ARLA/CLUSTER: Sistema Digital da Alinco - CVSD (Continuous Code Delta Codec)

[João Gonçalves Costa]

Digital Mode:
Q: What is the digital mode available on the DR-135/235/435T and DJ-596T ?
A: The EJ-40U/EJ-43U optional digital boards consist of a CVSD (Continuous Code Delta Codec), a CPU and GMSK-MODEM (Gaussian Minimum Shift Keying-Modem circuits. The analog voice signal from a transmitter's microphone is encoded by CVSD to a 14kbps digital signal while transmitting (Analog-Digital conversion). This digital signal is processed in the CPU using the publicly-open ITU-TV.32 protocol. This processed signal modulates the VCO in GMSK direct frequency modulation using a GMSK-Modem. It is then transmitted as a 20F3E conventional FM signal.

In the receiving mode, the GMSK modulated RX-AF signal comes out of a demodulation circuit and is processed at the GMSK-Modem. This signal is again processed in the CPU, then decoded in CVSD to obtain the original analog voice signal (Digital-Analog conversion). As in a normal FM receiver, the signal is amplified for output to a speaker.

Definitions: CVSD: Continuous Code Delta Codec. One of the delta modulations that is commonly used for A-D,D-A conversion. It is a relatively low error-rate pulse modulation system.
ITU-TV.32: Internationally published open modulation protocol for a data-modem circuit.
GMSK: One of the digital modulation systems. It uses a Gaussian filter to reduce side-lobes produced at the moment of modulation.

Q: Is it legal to use this device in Ham radio communications, because digital modulation might be interpreted as 'scrambled', and encrypted communications are prohibited in the amateur-use ?
A: It all depends on the decision of your local authority. Please be sure to consult the rules that apply in your country before you actually operate in the digital mode. This unit is neither available nor intended for use where the import, sale or use of such devices is prohibited by the government. Alinco declines any and all responsibilities for any illegal use, nor is it responsible for sales of our products by dealers to any party.
In Japan, this mode has been officially authorized by the Ministry of General Affairs for ham-radio use (actually the DR-135 with the EJ-40U Digital Board is the very first radio that the ministry has type-accepted for ham use), because the device uses a public, open protocol. Moreover, it is not the first use of DIGITAL communications in Amateur Radio; rather, amateur radio started with a digital mode, known as Morse Code or CW coded signals. Nowadays, digital modes are very popular, including PSK31, AX.25, Pactor, G-TOR, Hellschreiber and others. They are legal because everyone uses the same published protocol, available publicly. We ask, why not use digital technology for VOICE communications? The Alinco EJ-40U uses the established ITU-TV.32 protocol. From this point of view, after having contacted the FCC in the USA, we have become confident that the use and sale of this device in the USA is not a problem. It is important to note that the signal is not encrypted, nor is the equipment intended to obscure the true meaning of the communication. Anyone with similar equipment or a means to receive and process the signal can monitor the communication. Such is the case for AX.25 packet, PSK-31 and other digital communications. Unlike the digital mode used in some commercial radios, the EJ-40/43U has only one protocol and it is not user-programmable. Ham regulations in many countries only prohibit codes or ciphers intended to obscure the communication, not the digital modulation itself.

Q: What are the advantages and disadvantages of the digital mode ?
A: The advantages are low-noise and high-quality audio communication (and in commercial or government communications a higher level of security is possible)....in theory. However, for amateur use where there are restrictions such as bandwidth, cost of the radio etc, the technology applied to the EJ-40/43U is rather simple. We admit that the audio from the digital modulation mode in our radio definitely sounds 'processed'. Due to simplified circuit design, the error-correction rate is not as high as more expensive "cutting-edge" commercial rigs. However, using these boards, you can experience and experiment with the new world of digital audio communications in the amateur bands at a very affordable Alinco price. It is exciting to many to experiment with a new mode. And don't forget the analog FM audio quality of the DR-135/235/435 series and DJ-596 is superb. The Alinco radios allow you to easily select and use either mode.

Q: Is it possible to use this unit for amateur repeater-communications ?
A: If the system is very accurately aligned, theoretically yes. But in actual use, passing through a repeater increases the possibility of communication errors that may result in poor communication quality. Also, CTCSS is not available during digital operations.

Q: Is there anything I should be aware of before I use it on the ham bands ?
A: When you enter the digital mode, the analog signal becomes inaudible (it detects a GMSK modulated signal only through an open squelch. This is similar to a conventional Tone Squelch. Yes, digital communications also require a squelch circuit.). In order to avoid disturbing an on-going QSO on the same frequency or nearby, it is suggested that you first monitor the frequency in analog mode and make sure that the channel is clear. When your digital QSO is disturbed by analog QRM, you will experience the same effect as when analog FM QRM occurs; that is, stronger signals will block weaker ones. (This is known as the "capture effect".) Use this mode where simplex FM (20KHz bandwidth) is allocated in your local band-plan. Depending on regulations that apply in your country, an ID in the conventional FM analog mode, or in CW may be required during a QSO and at its conclusion.