NBC Chimes

Originating around 1929, the three-tone trademark sequence used by the National Broadcasting Company (NBC) and its acquiring entities has been among the most-recognized in history. The iconic arpeggio has the distinction of being the first audible trademark registered with the U.S. Patent and Trademark Office (U.S. Reg. TM 0523616; April 4, 1950). Its initial purpose was to cue affiliate radio and television stations to switch program sources, as well as to prompt those stations' announcers to give their local call-letter identification. Originally, the tone sequence was longer, but reduced to just three notes in an effort to attain consistency when played on acoustic "dinner chimes," which were mallet-struck metal plates mounted on a resonator box. To attain an even-more consistent sound, NBC employed the services of Richard Howland Ranger in 1932 to mechanize the sequence. The so-called "Ranger Machines," of which only about a dozen were built, contained a motorized drum that plucked sets of mechanical reeds in the same fashion as a music box. The reeds, of which there were eight per tone (one for the fundamental tone and seven for overtones) were coupled to a vacuum-tube amplifier that provided an electrical output. The Ranger Machines were gradually phased out between 1940 and 1950, supplanted by a fully-electronic vacuum tube circuit developed by NBC engineer Jarrett Lewis Hathaway, the essence of which is described in U.S. patent number 2292757, dated August 11, 1942.

The circuit illustrated below produces the NBC chime sequence. Circuit operation is as follows:

Counter IC U4, controlled by flip-flop U3A, produces 420mS logic-high (+5V) pulses at outputs 1 through 8 in sequence. The sequence commences upon closure of the OPERATE switch, and stops after one counter cycle. The pulses at outputs 1, 3, and 5 are used to trigger the tones G3 (192Hz), E4 (330Hz), and C4 (262Hz) in order, and the pulse intervals at outputs 2 and 4 provide appropriate rests between them. Outputs 6, 7, and 8 provide a delay that prevents restarting the sequence until the final chime fades. The 420mS pulse width is determined by U1's 600Hz clock frequency, that in conjunction with NAND gates U2A and U2B, ensures that the sequence always occurs immediately upon closure of the OPERATE switch, and that the sequence timing is even. The sequence duration is about 4.5 seconds from start to the final decay of the last tone. As shown, the circuit will ignore a maintained OPERATE switch closure. Alternatively, constant cycling may be obtained by closing the 600HZ CAL switch.

The first tone is generated by the section of the circuit consisting of quad operational amplifier U5 and SPST analog switches U6A and U6B. Operational amplifier U5C is configured as an astable multivibrator providing a square wave with a frequency of 196Hz. U5C's output drives the control of SPST analog switch U6A. The squarewave output of U5C is therefore present at U6A's pin 2 at an amplitude corresponding to the voltage at its pin 1. This voltage is provided by U5A. U5A is a unity-gain voltage follower, the input of which is controlled by a 1uF capacitor that charges and discharges according to the presence of the 420mS pulse at U4 output 1. The capacitor's charge rate is controlled by the 1000 ohm series resistor, and the discharge rate by the 1M shunt resistor. A series diode prevents the 1000 ohm resistor from providing an additional discharge path. The voltage at U5A pin 1 therefore rises and falls in accordance with the capacitor voltage; rising rapidly as it is charged by the 1000 ohm series resistor and falling slowly as it is discharged by the 1M shunt resistor. This provides an amplitude-modulated squarewave at U6A pin 2, fed to the input of a three-pole Sallen-Key low-pass filter (U5D and associated components) which removes most of the squarewave harmonics. The resulting waveform at U5D'S output is that of a chime.

Since U5A's output can go very close, but not completely to zero volts, a diode is placed series with its output. The diode stops conducting below its forward voltage threshold, and ensures that the waveform at the filter's input diminishes to zero. After the diode turns off, operational amplifier U5B, configured as a voltage comparator, senses when U5A's output is less than 240mV, turning on analog switch U6B, which shunts the multivibrator's negative input to ground, shutting it off. The shut-off scheme prevents the oscillator from producing bleed-through at the output when it isn't producing its chime interval.

The same scheme is repeated with separate sections for the second (E4) and third (C4) tones. The output of each tone section is summed by operational amplifier U10A which provides a final output of approximately 2.5V P-P, maximum.

Timing, 1uF amplitude-control, audio coupling, and filter capacitors are ±5% accuracy metalized polypropylene types. The IC decoupling capacitors and power-on-reset capacitor feeding the reset input of U3A are X7R ceramic types. The 22uF capacitors for U11 are aluminum electrolytics. Timing, filter, and summing-amplifier resistors are ±1% accuracy metal film types. Resistors elsewhere in the circuit are ±5% accuracy carbon film types. The potentiometers are 15-turn cermet trimmers.

Here is an .mp3 file of the circuit's output.

Here is a composition of unknown origin, circa 1930s, featuring the NBC chime theme.
(Kenny Baker may be the vocalist.)

Pictured below is a wirewrap assembly of the NBC chime generator. In this assembly, the four frequency-adjustment potentiometers were replaced with fixed-value resistors to obtain a compact form factor. The design permits this simplification, since long term stability in these types of oscillator circuits has been proven to be sufficient for an operational period of at least four decades, provided that the frequency-determining components (resistors and capacitors) are sufficently stable.

December 25, 2011
Updated January 17, 2017
Updated January 7, 2022

Text and images ©2011, 2017, 2022  by Arthur Harrison

NBC is a registered trademark of the National Broadcasting Company.

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