FED-STD 1023
September 25, 1989
FEDERAL STANDARD
TELECOMMUNICATIONS: INTEROPERABILITY REQUIREMENTS
FOR ENCRYPTED, DIGITIZED VOICE UTILIZED WITH
25 KHZ CHANNEL FM RADIOS OPERATING ABOVE 30 MHZ
This standard is issued by the General Services Administra tion pursuant to
the Federal Property and Administrative Services Act of 1949, as amended.
1. SCOPE
1.1 Description. This standard establishes interoperability requirements regarding the analog to digital conversion, encryption (with related synchroniza tion),
and modulation of encrypted voice associated with Frequency Modulation (FM) radio systems employing 25 kHz channels and operating above 30 MHz. In
this standard, voice is digitized using 12 kbit/s Continuously Variable Slope Delta -modulation (CVSD) and then encrypted using a National Security Agency
(NSA) Commercial COMSEC Endorsement Program (CCEP) Type I encryption algorithm.
1.2 Purpose. This standard is to facilitate interoperability between telecommunication facilities and systems of the Federa l Government.
1.3 Application. This standard shall be used by all Federal Departments and agencies in the design and procurement of digitized voice Type I encryption
equipment for use with nominal 25 kHz channel FM radio systems that operate above 30 MHz and digitize voice at greater than 4.a kbits/s and less than 16
kbits/s. All such equipment must be capable of digitizing voice using 12 kbit/s Continuously Variable Slope Delta -modulation (CVSD).
Note: This standard applies only to Type I (i.e., protection of classified information) systems and does not restrict the use of other systems, such as Data
Encryption Standard (DES) encryption or analog and quasi -analog scrambling systems.
2. REFERENCED DOCUMENTS
a. NSA Specification 86-33, INDICTOR Interface Control Document (FOUO)
b. NSA Specification 86-32, WINDSTER Interface Control Document (FOUO)
c. Communications Security Equipment System Document 14, TSEC/KY -57/58 (CQNFIDENTIAL)
Note: All references to the above document assume the KY -57/58 has been modified to operate at 12 kbits/s (i.e., 75 percent normal clock rate).
The above three documents are published by the National Security Agency (NSA), Fort Meade MD 20755, and can be made available to Government
departments and agencies and to manufacturers participating in the NSA Commercial COMSEC Endorsement Program (CCEP).
3. REQUIREMENTS
3.1 Overview. This standard describes interoperability-related requirements for the conversion of analog voice to digital form (section 3.2), its encryption and
related synchronization (section 3.3), and subsequent frequency modulation (section 3.4).
3.2 Analog to Digital Conversion
3.2.1 Digital Rate: Voice shall be converted, using Continuously Variable Slope Delta -modulation (CVSD), to a 12,000 bit s/s +.018 percent digital stream.
3.2.2 Block Diagram and General Description. The following diagram is a typical representation of the CVSD analog -to-digital conversion process.
In the typical CVSD representation above, the incoming analog voice signal is passed through a Voice Frequency (V.F.) Filter and then compared, by a
Comparator, with the output of the Principal Integrator. The previous bit output of this Comparator is used: (1) as the digital output of the CVSD encoder, (2) to
determine the po larity of the pulse generated by the Pulse Modulator, and (3) as input of the Modulation Level Analyzer. The Modulation Level Analyzer (MLA)
provides indication to the MLA Integrator whenever the last and previous two bits from the Comparator are either al l ONEs or all ZEROs. (This is referred to as
run-of-three coincidence coding). The MLA Integrator determines the step size, which is variable and based upon the MLA output, and provides this pulse FED -
amplitude information to the Pulse Modulator. The Pulse Modulator provides pulses to the Principal Integrator as the Principal Integrator attempts to follow the
shape of the input voice waveform.
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