Time compression/expansion with synchronized individual pitch correction of separate components

A time compression/expansion audio reproduction system of the type that provides pitch correction by repetitive variable time delay achieves improved performance by separating the reproduced signal from a recording into components which are separately delayed. For studio quality reproduction the signal is separated into contiguous frequency bands which are each delayed synchronously and the processing noise in each band is eliminated by filtering each band signal after delay to eliminate high frequency components. Bandpass filtering prior to recombination as well as blanking and amplitude compression and expansion are also disclosed.

BACKGROUND OF THE INVENTION 
Time compression/expansion systems are known in the prior art and in recent 
years versions employing repetitive variable time delay for pitch 
correction such as disclosed and claimed in U.S. Pat. No. 3,786,195 have 
become commercially available. In such systems, time compression refers to 
the playback of a recording at a speed higher than that at which the 
original work was recorded in order to complete the playback of the record 
in a shorter time than was taken for performance of the original work. 
Under such conditions, a normal record reproducer will produce an output 
audible signal which has its pitch frequencies transformed by the same 
factor as the time compression, i.e., the ratio of playback time to 
original recording time. In order to utilize time compression effectively 
where the factor is significantly greater than one such that a substantial 
saving in time is realized, pitch correction has been applied to the 
signal to return the pitch of the output signal to something approximately 
the normal pitch of the speaker or singer or other audible material 
originally recorded. Where approximately the original pitch is restored to 
the audible signal obtained with time compression it becomes possible to 
listen to recorded material at twice the speed of its original recording 
thus using only approximately one-half the time necessary to listen to a 
given speech, lecture or other recorded material and this speed listening 
is actually less tiring, if the pitch is properly corrected, than 
listening to the somewhat slow pace of delivery by the original speaker, 
for example. Time compression at greater than a factor of two is also 
possible with such systems although the greater the increase in speed more 
familiarity with such listening sensation is required for comprehension. 
Time expansion is actually the opposite of time compression where the 
record is played back at a slower speed resulting in a reduction of the 
pitch frequencies obtained from the record and requiring pitch correction 
which increases the frequencies of the signals obtained from the slowly 
played record to restore them to approximately normal frequency. Time 
expansion is useful where difficult material is being absorbed by the 
listener such as foreign language learning and other situations where 
comprehension of the text may be difficult or impaired by natural reasons. 
In the present invention, time compression/expansion is considered a 
generic term as involving compression or expansion or both within the 
capability of a given apparatus as disclosed or claimed. 
As described in considerable detail in U.S. Pat. No. 3,786,195, when 
repetitive variable time delay is used for pitch correction special 
measures are usually required to compensate for the discontinuity which 
occurs when the variable delay repeats. Thus filtering, blanking and gap 
filling with other signal components are suggested for use either alone or 
in combination in that patent. More sophisticated techniques for 
processing the signal including the discontinuity are found in U.S. Pat. 
Nos. 3,828,361; 3,846,827; and 3,869,708. While the techniques shown in 
these prior art patents produce very high quality audio reproduction at 
altered speeds with pitch correction, the complete elimination of the 
switching transient and the filling of the gap left by clearing and 
refilling the delay medium in certain forms of delay devices can result in 
still audible vestiges of the interfering transient signal. For extremely 
high quality record reproduction such as that required for radio and 
television broadcasting, for example, it is essential that the audio noise 
components be reduced to the level where they are imperceptible to the 
untrained ear. This quality is particularly required where the program 
material includes commercial messages which are presented on commercial 
radio and television broadcast stations at extremely high cost to the 
sponsor of the commercial message who thus will accept nothing short of 
perfection in the end product. On the other hand, the broadcast station is 
faced with the problem of presenting a pre-recorded message precisely 
within the interval of a predetermined time slot which is built into the 
recorded program material. If the recorded commercial message can be 
expanded or compressed exactly to fit the predetermined time slot the 
production and programming chores for network and studio broadcasting 
would be greatly simplified. 
SUMMARY OF THE INVENTION 
The present invention is related to extremely high quality audio time 
compression/expansion systems wherein record reproduction can be selected 
to occupy a time difference other than that at which the original work was 
recorded to meet various needs such as speed listening or slow learning 
and further to adapt recorded messages to predetermined time intervals in 
which the ultimate audio output of the system presents the original 
recorded material at any desired pitch frequency such as a frequency that 
approximates or equals the frequencies of the voice or musical instrument 
which performed the original work which was recorded. The invention 
further pertains to processing separate components of a signal 
representing a recorded work, such as the playback of a stereo recording, 
with the provision for pitch corrected compression or expansion while 
maintaining time and phase coincidence for the components of the separate 
stereo tracks. 
According to the present invention quality time compression/expansion 
systems are provided by separately treating the components parts of an 
electric signal derived from playback of a recording, such as the separate 
tracks of a stereo recording, or by the subdivision of the electric signal 
into frequency bands for separately processing two or more frequency bands 
of the signal representing the recorded work. These frequency bands are 
preferably contiguous such that the entire frequency spectrum of the 
electrical signal is processed for pitch correction and to assure time and 
phase coincidence the periodic delay variations for the subdivided bands 
are synchronized such that the delay introduced for each band is equal and 
the variation progresses equally such that the outputs of the delay 
devices for each band can be combined to reconstitute the complete signal 
after pitch correction without time or phase distortion between the bands. 
Further improvement can be obtained by amplitude compression prior to 
signal delay in each band with corresponding amplitude expansion in each 
band after delay and prior to summing the signals to obtain the composite 
output. Since the frequency band occupied by the electric signal obtained 
from the recording depends upon the time compression or expansion 
currently selected, the subdivision of the signal into contiguous 
frequency bands can be enhanced by having variable bandpass filters 
adjusted in accordance with the motor speed control for the playback 
device which in turn is coordinated with the required variable delay rate 
to obtain the desired pitch correction. 
Subdivision into frequency bands permits further improvement in signal 
processing for time compression/expansion in that the length of the delay 
line required for each band can be optimized. In other words, for a bucket 
brigade delay line, for example, the number of stages in the shift 
register delay for the higher frequencies can be increased to provide an 
optimum number of samples for the higher frequencies while the delay can 
be synchronized with the lower frequency bands which have fewer stages by 
a corresponding adjustment in the clock rate within the different bands. 
Further improvement in processing can be obtained with band splitting by 
utilizing lowpass filtering after the pitch correction delay has been 
effected. Such filtering eliminates switching transients and other 
processing noise while passing the pitch corrected information signal such 
that subsequent bandpass filtering operates on the delayed band signals 
prior to recombination without cross modulation from the higher frequency 
switching components and other transients. 
In accordance with the teaching of of the aforementioned U.S. Pat. No. 
3,786,195, the present invention provides for blanking by switching the 
output channel at the reset time for the delay lines in a analog shift 
register-type delay system. However, because of the improvements provided 
by the present invention with band splitting and lowpass filtering after 
the signals have been delayed, the present invention is capable of 
providing high quality output without blanking. The associated transients 
involved in switching the signal to fill the gap present when the signal 
channels are blanked are thus not present. 
The present invention further enhances the quality of time 
compression/expansion signal processing by utilizing amplitude compression 
prior to time delay with corresponding amplitude expansion of the fully 
processed and filtered signal prior to recombination of the multiple 
bands. 
Accordingly, it is the primary object of the present invention to provide 
high quality time compression/expansion signal processing with low level 
distortion and switching transients such that the fully processed audio 
signal is suitable for broadcast quality use or studio composite assembly 
of finished programming with variable time components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 a source of signal from a playback of a recording or other source 
is applied to an input stage 11, the output of which is applied to three 
bandpass filters 12, 13 and 14. Preferably the filters 12, 13 and 14 have 
continguous passbands to subdivide the audio signal from input stage 11, 
to include all the desirable signal frequencies and exclude frequencies 
outside the signal range which are not useful in the particular 
application for the sytem or use to which the apparatus is adapted. For 
high quality reproduction the overall passband of the filters 12, 13 and 
14 will be the full audio range whereas for a limited quality reproduction 
or other special applications a narrower band can be used. 
The individual frequency bands from the filters 12, 13 and 14 can be 
applied, if desired, to individual amplitude compressors 15, 16 and 17. 
Amplitude compression is useful with certain type delay devices to limit 
the dynamic range of the signal samples and avoid overload problems. The 
output of the amplitude compressors is applied directly and through 
inverting amplifiers 18, 19, 20, to matched pairs of bucket brigade device 
analog shift registers 21, 22 and 23. The outputs of the bucket brigade 
delay devices 21, 22 and 23 are applied to differential amplifiers 24, 25 
and 26 respectively. The technique for inverting the signal to be delayed 
prior to application to similar delay devices followed by application of 
the delay signals to a differential amplifier for the purpose of 
cancelling processing noise is described in U.S. Pat. No. 3,906,384 and 
its application for time compression/expansion systems is disclosed in 
U.S. Pat. No. 3,846,827. 
The outputs of differential amplifiers 24, 25 and 26 are applied 
respectively to lowpass filters 27, 28 and 29 which are arranged with 
their cutoff frequencies to pass their respective frequency band after 
pitch correction but to exclude the next higher frequency band. With this 
arrangement the lowpass filters also exclude the higher frequency 
components of the processing noise and switching transients to an extent 
that satisfactory aural reception of the ultimate signal is obtained. As 
described in the prior art particularly U.S. Pat. Nos. 3,786,195 and 
3,846,827, previously cited, it would be possible to utilize blanking and 
gapfilling switching at the location of lowpass filters 27, 28 and 29 
either with or without the lowpass filtering. 
The outputs of the lowpass filters are applied to amplitude expanders 31, 
32 and 33 respectively where the amplitude compression introduced by 
compressors 15, 16 and 17 is restored to reconstitute the amplitude 
character of the signals. The outputs of the amplitude expanders is 
applied to a summing amplifier 34 which combines the pitch corrected 
different frequency band signals into a composite output signal 
representing the original recorded work occupying different time duration 
but with pitch correction to approximate the original frequencies of the 
work as it was performed for recording. 
The swept pulsing of the analog shift register devices 21, 22 and 23 with a 
variable pulse period to provide progressive delay between samples and 
thus obtain pitch correction for the signal passing therethrough is 
described in detail in U.S. Pat. No. 3,786,195. In accordance with the 
present invention the bucket brigade delay pairs 21, 22 and 23 are all 
swept with a synchronized signal from clock 35 which applies a variable 
pulse period swept clock pulse sequence which is reset after a 
predetermined count by counter 36 which applies a reset signal to the 
clock 35 from the reset pulse generator 37. Details as to the length of 
delay, the rate at which the delay is varied and the generation of 
variable pulse swept signals are described in the previously cited patents 
with the linearly varying period for the pulses being further described in 
U.S. Pat. No. 3,840,814. 
For the purpose of adjusting the time duration of a given recording, a 
motor speed control 41 is provided which controls the speed at which the 
recording is played back. To synchronize the motor speed control and the 
corresponding pitch change which the electric signal derived from playing 
the recording at a different than recorded speed entails, the swept period 
of clock 35 is controlled from control 41 via line 42 to obtain a 
corresponding pitch correction which compensates for the selected motor 
speed for playback. Since the change in motor speed for playback of the 
recording changes the frequencies of the electric signals derived 
therefrom a synchronized control line 43 for the bandpass filters 12, 13 
and 14 is also provided to adapt the overall passsband and the individual 
passbands of filters 12, 13 and 14 to fit the actual band of the signal 
derived from the recording. 
FIG. 2 shows a system in all respects similar to FIG. 1 except that the 
outputs of the lowpass filters 27, 28 and 29 are applied through further 
bandpass filters 51, 52, 53 respectively. Since the restored pitch 
frequency after delay will generally correspond to normal speech or music 
the bandpass filters 51, 52 and 53 do not generally need to be modified 
with change in the pitch control 41 since they receive relatively constant 
band frequencies irrespective of the pitch correction adjustment and motor 
speed control adjustment. 
FIG. 3 shows a dual channel band splitting time compression/expansion 
system wherein two or more signal channels are processed synchronously. A 
first channel input signal is applied to an input stage 61 is split into 
two bands by bandpass filter 62 and 63. The outputs of these bandpass 
filters are amplitude compressed in compressors 64, 65, the output of 
which is applied directly and through inverting amplifiers 66 and 67 to 
the input of analog shift register bucket brigade delay devices 68 and 69 
which operate as pairs on the direct and inverted audio signal as 
previously described. The outputs of the pairs of delay lines 68 and 69 
are differentially combined in differential amplifiers 71 and 72, the 
output of which is applied respectively to lowpass filters 73 and 74. The 
output of the lowpass filters are applied to bandpass filters 75, 76, the 
outputs of which are amplitude expanded in expandors 77,78, with the 
expanded signals summed in summing amplifier 79 to produce a composite 
output of the pitch corrected signal for channel 1. The signal processing 
involved for channel 1 from input stage 61 to summing amplifier 79 
corresponds substantially identically with that described for FIG. 1 
except that the bandsplitting is confined to two frequency bands instead 
of three. 
The second channel is identical with the first channel and constitutes 
elements which have corresponding numbers primed functioning in the same 
way as described in channel 1. Thus input stage 61' has the channel 2 
signal subdivided into two frequency bands by bandpass filters 62' and 
63'. Corresponding elements process these frequency separated signals for 
pitch correction and filtering until the band separated signals in channel 
2 are combined in summing amplifier 79'. Thus channels 1 and channel 2 are 
separately available at the output terminals of summing amplifiers 79 and 
79' respectively. 
The paired analog shift registers 68, 69 and 68' and 69' in the two 
channels are all driven in synchronism by a common clock 35 operating in 
conjunction with a counter 36 and reset 37 to provide the repetitive 
linear periodic clock period variation for the delay variation of signal 
through the bucket brigade delays 68, 69, 68', 69'. As before, a motor 
speed control 41 may be provided to control the clock rate for the desired 
pitch correction along with adjustment of the bandpass for bandpass 
filters 62, 63, 62' and 63'. 
Referring to FIG. 4, a modified application of the invention is shown for a 
conventional two-track stereo system. The two stereo signals are applied 
respectively to input stage 81 and 82 and processed as single frequency 
band signals through bandpass filters 82 and 82'. Each channel of the 
stereo signal is passed respectively through amplitude compressors 83, 83' 
and directly and through inverting amplifiers 84, 84' to paired bucket 
brigade delay devices 85, 85', the outputs of which are combined by 
differential amplifiers 86, 86' and applied to lowpass filters 87, 87'. 
The pitch corrected signals pass through similar bandpass filters 88, 88' 
and through amplitude expanders 89, 89' to provide the two channel stereo 
output signals pitch corrected at the outputs of the expanders 89, 89'. 
Again, the paired analog shift registers 85, 85' are operated in 
synchronism from a common clock 91 which operates in conjunction with a 
counter 92 and a reset 93 to provide the variable pulse spacing repetitive 
clock signals for the desired progressive time delay as previously 
described. The clock and the bandpass filters 82, 82' are operated from a 
common control 41 which again may be used to control motor speed for the 
playback device. 
The general application of the invention to various systems will be 
understood from the foregoing description. A detailed description of a 
particular application of the invention will now be described with 
reference to composite FIG. 5 made us of FIGS. 5A, 5B, 5C, 5D and 5E 
assembled as indicated thereon. 
Referring to the assembled FIG. 5, there is shown in FIG. 5A thereof 
multiple input signal sources such as playback head 101 and input line 
102. Each input signal can be processed in a conventional manner such as 
through differential buffers 103 and differential preamplifiers 104. The 
signal from playback head 101 may be further processed through an 
equalizer 105 and in each signal channel the signal is subjected to an 
active attenuator 106 to provide a standardized signal level for further 
processing. The plural input signals are selectively available by 
connecting the output of the active attenuators 106 to switch 107 which 
selects either signal source to derive the signal for further processing. 
The signal selected by switch 107 is passed through an amplifier 108, the 
output of which is selected by a pitch correction on/off switch 109 for 
direct use, if desired. 
For pitch correction processing the output from switch 107 is applied 
directly to four highpass filters 111, 112, 113 and 114. These highpass 
filters form the low frequency cutoff characteristic for bandpass 
filtering of the input signal and operate in conjunction with a set of 
lowpass filters 115, 116, 117 and 118 which form the high frequency cutoff 
characteristic for the bandpass filters. In each instance the highpass and 
lowpass filters are designed as third order Butterworth filters with an 18 
db per octive roll-off with a 3 db frequency located as indicated in the 
individual filters on FIG. 5A. Thus filter 111 has its 3 DB frequency of 
80 Hz, while the lowpass filter 115 for that channel has its 3 DB 
frequency at 800 Hz. These two filters 111 and 115 define a passband for 
band No. 1 of 80 to 800 Hz with a center frequency f.sub.o at 253 Hz. 
Similarly, for the other three passbands the design criteria for the 
particular embodiment disclosed in FIG. 5 is shown for band 2 with f.sub.o 
at 1.27 kHz, band 3 with f.sub.o at 3.16 kHz and band 4 with f.sub.o at 
8.1 kHz. The 3 db cutoff points for the filter characteristics for each of 
these bands are indicated in FIG. 5A; the high and low 3 db frequency 
points for band 2 being 800 Hz and 2 kHz, for band 3 2 kHz to 5 kHz and 
for band for 5 kHz to 13 kHz. 
If desired the output from the bandpass filters can be amplitude compressed 
by a set of separate amplitude compressors 119 introduced in each band. 
The amplitude of the output signal from the amplitude compressor can be 
standardized through noninverting amplifiers 121 for each channel, the 
output of which is applied directly and, after inversion in inverting 
amplifiers 122, to the paired analog shift register bucket brigade delay 
lines 123. Up to the first bank of bucket brigade delay lines 123 the 
signal processing in each channel has been identical except for the band 
splitting provided by the different frequency ranges for the bandpass 
filters of bands 1, 2, 3 and 4. 
A feature of the invention provides for increasing the number of stages of 
shift register delay for the higher frequency channels. For this purpose 
the outputs of bucket brigade delays 123 in bands 3 and 4 are applied to 
current to voltage converters 125, the outputs of which for each band are 
applied to differential amplifiers 126. The outputs of the differential 
amplifiers 126 are the reconstituted total signal for bands 3 and 4 
respectively and these signals are again applied directly and through 
inverting amplifiers 127 to matched pairs of bucket brigade delay devices 
128. The signals emerging from the delay devices 128 for bands 3 and 4 
thus have passed through twice as many delay stages as the corresponding 
output signals from the pairs of delay devices 123 use to delay the 
signals in bands 1 and 2. In order for the signal in each band to 
experience the same time delay, the delay shift registers 123 and 128 for 
bands 3 and 4 are clocked at twice the clock rate of the delay registers 
123 for bands 1 and 2. This difference in clock rate is accomplished by 
deriving the basic clock frequencies for bands 3 and 4 from clock driver 
131 which clock rate is applied to separate bucket brigade delay drivers 
132 and 133. The driver 132 drives the BBD delay devices 123 and 128 for 
channel 3 and the driver 133 applies driving clock pulses to the BBD 
delays 123 and 128 for channel 4. The same clock frequency is divided by 
two in divider 134 and this half frequency clock pulse train is applied 
through BBD driver 135 to clock the BBD delays 123 in channels 1 and 2. As 
indicated, the BBD drivers apply opposed phase square driving pulses .0.1 
and .0.2 to the BBD delay lines as the conventional form of clocking for 
the particular devices. 
The derivation of the ramp control signals and the phased pulses to provide 
desired pitch change timing to the driver's 132 and 135 is achieved from a 
circuit 141 of the type described in detail in U.S. Pat. No. 3,846,827. In 
particular, FIG. 1 of that patent shows drivers 65 for the BBD delays 68, 
69, timed by a counter 81 and reset generator 52, designated "chip 2" in 
that patent. This combination corresponds generally with the circuit 131 
shown in FIG. 5E comprising the 256 stage counter, reset generator and 
clock driver. The elements of circuit 131 are controlled by the ramp 
generator and logic for producing V.sub.c, the voltage controlled period 
waveform for the clock drivers from a circuit 141 which corresponds 
generally with "chip 1" shown in FIG. 1 of U.S. Pat. No. 3,846,827. The 
ramp control signal is applied from a buffer 142 which has the pitch 
selection control input signal applied thereto from line 143 in the form 
of a DC control votage selected as the variable contact point on a voltage 
divider 144. 
For high quality time compression/expansion the present invention provides 
for selection of the relative length of both the reproduced portion of the 
signal and the gap discard interval controlled in coordination by a set of 
gang switches 145,146 and 147. Each of these switches provides two 
positions for segment timing T.sub.s or 2T.sub.s. Each of the switch 
contacts 145, 146 and 147 is connected respectively through slide switches 
148, 149 and 150 which selects a time interval 0.5 T.sub.s. T.sub.s is 
defined as to time interval for the variable delay period and equals the 
portion of the signal which is utilized plus the discarded portion which 
is discarded as the ramp voltage control reverses to reset for the next 
delay sweep. A typical value for T.sub.s is 175 ms which, for C=1.15, 
would result in a "keep interval" of 153 ms and a "discard interval" of 22 
ms. The resulting time reduction would be 13.1%. 
The adjustment of the timing is accomplished by changing the time constants 
for the ramp generator, the reset generator and the counter circuits of 
the chip 1 and chip 2 circuits of U.S. Pat. No. 3,846,827. For this 
purpose the switches 145, 146, 147 are ganged to operate together between 
either the T.sub.s or the 2T.sub.s position and the switches 148, 149 and 
150 are ganged together to select the 0.5T.sub.s position. For any 
selected segment duration interval the capacitors selected by the switches 
147 and 150 control the time duration of the ramp generated by ramp 
generator 141. The slope of the ramp to provide the proper pitch change is 
controlled by the voltage applied to the ramp generator selected from the 
potentiometer 144 at variable contact line 143, as previously described. 
When the duration of the ramp is selected by selecting one of the segment 
duration values using switch 147 and 150 a further control on the ramp 
generation is provided by the switches 147 and 150 to control the reset 
time by establishing the maximum excursion of the ramp thereby to control 
the maximum and minimum values for the progressively changing clock 
frequency. A similar set of capacitors controls the portion of the segment 
which initiates the gap or discard by means of the setting of switches 145 
and 148 to control the 256 stage counter in unit 131. Thus the segment 
duration selection by means of switches 146, 149, the ramp excursion set 
by switches 147, 150 and the counter output to control reset selected by 
the switches 145, 148 may be adjusted to optimize these parameters for any 
given program material. For example, where a video recorded commercial 
insert is adapted to fit a particular time slot on broadcast television 
the video tape can be played and the accompanying audio signal modified in 
pitch to achieve a normal or nearly normal sound. By adjusting the various 
parameters for the repetitive delay function a particular setting can be 
found which has the minimum noise artifacts or other annoying signal 
components due to the repetitive and blanking functions of the system. 
When the optimum parameters have been selected for a given time change for 
a given playback video recording the audio track can be rerecorded using 
the system of the present invention so that the commercial message can be 
played with a completely normal sounding and noise-free audio message. 
The blanking signal generated from the output of the 256 stage counter in 
unit 131 is converted into an RC timing waveform by adjusting the 
resistance with variable resistor 152. This waveform provides the blanking 
timing signal for controlling the interval during which blanking is 
applied if the system is conditioned to provide blanking as hereinafter 
described. 
The blanking timing signal on line 151 is applied through a buffer 153 to a 
voltage comparator 154 which produces a timed output pulse at a 
predetermined voltage comparison condition which is applied to a gallium 
arsenide infrared emitting diode (i.e., photo coupled) drivers 155, each 
of which controls separate photo coupled light sources 156 to produce a 
voltage controlled light pulse whenever switch 157 is connected as shown 
to provide blanking. The other position of switch 157 eliminates blanking 
by applying plus voltage to block the driver circuits 155. A switch 157 is 
provided in the plug 15 volt supply for the photo coupled light source 156 
to eliminate generation of the blanking light pulse in a given recording 
being reproduced. 
The delayed segment and gap signals from BBD delay lines 123 for band 1 
appear on line 161 as current signals which are applied to current to 
voltage converters 162, the outputs of which drive the opposite polarity 
inputs of a differential amplifier 163 to recombine the separately delayed 
phase opposed signals. Similarly, the outputs of the band 2 delay lines 
123 appear on lines 164 and are applied to current to voltage converters 
165, the outputs of which are applied to the positive and negative inputs 
of differential amplifier 166. 
The higher frequency bands which have been processed through tandem BBD 
delays 123 and 128 for both bands 3 and 4 are similarly processed. Thus 
for band 3 the phase opposed delayed signals from the tandem BBD delays 
123 and 128 in that band appear on lines 167 and these signals pass 
through current to voltage converters 168 which have their outputs applied 
to differential amplifier 169 as previously described for bands 1 and 2. 
The delayed outputs for band 4 after passing through BBD delays 123 and 
128 appear on lines 171 which after current to voltage conversion in 
converters 172 are applied to the inputs of differential amplifier 173. 
The outputs of the differential amplifiers 163, 166, 169 and 173, supply 
the reconstituted delayed signals for bands 1, 2, 3 and 4 respectively 
which are applied respectively to lowpass filters 174, 175, 176 and 177. 
Filter 174 is a third order Butterworth having a roll-off characteristic 
of 18 db per octave with the 3 db frequency of 1 kHz. The remaining 
lowpass filters are of similar design with the 3 db frequency points for 
filter 175 at 2.5 kHz, for filter 176 the 3 db point is at 6 kHz and for 
filter 177 the 3 db frequency is 15 kHz. These lowpass filters assure for 
each band that spurious noise and artifact signals above the passband of 
each respective filter is eliminated. With the previous band separation 
provided by bandpass filters in each channel, for example highpass filter 
111 and lowpass filter 115 for band 1, the addition of the lowpass 
filters, for example, filter 174 in band 1 is often adequate to provide 
high quality reproduction without blanking during the gap interval. Where 
such is the case the noise artifacts provided by blanking in each channel 
can be eliminated by positioning blanking switch 157 to the off position 
whereupon the output signals from lowpass filters 174, 175, 175 and 177 
are continuously coupled through the bilateral analog FET portion of the 
photo coupler normally gated by the photo coupled emitting diode 156. The 
photodiodes are indicated as 156' in each channel. The output of each of 
the lowpass filters 174, 175, 176 and 177 is applied respectively to an 
active attenuator 178 in each channel used to modify the amplitude 
excursion of the signal to a standardized level before application to the 
bilateral analog FET 156'. 
The outputs of the bilateral analog FETs 156' are applied through buffers 
179 in each band before the signals are applied through bandpass filters 
comprising tandem highpass and lowpass filters. In band 1, a high pass 
filter 181 is a third order Butterworth with a cutoff characteristic of 18 
db octave and a 3 db frequency of 37.5 Hz. The corresponding lowpass 
filter 182 in band 1 is a lowpass third order Butterworth with a 
characteristic 18 db per octave roll-off and a 3 db frequency of 800 Hz. 
Similar pairs of highpass and lowpass third order Butterworth filters are 
connected in tandem for each band with band 2 having highpass filter 183 
and lowpass filter 184 with 3 db frequencies of 600 Hz and 2 kHz 
respectively. Band 3 has a highpass filter 185 with a 3 db frequency of 
1.5 kHZ and a lowpass filter 186 with a 3 db frequency of 5 kHz. Band 4 
has a highpass filter 187 with a 3 db frequency of 3.7 kHz and a lowpass 
filter 188 with a 3 db frequency of 10 kHz. 
The outputs of the bandpass filters for bands 1, 2, 3 and 4 are applied to 
amplitude expanders 191 in each band where the signal is expanded with a 
characteristic complimentary to the amplitude compression provided by 
amplitude compressors 119. 
The outputs of the expanders 191, if amplitude compression/expansion is 
employed are summed in a summing circuit 192 to recombine the full band 
audio signal with pitch correction. The output of the summer is applied 
through a switch 109 to an output terminal 193. The output 193 also can be 
connected, as previously described, by throwing switch 109 to line 194 
which is directly connected to the output of amplifier 108. 
In the low frequency bands 1 and 2 and the high frequency band 3 the 
bandpass frequency characteristic of the Butterworth filters produces a 
phase difference of 180.degree. between the adjacent bands. For this 
reason a phase correction is introduced with 180.degree. phase shifter 195 
for the signal in band 2 just prior to summing in the summing amplifier 
192. Likewise, a phase correction is introduced with 180.degree. phase 
shifter 197 for the 180.degree. phase difference between high frequency 
bands 3 and 4. In addition, the signal in band 4 which has the higher 
frequencies is sometimes desired to be eliminated. For this purpose a 
muting switch 196 is provided on expander 191 and band 4 to remove band 4 
from the summed output. 
As previously described the system of FIG. 5 is ideally suited for adapting 
the audio portion of recorded messages to fit a particular time slot. A 
particular application of this technique is found in tailoring commercial 
messages to fit the preestablished time slot in a recorded TV or radio 
broadcast program. For this purpose the video tape can be processed 
running at a speed which will exactly correspond with the alloted time 
slot and the audio message accompanying the video signal can be analyzed 
for quality. With the system of FIG. 5 pitch correction can be 
accomplished and the paramaters adjusted until the audio portion of the 
signal is essentially noise-free. When this time duration pitch correction 
and noise-free quality is achieved a rerecording of the audio portion of 
the tape can be made so that the commercial recording has the exact time 
duration required and the high quality essential for broadcast use. The 
resulting audio reproduction is indistinguishable for the ordinary 
listener from the high quality reproduction of the original recording. 
Many applications of the band splitting and channel separation pitch 
correction techniques disclosed by the present invention will be now 
apparent to those skilled in the art by processing an audio signal as 
subdivided frequency bands or a high quality signals such as stereo into 
its separate channels with the techniques disclosed herein while 
accomplishing pitch correction in a synchronized manner the resultant oral 
signal quality after the signals have been recombined for reproduction is 
extremely high quality and to the ordinary listener completely free of any 
annoying sounds originating from the signal processing. The invention 
accordingly should be considered to include those modifications which fall 
within the scope of the appended claims.