Apparatus reduces Time Base Error /TBE/ in the playback of digital data from magnetic recording equipment. The apparatus uses a magnet which employs a servo position control of the tape by which the playback data clock is phase locked with a fixed frequency reference signal

Moore, J. M.

English

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August 1968	 Brief 68-10317 
NASA TECH BRIEF 
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Method of Reducing Time Base Error in Digital Magnetic Recorders 
Playback Data Clock (Freq. fc) 
From 
Playback I	 Playback	
__/_f Clock	 +17 Head —i Amp &	 Extractor 
Servo Sync L.—i Phase	 I-H LPF F1 
Compensation I
	 I Recorder 
Acquisition F-1 Detector	 0 C Amplifier	 Motor Circuits  and	 (___{Driv 
	
Magnetic Recorder 	
- -------
	
TBE Reduction Unit 	 Servo Reference Signal (Freq. fo = fc) 
4xf0	 T 
Osc.&	 0 
Shaper	 1
T
Buffer Clock 
BPF	 Shaper	 (Freq. fc) 
	
LPF	 & Network Delay 
N RZ 
	
Biphase	 /	 Buffer	 Biphase	 B Decoder	 Storage	 Encoder	 iphase Out 
	
An apparatus for the reduction of time base error	 bit for the apparatus to function properly. 
	
(TBE) in the playback of digital data from a certain 	 The accompanying figure shows a block diagram of 
	
class of magnetic recording equipment is described	 the system. The playback data is in biphase form and 
	
herein. The TBE of a signal is defined as the dy- 	 the clock is extracted from it. The TBE in the data 
	
namic phase deviation of the signal from an extrapo- 	 (and clock) is a result of angle modulation caused by 
	
lated long term average phase. The apparatus was	 tape speed variations. The clock signal is passed 
	
developed for a magnet which employs a servo posi- 	 through a very narrow bandpass filter which has a 
	
tion control of the tape by which the playback data 	 center frequency equal to the nominal (average) play-
	
clock is phase locked with a fixed frequency reference	 back data rate. The filter band limits the clock signal 
	
signal. The peak-to-peak TBE of the playback data 	 and in so doing effectively removes harmonic compo-
	
of the recorder must be less than the period of a data	 nents and the angle modulation sideband components 
(continued overleaf) 	 — 
	
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https://ntrs.nasa.gov/search.jsp?R=19680000317 2020-03-12T06:12:39+00:00Z
except those with frequencies very nearly equal to the 
nominal data rate. The resqlting signal has a lower 
degree of angle modulation and therefore a reduced 
amount of TBE. For effective TBE reduction, the 
bandwidth of the bandpass filter must be very small. 
Also it is essential that the frequency corresponding to 
the nominal playback data rate lie in the passband 
region of the filter where the attenuation and phase 
shift are negligible. The use of a fixed frequency filter 
is not practical due to the stringent requirements 
which would be imposed on the center frequency 
tolerance and the drift characteristic. 
The filter employed is one in which the center fre-
quency tracks the frequency of the servo reference 
signal. Since the extracted clock is phase locked to the 
servo reference signal, the nominal data rate lies in the 
center of the filter's bandpass region. The filter is of a 
type sometimes referred to as a digital filter. It 
employs a synchronous modulator—demodulator type 
of circuit which translates the amplitude and phase 
characteristics of a low pass filter which are sym-
metrical about zero frequency by an amount equal to 
the modulation frequency, which in this case is the 
frequency of the servo reference signal. The output of 
the filter is squared and delayed to obtain the proper 
phase Ibr redocking the data. The playback data is 
converted to NRZ and fed to a signal cell buffer stor-
age unit where it is held until transferred out by the 
processed clock. The TBE of the buffer output is equal 
to that of the processed clock. The data is then recon-
verted to biphase form.
The advantage of the system described over one in 
which the buffer output clock is obtained from the 
servo reference signal by delaying it is that the phase 
difference between the data and buffer output clock is 
independent of the phase difference between the play-
back data and servo reference signal. Any changes in 
the mechanical load on the servo motor resulting in a 
change in the phase difference between the servo ref-
erence signal and playback signal will not cause 
changes in the clocking points of the signal from the 
buffer in the system described. 
The advantage of this system as compared to others 
for achieving TBE reduction is that the system re-
quires less circuitry. In cases where the worst case 
peak-to-peak TBE is greater than the period of a data 
bit, a buffer of increased capacity is required. 
Note: 
Inquiries concerning this innovation may be di-
rected to:
Technology Utilization Officer 
Goddard Space Flight Center 
Greenbelt, Maryland 20771 
Reference: B68-10317 
Patent status: 
No patent action is contemplated by NASA. 
Source: John M. Moore 
of Electro- Mechanical Research, Inc. 
under contract to 
Goddard Space Flight Center 
(GSC- 10108) 
Category 01	 Brief 68-10317


Method of reducing time base error in digital magnetic recorders

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