Automatic audio mixing selector device

An automatic mixing selector device for use with an audio amplifier or tuner for automatic priority selection and digital fading of two or more audio sources using the amplifier and tuner controls. The circuit comprises a priority selector circuit which monitors the designated priority audio source for signal amplitude and duration, a micro-computer then operates a digital fader to phase in the priority signal and also to phase in the alternate signal in the absence of a sufficient priority signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to automatic audio detecting switching 
devices and more particularly, but not by way of limitation, to a mixing 
selector circuit having a micro-computer for the automatic fading and 
switching of a plurality of audio signals. The automatic switching 
selector device may be built into the amplifier and/or tuner or may be 
constructed as a black box which is connectable to the input and output 
jacks of existing sound systems. 
2. History of the Prior Art 
A typical home entertainment system comprises a tape deck, record player, 
AM/FM stereo tuner and amplifier and speakers. Often at parties or social 
events it is desirable to have background music which may consist of an FM 
radio station or records or both. 
The constant monitoring of a record changer can become a nuisance to the 
host in changing records and turning on the FM station when the records 
are finished playing. 
This problem not only exists in entertainment systems but also at 
commercial radio stations where it was desired to monitor a priority 
signal source. 
If a disc jockey is away from his station when a record finishes playing, 
it would be desirable to automatically fade in or switch in an alternate 
signal source. 
Several attempts have been made to solve this problem by rather elaborate 
programming devices such as taught in the patent to Smith, Sr., U.S. Pat. 
No. 3,809,812, issued in 1974 for a "Programming System". While such a 
device might be feasible for an unmanned FM station, it would be 
prohibitively expensive and complicated for home use. 
A more pertinent solution to the problem was provided by two patents to 
Gates, U.S. Pat. No. 3,952,154, issued in 1976 for "Automatic Audio Source 
Selector For Stereo Entertainment Centers" and the subsequent patent, U.S. 
Pat. No. 3,956,591, issued in 1976 for "Two-Input Automatic Source 
Selector For Stereo Entertainment Centers". 
While the two patents to Gates solved the basic problem set out above, the 
selection of audio in the Gates patents is by switching abruptly from one 
source to another when sound has been detected on the priority source. 
This solution to the problem could be more disconcerting than the problem 
itself and would give rise to switching back and forth during the pause 
between songs on a single record. Further, if a record or tape is the 
priority source, and switching is not made until the source is already 
playing, often the first notes of the musical composition are clipped and 
one does not hear the full record. 
SUMMARY OF THE INVENTION 
The present invention provides a solution to the problems hereinbefore set 
forth in a rather simple but sophisticated circuit including a 
micro-computer which analyzes the audio signals being monitored to 
determine if there is a priority signal present, the strength of the 
signal and whether or not the signal duration is sufficient to warrant 
fading out the alternate signal and fading in the priority signal. If the 
priority signal is a record player, the detection device monitors whether 
or not the needle is still in the groove when a song has finished in order 
to fade back to the alternate audio signal. 
Since the present invention detects when the needle enters a groove or when 
there is tape noise on a tape player of a priority source, the alternate 
music source is immediately faded out and then a determination is made as 
to whether or not there is music present on the priority source. If there 
is music present, the priority source is faded in rather than switched in 
abruptly. On the other hand, if the record player needle is still on the 
lead-in groove, the priority signal is switched in so that the first note 
is not missed thereby providing a minimum of interference with the 
priority audio source. 
As an example, consider the tuner as an alternate audio signal and a record 
as the priority audio signal. The tuner is coupled with the mixer which in 
turn is connected to the tuner input jack at the amplifier. The record 
player is connected to the phono input jacks while the mixer is connected 
to one set of the tape monitor jacks. Other audio sources such as tape 
decks may be connected to the input of the other tape monitor jacks. 
The tape monitor switch is then set to route the record player output of 
the mixer through the tape monitor jacks. Hence, the mixer receives its 
alternate audio signal from the tuner as input A and the priority audio 
signal from the phono through the tape jack as input B. The mixer is also 
connected to a switchable power outlet on the back of the amplifier so 
that the mixer is powered up with the amplifier. 
If there is no signal present at input A, the tuner signal is routed 
directly through an audio signal selector circuit and through output 
buffers to the tuner input jacks for broadcast by the stereo system 
speaker. 
When a record is played, as soon as the tone arm stylus makes contact with 
the record surface, an audio signal appears at input A which is fed 
through a mixer circuit to form a composite signal at both left and right 
channels which is in turn fed through a Schmitt triggering circuit which 
passes the signal to a micro-computer. The computer, through digital 
comparison with programmed amplitude levels and time lapses, determines 
that there is a priority signal present at input A and immediately 
activates the audio selector circuit and a plurality of digital faders to 
fade out the tuner signal. At this point the input at A is again checked 
to see whether or not there is music present. If there is music present, 
the priority signal at input A is faded in. On the other hand, if the 
needle is still in the groove rather than music playing, the priority 
signal is switched in so that the first note is picked up on the record. 
After the needle has lifted off the record surface, the computer will 
switch out the signal from input A and fade in the tuner signal at input B 
so that there is a smooth transition from one source to the other. 
A different priority signal may be selected such as a tape player or the 
like, by using the tape monitor selector switch located on the amplifier. 
The only replugging of the input and output jacks that is necessary is if 
the user wishes to select some source other than the tuner as the 
alternate audio source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings in detail, reference character 10, shown in FIG. 
1, generally indicates an automatic audio mixer selector hereinafter 
referred to as mixer which is operational with or may be made as an 
integral part of an amplifier 12 for use in priority selection of audio 
signal sources from apparatuses such as a record player 14, tuner 16 and 
tape recorder 18. When adapted for use with stereo systems, the mixer 10 
comprises a first pair of input jacks J1 and J2 which are operably 
connected to the output jacks of a first set of tape jacks T1 normally 
located on the back of the amplifier 12. 
The mixer 10 further comprises a second pair of input jacks J3 and J4 which 
are connected to what will be referred to as an alternate audio source 
shown as the tuner 16 in FIG. 1. 
The mixer 10 further comprises a pair of output jacks J5 and J6 which in 
this case are operably connected to the tuner input jacks on the amplifier 
12. The mixer 10 receives its electrical power from power outlet 20 
located on the amplifier 12, through a power supply 22 which in turn 
supplies DC power at the necessary voltage levels for operating the 
circuit components hereinafter set forth. For this particular circuit 
three power output levels are required consisting of a first positive 
level V1, a negative level V2 and a second positive level V3. 
The mixer 10, shown in FIG. 2, generally comprises a pair of stereo input 
buffers 24 and 26 referred to as audio A input and audio B input, 
respectively. 
The audio outputs of buffers 24 and 26 are provided to an audio selector 
switching mechanism 28, the output of which is passed through an output 
buffer 30 and back to the amplifier 12. The output of the stereo input 
buffer 24 is also applied to the input of a mixer detector circuit 32 
which is in turn operably connected to a micro-computer processor chip 34. 
The output of the computer process chip 34 is applied back to the mixer 32 
and is also provided to an output latching circuit 36. The latching 
circuit 36 is then operably connected to the audio selector 28 and to the 
output buffer 30 through a digital fader network 38. 
Referring to FIG. 3 of the drawings, the input buffers 24 and 26 which 
receive their inputs via jacks J1, J2, J3 and J4 provide the functions of 
signal and impedance matching, isolation of DC offset voltages and 
increasing the dynamic range of the audio signal. 
The input buffers 24 and 26 generally comprise operational amplifiers U1, 
U2, U3 and U4 which receive their audio input signal from the input jacks 
J1, J2, J3 and J4, respectively. 
A signal entering the jack J1 passes through the capacitor C1 which blocks 
any offset voltages and through the resistor R1 to the positive input of 
the operational amplifier U1, the capacitor C1 and resistor R1 forming a 
high pass filter to prevent low frequency noise from entering the mixer, 
while the resistor R2 provides an audio signal load to ground. The output 
of the operational amplifier U1 is fed back to the negative input thereof 
through the resistor R3, the resistors R3 and R4 serving as a voltage 
divider to control the gain of the amplifier U1. 
The buffers comprising operational amplifier U2, U3 and U4 are similarly 
constructed for a similar function, the components thereof being 
designated with subscripts a, b and c, respectively. 
The outputs of the audio amplifiers U1, U2, U3 and U4 designated as A, B, C 
and D of FIG. 3 are operably connected to the inputs of the audio signal 
selector 28. The audio selector 28 comprises four analog switches S1, S2, 
S3 and S4, each being operably connected to the outputs of operational 
amplifiers U1, U2, U3 and U4, respectively. Switches S1 and S2 are wired 
for simultaneous operation by a signal from the output latches circuit 36 
via line 40 while switches S3 and S4 are simultaneously operated by an 
output signal from the output latches circuit 36 via line 42 as shown in 
FIG. 5 of the drawings. The output of switches S1 and S3 are coupled and 
provided as a first input to the output buffer circuit 30 via line 44 
while the output of switches S2 and S4 are coupled and provided as a 
second input to the output buffer 30 via line 46. 
Hence, if switches S1 and S2 are closed, output signals from the input 
buffer 24 are passed directly to the output buffer 30. On the other hand, 
if switches S3 and S4 are closed, the outputs from input buffer 26 are 
passed directly to the output buffer 30. 
The output buffer 30 generally comprises a pair of operational amplifiers 
U5 and U6. The audio signal from line 44 enters the positive input of the 
operational amplifier U5 through a current limiting resistor R5. The 
positive input to operational amplifier U5 is also operably connected to 
one output of the digital fader circuit 38 for a purpose that will be 
hereinafter set forth. The output of the operational amplifier U5 passes 
through an impedance matching resistor R6 the output of which is provided 
as a negative input feedback to the operational amplifier U5. 
The output of the resistor R6 is then passed through a capacitor C2 which 
serves to block any DC offset voltages and in conjunction with the 
resistor R6 forms a high pass filter. This output is then provided to an 
output jack J5 as shown in FIG. 5. The operational amplifier U6 receives 
its input via line 46 and operates in essentially the same manner as that 
hereinbefore described, the output thereof being provided to an output 
jack J6. The components associated with the operational amplifier U6 are 
designated with subscripts a. As hereinbefore set forth, the positive 
input to the operational amplifier U5 is operably connected to one output 
of the digital fader circuit 38 via line 48 while the input to operational 
amplifier U6 is operably connected to a second output of the digital fader 
38 via line 50. 
The digital fader circuit 38 comprises a first set of analog switches S5, 
S6, S7 and S8, one side of which are connected to parallel load resistors 
R7, R8, R9 and R10, the said resistors being coupled together and operably 
connected to the output line 48 which is in turn connected to the input of 
operational amplifier U5. The switches S5, S6, S7 and S8 may be 
simultaneously or sequentially operated by output signals from lines 52, 
54, 56 and 58, respectively, from the output latching circuit 36. 
The digital fader comprises a second set of identical switches S5a, S6a, 
S7a and S8a, the operators of which are also connected to lines 52, 54, 56 
and 58, respectively, one side of said switches being connected to 
parallel resistors R7a, R8a, R9a and R10a, respectively. The said 
resistors are coupled together and attached to the input of operational 
amplifier U6 via line 50. 
Referring now to FIG. 4 of the drawings, the mixer circuit comprises a pair 
of staged operational amplifiers U7 and U8, coupled with a switching 
circuit comprising switches S9, S10, S11 and S12. 
The outputs from the input buffer 24 are coupled together through a pair of 
resistors R11 and R12, respectively, and provided as a composite input to 
the negative side of the operational amplifier U7. The positive side of 
the operational amplifier U7 is connected to ground through a resistor R13 
and is also connected to the micro-computer 34 through a resistor R14. The 
output of the operational amplifier U7 is fed back to its negative input 
through a resistor R15 to set the gain of the operational amplifier. 
The amplified output of the operational amplifier U7 passes through a 
resistor R16 and capacitor C3 to ground which forms a low pass filter. The 
junction between the resistor R16 and capacitor C3 then is provided as an 
input to the negative side of operational amplifier U8 through a resistor 
R17. The output of the operational amplifier U8 then is fed back to its 
positive input side through a resistor R18. The positive input to the 
operational amplifier U8 is also connected to ground through a resistor 
R19, the resistors R18 and R19 forming a voltage divider to set the 
reference voltage for the operational amplifier U8. 
This reference voltage applied at the positive input side of operational 
amplifier U8 is also connected to one side of both switches S9 and S10. 
The input to the negative side of the operational amplifier U7, is provided 
to one side of switches S11 and S12. Operation of the switches S9, S10, 
S11 and S12 is provided from the micro-computer circuit 34 via lines 60, 
61, 62 and 63, respectively. 
The opposite side of the switches S9 and S10 are provided with resistors 
R20 and R21, respectively, which are tied together and operably connected 
to the output of the operational amplifier U8. The opposite side of 
switches S11 and S12 are connected to resistors R22 and R23 which are then 
tied together and operably connected to the output of the operational 
amplifier U7. 
The output of the micro-computer circuit 34 is operably connected to 
switches S11 and S12 of the mixer circuit via lines 63 and 62, 
respectively. The output of the computer circuit 34 is also connected via 
lines 60 and 61 to the switches S9 and S10 of the mixer circuit 32. 
The output of operational amplifier U8 is connected to the base of an NPN 
transistor Q1 through a resistor R24. The base of the transistor Q1 is 
connected to ground through a resistor R25 and reverse biasing diode D1 
which are in parallel. The collector of transistor Q1 is provided as an 
input to the micro-computer circuit 34 while the emitter thereof is 
grounded. 
The circuit 32 therefore provides a combination mixer and Schmitt trigger. 
If the output voltage level from the operational amplifier U8 is above the 
reference voltage present at the input of the positive side of said 
operational amplifier, the Schmitt trigger is in a first logic state. If 
the voltage present at the output of operational amplifier U8 falls below 
the reference voltage level at the input of the positive side of the 
operational amplifier U8, the Schmitt trigger will change to the opposite 
logic state. 
If the output of operational amplifier U8 is a negative voltage, it is 
connected to ground through resistor R24 and diode D1 reverse biasing the 
transistor Q1. When a positive voltage is present at the base of 
transistor Q1, Q1 is forward biased and its collector input to the 
micro-computer 34 is grounded and becomes zero volts. Thus, if the output 
signal at U8 is outside the threshold band set by resistor R18, the 
computer 34, by a series of software routines, which will be hereinafter 
set forth, selectively provides suitable output voltages to a plurality of 
lines 64, 66, 68 and 70 which are operably connected to the output 
latching circuit 36 which in turn causes sequential signals to be provided 
to the digital fader circuit 38 via lines 52, 54, 56 and 58, respectively. 
The voltage levels present at the inputs of the operational amplifiers U5 
and U6 are directly controlled by the digital fader circuit 38 to 
sequentially attenuate or fade the audio signals present at those inputs. 
When the signal present at the inputs of operational amplifiers U5 and U6 
have been almost completely attenuated, the computer circuit 34 provides 
output signals along lines 72 and 74 to the output latching circuit 36 and 
subsequently to the audio signal selector circuit 28 via lines 40 and 42, 
respectively, to open switches 53 and 54 and to engage switches S1 and S2. 
The reference voltage threshold at U8 is set rather narrow as shown in the 
first curve of FIG. 11 at plus or minus three volts by reference 
characters 76 and 78. Hence, if the signal is greater than plus or minus 
three volts at the output of operational amplifier U8, the signal is 
sufficient to alternately forwardly and reverse bias transistor Q1 to 
provide a square wave input to the micro-computer circuit 34. 
Upon receipt of this signal from the phonograph 14, the computer circuit 34 
sets up a delay to assure that there is a signal present. If the signal 
holds for the required delay time, the micro-computer then provides a 
switching signal via lines 60 and 61 to the mixer circuit in order to 
close the contacts on switches S11 and S12. This places resistors R22 and 
R23 into parallel with resistor R15 thereby reducing the amplification of 
the operational amplifier U7. This will prevent operational amplifier U7 
from saturating and clipping the output wave form present at the output of 
operational amplifier U7. This feature is for signal processing accuracy 
and in no way affects the actual audio output present at the output buffer 
30. 
At the same time the computer circuit 34 has determined that there is still 
a signal at the input buffer 24, it sequentially provides signals along 
the lines 64 and 66, 68 and 70 to the output latches circuit 34 which in 
turn sequentially closes the switches of the digital fader circuit 38, 
switches S5 and S5a being simultaneously closed, then S6 and S6a and 
etcetera which provides a sequentially changing voltage at the inputs of 
operational amplifiers U5 and U6 thereby attenuating the signal from the 
tuner being provided thereat. The signal attenuation is shown by a 
representative graph of FIG. 12, it being recognized that four resistors 
in each digital fader circuit can produce up to sixteen fading steps. The 
dashed line of FIG. 12 represents a fade curve which is pleasing to the 
human ear. Naturally, the more resistors provided in the digital fader 
circuit, the closer the curve can be followed. When the signal from the 
tuner has been attenuated, simultaneous signals are provided along lines 
72 and 74 from the computer to the output latching circuit 36. The signal 
then is processed through lines 40 and 42 which serve to close switches S1 
and S2 and open switches S3 and S4 of the audio selector circuit. This, in 
effect, connects the stereo output from the input buffer 24 through the 
switches S1 and S2 to the output buffer circuit 30. 
The computer then provides a signal along lines 60 and 61, which closes 
switches S9 and S10 of the mixer circuit 32 thereby connecting resistors 
R20 and R21 in parallel with resistor R18 which serves to widen the 
threshold band at the feedback input to the operational amplifier U8. This 
increased threshold is shown in the second curve of FIG. 11 with the 
threshold levels being set at plus or minus 12 volts and indicated by 
reference characters 80 and 82. 
Therefore, if there is a signal above and below the threshold voltage 
levels 80 and 82, it would indicate that there is music on the record 
player and the computer sends out sequential signals along lines 70, 68, 
66 and 64 which through the output latching circuit 36 serves to 
sequentially open the switches of the digital fader circuit 38 thereby 
fading the music in from the record player. 
However, on the other hand, if after the threshold has been widened, there 
is no signal, this would indicate to the computer that although the needle 
is still in the groove, the musical recording has not been reached by the 
needle and hence there is no need to fade in the signal. In this case, the 
computer circuit simultaneously provides signals through lines 70, 68, 66 
and 64 to simultaneously unlatch or open all of the switches of digital 
circuit 38 thereby allowing the full signal from the phonograph 14 to the 
output buffer 30. 
Normally, the delay is provided after the initial sensing of a signal at 
the input buffer 24 is sufficiently small so that the switching can take 
place before the music is encountered by the needle on the record player 
so that the first note is not missed in bringing in the phonograph signal. 
It should also be pointed out that whereas the drawings and particularly 
the functional block diagrams indicate that the priority signal is a 
phonograph, it can just as well be a tape player whereas the tape noise 
between musical compositions will serve to activate the system just as it 
will the needle in contacting the record surface. 
After switching has occurred, the circuit will be tested to see if there is 
music present at the input buffer circuit 24. If there is, indicating that 
the signal level at the output of operational amplifier U8 is outside the 
threshold levels 80 and 82 as shown in the graph of FIG. 11, the computer 
will go into a half-second delay loop whereby every half second it will 
test to see whether or not there is music at the inputs of the input 
buffer 24. 
If at any time the check is made and there is no signal present with the 
expanded threshold limits of 80 and 82, the computer will send signals 
along lines 60 and 61 to open the switches S9 and S10 thereby reducing the 
threshold level to the levels 76 and 78 as also shown in FIG. 11. If there 
is a signal present at this point indicating that the needle is still in 
the groove, end of record (EOR) counter will be incremented and if this 
value is less than some arbitrary value indicated at 40 units in FIG. 8 of 
the drawings, the switches S9 and S10 will again be closed thereby 
broadening the threshold to levels 80 and 82. If there is then music at 
the priority input, an inner record gap time is detected and if the end of 
record increment is greater than a second arbitrary signal, the inner 
record counter is cleared. Again there is a test to determine whether 
there is music at the input buffer 24 and the circuit is completed. The 
circuit is repeated throughout the record to make sure that switching will 
not occur when the phonograph needle is still in place in the groove 
between songs on the record. 
On the other hand, if there is a determination that there is no music at 
the input, again the threshold level is reduced by opening switches S9 and 
S10 and a test is made to determine whether or not the needle is in the 
groove. If it is not in the groove, a check is made as to whether or not 
the inner record gap counter is below an arbitrary level. If the inner 
record gap counter is greater than this arbitrary level, then switches S1 
and S2 are opened and switches S3 and S4 are closed in the audio selector 
while simultaneously all of the switches of the digital fader circuit 38 
are closed which applies the input of the tuner 14 through the audio 
selector to the output buffer circuit 30. The computer then sequentially 
opens the switches of the digital fader circuit 38 thereby fading in the 
signal from the tuner through the output buffer circuit 30. The computer 
also provides a reset signal to the mixer circuit 32 through resistor R14 
to strobe the operational amplifier U7 which will provide a signal to the 
computer which will in turn open switches S11 and S12. 
From the foregoing it is apparent that the present invention provides an 
audio priority selection device which may either be built into existing 
stereo amplifier equipment or may be packaged as a separate piece of 
equipment whereby the tape monitoring jacks, tuner input jacks and the 
like on standard equipment may be utilized without any hardware 
modification thereto. 
It is further apparent that the present invention provides efficient and 
smooth transition to and from the priority signal and the alternate signal 
sources in a manner which is pleasing to the ear but at the same time 
reduces or eliminates any unnecessary signal loss during the transition. 
Whereas the present invention has been described in particular relation to 
the drawings attached hereto, modifications apart from those shown or 
suggested herein may be made within the spirit and scope of the invention.