The word "monophonic" is derived from two Greek roots: mono, meaning "one," and phone, meaning "sound." Thus, the combined roots mean "one sound," or as we use it in hi-fi parlance, "one-source sound" ( one amplifier channel, one speaker system). The word "stereophonic" is also derived from two Greek roots: stereos, meaning "solid," and phone, meaning "sound." Thus "stereo phonic" denotes "solid" or three-dimensional sound-sound coming from different sources, at different locations, with different volume levels ( two or more separate channels). Other terms used in describing monophonic and stereophonic sound are "monaural" and "binaural." Since mono means "one" and auris means "ear," the root of "monaural" is "one-ear." The term "monaural" was used for some time to describe what we now call "monophonic." However, due to the limited scope of the word "monaural," the word "monophonic," meaning one channel from start to finish, is now used instead.
The other term often associated with hi-fi systems is "binaural." This word is derived from two roots: bi, meaning "two," and auris, meaning "ear." Thus, "binaural" might be literally translated as "two-ear sound:'' The association of binaural sound with stereo sound arises from the fact that the binaural system was an early method of obtaining stereo effects. The basic idea behind binaural techniques is the fact that we, as human beings, have a sensation of direction in the sound we hear because our two ears work separately. The sound sensation to the brain from one ear is kept separate from the sensation from the other ear, and both are transmitted to the brain through separate auditory nerves. The brain compares the two auditory signals received by the ears and, from the differences between them, determines the direction from which the sound came. The theory behind binaural systems is that if two sound signals, which would be heard by a pair of human ears at the source, are trans mitted, reproduced in the same relationship, and applied to the corresponding ears at a remote location, all the directional effects of direct live listening will be preserved. Binaural systems are discussed later in this Section.
MONOPHONIC SOUND
In a monophonic sound system, the sound usually emanates from only one location when being reproduced. Dual speakers, large horns, and the location of the speaker system in a corner of a room can be used to spread the sound so that it is difficult to place the sound source at one point. However, there is no stereo effect until two separate channels utilizing separate microphones, amplifiers, and speaker systems are used.
While monophonic sound may be very pleasing to listen to, stereo phonic sound has advantages for the music lover that cannot be equaled by monophonic systems.
STEREOPHONIC SOUND
Modern stereophonic sound, with its directivity and depth properties, adds the third dimension to the sound. It makes clear distinction between foreground, middle, and background, as well as be tween right, middle, and left sound sources. Stereo has thereby been able to produce a greater amount of clarity and instrumental sound color than monophonic sound. The reception of complex sound sources is also made possible. The directional effects and small time delays from echoing and reverberation of the elemental sounds of music are separately channeled from source to ear, thus providing high-quality simulation of live music.
History of Stereo
The idea of stereophonic sound is not new. Ever since electrical sound systems were first devised in the form of telephone circuits, engineers realized the spatial characteristics of the reproduced sound are important. It is known that as far back as 1881 experiments with binaural sound were being made. In that year, performances from the Paris Opera were transmitted via a pair of tele phone lines to the Paris Exposition. Each telephone line constituted a channel, with the two channels corresponding to the two ears of the listener.
In the early 1920's, shortly after standard a-m broadcasting began in the United States, experiments were made with dual radio broad casts. However, the fact that few listeners had two receivers led to the discontinuance of these broadcasts.
On April 27, 1933, engineers of the Bell Telephone Laboratories transmitted the music of the Philadelphia Symphony from the Academy of Music in Philadelphia, to Constitution Hall in Washing ton, D.C. A three-channel system was used in this experiment.
Perhaps one of the most spectacular steps forward in the field of stereo sound reproduction was the introduction in 1940 of Walt Disney's Fantasia. This was a movie for which the accompanying sound approached true stereo characteristics. The sound was re corded using a large number of microphones, each feeding a separate recording channel. In the theater in which the picture was shown, each of the many speakers was placed in the same relative position as a microphone in the recording setup. Speakers were mounted in positions all around the perimeter of the theater, even in the back. To the audience, the sound could come from any direction, including the sides and the back. This production was a huge success, due in large degree to the realism and unusual nature of the sound. Today, practically all movies use wide-screen projection along with some form of stereo sound, but none approach the number of sound channels used in Fantasia.
All the foregoing progress was confined to the commercial theater or communications business. For a long time, people in the sound business realized that, until a simple and convenient method of recording stereo sound on discs was perfected, stereo could not be introduced into the majority of homes. In 1931, A. D. Blumlein obtained a British patent on a system for cutting and reproducing two-channel recording discs. Later, in 1936, Bell Laboratories engineers A. C. Keller and I. S. Rafuse obtained United States patents for two-channel disc recording. However, these early ideas did not blossom into commercial reality because materials, methods, and techniques had not advanced to the point at which production and distribution were feasible.
In 1952, another pioneer, Emory Cook, developed binaural disc records using two normal pickups spaced about two inches apart.
The outer portion of the record surface was used for recording one channel, and the inner portion for the other channel. Because two pickups were needed, there were tracking problems with this arrangement and it never gained wide acceptance. It was not until single-groove systems were developed that stereo discs became practical.
Single-groove stereo records became a reality to the public in 1956, when the London ( British Decca) Company developed its system. The next year the Westrex stereo-disc system was introduced in the United States. (The Westrex system has been standardized in this country, and it is the subject of detailed discussions in later Sections.) Since the advent of stereo discs in large quantities, other components of stereo systems have followed rapidly.
Today there is available to the public a wide variety of stereo tuners, amplifiers, speaker systems, and other accessories, all of which are discussed in the following Sections.
STEREOPHONIC SYSTEMS
In Section 6, it is shown that speakers have directivity which varies with frequency. This means that the frequencies heard from a speaker vary with the position of the listener with respect to the speaker. To put it more simply: If you're in front of the speaker, you hear highs, and if you're off to the side, you don't. This is a kind of spatial distortion. But there are several other kinds of spatial distortion which engineers strive to overcome with stereo systems.
If the foregoing type of distortion is eliminated, and all the frequency components of the sound are radiated equally in all directions, all of the reproduced sound will still emanate from the point at which the speaker is located. The fact that the instruments or voices which originally produced the sound were widely separated spatially means that the single-point speaker source is not realistic.
In other words, there is distortion; one might quite accurately call this "apparent-source direction distortion." For example, if we attend a concert and don't sit too far back from the stage ( or orchestra pit), we will be clearly aware that the piano is, say, to our left, the violins are to the right, and the drums possibly in the middle portion of the stage. Of course, our eyes tell us these things, but, when the orchestra begins to play, our ears will also tell us. As explained more fully later, the human auditory sys tem is keenly directional, with the slight difference between the sound components which enter the two ears indicating to the brain the direction of the source. If, in our example, we don't hear the piano, the violins, the drums, and the other instruments as though they are coming from their respective directions, we are not hearing an exact simulation of the source material. In this case, we cannot experience the realism of high fidelity. True stereophonic sound overcomes this lack and restores to the listener a sense of the direction of the original source of the instruments creating the sound.
Recording Techniques
Realism is preserved in stereophonic sound by picking up and re producing sounds at different depths (distances) as well as from different directions in proper or exaggerated relationships. Delays to make it seem that one sound is deeper ( farther away) in relation to another may be introduced artificially within the recording equipment. This method is known as delay stereophony.
An old trick in the recording business is to add an artificial second channel having the same material as the first, but with a time delay introduced in it. This provides an artificial stereophonically enhanced sound. The most effective time delay is believed to be between 8 and 12 milliseconds.
Variations in intensity of the same sounds will provide a stereo phonic effect if reproduced at properly spaced positions within the room. This is easily demonstrated by the ping-pong effect, where the sound of a ping-pong ball bouncing back and forth is reproduced.
Recording music from an orchestra is a refined process of organization, placement, separation, emphasis, combination, equalization, reverberation, limiting, rolling off, takes, inserts, overdubs, and editing. Usually the orchestra is arranged in a circular manner around the conductor, spaced in a manner suited to recording-which is different from arrangements made for performing for an audience.
There may be an isolation booth for a singer who hears ( and keeps time with) the orchestra through a low-level speaker or headphones.
Each musical instrument or group of the same instrument may have one or more microphones at strategic locations to take best advantage of the output of each instrument or instrument group. The object is to separate sounds and to reduce "leakage," which is any sound picked up by a microphone but not intended to be picked up. Separation is essential to achieve clarity, or emphasis, and balance control for the production of a good record. Low-volume instruments such as flutes must be separated from louder instruments, or their presence will be obscured.
For these reasons, the musicians are sometimes separated by space or by sound-reflecting walls. Microphones are placed closer to softer musical instruments to increase their relative outputs. On occasion an instrument such as a bass viol is placed behind a V-shaped wall to prevent its high-level sound waves from spreading around the room. Dozens of microphones may be used with artful placement to develop the desired separation, presence, or intimacy. Hard music, such as rock-and-roll, requires closer "miking" and more micro phones. For violins, several microphones may be placed over a group and the heights varied to change the effect during the program.
All of the audio signals thus obtained are combined in the recording room on tape. The output of each microphone is adjustable on the control engineer's panel so that the separate sounds can be narrowed, broadened, increased, decreased, and altered in frequency response, according to the art of creating a beautiful and dynamic combination of the input elements of sound. It can be seen that the engineer must be a talented artist, as well as a good technical man, to be able to produce a creative "mix." There is a striking difference between the sounds in the orchestra room and the sound output of the final recording.
The control room is essentially concerned with mixing, emphasis, reverberation, and limiting. Since each mike has its own control, the mix is accomplished in the control room, not in the orchestra room nor by the conductor. Emphasis or de-emphasis is the increase or decrease of the relative amplitude of selected portions of the AF spectrum used to enhance the sound of an instrument or to delete its lesser outputs by means of electronic equipment. High frequencies are boosted to bring out the overtones of violins; this makes them "brighter." The midrange frequencies are often boosted in the re cording of guitars, drums, and percussion instruments to create the hard sounds of rock-and-roll records.
Reverberation is applied by the electronic equipment to sustain musical sounds. This has the same effect as enlarging the room containing the orchestra or the output of the specific instrument so treated. It is said that this adds excitement but it is often used to "cover up" poor musical performance. Reverberation of sounds can also be created in specially designed, small echo chambers or with electromechanical devices.
Limiting is provided by automatic electronic equipment designed to instantaneously reduce volume peaks that exceed the capabilities of the electronic systems used. This protects the recording against damage and distortion due to overloading.
The take is the actual recording of the live music, mixed, emphasized, balanced, etc.
The insert is a take of that part of the program that is desired to be improved. This is spliced in place on the original master tape.
Good inserts are hard to achieve because it is difficult to repeat the same texture of performance from any group at a separate time from the first or "master" play. Tempo, mix, balance, and emphasis must come together to sound the same.
The overdub is a technique which is used to apply the vocal performance onto the master tape after the orchestra is recorded. The master tape of the orchestra performance is played back while the vocalist accompanies this playback. The orchestra playback is wired directly from the recorder to another tape deck to produce a copy of the orchestration while the new vocal performance is added to the vocal track of the copy. This type of operation is used to improve an original poor vocalization and/ or to reduce the cost of recording.
However, there must be available a master tape of the orchestra without vocal. Two tapes are usually made: one with the vocal and one without it. In order to achieve this, an isolation booth is required for the vocalist. Overdubs are also used to create special effects, such as doubling the number of strings in an orchestra.
Editing is the elimination of lesser parts of the performance or reduction of length to fit standard record or tape-cartridge sizes. The best parts are pieced together to make a finer performance, or a se lection may be shortened, or "clams" and studio noises eliminated.
Following the editing process, master records or master processing tapes are made from the edited master. The master records and tapes are used to reproduce records, tapes, and tape cartridges at a small fraction of the cost of making the original master.
Channels In the course of our discussions of stereo, we often use the word "channel." As applied to this subject, "channel" means a separate and distinct path for an electrical or acoustic signal. For example, an ordinary monophonic audio-frequency amplifier is part of a single channel, because it can carry only one signal at a time. If we at tempt to use the amplifier for more than one signal at the same time, the two signals will interfere with each other and cannot be separated at the output. In non-stereo systems, only one channel is used.
One microphone picks up the sound, which is amplified in one amplifier of conventional design, and fed to one speaker system. In a two-channel. system, two microphones are used; the sound signal from each microphone is passed through a separate amplifier sys tem, so that one sound signal has no effect on the other. As we shall see later, three amplifiers are not always necessary for three channels, because there are techniques for providing three or four electrical channels in two amplifiers, although the effect is not exactly the same as though three or four complete amplifiers were used.
One or two of three or four channels can be simulated in a two channel amplifier by additive or subtractive methods to gain the practical effect of a third or fourth channel.
Multichannel Systems
As was said, stereo sound is "solid sound," that is, sound which, even though it is artificially reproduced, seems to the listener to come from the same directions that it would if he were present at the source. One theoretical way to reproduce stereo sound would be to mount speakers all around the listener. Then we would design the system so that the speaker or speakers in the respective directions are activated at the right times with the right portions of the sound program.
Let us imagine that one listener is listening to a live performance of an orchestra in a concert hall, and a second listener is in a similar position in a similar room in a remote location to which the sound is being transmitted by a stereo system. If the piano is playing directly in front of the concert-hall listener, then the stereo system should reproduce the piano primarily from a speaker directly in front of the remotely located listener. If another instrument is located to the left side of the local listener, the listener in the remote location should hear it from the speaker or speakers to his left, and so on.
This kind of arrangement is employed in elaborate systems such as were used for Fantasia, and, to a lesser degree, is being used today in movie theaters and in home systems with four discrete or matrixed channels. The method is depicted in the diagram of Fig. 2-1. Theoretically, for "perfect" stereo effect, we would need an in finite number of speakers, so that there will be no "holes" in our reproduction. However, as we shall see, this is no problem because the same effect as additional speakers can be obtained by "sharing" between adjacent units. As can be seen from this illustration, such an elaborate system is very expensive, and there are many electrical problems that are not immediately apparent.
Binaural System
The foregoing method operates on the principle of providing actual sound from all the directions involved. Obviously, this method is far too complex to be practical for home use. Other methods, including the stereo methods now in general use, provide a sense of direction and depth even though as few as two channels are used.
Fig. 2-1. An elaborate stereo system.
The binaural system illustrated in Fig. 2-2 is an example of a two-channel system. Rigidly correct binaural systems use an actual "dummy" head at the source location as shown. A microphone is mounted at each ear of the dummy. Thus each microphone should "hear" exactly what each ear would hear if the dummy were a human being. The whole head is used, instead of just placing microphones at the two ear locations, to simulate the effect of the human head on the sound waves. Each microphone is connected to a separate amplifier, and two audio signals are transmitted to the listener's location. There, each channel terminates in an earphone on the same side of the head as the source microphone for that channel. If we assume that the microphones, amplifiers, transmission systems, and earphones have a high degree of fidelity, each ear of the listener should receive the same sound as the corresponding ear of the dummy at the source. The listener's auditory system puts the two sounds together to provide indication of the directions and depth from which the sound components are reaching the dummy at the source.
Fig. 2-2. Simple representation of a binaural system.
Multiple-Channel Stereo
Binaural systems give a good degree of realism, but most people don't want the inconvenience of wearing headphones. That is why the stereo system of today uses speakers. However, the two channels of a stereo system operate on the principle that if the two speakers are located the same distance apart as two pickup micro phones at the source, the outputs of the speakers will combine in such a way that the listener is given the sensation of sound direction and depth comparable to that of a listener at the source. A simple diagram of this kind of stereo system is shown in Fig. 2-3.
The system is designed to have each speaker reproduce the sound which is present at its corresponding position at the source.
Speaker and Microphone Placement--The obvious advantage of stereo reproduction is that, added to other high-fidelity characteristics, it gives the listener greater enjoyment through the sensation of the relative direction, depth, and intensity of the various parts of the program. It affords a greater satisfaction because it gives an added dimension of realism.
However, there is difficulty in locating the listener in the same relative position as the "assumed listener" at the source. The listener ( at the source) is considered to be sitting along the center line of the hall, and about equidistant from the two microphones. But suppose the remote listener's setup is such that he can't place himself at the same distance from the speakers as the assumed listener is from the microphones at the source. It has been found that small differences have little negative effect, but if the listener is sitting directly in front of one of the speakers, he hears practically nothing from the other speaker, and his sensation of sound direction is spoiled. Therefore, proper positioning of equipment and the audience is an important consideration.
Fig. 2-3. Simple representation of the stereophonic principle.
Another important consideration is how far apart the microphones should be from each other when recording, and how far the speakers should be from each other when reproducing the recording. It is logical, and generally accepted, that the speakers should have the same spacing as the microphones. To understand better the problem of deciding what this spacing should be, let us consider the extreme cases. If the microphones and speakers were as close as they could be to each other, they would appear as one, and we would have a monophonic system. But let's assume that they're just a few inches apart. Then if the listener stands twenty feet away they will still effectively appear as one source. If he moves closer, until his distance from the speakers is comparable with the distance between them, the stereophonic effect is reestablished.
Now suppose that we spread the microphones and speakers very far apart. In the extreme, they would be so distant that they would not pick up sound or reproduce it so it could be heard. When they are closer together, but still widely spaced, the reproduced sound would be heard from two separate and distinct sources. This effect is known as the "hole in the middle," because nothing seems to be coming from the area between the two speakers, whereas the center of the orchestra would be at this point at the source.
To overcome the "hole-in-the-middle" effect, stereo may employ any number of channels. An arrangement may provide for pickup of an orchestra with six or even twelve microphones proportionally spaced around the orchestra. A corresponding number of amplifiers are used to reproduce the program. The amplifiers feed the speakers which are placed at the points where microphones were located in the original recording. Certain motion-picture extravaganzas have used these techniques.
The complete stereo system just described produces a very realistic stereophonic effect, imparting breadth, depth, and even height to the sound output. In addition, many detailed directional effects are possible. However, three or four speakers with two channels can provide nearly the same effect. Improved techniques of recording microphone placement, channel intensity control, and speaker placement--have made it possible to approach the quality of discrete four-channel stereo with two channels ( and a third and fourth speaker). This is accomplished by combining certain portions of the right- and left-channel signals and feeding the resulting signals to the other speakers.
To provide signals for the other two speakers, two additional microphones, properly positioned at right rear and left rear, may be employed at the sound source. The signals from the added micro phones are mixed with the signals from the left and right front microphones, and the resultant signals are fed to the left and right channels, respectively. Thus, the left and right channels contain a certain proportion of the effects of the outputs of the other microphones.
The signals at the outputs of the preamplifiers may be divided so that the majority of the output from either channel is fed to its respective left or right speaker, but a small portion ( less than a third) of the output of each channel is fed to the rear speakers. Therefore, the rear speakers have a lower input level than do the other two.
Only a small difference in the intensity of the sound from the two front speakers is sufficient to give the desired directional effect.
However, the stereo effect is produced over a greater area by the other speakers. The listener can sit nearer or farther from the speakers, or move more to the right or left without losing the stereo effect.
The illusion of a "curtain of sound" spreading across the room is developed.
It can be seen from the foregoing that adding the third and fourth speakers involves more than parallel connections. Technical instructions for setting up three- and four-speaker stereo are given later in this guide.
Four-Channel Stereo
Stereo systems with four discrete channels are now available.
Such a system has four completely separate channels, with four separate input signals passed through four preamps, four amplifiers, and at least four separate speaker systems. To many music lovers, the dramatic effect that can be produced with well engineered and matched equipment and expert installation is worth the additional cost of such a system. In addition, excellent low-cost four-channel systems are becoming available, with the result that four-discrete channel sound (sometimes called surround sound) can be enjoyed by almost every family.
These systems should not be confused with four-speaker systems that reproduce derived or matrixed four-channel sound. Similar methods of using derived sound have been suggested for removing the "hole-in-the-middle" effect, discussed elsewhere in this Section.
Four-channel matrixing presently is accomplished by several methods. The simplest is the Dynaco system that permits three- or four-directional (simulated channel) information to be carried on a presently compatible two-channel disc, tape, or fm broadcast. The effect of four-directional sound playback can be enjoyed on a conventional stereo system with the addition of only one or two loud speakers and simple interconnecting components and cables.
The advantages of this system outweigh its disadvantages. Present stereo recordings can be used in this system. Some present records inadvertently have extra channel information because of peculiarities of the recording system, which actually produces left and right signals which can be vectorially added or subtracted to create a matrix of signals. This process provides several additional distinguishable signals, two of which should be discrete signals and one or two of which may be ambient signals or sub-signals--or you may call them derived.
This kind of matrixing can be accomplished purposely on any two-channel stereo medium by recording the signals from the rear two microphones 180° out of phase with the recording of the signals from the two front (main-channel) microphones. This operation would be carried out while recording four discrete channels of in formation in preparation for encoding the four channels to provide two channel matrixes for fm stereo transmission or recording of a stereo disc or tape.
By decoding whatever portions of these additional signals ( L-R and R-L) that exist in any two-channel stereo medium, one or more additional channels may be derived from the unaltered original program material containing recorded information 180° out of phase.
When the signals from rear microphones are fed to the encoder 180° out of phase, these signals are subtracted rather than added, thereby creating additional signal components. They are supposed to be added for mono, and, therefore, under this form of processing, these rear signals will not show up in a mono playback, but will reduce whatever portions of the rear signal components are in the sum of all the sound signal components in the mono playback. This reduces the compatibility of any medium so recorded. For mono, in some cases, the program quality will be reduced to an undesirable degree.
Regardless of which kind of processing of the original program signals is used, the effect of three- or four-channel sound can be derived to a sufficient extent that acceptable and enjoyable simulated three or four-channel stereo can be produced from an existing two-channel system. A description of the Dynaco equipment and installation in formation is given elsewhere in this guide.
The Electro-Voice system for reproducing four-channel stereo sound utilizes all phases of any four-channel recording by encoding four separate input signals into two complex matrixed signals which can be stored on tape, processed onto discs, or broadcast over fm stereo.
A portion of the signal components of each of the four original discrete channels is encoded in each of two matrixes in different proportions according to the amplitude and frequency of the components in relation to phase ( or bearing) of the original discrete but composite signals. Therefore, each matrix contains signal components from each of the original four discrete channels, and the two matrixes contain all of the information from the original four discrete channels but with less separation.
At the receiving or listening end of the system, the two matrixed signals must be de-matrixed, or decoded, by an opposite process, thereby deriving the original signal components for each of the four original channels of information with acceptable fidelity. The distribution may not be identical, but the overall quality should be equal to that of the original.
In similar systems used by other manufacturers, the best distribution of power to the four (simulated) channels at the output may be adjusted with individual gain-control circuits by means of techniques and equipment described elsewhere in this guide.
In these systems, there is a reduction in the degree of separation available in the four-channel sound output, but the accumulated separation among all four channels should be equal to the separation between the two channels in the system in which the signals are processed. This means that if you can achieve 30-dB separation be-tween two channels on a medium such as tape, you should be able to obtain at least 15-dB separation between any two of the four channels decoded from two properly matrixed channels on this medium.
COMPATIBILITY
It is likely that monophonic equipment will be used to some degree for some time to come. It is therefore important that stereo systems be compatible with monophonic systems. By "compatible" we mean that one should be able to play a monophonic record or broad cast tuner on a stereo system and be able to play a stereo record or tuner on a monophonic system. If any part of the system-the record, the tuner, or the amplifier-is limited to monophonic performance, the result is monophonic reproduction. However, we say a system is "compatible" if monophonic and stereophonic reproduction is practical and normal in the same system. Design for compatibility has been mainly one involving records, phono pickups, and broad casting techniques; it is relatively easy to combine or separate the two channels of the remainder of a stereo system to produce mono phonic or stereo output.
The Westrex stereo recording method, now standard, is designed for near-perfect compatibility in record reproduction, as will be shown in Section 3. Unfortunately, in practice, true compatibility does not yet exist because of mechanical limitations of the older monophonic pickups. As a result, stereo records should never be played with monophonic pickups. However, monophonic records can be played with stereo pickups, and often the reproduction is more pleasing than when the record is played on a monophonic system. Of course, there is no stereo effect. Details about compatibility and difficulties in attempting to play stereo records with mono phonic pickups are discussed in Section 3.
Compatibility is the chief obstacle to the development of commercially successful four-channel sound. All problems in this regard could be solved eventually, given enough time and money. But, until the cost and time of such developments can be reduced and agreements are made between the various segments of the industry and the Federal Communications Commission, four-channel sound is likely to continue to develop in the present direction: specialized according to use rather than compatible in all regards.
The main reason for the present problems centers around the success of the disc in its present form and the difficulty and costs involved in developing a new process and system for making and using discs that could be played on monophonic and two-channel equipment as well as on four-channel equipment. It is interesting to note that arrangements have been proposed that would provide compatibility between two- and four-channel discs, but without the capability for monophonic playback. This would be a satisfactory solution for the future, but at present has the disadvantage that much home equipment is still monophonic.