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Document 24
Attachment A
Second Report of the ARRL Technology Working Group on Digital Voice
to the ARRL Technology Task Force
July 15, 2001
1. Introduction
The Digital Voice Committee held a teleconference on June 16, 2001 to summarize our work thus far and to discuss a second report and plan. Most of the following is based on talking points of the teleconference.
2. Advantages of Digital Voice for Amateurs
Advantages of digital voice for amateurs include:
A. high recovered SNR,
B. high immunity to QRM,
C. error detection and correction,
D. voice messaging, forwarding, trunking,
E. simultaneous voice and data,
F. selective calling and automatic link establishment,
G. embedded identifiers may include protocol, version,
areas of interest, etc., and
H. long-time integration and reduced data rates make
voice contacts possible that were previously impossible.
3. Disadvantages for Amateurs
Significant disadvantages are:
A. new equipment is required,
B. "digipeaters" are required to extend range
on VHF and above,
C. initial negative reaction to digital signals on the
phone bands is possible, and
D. a marked trade-off between voice quality, data rate
and occupied bandwidth exists.
4. Operating Modes, Bands and Technical Standards
A. Digital voice on HF. Simultaneous voice and data is not allowed under current US rules, except on 10 m. That may not be a problem, since such operation may not be desirable on HF. Even then, though, Novice and Technician Class licensees are excluded from digital voice on 10 m. One correspondent suggests re-farming the Novice bands for digital voice operation. In any case, it seems that licensing requirements may limit the popularity of digital voice on HF.
It's very likely that HF systems will have to employ error-correction and interference-avoidance algorithms that are quite different from those used at VHF and above. Data rates, and therefore speech quality, will be severely limited by available bandwidth (3 kHz) and by the propagation medium. On the other hand, the HF range is ripe with opportunities for experimentation with relatively high-speed modems.
Digital audio broadcasting standards have been adopted by the ITU for use below 30 MHz (see Jul/Aug 2001 QEX). Those standards allow simultaneous analog and digital transmission in a single channel. We think there is something to learn from those operations.
B. Digital voice on VHF/UHF. Here, higher bandwidths and data rates are easily possible. Simultaneous voice and data operation is legal, except on 219-220 MHz in the US.
C. Digital voice on microwave bands. We don't see a lot of digital video in Amateur Radio as yet. We might have a chance to learn something from commercial broadcasters, almost all of whom are converting to digital formats. The possibilities for high-quality digital voice, data and video are virtually unlimited. Low-cost commercial equipment (802.11a, b) may be used to occupy bands in which commercial interests are currently very interested (13 cm and 5 cm). That might also be a good way into schools, many of which are in need of networking upgrades.
D. Digital voice via satellites. Digital audio broadcasters have already begun operation via satellite (XM Radio, Sirius). We may learn something from their experience. We think a dialogue with AMSAT and others is needed to coordinate opportunities.
E. Digital voice in laser communications. Not much has been published on this, but high speeds and bandwidths are obviously possible.
F. Channel widths and spacing. A definite trade-off exists between narrow channels with poor voice quality and low data rates, and wide channels with better speech quality and faster data.
G. Occupied bandwidth and QRM. Occupied bandwidth largely depends on the modulation mode chosen. Certainly, QRM is an important issue and systems have to be tested in the presence of both digital and analog QRM, as well as for the QRM they cause. It would be possible to define occupied bandwidth standards, perhaps based on national or international norms.
H. Modulation modes GMSK, offset QPSK, OFDM, CPM, others. Modems may be readily built using off-the-shelf chips. They may also be built in software on a DSP development platform that includes data-conversion capability. Both avenues are currently being explored. The flexibility offered by software-based modems is hard to ignore.
I. Evaluation of voice quality. Because what someone hears (or doesn't hear) cannot be evaluated directly, but only by asking questions of the observer, it's quite important to adopt a criterion-free model for subjective voice-quality evaluation. A simple scale such as mean-opinion score (MOS) from 0-5 may be good because it roughly corresponds to the readability measure commonly used in signal reports. Such MOS evaluations may supplement quantitative data taken during development and testing.
IEEE and others have undertaken the development of standards for quality evaluation. Those are being examined for possible use in Amateur Radio.
5. Commercial Digital Voice Systems
Contact has been made with Dr. John Hardwick, President of Digital Voice Systems, Inc. (DVS). DVS have the multi-band excitation coding (MBE) technology used to digitize and recover voice in the G4GUO/TAPR, APCO 25 and Iridium systems. He has kindly invited ARRL to visit their headquarters in Massachusetts. He confirms that aside from selling chips, they also license firmware versions of their algorithms. They are interested in supported ARRL efforts for digital voice and a meeting at the executive level might be useful in further defining how that could happen (see DV reflector for more detail).
Since ARRL and APCO have MOUs pledging mutual cooperation and training, we think instruction of ARRL personnel on the operation of APCO 25 units could achieve two goals: 1) improving the readiness of our operators and 2) evaluating the speech quality and other features of the radios. We seek contact with those who are working with APCO on cross-training to assist with the latter goal. We seek hands-on experience with APCO 25 radios.
A. Advantages of adopting commercial standards. Adoption of a commercial or governmental standard would appear to make it easy for radio manufacturers to offer an amateur version of their hardware. Some of those manufacturers already make ham gear, and some do not; some ham-radio equipment manufacturers don't currently plan to make APCO 25 gear.
The DVS chips have a feature that detects when the speaker isn't speaking. That could be very handy for "simultaneous" voice and data, since data could be sent during silent periods. APCO 25 offers an advantage in that much of the hard work has already been done; but there are some disadvantages to that approach, as well.
B. Disadvantages of commercial systems. We have no indication yet that APCO 25 radios can reach the ham bands or that direct interoperability is necessary or even desirable. Adopting a fairly rigid standard at this point may stifle innovation among amateurs. The use of DVS chips is among the most rapid and efficient ways to get equipment on the air, but it doesn't bristle with possibilities for future improvements by experimenters.
C. Similarities and differences among commercial, public-service and amateur practice.
Both commercial and amateur operators may have a need for simultaneous voice and data. Embedded identifiers and message codes can indicate ancillary information for all services. Message store-and-forward services are universally useful. Trunking systems are finding widespread use in commercial and government services.
Many amateur contacts are casual in nature, which may obviate the need for rigid networking architectures. Voice quality is less important in commercial and public-service communications than in Amateur Radio. So it may be that amateur systems will dedicate more throughput capacity to voice than to simultaneous data transmission. Amateurs may also be more interested in video and other modes than our commercial counterparts.
6. New Voice and Data Services
The ability to send and receive voice and data simultaneously opens some very interesting possibilities:
A. Text messaging plus voice.
B. Interest broadcasting ("I am interested in rag-chewing and DX.")
C. APRS (Geo-location)
D. Voice mail.
E. Selective calling.
F. Virtual trunking systems.
G. Smart, environment-aware radios.
H. Possibility of digital-voice, licence-free services based on amateur work, perhaps on a few channels at UHF.
I. Bringing in the computer crowd. Possibilities for TCP/IP and other protocols exist, especially at higher frequencies and bandwidths. A definite tie-in with the League's high-speed networking efforts is evident.
J. Regulatory issues. Current licensing requirements may limit the use of digital voice on HF. The rules in different countries vary widely when it comes to digital phone there, apparently. Licensing requirements must be minimized to attain critical mass for digital voice and the other ancillary services mentioned above.
7. Basic Equipment Requirements
A. Radio or external box available as kit or ready-built. The G4GUO/TAPR codec is in beta test. Some good UHF radio kits have been identified and are available at reasonable cost. Digipeaters will be required to extend range on VHF and above. Development of those logically follows that of DV transceivers.
B. Inexpensive development environments. Having digital-voice algorithms under software control is a very attractive arrangement. When new software becomes available as improvements are made and bugs are fixed, it may be downloaded to the target radio or DV box. Jesse Morris has been very helpful in providing Analog Devices' development platforms to committee members.
C. Open source code. As evidenced by the popularity of Linux and other software, open, well documented source code is the best way to encourage further experimentation. That is not necessarily an attractive possibility for equipment manufacturers, though.
D. Bringing down the cost. Free-market issues are at play here: supply and demand.
E. Sales channels; i.e., kit manufacturers. Kits are getting more difficult to offer successfully because of the proliferation of surface-mount technology (SMT) and the decline of through-hole components. As against that, SMT means circuits can be built through automated processes at reduced cost.
F. Elmering of new digital radio users. Lots of useful information for interested parties has been posted to the TIS digital voice page. It's quite possible we can arrange for a news or chat group for those wanting additional support as we get rolling. Visibility will be continually needed in ARRL publications through articles outlining goals and progress made, along with contact information.
G. Vocoder technology. Intellectual property issues tend to limit what's available for amateur experimenters and those wishing to market finished products that include patented technologies. Amateur Radio may have to be willing to work with a wide variety of voice coding schemes. Some of those, fortunately, are in the public domain as international standards.
8. Further Work
A. Generating some interest. We have established the TIS Digital Voice page as the place amateurs can go to get the ungarbled word on the state of the art. We'd like to generate some additional tutorial information and add to it as progress is made. An article in a major ham publication would give some visibility and may generate additional interest and feedback. One article on DV has been circulated and may become part of a series. (It's on the DV reflector and a revised version with less technical content has also been done).
B. Feedback. Such an article series would be a great chance to get input from interested parties. One big concern for us is to find volunteers to get involved with hardware and software development and testing of systems. We'd like to make contact with amateurs in other countries who are interested in DV and who are willing to help us evaluate regulatory and technical issues.
DV should be a discussion point wherever meetings between ARRL and other bodies around the world occur. The business of simultaneous voice and data may serve as a starting point for such discussions, since it's not clear to us yet how regulations affecting that differ across the globe.
C. Conclusions. While we acknowledge similarities in the technical and philosophical goals between commercial and amateur use of DV technology, we also see significant differences. Amateur goals do not center on pecuniary gains, but on the continuing freedom to make a significant impact on the art and on serving the public.
The attainment of those goals depends on what DV does for us. Some of its technical advantages have been outlined above. Adopting an existing standard is not bad, but it doesn't satisfy our stated goal of advancing the state of our art.
We're counting on certain dedicated individuals to come up with something new and better than what's already been done. Better, in this case, may mean something that brings the technology within reach of thousands of hams around the world; or, it may mean bringing us up to speed on methods that have already seen success elsewhere. It may also mean something that achieves a technical breakthrough. A lot of work needs to be done on all those fronts. One of our pressing goals should be to communicate our needs and desires to as many hams as possible. That way, we can be better satisfied that we haven't missed something or someone.
Respectfully submitted,
ARRL DV Committee:
Gary Barbour, AC4DL
George Bednekoff, AC5WO
Charles Brain, G4GUO
John Gibbs, KC7YXD
Jesse Morris, KC5GTK
Doug Smith, KF6DX, Chair
Mike Tracy, KC1SX, HQ Liaison
7-15-2001
