Method of stabilizing the level of an audio signal in television broadcasting

To prevent particular transmission equipment from artificially increasing low level audio signals to a predetermined allowable level, such signals are first summed with a stabilization signal having a predetermined frequency to form a composite audio signal. Moreover, the stabilization signal has a level that prevents any increase in signal level that may be artificially imposed on the composite signal during the processing of the composite signal by the transmission equipment. Once the composite signal has been so processed, then the stabilization signal is removed therefrom and the original audio signal is outputted to associated transmission media.

TECHNICAL FIELD 
The invention relates to the transmission of audio signals. 
BACKGROUND OF THE INVENTION 
The AGC circuit and/or limiter circuit in the transmitter section of a TV 
broadcasting system artificially raises low levels of audio signals of a 
broadcast commercial to an allowable maximum level and then transmits the 
resulting signals over associated transmission media, e.g., antenna, coax 
cable, fiber, etc. The effect of artificially raising the level of such 
audio signals may create a result not intended by the producer of the 
commercial. For example, assume that a television commercial as produced 
purposely starts with a low-level audio signal, e.g., -20 VU audio signal, 
then slowly increases the volume of the audio signal over a predetermined 
period of time, e.g., 10 seconds. However, when the commercial is 
processed by particular broadcast television equipment for transmission 
over associated transmission media, the AGC and/or limiter circuits in 
such equipment artificially increases the -20 VU (low-level) audio signal 
portion of the commercial to, for example, -5 to 0 VU, thereby diminishing 
the desired effect intended by the producer of the commercial. 
SUMMARY OF THE INVENTION 
The relevant art is advanced, in accord with an aspect of the invention, by 
controlling an audio signal in such a way that the desired effect intended 
by the producer of the commercial is not diminished. More specifically, 
such an audio signal is first summed with a stabilization signal having a 
predetermined frequency to form a composite audio signal in which the 
stabilization signal has a level that prevents any increase signal level 
that may be artificially imposed on the composite signal by transmission 
equipment processing the composited signal. Once the composite signal has 
been so processed, then the stabilization signal is removed therefrom and 
the original audio signal is outputted to associated transmission media.

DETAILED DESCRIPTION 
It is noted that although the following discusses the claimed invention in 
the context of television broadcasting, that should not be construed as 
being a limitation of the claimed invention, since it is apparent that the 
invention may be practiced in any application where there is need to 
control the level of an audio signal that will be processed by AGC or 
limiter circuits. (It is noted that the terms AGC and limiter circuits as 
used herein include any type of circuit and/or device which increases the 
level of a signal to a level prescribed by a predetermined requirement, 
e.g., a transmission requirement.) Moreover, the following discussion uses 
the term "stabilization signal" which is defined herein as being a signal 
that remains at a constant level, for example, a level of 0 VU, in which 
the frequency of the stablization signal is selected for a given 
application. In certain applications, the constant level will be an 
allowable level set by the respective application. 
With that in mind, the following first discusses the way in which the 
stabilization signal is generated and added to a commercial broadcast 
soundtrack and then discusses the processing of the resulting signal at a 
broadcast facility. Specifically, recording studio 100 shown in FIG. 1 
includes audio playback device 5 for reproducing in a conventional manner 
the original sound track of a commercial broadcast that is recorded on 
storage media 6, e.g., magnetic tape. Audio playback device 5 includes 
control 7 for adjusting the level of the audio signal that device 5 
outputs to VU meter 10. The level of the audio signal that device 5 
outputs is thus monitored by VU meter 10 to ensure that the level does not 
exceed, for example, 0 VU. The output of VU meter 10 is then presented to 
input 11 of audio device 15, which may be, for example, a conventional 
audio mixer. 
In accord with an aspect of the invention, a stabilization signal is 
combined with the audio signal in a conventional manner to prevent the 
level of the latter signal from being artificially increased when it is 
being processed by particular broadcasting equipment. Specifically, 
stabilization signal 32 is generated by a conventional sine wave generator 
30 which includes volume control 31 for adjusting the level of 
stabilization signal 32. In accord with an aspect of the invention, the 
level of the stabilization is monitored by VU device 35 and adjusted using 
volume control 31 to ensure that the level of the stabilization signal 
does not exceed a predetermined transmission level, e.g., a level of 0 VU. 
(For the purpose of the present application and not by way of limitation, 
as noted above, the frequency of stabilization signal 32 in accord with 
the present illustrative application of the invention is set to, for 
example, 18 kHz. The reason for selecting that frequency will become 
apparent below.) 
Continuing, VU device 35 then presents the stabilization signal to input 12 
of audio device 15. Audio device 15, in a conventional manner, sums the 
input signals appearing at inputs 11 and 12 to produce a single, output 
signal (i.e., a composite signal), which is presented to path 16. 
Similarly, audio device 15 includes volume control 17 for ensuring that 
the level of the "mixed" audio signal that is outputted to path 16 does 
not exceed 0 VU as monitored by VU device 20. The mixed audio signal is 
then presented to recording device 25 where it is recorded onto storage 
media 26, which may be, for example, video tape. In an illustrative 
embodiment of the invention, device 25 may be, for example, a so-called D2 
digital video tape recorder. The recording produced by device 25 may then 
be presented to a television broadcast facility, e.g., a television 
station, for commercial broadcast. 
A broad block diagram of such a facility is shown in FIG. 2 and includes, 
inter alia, playback device 205 which outputs to path 206 for presentation 
to audio/video processing equipment 210 the information that has been 
prerecorded on storage media 26. Processing equipment 210 includes 
conventional television broadcasting equipment such as AGC and limiter 
circuits, amplifiers, audio compressors, signal modulators, signal 
combiners, etc. Such equipment processes the signal that is received via 
path 206 such that the AGC circuit of equipment 210 is stabilized 
(controlled) by the stabilization signal to prevent the AGC circuit (not 
shown) from raising the level of the original audio signal stored on 
storage media 6 (FIG. 1) and which forms part of the composite signal 
recorded on media 26. 
Equipment 210, more particularly, processes the signal that it receives via 
path 206 in accord with a plurality of well-known transmission 
requirements. One such requirement, in this case broadcast television, 
band-limits the audio signal that is outputted to path 211 to a frequency 
range of 50 Hz to 15 kHz. Since the frequency of the stabilization signal 
is not within the required band-limited audio range, it is removed 
(filtered out) by equipment 210 leaving, in accord with the aspect of the 
invention, an audio signal that is virtually identical to the original 
audio signal 8 (FIG. 1). Thus, in accord with an aspect of the invention, 
stabilization signal 32 prevents equipment 210 from artificially 
increasing the commercial audio signal. 
Following the processing of the commercial broadcast signals that it 
receives via path 206, equipment 210 outputs a processed version of signal 
8 to path 211 for presentation to conventional transmitter equipment 215. 
Equipment 215, in turn, presents the commercial broadcast signal to an 
associated transmission media, e.g., a transmitting antenna for 
over-the-air transmission. (It is noted that for the sake of clarity and 
conciseness, the processing of video signals has been omitted from the 
Figs. and the foregoing). 
The foregoing is merely illustrative of the principles of the invention. 
Those skilled in the art will be able to devise numerous arrangements, 
which, although not explicitly shown or described herein, nevertheless 
embody those principles that are within the spirit and scope of the 
invention. For example, although the invention has been discussed in the 
context of commercial broadcast television, it may also be practiced in 
any application where there is need to control the level of an audio 
signal that will be processed by AGC or limiter circuits, as mentioned 
above. For example, such a need may be found in such applications as 
commercial radio, (AM and/or FM), cellular and/or wired telephone 
transmission, film industry, satellite, fiber optic transmission, infrared 
as well as high-definition television.