Frequency Ringer Identification Codes

[unbeldi]

Frequency selective ringing systems were first deployed for party-line telephone signaling in 1882 in Massachusetts [Bell Laboratories Record, Vol. 12 (January 1934)].

Early frequency ringing systems used sets of four different frequencies of ringing current. These frequencies were generated by generators running at 1000, 2000, 3000, and 4000 revolutions per minute, resulting in a base frequency of 16 2/3 (=1000/60) cycles per second and its 2–, 3–, and 4–times multiples.  The same set of frequencies could be generated with one generator by placing multiple armatures onto the same motor spindle with one, two, three, and four pairs of coils, respectively.  Because the system used integer multiples (harmonics) of the base frequency, it was called the harmonic ringing system.
This provided for 4-party bridged metallic ringing service on one line, or fully selective 8-party service when using divided ringing via a ground connection.

The problems with this system were technically cumbersome, which ultimately let Western Electric to abandon frequency-selective ringing, as ringers have the tendency to respond not only to the tuned frequency, but also to the harmonic multiples and sub-multiples, as the frequency ratios between most pairs were integers. The problems were mitigated by independent companies, led by the Kellogg Switchboard and Supply Company, by using other frequency schemes that did not have common multiples, or integral frequency ratios.

By the 1920s, two systems were in use, the multiple harmonic system and a non-multiple system [Smith, 1924]. The non-multiple system used 30, 42, 54, and 66 Hz, with has increments of 12 Hz between adjacent frequencies. Although this can also be interpreted as using odd-numbered multiples of 6 Hz, namely 5x, 7x, 9x, and 11-times as shown in the table, the frequency ratios between any two stations on the line were not whole numbers, and thus the ringers only responded to their designated frequency.

Eventually, three major frequency systems were distinguished, typically identified as harmonic, synchromonic, and decimonic, differentiated by the relations between the frequencies.

The decimonic system used increments of 10 Hz, resulting in the range from 20 to 60 Hz.

While these systems started out having four frequencies each, Kellogg advocated and implemented a five-frequency system already in 1910s [See KSS&S Bulletin No. 12 (1918)] for operation of four-party lines to mitigate ringing errors by operators. The fifth frequency was close to the straight-line frequency.  Other independent equipment makers followed suit. As can be seen from the tables, the extra frequency always has the highest code number, e.g.: HA5, HB5... WA5.

The coding sequence of the Kellogg system also shows that the lowest frequency was actually the last used frequency in the harmonic system, and the ringer for it was typically wound to a much higher impedance than the higher frequencies.  For example, the harmonic coding sequence in the Kellogg system was HA1 (33 Hz), HA2 (50 Hz), HA3 (66 Hz), HA4 (16 Hz), and finally HA5 (25 Hz).


Ringing key designations
Exchanges had multiple ringing keys, one for each frequency. These may have been either letter-coded (WRJM) or color-coded. The colors and letters were included in telephone number listings, e.g. Red 123, or  123M.
*Western Electric: W, J, R, M  (see also type-E dial face plates), [BLR 1953-03 v29 p.115, R.B. Hill W-R-J-M].
*Kellogg: red, green, blue, and black.

Frequency Ringer Markings
Western Electric
After having initiated the practice in the 1800s, Western Electric and the Bell System only used frequency ringers in a limited manner later, implementing instead a system that enabled selective service by variations in the central office and the wiring plant, by using various wiring schemes and signaling methods.  This had the advantage that the variety of necessary ringer types was minimized, as frequency ringers must be individually tuned after assembly.  In addition, this same method permitted message rate service and station identification for billing of toll calls. The harmonic system that Western Electric initially used consisted of four frequencies of 1x, 2x, 3x, and 4x multiples of 16 2/3 Hz. The system performed poorly by being susceptible to false ringing due to overtones of the base frequency.

Kellogg/ITT
Kellogg was the pioneer of alternate ringing system in the 1910s and 20s.
In the 1940s, Kellogg 1000 Masterphones and later ITT telephones were available with a variety of ringer assemblies with biased, harmonic, selective frequency, and straight-line ringers.  The codes for straight-line devices started with an S, and frequency ringers with H.

Stromberg-Carlson
Stromberg-Carlson used letter suffixes to the ringer number to indicate the frequency.
Harmonic ringers were only specified to the integer value in Hz, e.g. a 16.6 Hz ringer is listed as 16 Hz.

Automatic Electric
Early Automatic Electric catalogs do not list codes for the various frequencies. The ringer boxes are described as supporting any standard ringing system. The ringers themselves are identified by individual part numbers. Later catalogs do have codes (tbd).
The general trend is that ringer resistance is decreasing for the ringers with higher frequencies. A 66.6Hz ringer only has 476 ohms resistance, while 16.6 Hz ringers have 2760 ohms.[Automatic Electric Cat 4055 (1934)]
The straight-line ringers are provided with a biasing spring permitting the use of pulsating ringing current and may therefore be used for biased bell ringing on party lines.
Harmonic ringers use a clapper arm fastened to a steel reed which is adjusted to operate at one of the standard frequencies.
In 1957 (Catalog TA-57) order numbers are suffixed with a letter for the frequency. The descriptions also use terms harmonic and decimonic. Ringers are primarily classified as high and low impedance units.


Ringer Identification Codes

Ringing Frequency	Stromberg- Carlson	KS&S, ITT	Western Electric	Automatic Electric	ratios	Ringing Voltage
Straight-line
20 Hz	A	Impedance SA (high) SB (medium) SC (low)	A	SL		90 or 110
Polarized/Biased
20 Hz	BT (with tube)	Impedance BA (high) BB (medium) BC (low)
Harmonic			[16 2/3]		(8 1/3 Hz)
16 2/3 Hz	E	HA4	R	+	2
25 Hz	N	HA5		+	3
33 1/3 Hz	F	HA1	S	+	4
50 Hz	G	HA2	T	+	6
66 2/3 Hz	H	HA3	U	+	8
Synchromonic					(6 Hz)
16 Hz	R	HB5
30 Hz	K	HB1		+	5
42 Hz	L	HB2		+	7
54 Hz	M	HB3		+	9
66 Hz	P	HB4		+	11
Decimonic					(10 Hz)
20 Hz	I	HC1	W	+	2	105
30 Hz	K	HC3		+	3	125
40 Hz	Q	HC4			4	135
50 Hz	G	HC5	T	+	5	145
60 Hz	J	HC2	Y		6	155

This table was assembled from various sources and catalogs.

For AE: + means available acc. to catalog


PS: This table has been updated with image copies of my master table.

[Scotophor]

Was 44 1/4 Hz a standard selective-ringing frequency, and if so, which system did it fit in?

[unbeldi]

Was 44 1/4 Hz a standard selective-ringing frequency, and if so, which system did it fit in?

Haven't run across any mentioning of that frequency, I don't think.
It also doesn't fit into the three standard systems shown here, because none of the three use increments, or common multiples, with which one could arrive at that frequency.

[Scotophor]

Something else odd I just noticed in your description of the Synchromonic system. You mention it being based upon an increment of 6 Hz. The figures shown fit by using the ratio from the last column as a multiplier, except for 16 Hz. Is that a typo, which should have been 18 Hz (ratio = 3) instead of 16, or did they really somehow use this one oddball frequency? If so, how was it generated?

[unbeldi]

Something else odd I just noticed in your description of the Synchromonic system. You mention it being based upon an increment of 6 Hz. The figures shown fit by using the ratio from the last column as a multiplier, except for 16 Hz. Is that a typo, which should have been 18 Hz (ratio = 3) instead of 16, or did they really somehow use this one oddball frequency? If so, how was it generated?

Yes, the 16 Hz frequency is stated in the Kellogg catalogs of the 1940s. It was a later addition to the set, as it is labeled HB5, the original set started at 30 Hz (HB1). Only other company that classified it as synchromonic was Stromberg-Carlson, and also added last (R).

Originally each set of frequencies had only four members.

[unbeldi]

Here is yet another update on the frequency ringer codes table, as an image from my master source.

I'll update the text-based table some other time. (perhaps).

[previous updates are deleted]

[unbeldi]

The Federal Communications Commission (FCC) uses the following identifications and definitions for telephone ringers.  This is codified in Part 68 of Title 47 Code of Federal Regulations of the United States which regulate the electrical interfaces and connections of telecommunications equipment with the public switched telephone network.

References:
–FCC, CFR 47, Part 68.312 (1996)
–Reeve, W.D., Subscriber Loop Signaling and Transmission Handbook—Analog, IEEE New York (1992)