent volume control is placed a the input of the amplifier so the user can adjust the output level. A center-detent control (in the detent position) is used to allow for an approximate 1-to-l relationship of the right or left input

level to its corresponding output level when in the speaker-level operation. However, in line-level operation (switch-selectable on the transmitter), the volume control allows adjustment to obtain desired listening levels.

B. Transmission of Monophonic Audio Signals

Referring to Figure 12, a block diagram of a preferred system for the transmission

of monophonic audio signals over power lines is provided. A preferred monophonic

audio system 200 is depicted as including a transmitter component 201 and a receiver

component 202 connected via an RF link 203.

The transmitter component 201 includes an audio input stage 204 which receives

left and right inputs and has a line level/speaker level select. A circuit diagram of a

preferred audio input stage 204 is provided in Figure 13. The audio input stage is intended to be connected to a 4-wire (2-channel) audio source in which the two channels are combined into a mono signal. These inputs preferably contain protection circuitry that prevents shorting speaker wires together which could cause damage to the amplifier that

is driving them. The audio source is assumed to be either line-level (smaller amplitude) or speaker-level (larger amplitude) audio. A switch is used to select between these types

of audio levels. In "speaker level" mode, the transmitter's audio inputs are expecting to be driven by a power amplifier that would normally drive 8-ohm speakers. The system is designed to produce the same level across the wireless receiver's audio output, thus making the system transparent to the power amplifier source. Further output level adjustment is obtained by the use of the center-detent volume controls (normally in the

detent position to produce a 1-to-l relationship) on each of the wireless receivers. In "line level" mode, the transmitter's audio inputs are expecting signal levels at +4dBu

(nominal). These are the typical signal levels that are used to connect between audio components in a home stereo system. Line level mode is used in conjunction with the center-detent volume controls in a home stereo system. Line level mode is used in conjunction with the center-detent controls on the wireless receivers to obtain desired

listening levels.

The output from the audio input stage 204 is provided to a compressor 205 which is preferably a hi-fi compressor/expander combination such as Vi NE571 as described in

Philips application AN174. This component provides noise reduction and uses pre-

emphasis and de-emphasis to reduce unwanted FM hiss.

The compressed mono audio output from the compressor 205 is provided to a

voltage-controlled oscillator (VCO) 206 which frequency modulates the oscillator. The oscillator has a nominal frequency of 4.9MHz (carrier frequency). The VCO is buffered via the transmitter amplifier 207 and is filtered through a 4.9MHz band pass filter 208

(see Figure 14 for a diagram of a preferred circuit). The carrier is then injected into the power line interface 209 using a filter/safety capacitor and a balun transformer. Power

supply 210 is also depicted. The carrier thus being present on the power lines it can be received by both receiver/amplifiers. There is no limit to the number of receivers that can be used when the inventive concept is employed.

Again with referenced to the appended drawings, a discussion of the preferred receiver component 202 is provided. The receiver component 202 receives a signal over the AC power lines by RF link 203. The AC line interface 211 is, similar to the transmitter component 201, a circuit that includes a filter/safety capacitor and a balun transformer. Figure 15 depicts a preferred AC line interface 211 and power supply 212. The preferred power supply 212 is depicted in greater detail in Figure 16. The receiver circuit 214 demodulates the FM signals and produces monaural audio which is passed to a

16kHz low pass filter 215. Greater detail of the receiver circuit 214 is provided in Figure

17. The FM receiver used is preferably TDA 7021 commercially available from Philips

Electronics. The filter 215 is used to suppress local-oscillator components from the FM receiver circuit and to supply gain which restores the compressed audio's amplitude

corresponding to the level produced by the compressor in the transmitter. This allows for

proper matching with the expander circuit.

A mute function 217 is provided by mute switch 216 and field strength indicator

(FSI) 221 from the FSI pin of preferred TDA7021 receiver. Reference is made to Figure

18 for greater detail of a preferred circuit. The mute function is used to eliminate any audio noise that may result from a weak or absent transmitted carrier. The FSI presents a dc level that corresponds to the strength of the received carrier and thus can reach a

threshold where an audio pass transistor is turned off.

The audio presence detector 218 operates by charging a capacitor when audio is present. The voltage across this capacitor decreases slowly when no audio is present. If

no audio is present for about 90 seconds, the voltage reaches a threshold where the standby pin of the audio amplifier is set high. This will place the audio amplifier in "standby" mode which allows no audio to reach the speaker and draws little power so the

heat sink of the amplifier will cool down. An expander 219 is provided that is preferably a hi-fi compressor/expander combination (such as the Vi NE571 disclosed in Philips application note AN 174). The expander 219 provide noise reduction and uses pre-emphasis and de-emphasis to reduce unwanted FM hiss.

An audio amplifier circuit 220 is provided that is intended to drive a low impedance speaker (8-ohm) and has additional filtering to eliminate high-frequency

oscillations. A preferred circuit is depicted in Figure 19. A center-detent volume control is placed at the input of the amplifier so the user can adjust the output level. The reason

for a center-detent control (in the detent position) is to allow for an approximate 1-to-l

relationship of the right or left input level to its corresponding output level when in

speaker-level operation. However, in line-level operation (switch-selectable on the

transmitter), the volume control allows adjustment to obtain desired listening levels.

While the present invention (including the inventive concept) has been described

and illustrated in conjunction with a number of specific embodiments, those skilled in the art will appreciate that variations and modifications may be made without departing from the principles of the invention as herein illustrated, described and claimed.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be

considered in all respects as only illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

I claim:

1. A system for the transmission of stereophonic audio signals over a power line carrier, the system comprising: a transmitter including:

an audio input stage for receiving a 2-channel audio signal, a compressor connected to receive audio signals which have been output from said audio input stage,

a stereo modulator connected to receive audio signals which have been output from said compressor,

a transmitter amplifier connected to receive audio signals which have been output from said stereo modulator, and an power line interface for injecting audio signals from said transmitter amplifier onto a power line; and at least one receiver including:

a power line interface for receiving audio signals from a power line,

a receiver connected to receive audio signals which have been output from said power line interface,

a stereo decoder connected to receive audio signals which have been

output from said receiver, an expander connected to receive audio signals which have been output

from said stereo decoder, and an audio amplifier connected to receive audio signals which have been output from said expander, said audio amplifier being capable to outputting a signal usable as an audio output to a speaker.

2. A system as recited in claim 1 wherein said audio input stage is selectable between line-level and speaker-level audio.

3. A system as recited in claim 1 wherein said stereo modulator is a multiplixer which outputs a multiplexed stereophonic audio signal.

4. A system as recited in claim 1 wherein said compressor provides a noise

reduction function.

5. A system as recited in claim 4 wherein said compressor serves to reduce

unwanted FM hiss.

6. A system as recited in claim 1 wherein said compressor provides pre-

emphasis and de-emphasis.

7. A system as recited in claim 1 further comprising at least one filter serving to filter unwanted components from an audio signal received from said stereo modulator before the audio signal is provided to said transmitter amplifier.

8. A system for the transmission of stereophonic audio signals over a power

line carrier, the system comprising: a transmitter including: an audio input stage for receiving a 2-channel audio signal,

a compressor connected to receive audio signals which have been output

from said audio input stage, said compressor serving to reduce unwanted noise in said

signal, a stereo modulator connected to receive audio signals which have been output from said compressor, said stereo modulator producing a pilot signal output and a multiplexed audio output,

a band pass filter for filtering said pilot signal, a low-pass filter for filtering said multiplexed audio output,

a summing amplifier to combine output signals from said band pass filter and said low-pass filter into a composite signal,

a VCO for creating a frequency modulated signals from said composite signal,

a transmitter amplifier connected to receive said frequency modulated signal, and an power line interface for injecting audio signals from said transmitter amplifier onto a power line; and a receiver including:

a power line interface for receiving audio signals from a power line, a receiver connected to receive audio signals which have been output from

said power line interface, said receiver serving to demodulate said audio signals in order to produce a composite multiplexed signal,

a stereo decoder connected to receive audio signals which have been

output from said receiver, said decoder serving to demultiplex said signals into a left

channel signal and a right channel signal,

an expander connected to receive audio signals which have been output

from said stereo decoder and which includes a noise reduction function, and

an audio amplifier connected to receive audio signals which have been output from said expander, said audio amplifier being capable to outputting a signal usable as an audio output to a speaker.

9. A method for transmitting stereophonic audio signals over a power line carrier comprising the steps of: providing a left and a right audio input signal,

modulating said audio signals to provide a multiplexed output signal and a pilot output signal,

summing said pilot and multiplexed signals to produce a composite signal, frequency modulating said composite signal to produce an FM signal, injecting said FM signal onto a power line carrier,

receiving said FM signal from the power line carrier at a remote location, demodulating said FM signal into a composite signal,

decoding said composite signal into a left and a right channel audio signal,

amplifying said audio signal, providing said audio signal to a speaker device to produce sound.

10. The method recited in claim 9 further comprising the steps of:

compressing said left and right audio input to reduce noise prior to modulating

said signals, and expanding said decoded left and right channel signals.

11. The method recited in claim 10 further comprising the step of filtering said

multiplexed output signals and said pilot signal.

12. The method recited in claim 11

wherein said filtering of said pilot signal is performed with a band pass filter and wherein said filtering of said multiplexed output signal is performed with a low pass filter.

13. The method recited in claim 12

wherein said band pass filter is about a 19kHz band pass filter and wherein said low pass filter is about a 57 kHz low pass filter.

14. The method recited in claim 9

wherein said modulating step is performed by a modulator circuit that produces multiplexed audio output at about a 38 kHz sampling frequency.

15. The method recited in claim 14 wherein said modulator circuit produces a pilot output with a 19kHz square wave signal and which has a direct phase relationship with said multiplexed audio output.

16. The method recited in claim 9 wherein said frequency modulating step is performed is performed using a voltage

controlled oscillator. 17. The method recited in claim 16 wherein said voltage controlled oscillator utilizes a 4.9 MHz carrier frequency.

16. A method as recited in claim 9 wherein FM signal is received from the power line carrier using a filter/safety capacitor and a balun transformer.

17. A method as recited in claim 9 wherein said FM signal received from the power

line is filtered through about a 4.9MHz band pass filter.

18. A method as recited in clam 9 wherein said FM signal is received from the power

line by a receiver which demodulates said FM signal to produce a composite

multiplexed signal and a field strength indicator signal in order to perform a mute function.

19. A method as recited in claim 18 further comprising the step of selecting a left or a

right channel from said composite multiplexed signal from said receiver as a single channel audio signal.

20. A method as recited in claim 19 further comprising the step of filtering said single

channel audio through a low pass filter in order to suppress local oscillator components from said FM receiver and to supply gain in order to restore audio amplitude.

21. A method as recited in claim 18 further comprising the step of activiting a mute switch by use of said field strength indicator signal in order to reduce noise from a weak or absent carrier.

22. A method as recited in claim 9 further comprising the step of detecting the

absence of an audio signal by charging a capacitor when audio is present, and when said capacitor discharges indicating the lack of an audio signal, placing said

amplifier in standby mode.

23. A method as recited in claim 9 wherein said amplifying step is performed by an

amplifier and further comprising the step of ceasing to provide an audio signal to said speaker when no audio is being received by said amplifier.