the method of adjusting a local oscillator to zero beat with the frequency of the received signal, the frequency of the local oscillator being measured in terms of a temperature-controlled piezo-oscillator calibrated in terms of the national frequency-standard. This method is very satisfactory, provided care is exercised in making the measurements and well designed and constructed apparatus is employed. Full information on such methods is given in articles listed at the end of this report.

It is recommended that each national administration monitor its own transmitting stations, taking the necessary precautions to make the frequency measurements accurately in terms of the national standard. In case of complaints regarding a station's frequency, the national administrations concerned shall, so far as possible, cooperate in monitoring its frequency; in case of disagreement appearing in the results of such monitoring, the national standards shall be intercompared by one or more of the methods discussed under "National Standards and Their Intercomparison."

It is recommended in addition that, wherever congested radio. conditions warrant, the nations of a region establish a joint monitoring system to monitor stations likely to cause regional interference.

5. Synchronization of Transmitting Stations

This section states the conditions under which synchronization of broadcasting stations may be considered as an international problem, together with suggested methods of regulation. Synchronization has not up to the present been undertaken at other than broadcasting frequencies.

Since broadcast station-bands are limited in number and the demands for them are constantly increasing, synchronization has been repeatedly suggested as a possible method of relieving this congestion and increasing the net program service area in each country.

Inasmuch as broadcasting frequencies are continental in distance range, synchronization should be considered a continental problem. When the power used is such that the coverage may be considered continentally international, then synchronization is a matter for agreement among the nations of the continent concerned.

In order that one nation may not impose the results of a partial experiment on neighboring nations, it is suggested that the synchronization methods or schemes be amply proven first. Even after the system has been pronounced a success in a local way, it should be recognized that additional problems may arise with changes in loca

tions, distances, power, etc. Therefore an experimental period should be designated and sufficient data gathered, in order that the net increase in program area be ascertained before final approval is given by the nations concerned.

Suggestions as to the manner in which the problem of synchronization may be considered are contained in Annex 3. In brief, the suggestion is to provide four experimental periods during which stations proposing to synchronize will demonstrate that they can accomplish the successive steps necessary to make synchronization

a success.

Up to the present, there has been some success with one type of synchronization, namely, by sending a master frequency to the two stations over a wire line. Even that type of synchronization has not been employed by more than two stations nor by stations of a wide variety in relative powers and distances between the stations. No engineering data are available as to the interference effects resulting from this synchronized operation at distances of reception in which fading ordinarily occurs.

For stations desiring to synchronize, except stations that are duplications of proved examples of synchronization, test experiments and demonstrations covering a considerable period of time and outside of broadcasting hours should be required before the applicants are permitted to operate during broadcasting hours and then only after the administration is satisfied that the synchronization will result in better service than can be attained without such synchronization.

IV. STATION BAND-WIDTHS

The necessary separation between adjacent frequency assignments, or the necessary width of station-bands, depends upon a number of factors, including the frequency tolerances, the communication bandwidths required by various types of transmission and speeds of signalling, receiver-selectivity, geographical separations, etc. Frequency tolerances have been specified above. Some of the other factors affecting station band-widths are discussed below. A proposed standard system of station-bands for international use is then set forth. In cases where the "overall communication and tolerance band" of a station exceeds the standard station bandwidth, more than one standard station-band must be assigned the station.

1. Communication Band-widths Required by Various Types of Transmission and Speeds of Signalling1

Paragraph 4 of Article 4 of the International Radio Regulations of Washington, 1927, requires that the width of the communicationband be reasonably consistent with good current engineering practice for the type of communication carried on. The process of modulating a carrier wave by a signalling wave having a single frequency, in actual practice, results in the production of two series of side frequencies, one series on either side of the carrier, of which side frequencies the most important is that pair which is nearest to the carrier. These side frequencies are separated from the carrier by frequency intervals which are successive multiples of the frequency of the signalling wave. For example, in the case of a 100 kc carrier which is modulated by a signalling frequency of 1 kilocycle, side frequencies occur in pairs at 101 and 99, 102 and 98, 103 and 97 kilocycles, etc. The side frequencies farther removed from the carrier are normally of substantially lower amplitude than the first. The extent to which they may be further reduced depends largely upon the operating characteristics of the transmitter and upon the degree to which it is practicable to include circuits of suitable selectivity. For example, the second, third, and fourth side frequencies resulting from square-wave keying may, in practice, be produced at levels 10, 14 and 17 decibels, respectively, below the first side frequency. These levels may, fairly readily, be further reduced (for example, by the use of two coupled circuits of power factor of 0.3 per cent in the output portion of a radio-transmitter) by approximately 3 decibels for each 0.1 per cent by which the side frequency differs from the carrier frequency.

Similarly, multifrequency modulation of a carrier wave results in a multiplicity of side frequencies or a "side band" on each side of the carrier.

Radio emissions are classified in Paragraph 1 of Article 4 of the International Radio Regulations of Washington, 1927. These classes, together with certain interpretations made necessary by recent radio developments, are listed below. Under each, there is given a statement of the frequency-band required by the signalling waves corresponding to various methods and speeds of signalling.

Class A.-Continuous waves.

Waves the successive oscillations of which are identical under permanent conditions.

'References: See Annex 1, B, 1, 2, 3, 4 and 5.

Type A 1.-Unmodulated continuous waves.

Continuous waves, the amplitude or frequency of which is varied by means of telegraphic keying.

While the foregoing definition specifies telegraphic keying, it seems reasonable that other methods of communication involving interruption of a carrier wave as in some systems of facsimile or picture transmission and of television may appropriately be included under Type A1.

Telegraph transmission.-The speed of telegraphic signalling may be conveniently expressed as a number of dots per second,1 and is defined as the number of signal elements per second divided by two. (For example, a telegraphic dot and space together comprise two signal elements and are counted together as one "dot" (i. e., dot cycle) in determining the speed of signalling.)

The frequency-band occupied by the signalling wave is proportional to the speed of signalling in dots per second and, as an ideal limit, the frequency-band in cycles is equal numerically to the number of dots per second. When, as is ordinarily the case in present practice, the transmission is on a double side-band rather than a single side-band basis, the ideal limit becomes twice the number of dots per second.

The fact that practical telegraph systems do not utilize the range with ideal effectiveness requires that these ideal limits be multiplied by a factor which, for example, in the case of wire circuits, is usually about two. This implies the transmission of a wider frequencyband which results in a closer approximation to the rectangular wave-shape which is usually very desirable.

The following table gives pertinent information relating to the signalling band-width required for several telegraph systems on the foregoing basis:

Facsimile transmission.-For facsimile transmission or telephotography, the width of the frequency-band in cycles (for ordinary

1

'One dot per second=1 mark-space cycle per second=2 bands.

double side-band transmission as an ideal limit) is equal to the number of picture elements per second. Assuming for present purposes that this method of transmission permits a close approximation to this ideal limit, the frequency band-width required for the transmission of a given picture is roughly equal to the number of elements in the picture divided by the number of seconds occupied by the complete transmission. For example, a picture of 360,000 elements occupying 72 minutes in transmission would use a frequencyband of 800 cycles on a double side-band basis.

In the facsimile transmission of written material, the number of words which can be transmitted per second within a given frequencyband and with a given amount of detail depends upon the character of the printing or writing, the degree of legibility desired, the way the words are arranged on the sheet, and on other factors. If it is assumed that from 50 to 200 picture elements are required for the satisfactory transmission of each typewritten character, depending on the system employed and the transmitting medium involved, the frequency range on a double side-band basis is given by multiplying this number by the number of characters transmitted per second. For example, if 100 words, each consisting of 5 letters and a space, are to be transmitted per minute, the number of characters per second would be about 10 and the frequency-band required would be about 500 to 2,000 cycles, transmitting only the pair of side-bands closest to the carrier.

Television. In television as in facsimile there is a fixed limiting relation between the width of the frequency-band actually occupied and the amount of detail which is desired in the picture or image. In practically all present television systems the picture is, in effect, divided into a number of small elements. For ordinary double sideband transmission, the width of the frequency-band in cycles is roughly equal to the number of pictures transmitted per second times the number of picture elements per picture. For example, if the picture is divided into 4,320 elements and 20 pictures are transmitted per second, the width of the frequency-band required is about 86,400 cycles, transmitting only the pair of side bands closest to the carrier. Television would, at the present time, appear to require a frequencyband at least 100 kilocycles wide.

Type A2.-Continuous waves modulated at audible frequency. Continuous waves, the amplitude or frequency of which is varied in a periodic manner at audible frequency, combined with telegraphic keying.

The width of the frequency-band occupied in this case is equal to that employed in case A1 plus twice the maximum modulating frequency.