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ST-20B/C Speech Inversion Scrambler
ST-25B/C Rolling Code Encryption Modules

Private Collection

Growing congestion of the airwaves has led to increasing concern for the security of wireless communications. With the widespread availability of RF scanners, anyone from the idle hobbyist to your direct competitor to the most determined criminal can easily record your most sensitive conversations.

First introduced in 1989, the Selectone Private Collection� has become the industry�s leading solution for secure voice applications over two-way radio systems. With three different levels of security, the Private Collection offers the right amount of protection for nearly any application. With tens of thousands of units in daily service, each backed by a two year factory warranty, you can count on the Private Collection for years of reliable service.

  • Three Levels of Security � suitable for most secure speech applications.
  • Miniature Size - for easy mounting in most portable and mobile radios.
  • High Quality Audio � input and output audio processing filters provide high quality, low distortion audio recovery in encrypted mode.
  • Fully Compatible � with nearly all radio systems, including repeaters and trunked systems.
  • Over the Air Programming (ST-50 series only) � allows system manager to change code keys or disable lost or stolen units over the air.
  • Full Applications Support � with detailed installation instructions for many popular mobile and portable radios.


For any voice encryption application, the level of security required is a function of the sensitivity of the information being transmitted and the sophistication and determination of the persons trying to intercept that information. The Selectone Private Collection features three different levels of security for a wide range of applications: simple inversion, rolling code inversion, and time domain digital encryption. As with all encryption devices, there is a tradeoff between cost, the level of security and the quality of the recovered audio.

  • Inversion Scrambling
    Simple frequency domain inversion scrambling is the most popular and lowest cost approach to speech security. The cipher process inverts speech frequencies in reference to one of eight possible inversion carriers. The scrambled speech is unintelligible to the casual listener, but the recovered audio is very clear. This type of encryption is suitable for many business and commercial applications to discourage eavesdropping from co-channel users or RF scanners.
  • Rolling Code Encryption
    The next highest level of encryption is known as frequency domain rolling code. In this method, the cipher process uses a proprietary, microprocessor controlled analog scrambling algorithm with over one trillion available code keys. The encrypted speech is completely unintelligible, but audio processing filters provide high quality recovered audio. While it is possible to "break" an encrypted transmission, to do so requires sophisticated equipment, knowledgeable technical personnel, and considerable time. Therefore, rolling code encryption is suitable for tactical communications typical of most public safety and more critical commercial applications.
  • Time Domain Digital Encryption
    The highest level of security available for conventional, analog radios is time domain digital encryption. The cipher process uses Digital Signal Processing (DSP) techniques to produce a time domain encryption algorithm. The encryption module digitally records microphone audio for a small time segment. The segment is then subdivided into smaller blocks within the segment. The blocks within each segment are then rearranged in time based on a 72 bit encryption algorithm. The rearranged blocks are then played back for transmission, with playback being in a reverse order. Encrypted audio is completely unintelligible and virtually impossible to break, making time domain encryption suitable for the most sensitive strategic communications. Despite the complexity of the encryption, DSP audio filters provide high quality, low distortion recovered audio.


The ST-20B/C is a miniature, low cost inversion scrambler designed for voice grade communications systems. Applications include commercial shared repeaters and other business and marine radio systems where protection is desired from co-channel users and casual eavesdroppers. The ST-20B/C does not require a license for export.

The unit measures only 0.9"W X 1.5"L X 0.18"H for easy mounting in most portable and mobile radios. Operation is virtually transparent to the user. The Clear/Cipher input, used to enable or disable the cipher mode, is the only user control. A choice of eight field-selectable inversion codes allows compatibility with most commonly used inversion formats.

The standard model for most applications is the ST-20B. The ST-20C is identical but provides a low pass filter in parallel to the normal receive audio path. This path is used for special applications where CTCSS detection must occur after the application point in the host radio. The following Motorola radio models require the use of the ST-20C: M1225, P1225, MT2000 and MT2100.


The ST-25B/C Rolling Code Encryption Module is a miniature, high security voice encryption device especially designed for tactical communications. Up to four user programmable code keys, each selected from over four billion possible keys, may be used to communicate in a secure mode with up to four different groups on the same radio channel. To maintain security, code keys are stored in a non-volatile EEPROM and are never transmitted over the air.

The Clear/Cipher select line is the only user control. Receive mode is automatic, so that clear speech or encrypted speech with the proper code key will be received regardless of the mode selected. Once installed, the ST-25B/C can be enabled or disabled as required. Therefore, it is not necessary to equip an entire fleet with encryption, but only those units where high voice security is required.

Special factory set master code keys may be reserved to provide extra security for special applications. Each master code key group has over 268 million possible code keys. User selectable code keys and other operating parameters are field programmable with a PC using the ST-25PM Product Manager Software.

The standard model for most applications is the ST-25B. The ST-25C is identical but provides a low pass filter in parallel to the normal receive audio path. This path is used for special applications where CTCSS detection must occur after the application point in the host radio. Radios which require the use of the ST-25C are listed above (same as ST-20C).



Encryption Frequency Inversion 24 bit algorithm with
32 bit code key
Operating Voltage 5.2 Vdc to 16.3 Vdc
Operating Current <8mAdc

Clear Rx & Standby


Ciphered Rx or Tx

Available Carrier Frequencies 8 n/a
Available User Code Keys n/a Greater than 4 billion
Cipher Algorithm n/a Real Time Frequency Domain
Synchronization n/a Initial & Maintenance
Usable Audio Level Rx and Tx 25 mV p-p to 2.0 V p-p
Input to Output Gain <�1dB
Frequency Response (ST-20B) 300Hz to 2600Hz
(ST-20C) 67 to 250 No cipher
300Hz to 2600Hz
(ST-25B)300Hz to 2600Hz
(ST-25C)67 to 250 No cipher
300Hz to 2600Hz
Temperature Range -30�C to +60�C

18" flying leads terminated in a 13 pin low profile connector

Size 0.90"W x 1.5"L x 0.18" H
(22.8mm x 38.1mm x 4.6mm)

Specifications and descriptions subject to change without notice

WISCO International, Inc. - MIAMI, FL U.S.A.
Fax: (954) 370-3997 | E-mail: wiscointl@wiscointl.com

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