The ARRL Handbook for Radio Communications; Volume 3: Practical Design & Principles PART 2 by ARRL Inc

Author:ARRL Inc. [Inc., ARRL]
Language: eng
Format: azw3
Publisher: ARRL, the national association for Amateur Radio®
Published: 2018-10-07T16:00:00+00:00

2) The FreeDV API: An open source, C-callable software library, that has interfaces for audio and modem samples. The API can be linked into third-party SDR programs that wish to support FreeDV.

A standalone hardware implementation of FreeDV (SM1000) is also available that does not require a PC host. (See freedv.org for more information about FreeDV software and accessories.)

FreeDV Architecture

Figure 15.7B shows a typical FreeDV system. Voice signals from a microphone are sampled by the ADC, typically at 8 kHz with 16-bit resolution. A speech encoder (Codec2) then compresses the speech to a low bit rate, for example 1300 bit/s. FEC bits are then added to protect against errors encountered in the channel. This may increase the bit rate to 1600 bit/s. A modulator (labeled “mod”) then converts the bit stream to tones that can be passed via a radio channel. Over the channel the digital speech signal will encounter noise and other impairments such as fading and frequency offsets.

On the receive side, the demodulator (labeled “demod”) then converts the tones back into a bit stream. The FEC decoder attempts to correct channel errors before passing the payload compressed voice information to the voice decoder (labeled “codec 2 dec”). The output of the voice decoder is an 8 kHz, 16-bit/sample sequence that is converted back into analog speech by the DAC and played through a speaker or headphones.

More details and the software can be found at freedv.org as well as written and video setup guides. A coordinating website for FreeDV QSOs is available at qso.k7ve.org.

15.4.6 ALE

Automatic link establishment (ALE) was created as a series of protocols for government users to simplify HF communications. The protocol provides a mechanism to analyze signal quality on various channels/bands and choose the best option. The purpose is to provide a reliable rapid method of calling and connecting during constantly changing HF ionospheric propagation, reception interference and shared spectrum use of busy or congested HF channels. It also supports text messages with a very robust protocol that can get through even if no voice-quality channel can be found.

Each radio ALE station uses a call sign or address in the ALE controller. When not actively in communication with another station, each HF SSB transceiver constantly scans through a list of frequencies, listening for its call sign. It decodes calls and soundings sent by other stations, using the bit error rate to store a quality score for that frequency and sender call sign.

To reach a specific station, the caller simply enters the call sign, just like dialing a phone number. The ALE controller selects the best available frequency and sends out brief digital selective calling signals containing the call signs. When the distant scanning station detects the first few characters of its call sign, it stops scanning and stays on that frequency. The two stations’ ALE controllers automatically handshake to confirm that a link of sufficient quality is established and they are ready to communicate.

When successfully linked, the receiving station which was muted will typically emit an audible alarm and visual alert for the receiving operator of the incoming call.

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