While advanced encryption schemes can deliver a high degree of link security, a less-complex approach can also provide basic security with far greater simplicity and at lower cost, and be well-matched to the end user’s unique application. For these reasons, it may make sense to scramble an audio signal close to its source, then reverse the process near to the output transducer. As an added benefit, this type of analog scrambling/descrambling-based security is largely independent of the channel type, frequency, format, protocol, or any additional encryption.
To fill these needs, CML Microsystems plc has introduced the CMX7158, a full-duplex, frequency-inversion audio-scrambler IC that can be used in multi-function audio systems as well as mobile and handheld radio systems (Fig. 1). It can be easily switched between clear and scrambled voice modes and allows for user-set frequency “inversion point” in addition to the default value. It’s designed to be easily integrated into existing systems or used in simple accessories.
1. CML Microsystems’ CMX7158 is a full-duplex audio-scrambler IC that “flips” the voice-band frequencies around an inversion point for basic security. It also handles signaling and coded tones.
On power-up or system reset, the CMX7158 is initialized using CML’s proprietary FirmASIC component technology. At power up, what’s called a Function Image is downloaded either from a host controller connected over a basic interface or an attached serial flash-memory device. This defines the device’s functionality by assigning internal device resources and determining device features.
The CMX7158 has fully independent transmit (Tx) and receive (Rx) paths, and incorporates a frequency-inversion descrambler in receive mode that can be toggled on/off as needed by software. This function in the receive path descrambles the received audio-band signals that have been scrambled at the transmitter. The inversion frequency defaults to 3300 Hz, but may be modified via software. The command, status, and data are transferred between the CMX7158 and the host microcontroller using a C-Bus serial data interface (C-BUS); this interface is compatible with Microwire and SPI interfaces.
In addition to voice transmission, most audio-based systems use in-band signaling to implement additional functions. These include the well-known DTMF (dual tone multifrequency) technique as well as Selective Calling (SelCal), which is a widely used, tone-based system with the ability to call selected radios using a tone sequence. The CMX7158 can generate these tones as needed and they, of course, should not be “inverted.” Thus, the IC includes an option to automatically turn off the scrambling function for these tones.
Other built-in functions are user-programmable bandpass and other filters, microphone-path automatic gain control (AGC) to level out the voice-signal power despite continually varying conditions, and VOX (voice operated exchange) detection. That last feature is used to turn on a transmitter when someone speaks and turn it off when they stop speaking, by comparing the associated audio signal strength to a preset threshold.
For hardware design and development support, CML offers its PE0402 evaluation board (Fig. 2).
2. Designers can use the PE0402 evaluation board, with its many easy-to-access connection points, for hardware and software development.
The CMX7158 includes a crystal-based clock generator with buffered output, which can provide a common system clock if required. The IC operates from a 3.0- to 3.6-V supply and is housed in a 7- × 7-mm, 48-pin VQFN package.