CA1203020A - Audio system having a time base error correction arrangement - Google Patents

Abstract

ABSTRACT
AUDIO SYSTEM HAVING A TIME BASE ERROR CORRECTION ARRANGEMENT

A system for reproducing and/or transmitting analog audio information is disclosed which provides for the cor-rection of time base errors. An inaudible pilot signal accompanies the audio information. Correction circuitry examines the frequency and phase characteristics of the pilot signal to develop signals for effecting correction of the errors.

Description

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AUDIO SYSTEM HAVING A. TIME BASE~ ERROR CORRECTION ARR~N(',E:MENT.

BACKGROUND OF THE INVENTION

The present invention relates to an arrangement for effecting corrections of time base errors present in repro-duced and/or transmitted audio information of the analog type.
For purposes of explanation, reference will be made to errors of the sort which are associated with reproductions from mechanical recording devices. Typically, such errors are in the form of speed, flutter and multichannel delay errors.
An example of speed error would be the playback of a magnetic tape at a speed slightly faster or slower than the speed at which it was recorded. This could be the resulk of rotational speed error o the capstan motor, incorrect capstan diameter (due to wear, for example), or slippage between the magnetic tape and the capstan due to an imbalar.ce of tape tension. Speed error results in a change of pitch in a recording as it is played back. A speed error of as little as 0.1% can result in a change of pitch detectable 2Q to a trained musician. A speed error of 1% results in a noticeable pitch change. Speed error also causes a record-ing to play back over a different period of time. For ex-ample, a recording that was made over a period of 60 minutes would play ~ack over 59 minutes and 24 seconds, in the case 25~ of a 1% error.

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Flutter is a form of speed error and is an alter-.. :: :::: .
natingly increase and decrease in instantaneous speed at a cyclic rate, typically between 2 and 10 Hz. Flutter re-sults from mechanicam imperfections of all recording de-vices. A capstan that is not perfectly round (elliptical)will cause flutter in a magnetic tape recorder. Mechanical "play" in bearings and drive motors with non-uniform torque are other examples of sources of flutter. The primary ob-jection to flutter is its audible effect which, in severe cases, sounds like a "fluttering" or vibrato sound. Flutter is measured by recording a pure tone and then measuring the ~re~uency modulated component of the tone upon playback.
A weighting curve is usually applied which takes into account the psychoacoustic effect of the flutter rate and the result is expressed in a percentage. Typical flutter measurements for professional tape machines ~ary from about 0.03% to 0.15%.
Delay in error is a problem associated with recording devices (normally magnetic tape recorders) which have more than one audio channel cr "track". Mechanical imperfections, such as an error in tape head azimuth, can cause the play-back of one or more channels to lead or la~ a reerence ch~nnel in time. Tape skew is another potential source of this error.
Delay error is seldom more than 300 microseconds and usually is of no conse~uence until two or more tracks are electrically mixed. I both tracks contain components of the same audio signal, a nulling of this signal will result at a frequency ... .
whose period i9 equal to twice the delay error and at inte-gral multiples of this frequency. For example, assume that a lZai3QZO

stereophonic (two track) tape machine has a delay error of 50 ~s. If the outputs of this machine are electrically mixed to form a monaural (one channel) signal, elements of the audio which are present on both the original two tracks at about the same level will cancel each other in the vicinity of 10 kHz. This is because ~0 ~s corresponds to 180~ of phase at 10 kHæ. This effect is particularly troublesome in the field of FM broadcasting. In this case, a signiicant number of listeners are listening to stereo broadcasts on monaural receivers and are hearing the elec-trical sum of both stereo ch~nnels. Delay error is no~
audible to the stereo listener but the monaural listener hears a very degraded high frequency response of the center channel components of the audio.
In the past, eforts have been made to m;n;m;ze time base errors in audio equipment. An example is the arrange-ment for automatic alignment of the record head azimuth for least delay error for a magnetic tape recording and repro-ducing r-ch;ne which is disclosed in United States P~tent 4,101,937, issued to John P. Jenkins on July 18, 1978. This known arran~ement employs audible tones which must be erased before audio is recorded on the cartridge. Such a system corrects only for delay error contributed by the record head and only as that error is measured by the playback head of that same machine. It is not a real time continuous cor-; rection system which corrects for any error, regardless of source, each time the tape is played.

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SUMMARY OF THE INVENTIO~

The present invention provides time base corrections by me~ns of an inaudible pilot signal which is recorded (or transmitted) at a low level along with the analog audio in-formation. At the output of the reproducer (or at the re-ceiver in the case where the information is transmitted), the pilot signal is extracted and decoded. The information derived as a result o the decoding is used as a reference for time ~ase correction of the audio signal.
Details of the invention now will be discussed in connection with the accompanying drawings wherein:
FIGURE 1 is a block diagram of the invention as in-.corporated with an analog audio recorder/reproducer;
FIGURE 2 is a block diagram of details o the pilot encoder orming a portion of FIGURE l; and ~ IGURE 3 is a block diagram of details o the pilot decoder and error generator forming a portion of FIGURE 1.

DETAILS OF THE lr~ v~ ION

For purposes of disclosing the invention in detail, `20 re~erence primarily is directed to its incorporation in an analog recorder/reproducer system~ However, the invention also may be employed in a system which transmits audio in-formation. Consequently, in the discussion to follow, parenthetical expressions are utilized at appropriate points reerring to the latter type of system.

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In Figure 1, there is illustrated a recorder/repro-: ::
duc~r which comprises a recording device 10 and a playback device 12. Input to the recording device 10 is supplied from separate sources. As indicated in the dra,wing, these sources include left and right audio inputs which designate that the recordiny is to be done in separate channels as, for example, on a magnetic tape in device 10. The left and right audio inputs may be from various sources, such as separate microphones. Additionally, the recording de-vide is supplied with an inaudible pilot signal which isrecorded in each of the channels of device 10. The pilot signal will be described in detail hereinafter in connection with the discussion of Figure 2.
During reproduction, the information recorded on device 10 is directed by playback device 12 to a time base corrector which includes voltage-controlled delays 14 for each o~ the ch~nnels of reproduced information. As in the case of all the circuit components which are illustrated in block diagram form in this disclosure, the delays 14 are conventional circuits well known to those of ordinary skill in the art and commercially available from a variety of sources.
The outputs of delays 1~ are applied,to a pilot de-coder and error generator 16. As will be described in detail in connection with Figure 3, the circuitry arrange-ment 16 extracts time base information from the pilot signal.
... .
This information is utilized to develop error correction signals. In the case of 1utter and multichannel delay error, the error correction signals are fed back to the voltage-controlled delays 14 to alter their delay periods ~ ~3~20 thereby effecting corrections. In the case of speed error, the appropriate correction signal developed by the pilot decoder and error generator 16 is utilized to regulate capstan motor speed thereby correcting the error.
S The outputs of the delay circuits 14 also are passed through respective notch filters whereby the pilot signal information i5 eliminated as the recorded information passes to left and right audio outputs from the system.
The overall structure and operation of the invention having been described, details thereof now will be presented.
The pilot signal which is employed takes the form of a 19.000 kHz carrier which is 65% amplitude modulated by a 296.875 Hz sine wave. The pilot is recorded (or trans-mitted) at a level o~ -25 dbVU along with the audio signal.
There is nothing inherent in the invention that requires these specific parameters. For example, 20 kHz could be used for the carrier, 80~ could be used as the modulation level, 400 Hz could be the modulating frequency and -30 db W
could be the injection level,with equal results. The par-ameters of the preferred em~odiment of the invention were chosen to simplify the design and operation of the syfitem.
The carrier frequency was chosen so as to be supersonic but not so high in frequency that it would not be easily recordable on any tape recorder normally in use at a broad-cast station. The specific frequency of 19.000 kHz was chosen ~o prevent an audible heterodyne rom occurring in the event that slight leakage occurred through to the stereo generator of an FM broadcast station which also uses a 19 kHz pilot. The modulating requency was chosen because it ~Zl3t31~?ZQ

is 1/64th of the carrier frequency and therefore easily _.
and precisely generated by a divider. The frequency is high enough to resolve accurate delay information yet low enough that the lower sideband produced by modulation, while lower than the carrier, is still supersonic. The modulation depth is not critical and reliable results are obtained anywhere ~rom 50% to 75~. A modulation level of higher than 75% is not used to simplify the process of xegenerating the 19 kHz carrier. The pilot injection level o~ -25 dbVU was selected after tests were made to determine the m; nim~m pilot signal needed for proper operation of the correction unit, and the m~x;mllm level possible before modu-lation noise caused a measurable increase in noise floor.
A block diagram of the encoder is shown in Figure 2.
A 19.000 kHz frequency standard 20 is fed to both a switch-ing transistor (Q-l) and to a divider 22 to obtain a 296.875 square wave. This is fed to a 296.875 Hz bandpass filter 24 to obtain a 296.875 Hz sine wave which then is fed to modu-lating transistor Q-2. The modulation is set to 65~ with a potentiometer 26. The output of the modulated stage then is fed to a 19 kHz bandpass filter 28 which passes the carrier and both sidebands and rejects 296.875 Hz leakage and har-monics of 19 kHz. The bandpass filter output then is fed to a mixing network (not shown) and on to the audio input con-nections of the recorder (Figure 1).
The three distinct types of time base errors whichhave been mentioned (i-.e., multichannel delay error, flutter and speed error) can be corrected separately or concurrently utilizing the pilot decoder and error generator 16 which is illustrated in ~igure 3. However, for convenience of 1203~zo presentation, the correction arxangement for each of the above three types of errors will be discussed individually.
For multichannel delay error correction, samples of the output of each of the two digital delay~ 14 are fed to the inputs of respective 19 kHz bandpass filters 32. These filters serve to pass the modulated pilot sig-nal and reject the audio signal. Two automatic gain con-trolled (AGC) stages 34 follow which stabilize the ampli-tude of the pilot signal and compensate for variations in playback level. A detector, generally indicated as 36, follows each AGC. Each detector recovers the 296.875 Bz signal which is modulated onto the 19 kHz carrier. Bandpass filters 38 which reject noise ollow the detectors. The two 296.875 Hz signals are fed to a phase detector 40 which produces an output voltage having a magnitude proportional to the phase difference of the two signals and a polarity dependent on which channel leads in phase. If both signals are in perfect phasei there is no output. A DC coupled error amplifier 42 follows, which in turn is followed by an inte-grator 44 which averages instantaneous errors, and by a buffer amplifier 46. The output of the buffer amplifier comprises a delay error negative eedback signal, and it is ed to the control voltage input of the appropriate one of the two voltage-controlled delays 14. In this way, the delay parameter of one of the two delays 14 is servoed in such a way as to xeduce and minimize delay error.
It should be mentioned that a system based on a 296.875 Hz modulating frequency cannot handle a delay error of yreater than 1684 microseconds, which corresponds to 180 of phase of 296.875 Hz. However, this is far greater lZC~3~Z~

than the 200 ~s or 300 ~s maximum delay error of a typical tape recorder.
For applications that require an extremely accurate delay error correction, additional circuitry could easily be added which compares the phase o the two 19 kHz carriers.
In this case, two paralleled feedback loops would exist.
The first would be the loop derived from the 296.875 Hz in-formation and would bring the delay error down to less than 26 ~s (180 of phase of 19 kHz), and another "fine adjus~ment"
feedback loop derived from the phase of the 19 kHz carriers.
With respect to flutter cbrrection, the nature o flutter is such that all channels o~ a multichannel device will ha~e the same flutter error. For this reason, the pilot signal can be sampled from either (or any) o~ the c~annels, and the eedback signal can be applied to both (or all) of the channels. The circuitry begins with the 19 kHz bandpass filter 32 and the AGC s~age 34 which is shared with the de-lay error correction circuit just described. The modulated pilot then is fed to a limiter 48 which removes the ampli-tude modulation and leaves only the 19 kHz carrier. Thiscarrier then is filtered by bandpass filter 50 to remove noise and thereafter is applied to 2 frequency-to-voltage converter 52 which produces an output voltage proportional to the instantaneous fre~uency of the 19 kHz carxier. The AC component of this signal then is amplified by circuit 54 and is fed back to the control voltage inputs of both (or all) of the ~oltage~controlled delays 14. Such an arrange-ment comprises a negative feedback loop which tends to re-duce instantaneous speed error (flutter) by an amount pro-portional to the loop gain.

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When intially considered, it may be difficult tounderstand how a voltage-controlled delay can operate to correct instantaneous speed error. To facilitate an un-derstanding of this, an example will be presented. Assum-ing a voltage-controlled delay with a lO00 Hz tone at its input, the control voltage is set to a value which causes a delay of lO0 milliseconds (ms). At this constant value of delay, a lO00 Hz tone will appear at the output of the de-lay. However, if the control voltage is ramped smoothly upward for a period of one second, at the end of that second the absolute delay of the device is 200 ms. In other words, the delay has been increased at a rate of lO0 ms/s for a period of one second. At the end of the ramp, the delay onc~ again is stable and there is a 1000 Hz tone at both the input and output of the delay. However, during the one sec-ond ramp, the tone at the output of the delay had a frequency of 900 Hz. This is because the delay was being increased at a rate of 100 ms/s. The "lost" 100 cycles are contained in the delay unit. If the control voltage now is decreased smoothly back to lO0 ms delay ovex a period of one second, the tone at the output will have a frequency of llO0 Hz for a period of one second.
Flutter is caused by an alternatingly increased and decreased tape speed around a center value which causes an alternatingly increased and decreased pitch (frequency) around a center value. In view of the discussion just pre-sented, it can be seen why an AC signal impressed on the DC
control voltage of a voltage-controlled delay 14 can increase ~Z~3~Z~

and decrease the pitch of the audio coming out of the ...... .
- delay in a cyclic way. The feedback loop of the flutter corrector is set up in such a way than when the frequency of the 19 kHz carrier appearing at the output of the delay 14 tends to increase, a control voltage is generated that tends to increase the amount of delay which tends to de-crease the frequency of the 19 kHz carrier. Since the in-stantaneous fre~uency of the 19 kHz carrier tends to be corrected, so does the audio signal as well.
In order to effect speed correction, the regenerated 19 kHz carrier obtained at the output of the previously discussed bandpass filter 50 is fed to one input of a fre-quency comparator 56. The other input of the comparator is connected to a 19 kHz frequency standard (crystal oscilla-tor) 58. The output of the requency comparator is a volt-age proportional to the frequency difference between the two inputs and its polarity is determined by which frequency is higher. The output of comparator 56 is amplified by a DC coupled amplifier 60, and this DC voltage i5 fed to the capstan motor of the playback device 12 in such a way as to alter its speed over a given range. For example, if the 19 kHz carxier frequency tends to drift higher than the 19 kHz reference frequency, the feedback voltage tends to reduce the capstan motor speed since speed error, relative to the reference frequency, is inversely proportional to the loop gain. Also, as is obvious to a person of ordinary skill in the art, a phase comparator could be added to the fre~uency comparator to reduce the error to æero, relativa to the reference fre~uency.

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The foregoing discussion describes how various time base errors are corrected utilizing a system in which an inaudible pilot signal is provided at the input to the correction circuitry. However, it is apparent that to also accommodate inputs in which there is no accompanying pilot signal, the correction circuitry auto-matîcally can be electronically bypassed so that the audio inormation is reproduced tor transmitted) with no ill efect.

Claims (8)

WHAT IS CLAIMED IS:

1. An arrangement for correcting time base errors in an analog audio system, comprising:
a source of combined analog audio and inaudible en-coded pilot signals supplied on separate channels, said pilot signal being a supersonic carrier which is amplitude modulated by a signal of predetermined frequency;
voltage-controlled delay circuit means in each of said channels for delaying said signals;
means connected to said delay circuit means for separat-ing the respective delayed audio and pilot signals in each channel;
means joined to said separating means for decoding the pilot signals to develop an error correction signal, said de-coding means including:
(1) means for separating the supersonic carrier from its modulating signal in each channel;
(2) a phase detector; and (3) means for connecting the separated modulating sig-nals in each channel to said phase detector for developing the error correction signal as a function of the phase difference between the separated modulating signals; and means for applying said error correction signal to one of said delay circuits to adjust the delay thereof.

2. An arrangement as set forth in Claim 1, wherein the error correction signal has a magnitude which is a function of said phase difference and a polarity dependent on which of the separated modulating signals leads in phase.

3. An arrangement as set forth in Claim 1, wherein said source is a magnetic tape playback device having a speed-controllable capstan for moving the tape, and wherein said decod-ing means further comprises:
(1) a frequency comparator;
(2) a source of a standard frequency signal; and (3) means for connecting the separate carrier and said standard frequency signal to the comparator for developing a further error correction signal as a function of the frequency difference between the carrier and said standard frequency signal; and means for applying said further error correction signal to the playback device to adjust the capstan speed.

4. An arrangement as set forth in Claim 3, wherein the fur-therefor correction signal has a magnitude which is a function of the frequency difference and a polarity dependent on which frequency is higher.

5. An arrangement for correcting time base errors in an analog audio system, comprising:
a source of combined analog audio and inaudible encoded pilot signals supplied on separate chanels, said pilot signal being a supersonic carrier which is amplitude modulated by a signal of predetermined frequency;
voltage-controlled delay circuit means in each of said channels for delaying said signals;
means connected to said delay circuit means for separat-ing the respective delayed audio and pilot signals in each channel;
means joined to said separating means for decoding he pilot signals to develop error correction signals, said decoding means including:
(1) means for separating the supersonic carrier from its modulating signal in each channel;
(2) a phase detector;
(3) means for connecting the separated modulating signals in each channel to said phase detector for developing a first error correction signal as a function of the phase difference between the separated modulating signals;
(4) a frequency-to-voltage converter; and (5) means for connecting the separated carrier in one channel to said converter for developing a second error correc-tion signal as a function of the instantaneous frequency of the carrier; and

Claim 5 (cont'd.) means for applying said first error correction signal to one of said delay circuit means and said second error correc-tion signal to the delay circuit means in each of said channels, thereby adjusting the delays thereof to effect time base correc-tions.

6. An arrangement as set forth in Claim 5, wherein the first error correction signal has a magnitude which is a function of said phase difference and a polarity dependent on which of the separated modulating signals leads in phase.

7. An arrangement as set forth in Claim 5, wherein said source is a magnetic tape playback device having a speed-controllable capstan for moving the tape, said decoding means further including:
(1) a frequency comparator;
(2) a source of a standard frequency signal; and (3) means for connecting the separated carrier and said standard frequency signal to the comparator for developing a third error correction signal as a function of the frequency difference between the carrier and said standard frequency signal;
and means for applying said third error correction signal to the playback device to adjust the capstan speed thereby effect-ing an additional time base correction.

8. An arrangement as set forth in Claim 7 wherein the third error correction signal has a magnitude which is a function of the frequency difference and a polarity depend-end on which frequency is higher.