Technique uses a helical-scan video recorder with auxiliary signal-conditioning equipment to provide an inexpensive, high-capacity magnetic tape recording of a 112 channel, phase-locked, multiplexed, baseband signal

Heckman, D. C.

English

NASA Technical Reports Server

June 1970
	
-	 Brief 70-10069 
NASA TECH BRIEF 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech-Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Radio Frequency Baseband Recording Technique 
.
The problem: 
To provide inexpensive magnetic tape recording, 
for historical and analytical purposes, of a 112-
channel, phase-locked, multiplexed, baseband signal. 
Prior techniques for recording this type of signal use 
a modified transverse scan video recorder, a device 
which is very complex and expensive and which re-
quires highly skilled operating and maintenance per-
sonnel. 
The solution: 
A new technique that uses a helical-scan video re-
corder with auxiliary signal-conditioning equipment. 
This technique provides a high-capacity recording sys-
tem at less than one-fourth the cost of the older 
method. 
How it's done: 
The baseband signal being recorded is a frequency-
division multiplexed, single sideband (SSB) signal, 
transmitted on 75-ohm coaxial cable. The signal is 
derived by 0.3 to 3.0-kHz audio modulation of a 
500-kHz signal which results in a 500.3-to 503-kHz 
i.f. signal after filtering to remove the unwanted 
lower sideband. This i.f. signal is then mixed with a 
locally synthesized frequency in the 516-to 906-kHz 
range and filtered to remove the upper sideband to 
result in a 16-to 460-kHz transmitted SSB frequency. 
The precise synthetic frequency within this 516-to 906-
kHz range that is used determines the transmission 
channel. The 500-kHz fundamental used to derive the 
i.f. and the synthesized carrier modulation frequency 
are both derived from a 4-kHz signal carried with the 
SSB to phase lock all the mixing frequencies and pro-
vide system coherence. The SSB frequency of 16 to 
460-kHz provides 112 channels of 4-kHz bandwidth
of which 3 kHz is in audio information and 1 kHz is in 
the guard bands. The composite signal amplitude is 
180 mV, p-p, of which 30 mV is rf baseband. - 
The property of the signal which contributes the 
major problem is the difference in amplitude between 
the rf portion of the signal which contains' all ' the 
intelligence and the 4- kHz reference signal which 
provides phase lock to the system. Recording the 
signal as transmitted results in a poor signal-to-
noise ratio (SNR) between the rf part of the signal 
and the recorder noise level even though the SNR of 
the composite signal is more than 35 db. 
Signal processing has been developed to overcome 
the problem in the following manner. 
The signal is processed first by a signal separator 
unit which separates the 4-kHz reference signal and 
the rf signal. The signal separator consists of a 
linear-phase, high-pass active filter and a low-pass 
active filter followed by driver amplifiers to amplify 
the signals. The 4- kHz reference signal is sent directly 
to the recorder where it is recorded on one of two 
auxiliary audio-bandwidth edge tracks. The rf signal is 
further amplified by an adjustable linear amplifier with 
built-in overload protection and indication. The ampli-
fied signal is . recorded then on the helical-scan video 
track of the recorder. 
In the reproduce mode the 4- kHz reference signal 
from the auxiliary track is used to trigger a variable 
phase signal generator to produce a new 4- kHz ref -
erence signal which is adjustable in phase through 
180°. The rf repro signal is sent to one input of a 
wideband summing amplifier with adjustable gain 
or attenuation. The regenerated 4-kHz reference signal 
is sent to the other input of the summing amplifier 
where the two separate signals are adjusted to correct 
(continued overleaf) 
An
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	
information contained in this document, or warrant that such use 
Government nor any person acting on behalf of the United -States 	 will be free from privately owned-rights.
https://ntrs.nasa.gov/search.jsp?R=19700000065 2020-03-17T00:02:25+00:00Z
amplitudes and added together to result in a composite 
reproduction of the original signal. Recording the 
4-kHz reference signal on the auxiliary track provides 
coherence to the reproduced composite signal when 
the reference signal frequency drifts or is degraded 
by the transmission media. In cases where the received 
reference signal is stable, the signal generator may be 
used in free-running mode to generate the new ref-
erence signal. 
To enhance the long-term, time-displacement error 
and flutter characteristics of the recorder, which is 
referenced to the 60-Hz ac power input, a precision 
ac power source is used which provides 1% line and 
load regulation and crystal-referenced 0.01% frequency 
stability. This enables the helical-scan recorder to 
perform to much closer tolerances than would nor-
mally be expected from this type recorder. The re-
corder used is selected because it is the only helical-
scan wideband recorder available using a two-head 
scanning technique, to provide overlap between suc-
cessive tracks being scanned. Other helical-scan re-
corders available are designed for video recording only 
and, as a result, use .a single head scanning technique, 
relying on the blanking period between video picture 
frames to mask the gap between one helical track and 
the next.
Notes: 
I. This technique is advantageous for recording any 
rf baseband signal comprising closely spaced 
channels of information (e.g., telecommunications 
logging and historical reference). By use of multi-
channel multiplexers the recording of many indi-
vidual voice channels (e.g., aircraft and marine 
communications bands) can be added. 
2. No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Headquarters 
National Aeronautics 
and Space Administration 
Washington, D.C. 20546 
Reference: B70-10069 
Patent status: 
No patent action is contemplated by NASA. 
Source: Donald C. Heckman of 
General Electric Company 
under contract to 
NASA Headquarters 
(HQN-10317)
. 
Brief 70-10069	 Category 02


Radio frequency baseband recording technique

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server