EP1041758A2

EP1041758A2 - Method and system for extending broadcast coverage on a single frequency network

Info

Publication number: EP1041758A2
Application number: EP00105296A
Authority: EP
Inventors: William Rollins; Junius Kim
Original assignee: Harris Corp
Current assignee: Harris Corp
Priority date: 1999-03-31
Filing date: 2000-03-14
Publication date: 2000-10-04
Also published as: EP1041758A3; CN1273465A; TW507428B

Abstract

A system for aligning the phase characteristics of N identical digitized audio program signals distributed from a studio to N broadcasting stations via N time division multiplex (TDM) communication signals includes N TDM multiplexers, each of which (i) multiplexes a digitized audio program signal into at least one channel of a TDM communication link established between the studio and one of the N broadcasting stations, and (ii) multiplexes a timing signal into another channel of the TDM communication signal. A TDM de-multiplexer at each of the broadcasting stations extracts the digitized audio program signal and the timing signal from the corresponding TDM communication signal. A timing signal comparator compares the extracted timing signal to a local timing signal that is delayed by a precise and adjustable amount, and produces an offset time corresponding to the comparison result. The delay adjust circuit dynamically adjusts the delay of the TDM signal as a function of the offset time, preferably so as to drive the offset time to substantially zero. The timing signal at the studio and the timing signals at each of the broadcast stations are provided by an existing time distribution network.

Description

The present invention relates to over-the-air electromagnetic signal transmission systems, and more particularly, to systems for providing simultaneous signal transmissions from at least two spatially separate transmission sites.
In order to provide adequate over-the-air signal broadcast coverage to a given geographical area, a provider may, in general, utilize either a single transmission site with sufficient power to reach all parts of the geographical area, or multiple sites that cover all parts of the area in the aggregate. Although there are disadvantages to both methods, there are economic as well as technical reasons why a multiple site system is more desirable. For example, at many frequencies of interest, the transmitted signal behaves in a "line of sight" mariner. This means that in general, the signal cannot be transmitted over the horizon (relative to the transmitting site). Further, geographical obstructions such as buildings, bridges, land masses, etc., will shadow regions of the geographical area from the transmitting site. Economically, a number of smaller, lower power transmitting sites are often less expensive to own and operate than a single site producing the same amount of power and/or providing the same broadcast coverage. Also, the U.S. Federal Communication Commission views with favor applications to convert existing, dissimilar frequency transmitting sites into sites that all operate at the same frequency, because doing so frees up bandwidth in an already crowded commercial frequency band. FIG. 1A illustrates a single, high power transmitter covering a region of interest R, and FIG 1B illustrates six lower power transmitters, each receiving a program feed from a central studio site, covering approximately the same region of interest R. Each lower power transmitter TXn covers a region Rn (for all n from 1 to 6).
One disadvantage to the multiple-transmitter architecture of FIG. 1B is that receivers within an overlap region (i.e., a region covered by one or more transmitters, such that signal levels from different transmitters are similar) tend to receive multiple signals. FIG. 2 illustrates a receiver M situated near the overlap region between two transmitters TX1 and TX2. In this illustration, each transmitter receives an audio program from a central studio X via a studio-to-transmitter link (hereinafter referred to as STL) implemented with a T1 (or E1) digital circuit in the Public Switched Telephone Network (hereinafter referred to as PSTN). Since the network path of the STL for TX1 may be longer or shorter than the network path of the STL for TX2, the audio programs transmitted by TX1and TX2 may not be contemporaneous. The consequence to the receiver near the overlap region is that as it receives signals from multiple transmitters, the demodulated output will "jump" between two time-delayed versions of the same audio program. Some prior art systems insert fixed delays into the STL paths to equalize the path delays. However, when implementing an STL via the PSTN (rather than via a dedicated line), the STL path delay may not remain constant due to various causes such as rerouting communication links, changes in traffic conditions, data buffering, and the variable processing delay of PSTN switches.
An object of the present invention is to substantially overcome the above-identified disadvantages and drawbacks.
The present invention includes a system for aligning the phase characteristics of N identical digitized audio program, signals distributed from a studio to N broadcasting stations via N time division multiplex (TDM) communication signals, said studio and said N broadcasting stations each receiving an accurate timing signal, comprising N TDM multiplexers at said studio, wherein each of said N multiplexers (i) multiplexes said digitized audio program signal into at least one channel of a TDM communication signal transmitted from said studio to one of said N broadcasting stations, and (ii) multiplexes said accurate timing signal received by said studio into another channel of said TDM communication signal, for each one of said N broadcasting stations,
(i) a TDM de-multiplexer for removing said digitized audio program signal and said studio timing signal from a corresponding one of said N TDM communication signals,
(ii) a timing signal comparator for comparing said studio timing signal to a local timing signal delayed offset to produce a comparison result and generating an offset time corresponding to said comparison result; and,
(iii) a delay adjust circuit for dynamically adjusting a delay of said TDM communication signal as a function of said offset time, and preferably in which said delay adjust circuit is substantially hitless.
The invention also includes a method for aligning the phase characteristics of N identical digitized audio program signals distibuted from a studio to N broadcasting stations via N time division multiplex (TDM) communication signals, said studio and said N broadcasting stations each receiving an accurate timing signal, comprising:
multiplexing said digitized audio program signal into at least one channel of a TDM communication signal transmitted from said studio to one of said N broadcasting stations;
multiplexing said accurate timing signal received by said studio into another channel of said TDM communication signal; and,
for each one of said N broadcasting stations,
removing said digitized audio program signal and said studio timing signal from a corresponding one of said N TDM communication signals;
comparing said studio timing signal to a local timing signal to produce a comparison result, and generating an offset time corresponding to said comparison result; and,
dynamically adjusting a delay of said TDM communication signal as a function of said offset time, so as to drive said offset time to substantially zero, and including the step of dynamically adjusting said phase delay of said audio program in a substantially hitless manner, in which a further step includes receiving each of said accurate timing signals from a pre-existing time distribution network.
Advantageously, a system for aligning the phase characteristics of N identical digitized audio program signals distributed from a studio to N broadcasting stations via N time division multiplex (TDM) communication links. The studio and the N broadcasting stations each receive an accurate timing signal. The system includes N TDM multiplexes at the studio. Each of the N multiplexers (i) multiplexes the digitized audio program signal into at least one channel of a TDM communication link established between the studio and one of the N broadcasting stations, and (ii) multiplexes the accurate timing signal received by the studio into another channel of the TDM communication link. Each of the N broadcasting stations includes a TDM de-multiplexer, a timing signal comparator and a delay adjust circuit. The TDM de-multiplexer removes the digitized audio program signal and the studio timing signal from a corresponding one of the N TDM communication links. The timing signal comparator compares the studio timing signal to a local timing signal and produces an offset time corresponding to a comparison result. The delay adjust circuit dynamically adjusts a delay of the digitized audio program signal as a function of the offset time. In one embodiment, the delay adjust circuit dynamically adjusts the delay of the digitized audio program signal so as to drive the offset time to substantially zero.
In another embodiment of the invention, the delay adjust circuit is substantially hitless. The term "hitless" means that a delay change can be effected without losing data or creating a gap in the output series of delayed digital data elements.
In another embodiment of the invention, the accurate timing signal is provided by a Global Positioning System (GPS) receiver.
In another embodiment of the invention, the TDM communications link includes a robust framing circuit for a T1 communications link.
In another embodiment of the invention, the TDM communications link is substantially unidirectional, such that timing signal and program audio signal data flows from the studio to the corresponding one of the N broadcasting stations.
In another embodiment of the invention, the system further includes an audio card at the studio and a substantially identical audio card at each of the N broadcasting stations, wherein the studio audio card digitizes an analog audio program, and each of the broadcasting station audio cards converts the digitized audio program to the analog audio program.
In yet another embodiment of the invention, each of the audio cards maintains a consistently linear phase delay of the audio signal over an audio signal frequency band of interest, thereby maintaining a constant audio signal group delay.
In another embodiment of the invention, the audio card includes at least one FIR filter, so as to assure the constant group delay.
The invention will now be described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1A illustrates a single, high power transmitter covering a region of interest R;
FIG. 1B illustrates six lower power transmitters combining to provide approximately the same coverage as the transmitter of FIG. 1A;
FIG. 2 illustrates a receiver situated near the overlap region between two of the transmitters shown in FIG. 1B; and,
FIG. 3 shows a block diagram view of a signal distribution system.
FIG. 3 shows a block diagram view of one preferred embodiment of a signal distribution system 100 for aligning the phase characteristics of N identical digitized audio program signals distributed from a studio to N broadcasting stations via N time division multiplex (TDM) communication links. The illustrated embodiment of the system 100 includes a studio site 102, a first broadcast site 104 and a second broadcast site 106. Although other embodiments of the system 100 may include a plurality (i.e., some integer N) of broadcast sites. The studio site 102 includes a central time reference 110, an audio program source 112, a first TDM multiplexor 114, a first transmit (hereinafter TX) audio circuit 116, a second TDM multiplexor 118 and a second TX audio circuit 120. A generalized embodiment of the system having N broadcast sites includes N TDM multiplexors and N TX audio circuits, such that each broadcast site includes a TDM multiplexor/ audio circuit pair at the studio site 102. Thus, in the illustrated embodiment, the studio site 102 includes two TDM multiplexor/audio circuit pairs: 114/116 and 118/120. Each TDM multiplexor/audio circuit pair receives a central timing signal 122 from the central time reference 110 and an audio program signal 124 from the audio program source 112. In the first TDM multiplexor/audio circuit pair 114/116, the TDM multiplexor 114 received the timing signal 122 and the TX audio circuit 116 receives the audio program signal 124. In the second TDM multiplexor/audio circuit pair 118/120, the TDM multiplexor 118 receives the timing signal 122 and the TX audio circuit 120 receives the audio program signal 124. For each TDM multiplexor/audio circuit pair, the TX audio circuit digitizes and processes the received audio program signal 124, and provides a digitized signal 126 to the corresponding TDM multiplexor. The TX audio circuits 116 and 120 utilize digital signal processing (hereinafter referred to as DSP) techniques, e.g., finite impulse response (hereinafter FIR) filters, to maintain a constant group delay, and thus accurate and consistent phase alignment, through the audio circuits 116 and 120. The TDM multiplexor distributes the digitized signal 126 and the timing signal 122 among a plurality of timeslots available in an output TDM signal. The first TDM multiplexor 118 produces a first TDM transmission signal 128, and the second TDM multiplexor 120 produces a second TDM transmission signal 129. Thus, the data packets transmitted via the TDM transmission signals 128 and 129 convey the digitized program signal 126 corresponding to the audio signal program signal 124, along with a timing signal 122 that "time-tags" the digitized signal 126. In other words, the timing signal 122 sent along with the digitized signal 126 in the TDM output signal provides information relating to when the studio site 102 originated the digitized signal 126.
The first broadcast site 104 includes a first TDM de-multiplexor 130, a first local time reference 132, a first timing signal comparator 134, a first delay adjustment circuit 136, a first delay offset 137, a first receive (hereinafter RX) audio circuit 138 and an exciter/transmitter assembly 139. The first delay adjustment circuit 136 receives the first TDM transmission signal 128 generated by the TDM multiplexor 116 at the studio site 102, and provides a delayed version of the TDM signal 128 to the TDM de-multiplexor 130. The TDM de-multiplexor 130 removes the digitized signal 126 and the timing signal 122 from the appropriate time slots in the TDM transmission signal 128 to produce a recovered digitized audio signal 140 and a recovered timing signal 142, along with a local timing signal 144 generated by the local time reference 132 and subsequently delayed by the delay offset 137. The timing signal comparator 134 produces a time offset signal 146 corresponding to the temporal difference between the recovered timing signal 142 and the local timing signal 144. The delay adjustment circuit 136 receives the time offset signal 146 and adjusts the amount of delay it applies to the TDM signal 128 so as to drive the time offset signal 146 to zero. In order to drive the time offset signal 146 to zero, the following equation must be satisfied: [delay of delay offset 137]=[delay of delay adjustment circuit 136]+[PSTN delay] The delay of the delay offset 137 must be larger than the largest expected PSTN delay. If all broadcast stations are to deliver the studio station program audio 126 simultaneously, the delay offset 137 in all of the broadcast stations must be identical. In alternative embodiments, the delay offset 137 may be set differently for different broadcast stations, delivering the program audio 126 to different broadcast stations at different times. This arrangement allows the overlap region between adjacent broadcast stations to be tailored somewhat. For example, the overlap region is preferably located at the location of equal power between two adjacent broadcast stations. If two adjacent broadcast stations have unequal power, the equal power location is not equidistant from each broadcast station, but lies closer to the lower power broadcast station. By adjusting the delay provided by the delay offset 137, the overlap region can be made to occur precisely at the equal power location.
One example of a delay adjustment circuit 136 is disclosed in the specification of U.S. Patent No. 5,818,769. The delay line effects a change to the through-put delay of a series of digital data elements, preferably in a hitless mode. The term "hitless" means that a delay change can be effected without losing data or creating a gap in the output series of delayed digital data elements. The delay line can also operate in "non-hitless" mode, during which a delay change can be initiated immediately when a loss of data in the series of digital data elements or a gap in the series of digital data elements can be tolerated. The audio circuit 138 receives the digitized audio signal 140 and produces a processed audio signal 148 that is a function of the digitized audio signal 140. The exciter/transmitter 139 receives the processed audio signal 148 and produces a modulated carrier signal 150 to be transmitted by a collocated antenna assembly 152.
The second broadcast site 106 includes a second TDM de-multiplexor 160, a second local time reference 162, a second timing signal comparator 164, a second delay adjustment circuit 166, a second delay offset 167, a second receive (hereinafter RX) audio circuit 168 and an exciter/transmitter assembly 169. In general, the components of the second broadcast site 106 operate and interact in substantially the same manner as the components of the first broadcast site 104. The second delay adjustment circuit 166 receives the second TDM transmission signal 129 generated by the TDM multiplexor 120 at the studio site 102, and provides a delayed version of the TDM signal 129 to the TDM de-multiplexor 160. The TDM de-multiplexor 160 removes the digitized signal 126 and the timing signal 122 from the appropriate time slots in the TDM transmission signal 129 to produce a recovered digitized audio signal 170 and a recovered timing signal 172, along with a local timing signal 174 generated by the local time reference 162 and subsequently delayed by the delay offset 167. The timing signal comparator 164 produces a time offset signal 176 corresponding to the temporal difference between the recovered timing signal 172 and the local timing signal 174. The delay adjustment circuit 166 receives the time offset signal 176 and adjusts the amount of delay it applies to the TDM signal 129 so as to drive the time offset signal 176 to zero. In order to drive the time offset signal 176 to zero, the following equation must be satisfied: [delay of delay offset 167]=[delay of delay adjustment circuit 166]+[PSTN delay] The delay of delay offset 167 must be larger than the largest expected PSTN delay. If all broadcast stations are to deliver the studio station program studio 126 simultaneously, the delay offset 167 in all of the broadcast stations must be identical. In alternative embodiments, the delay offset 167 may be set differently for different broadcast station, delivering the program audio 126 to different broadcast stations at different times (as is described herein for the first broadcast site 104). The audio circuit 168 receives the digitized audio singla 170 and produces a processed audio signal 178 that is a function of the digitized audio signal 170. The exciter/transmitter 169 receives the processed audio signal 178 and produces a modulated carrier signal 180 to be transmitted by a collocated antenna assembly 182.
The RX audio circuits 138 and 168 at the broadcasting sites 104 and 106, respectively, each utilize DSP techniques, e.g., FIR filters, to maintain a constant group delay though the PS audio circuits. A constant group delay through the RX audio circuits serves to preserve accurate and consistent phase alignment of the audio program signal for broadcast.
Each of the N TDM signals produced by the studio site 102 is conveyed to one of the N broadcast sites via a digital transmission channel. In a preferred embodiment, the public switched telephone network (hereinafter PSTN) provides the digital transmission channel, although other transmission media known to those on the art may provide such a channel. In a preferred embodiment a T1 circuit in the PSTN provides the digital transmission channel, although in alternative embodiments other communications transport mechanisms such as E1, ATM, TCP/IP, Ethernet, ISDN, or other may be used. A T1 ditigal transmission link from the studio site 102 to a broadcast site 104 or 106 defines the TDM protocol and implements a robust framing algorith to prevent signal interruption due to network transmission bit errors. In general, a T1 link includes 24 distinct TDM time slots, each conveying eight bits of information. A single framing bit is included with each group of 24 TDM time slots to form a frame of 193 bits. The framing bits of 12 consecutive frames form a unique pattern, which is used by the receiving equipment to establish frame synchronization. The transmission rate of a T1 link is 1.544 million bits per second (hereinafter MBPS), so the maximum available data rate for any single time slot is 64 thousand bits per second (hereinafter KBPS). With the T1 TDM architecture, 24 distinct 64 KBPS channels can be transmitted over a single T1 link. To increase bandwidth, a single channel can utilize more than one time slot. For example, if two time slots are used for a single channel, the channel capacity increases to 128 KBPS. If all 24 time slots are used for a single channel, the system achieves the maximum channel capacity of 1.536 MBPS.
The central time reference 110 at the studio site 102 and the local time references 132 and 162 at the broadcast sites 104 and 106, respectively, are synchronized so as to provide substantially the same time code at each site. In other words, the timing signal 122, the local timing signal 144 and the local timing signal 174 are substantially identical at all times. In a preferred embodiment, this time-synchronization is accomplished by utilizing a Global Positioning System (GPS) receiver at studio site 102 and each of the broadcast sites 104 and 106. The GPS system is a network of geosynchronous-orbit satellites that are tightly synchronized in time. The satellites continuously broadcast positioning and time-of-day information to be utilized by terrestrial receivers, primarily for navigational purposes. However, because the satellites are synchronized in time and each satellite broadcasts its local time information, the GPS network may also be used to effectively time-synchronize widely located terrestrial sites. In one embodiment of the invention, each GPS receiver is accompanied by a local time source referenced to a local frequency source. The local frequency sources may include a temperature-compensated crystal oscillator, an ovenized oscillator, a rubidium oscillator, a cesium oscillator, a hydrogen maser or other frequency source. In situations where the GPS signal is lost for a short period of time, the local time source can act as a "flywheel" to maintain system operation until the GPS signal is restored.
Further, GPS satellites include extremely precise frequency references, and GPS receivers can recover a precise reference signal from the satellite broadcast. Thus, a GPS receiver can provide a precise frequency source for use by collocated equipment, as long as the receiver maintains the link to the satellite. One receiver at each broadcast site to lock the carrier frequency, thus locking each broadcast site to a common reference.
The main utility of the present invention is its ability to dynamically compensate for small delay changes in the distribution network (e.g., the Public Switched Telephone Network) between the studio site 102 and the broadcasting sites 104 and 106. Ideally, the path delay from the central studio site 102 to each of the broadcasting stations should be equal. Because the delay adjustment circuit in each of the broadcasting sites can change its throughput delay without losing data or inserting gaps in the data series, the system can maintain the desired equal path delays without corrupting or interrupting the audio program. The accuracy of the system is primarily limited by the time resolution of the TDM communications link between the studio and the broadcast stations. For example, using a T1 TDM communications link allows the system to control the path delay from the studio site to each of the broadcasting sites (with respect to an absolute reference) to within plus or minus two microseconds. The accuracy may be increased by utilizing a communications link with greater resolution. In one preferred embodiment, the system resolution is 0.1 microseconds, i.e., the individual path delay between the studio site and a broadcasting site can be offset in steps of 0.1 microseconds to optimize performance of the system in broadcast overlap regions. In other embodiments, the delay circuit may be modified (as is described in detain in the '769 patent) to increase or decrease the system resolution depending on the system performance requirements.
A system for aligning the phase characteristics of N identical digitized audio program signals distributed from a studio to N broadcasting stations via N time division multiplex (TDM) communication signals includes N TDM multiplexers, each of which (i) multiplexes a digitized audio program signal into at least one channel of a TDM communication link established between the studio and one of the N broadcasting stations, and (ii) multiplexes a timing signal into another channel of the TDM communication signal. A TDM de-multiplexer at each of the broadcasting stations extracts the digitized audio program signal and the timing signal from the corresponding TDM communication signal. A timing signal comparator compares the extracted timing signal to a local timing signal that is delayed by a precise and adjustable amount, and produces an offset time corresponding to the comparison result. The delay adjust circuit dynamically adjusts the delay of the TDM signal as a function of the offset time, preferably so as to drive the offset time to substantially zero. The timing signal at the studio and the timing signals at each of the broadcast stations are provided by an existing time distribution network.

Claims

A system for aligning the phase characteristics of N identical digitized audio program, signals distributed from a studio to N broadcasting stations via N time division multiplex (TDM) communication signals, said studio and said N broadcasting stations each receiving an accurate timing signal, comprising N TDM multiplexers at said studio, wherein characterized in that each of said N multiplexers (i) multiplexes said digitized audio program signal into at least one channel of a TDM communication signal transmitted from said studio to one of said N broadcasting stations, and (ii) multiplexes said accurate timing signal received by said studio into another channel of said TDM communication signal, for each one of said N broadcasting stations,

(i) a TDM de-multiplexer for removing said digitized audio program signal and said studio timing signal from a corresponding one of said N TDM communication signals,

(ii) a timing signal comparator for comparing said studio timing signal to a local timing signal delayed offset to produce a comparison result, and generating an offset time corresponding to said comparison result; and,

(iii) a delay adjust circuit for dynamically adjusting a delay of said TDM communication signal as a function of said offset time, and preferably in which said delay adjust circuit is substantially hitless.
A system as claimed in claim 1, wherein said accurate timing signal is provided by a pre-existing time distribution network, and said pre-existing time distribution network includes a Global Positioning System (GPS) satellite network, and said accurate timing signal is provided by at least one GPS receiver.
A system as claimed in claim 1, wherein said TDM communications signal includes a T1 communications signal, in which said TDM communication signal also includes an E1 communications signal.
A system as claimed in claim 3, wherein said TDM communications signal is substantially unidirectional, such that said accurate timing signal and said program audio singal flow from said studio to said one of said N broadcasting stations.
A system as claimed in claim 4 wherein each os said digitized audio program signals includes a frequency band sufficient to accurately convey a high fidelity music program, and includes a dynamic range sufficient to accurately convey a high fidelity music program.
A system as claim in claim 5, wherein each of said digitized audio program signals conveys at least one channel of monophonic information, or each of said digitized audio program signals conveys at least two channels of stereophonic information.
A system as claimed in claim 1, characterized by an audio card at said studio and a substantially identical audio card at each of said N broadcasting stations, said studio audio card digitizes an analog audio program, and each of said broadcasting station audio cards converts said digitized audio program to said analog audio program, and each of said audio cards maintains a consistently linear phase delay of said audio signal over an audio signal frequency band of interest, thereby maintaining a constant audio signal group delay, in which said audio card includes at least one FIR filter, so as to maintain said constant group delay.
A system as claimed in claim 1, wherein said delay adjust circuit dynamically adjusts said delay of said TDM communication signal so as to drive said offset time to substantially zero.
A method for aligning the phase characteristics of N identical digitized audio program signals distributed from a studio to N broadcasting stations via N time division multiplex (TDM) communication signals, said studio and said N broadcasting stations each receiving an accurate timing signal, characterized by:

multiplexing said digitized audio program signal into at least one channel of a TDM communication signal transmitted from said studio to one of said N broadcasting stations;

multiplexing said accurate timing signal received by said studio into another channel of said TDM communication signal; and,

for each one of said N broadcasting stations,

removing said digitized audio program signal and said studio timing signal from a corresponding one of said N TDM communication signals;

comparing said studio timing signal to a local timing signal to produce a comparison result, and generating an offset time corresponding to said comparison result; and,

dynamically adjusting a delay of said TDM communication signal as a function of said offset time, so as to drive said offset time to substantially zero, and including the step of dynamically adjusting said phase delay of said audio program in a substantially hitless manner, in which a further step includes receiving each of said accurate timing signals from a pre-existing time distribution network.
A system for aligning the phase characteristics of N identical digitized audio program signals distributed from a studio to N broadcasting stations via N time division multiplex (TDM) communications links, said studio and said N broadcasting stations each receiving an accurate timing signal, comprising:

N TDM multiplexers at said studio, wherein each of said N multiplexers (i) multiplexes said digitized audio program signal into at least one channel of a TDM communication link established between said studio and one of said N broadcasting stations, and (ii) multiplexes said accurate timing signal received by said studio into another channel of said TDM communication link;

for each one of said N broadcasting stations,

(i) a TDM de-multiplexer for removing said digitized audio program signal and said studio timing signal from a corresponding one of said N TDM communication links,

(ii) a timing signal comparator for comparing said studio timing signal to a local timing signal to produce a comparison result, and generating an offset time corresponding to said comparison result; and,

(iii) a delay adjust circuit for dynamically adjusting a delay of said digitized audio program signal as a function of said offset time.