US2517819A

US2517819A - Electroacoustic delay system

Info

Publication number: US2517819A
Application number: US24391A
Authority: US
Inventors: Young Charles Jacob
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1948-04-30
Filing date: 1948-04-30
Publication date: 1950-08-08
Anticipated expiration: 1967-08-08

Description

Aug. 8, 1950 c. J. YOUNG ELECTROACOUSTIC DELAY SYSTEM 2 Sheets-Sheet 2 Filed April 30. 1948 III/[IA lNVENTOR ATTORNEY Patented Aug. 8, 1950 Charles Jacob Young, Princeton, N. J., assig'nor to Radio (lorporation oi America, a. corporation of Delaware Application April to, 1948, serial No. 24,391

This invention relates to apparatus and method Q for using acoustic time delay in connection with electric signals, and more particularly to correcting delay distortion in facsimile and program tr nsmission over long wire lines.

If a "wire circuit is more than 50 miles long, some frequencies are delayed more than others, thus causing variations in envelope delay. Such variations must be ke t small to prevent distortion. For instance, in facsimile transmission the variations in delay time throughout the transrnitted band should not exceed the time to scan one picture element, which ma be .12 milli-second (ms) when sending at a rate of about 28' square inchesper minute. The envelope delay over a '750-mile line may avera e 60 his, so it is essential that accurate correction be applied to keep variations'i'n such a long delay time within such a Small limit,

Time dela s selective as to frequency can be obtained with electrical filter networks, but when the delay correction is of the order of milli-secends rather than of micro-seconds, such net-- Works are very elaborate, expensive and troubleso e, not readily adjustable, and must be distributed along the line instead of being disposed in one location.

The primary object of this invention is to use the relatively low velocity of sound to obtain relatively long time delays.

Another object is to provide the entire delay correction at one point of a line with simple and compact apparatus.

A further object is to make such apparatus readily adjustable to various lines or conditions;

Still another object is to obtain the desired quality of reception .by manual adjustment while observing the effect thereof upon the reception.

Other objects and advantages of the invention 2 Claims. (Cl. 179 1) will become apparent as the description proceeds.

Figure 1 shows a delay curve of atypical pro?) i 1 tube is attached to the sound outlet of transgrain circuit which has been corrected to uniform amplitude only for broad-cast use;

Figure 2 is a schematic diagram of a preferred embodiment of the invention;

Figure 3 is a partly structural and partly sche inatic illustration of one of the electro-acoustic devices shown in Figure 2; and

Figure 4 shows a series of selectivit curves for a number of electro-ac'oustic' devices such as shown in Figures 2 and 3.

Similar parts are indicated by like reference characters in Figures 2 and 3.

Referring to the curve of Figure 1 which shows variations in time delay for various frequencies from about 700 to 8000 cycles, it Will be noted that such variations amount to about 2 ms. even though the telephone circuit or wire line was amplitude corrected for broadcast use. Such a line would not be suitable for high-quality facsiinile transmission which may, for exam le, re quire that the delay variation he not greater than .12 ms. 0n the other hand, facsimile transmission does not require a flat amplitude characteris'tic, especially when using sub-carrier frequency modulation.

The arrangement of Figure 2 illustrates an example of apparatus according to the present inveiition, by which a line as above described can have its delay distortion corrected. to be more suitable for facsimile and also more suitable for program use if the line is very long, such as a carrier frequencies and modulators thereof. The

transmitter 10 is connected to a long line H, which be connected at the receiving end to an amplifier l2 and from the output of said amplifier through a wire l3 to a number or electroacou'stic devices or acoustic links indicated generally by reference characters M to l9'inclu-' sive. More of such devices may be added as indicated at 2B, and they may be connected in series to the output of amplifier 12 instead or in parallel as shown.

considering the acoustic link 14, it may comprise a transducer 24 indicated schematically, but comprising means for translating electric waves into'sound waves. One part 34 of a telescope ducer 24. Another part 44 of said tube has its cross-section modified at point 54 to accommodate a transducer for translating sound waves into electric waves. The tube also has a third 3 part as containing material 63 which provides acoustic resistance, such as tufts of felt or other sound-absorbing material. The free end of the telescope tube is preferably left open.

The other acoustic links I to H] are provided with

similar telescope tubes

35 and 39 and 45 to 49 with modified cross-sections at 55 to 59 and terminations 65 to 69 containing acoustic resistance as described.

The acoustic link I4 is illustrated in more detail in Figure 3 as comprising a magnetic field structure if! providing an air gap 'H' in which is disposed a voice coil 12 mounted on the neck of a cone 13. The cone is supported at its periphery by frame i i, a corrugation such as it being provided to permit the cone to have limited movement longitudinally. A pair of leads 1! provides electrical connections to the voice coil l2 which may be connected directly or through a transformer to amplifier l2. One end of the telescope tube 3 3 may be flanged outwardly as indicated at E8 to form a substantially air-tight attachment to the frame M. Part M.- of the telescope tube has its cross-section modified at 54 as above mentioned to provide an elongated space 80 in which is dispose-d a transducer M which may be a ribbon microphone having connecting leads 83 and 84. It should be understood that the microphone BI is disposed in a magnetic field in known manner, the field structure not being, shown in the drawing.

For ease of assembly the telescope tube at point 50 may be divided and the part 3 3 attached to part 44 by suitable flanges 90 and 9!, with the microphone 8! secured therebetween. Parts 34 and 04 should have a sliding fit with little air leakage.

The acoustic link Hi thus provides an enclosed path for sound to travel from speaker as, through tube 34-44, to microphone Bl; the length of said path being adjustable. 'Assuming the velocity of sound in air to be 1100 feet per second, each foot of path will introduce a delay of .909 milli-second. Other rates of delay can be obtained, if

4 ment for eight transducers which are tuned broadly to component frequency bands centering respectively on 1000, 2000, 3000, etc. cycles. The breadth of tuning is such that the curves overlap to an extent which will give a substantially fiat characteristic to the combined bands over the signal range. Loud speakers are more efficient when thus tuned, so there is less loss than if'each speaker had a flat characteristic. The number of component frequency bands is not fixed; if desired they may be spaced somewhat irregularly, and their breadth of tuning may be altered, depending on the nature of the delay correction required in a given case.

desired, by filling the device It with other sound carrying media, including liquids, or by using solid sound conductors.

Referring again to Figure 2, each of the other acoustic links l5 to l9 inclusive may be con structed similar to link It. The output leads 84 to 89 from the respective microphones are connected by flexible leads to a common terminal 02 which is connected to the input of an amplifier 93 and thence to receiving or recording apparatus 94. Such apparatus may comprise all apparatus required for recording facsimile; including demodulators and arrangements for restoring frequency bands to their proper relative positions in the event that the'transmission was performed by the use of'separated and transposed frequency bands; The apparatus 94 may also comprise all apparatus required for deliveriijng the received signals to a broadcast transmiten 7 Y The electrical connections in Figure 2 are shown by a single-line diagram, it being-understood that the return connections are common and preferably grounded.

. Each of the transducers 24 to 29 may be made selective-to a component band of frequencies contained within the frequency band of the signals. Such selectivity may be obtained by the naturalresonance of the cone structure 13 in Figure 3, or av loud-speaker diagram may be tuned by means well understood in the art. Figure 4 illustrates a possible tuning arrange- Selectivity may also be-obtained by tuning the receiving transducer such as microphone 8|; or both transducers at opposite ends of an acoustic link may be tuned to the same band of frequencies. Acoustic filters may be substituted for the telescope tubes.

The operation of the arrangement shown in Figures 2 and 3 is as follows. The signals in line i I are amplified at l2 and divided into component frequency bands by the devices M to l9 and transmitted as sound bands through the parallel, but separate, acoustic paths provided by said devices. After translation back into bands of electricfrequencies at points 50 to 59, said bands are combined in amplifier 93 to obtain the original signals with relative time delay of component bands. If-delay distortion is present at the receiver 94:the acoustic paths provided by the links M to l9 are adjusted in length by sliding the-movable parts 44 to 49 of the telescope tubes in or out of the fixed parts 34 to 39 respectively. The effects of these adjustments can be observed at the receiver 94 or otherwise communicatedto the person manipulating the apparatus. Inasmuch as each telescope tube has an effect on only one component band of frequencies, it can be independently adjusted until that bandis properly received. Thus the band which is delayed the least in transmission over the line- I i will have a relatively long delay added by. extending its telescope tube. On the other hand, the band which is most delayed in transmission will have its tube almost completely telescoped so as to add a minimum of acoustic delay. Proper adjustmentwill result in adding sufilcient acoustic delay to each component band so that the sum of the line delay and the acoustic delay will be substantially thesame for all of the bands, thereby correcting the delay distortion and giving high-quality reception of signals.

In Figure 2 various groups of the telescope tubes areshown as having different lengths. Fo'r'instance, tube 34 is the longest and the group of tubes 31 to 39 is the shortest. This is convenient because a greater range of maximum to minimum delay is obtained. However, all of tionto delay systems and, therefore, is notv limited to facsimile systems or to the apparatus selected as an example.

;What is claimed is:

1. In an imagetransmission system in which image signals aretransmitted over a communication channel which causes different delay times for difierent frequencies of the image signals, a receiving'station for said image signal at the receiving end of said transmission channel, apparatus at said receiving station to compensate for the different delay times for difl'erent frequencies comprising a plurality of sound reproducers each tuned to a component of said image signals, a microphone for each sound reproducer,

means connecting each sound. reproducer to its associated microphone and providing a sound path of'adjustable length therebetween, each of said path providing meansbeing adjusted to provide a'longer delay time locally for the less delayed image signal component and a minimum image signals are transmitted over a communication channel which causes, different delay times for different frequencies of the image signals, a

receiving station for s'aidimage signal atthe receiving end of said transmission channel, apparatus at said receiving station to compensate for the different delay times for different frequencies comprising a plurality of sound reproducers each tuned to a component of said image signals, a microphone for each sound reproducer, a telescope tube connecting each reproducer to its associated microphone and providing a sound path of adjustable length therebetween, each tube being adjusted to provide a longer delay time, locally, for the less delayed image signal component and a minimum of local delay for the most delayed component of the image signal, and means for combining the output of said microphones to form the original image signal so that the sum of the line delay and the acoustic delay will be substantially the same for all of the components.

CHARLES JACOB YOUNG.

REFERENCES CITED The following references are of record inathe file of this patent:

UNITED STATES PATENTS Number Name Date 2,098,956 Dudley Mar. 16, 1937 2,209,326 DI-Iumy et a1 July 30, 1940 2,230,836 Hammond Feb. 4, 1941 2,237,298 Bauer Apr. 8, 1941 2,292,745 Davis Aug. 11, 1942 2,318,417 Phelps May 4, 1943 2,403,231 Parisier July 2, 1946 2,403,232 Parisier July 2, 1946