 Welcome to the session of the multi-channel audio or 5.1 channel Dolby digital system. Learning outcome of the session at the end of the session students will be able to differentiate in between mono and the stereo audio they will understand what is the surround sound they can explain 5.1 channel Dolby digital system they can distinguish between the surround sound and 5.1 channel Dolby digital system before proceeding let me ask you one question how the analog audio is converted into the digital audio you can pause the video think about the question and write down your answer in your notebook after writing the answer you can resume the video to see the answer let me give you the answer there are three processes in order to convert the analog signal into the digital one that is a sampling quantization and encoding the sampler will accept the input analog signal and convert into the discrete time signal the quantizer will round off the sampled value of discrete time signal into the nearest quantization level and these sampled values or the amplitude are converted into a digital data or the binary bit streams by using the encoder what is the mono and the stereo sound the recording used in the earliest sound movies is called as a monoral or simple mono the mono means that all the sound is recorded onto the one audio channel which is typically played onto the one speaker in case of the two channel recording the in which the sound is played on the both side of the speaker which is also called as a stereo a stereo recording is also referred as a stereophonic recording in the figure you can see that the mono audio is recorded from the single microphone and it is played onto the single speaker if you give the mono audio into the two speakers they both the speakers will play the same audio whereas in case of the stereo recording the two audio recorded for the two separate channels and when they are played onto the two speaker they will sound different. So, the two channel sound is a standard format for the home stereo receivers televisions and the FM broadcast systems which are produced with the two microphone setup at the live event when you listen these two channels on the separate speaker it will recreate the experience of being present at the live event as shown in figure there are two types of the recording plus one is used for the mono recording and this is used for the stereo recording in case of the mono recording there is a single microphone is used that is why it has only one mono input and another is a ground whereas in case of the stereo recording there are two channels one is for the left another is for the right and one is for the ground. So, basically the audio signals are routed through the single channel in case of the mono audio whereas in case of the stereo recording the audio signals are routed through the two or more than two channels in order to simulate the depth or the direction perception like in the real world. The cost of the mono recording is less expensive whereas in case of the stereo recording it is more expensive. In case of the mono recording it is very easy to record and require only basic equipments for the mono recording whereas in case of the stereo recording it requires a technical knowledge and skill to record apart from the equipment and it is it is important to know the relative position of objects and the events. Generally the mono recording is used in case of the public address system radio talk shows, hearing aids, telephones, mobile communication, some AM radio stations whereas the stereo recording is widely used for movies, televisions, music player, FM radio stations. Let us see how the mono recording and stereo recording sounds. In order to experience this please use the headphones. Let us see how the stereo recording sounds on to the surround sound. The term surround sound technically refers to the specific multi-channel system which is designed by the Dolby laboratories. It is most commonly used as a generic term for the home theater multi-channel sound system. There are special microphones that will record the surround sound. By picking up the sound into the three or more directions the recording is done. But this is not the standard way to produce the surround sound track. Almost all movies surround sound tracks are created in a mixing studio. Sound editors and the mixers take number of different audio recording such as dialogue recorded on the movie sets, sound effects recorded in a dubbing studio or which are created on a computer, a music scores and decide which audio channel or channels to put on them. A stereophonic sound. In a stereophonic sound system three or the five channel drew the speakers behind a movie screen. The popular four channel system included in one channel driving a speaker on to the left, one channel driving the speaker on to the right, one channel driving a center speaker and one channel driving surround speaker along the sides and the back of the theater. Some systems boosted the five separate channels behind the screen and one surround channel. So as you can see from the diagram stereo sound consists of one left speaker, one right speaker, one center speaker and the surround sound. In the in the movies the most of the sound is recorded on to the front channel so that the words seems to come from the screen. When an actor speaks on to the left side of the screen the dialogue sound comes from the left speaker. When the actor speaks on the right side the sound comes from the right speaker. Most dialogue is also channeled to the center speaker which will serves as an anchor or focuses the sound on to the screen. The rear track are typically reserved for the effect sound such as ambient background noise or the voice coming from off screen. Now what is a Dolby digital? A Dolby digital is advanced the form of the digital audio coding that make it possible to store and transmit the high quality digital sound far more efficiently that was previously possible. The first it is used in the movie theaters in 1992. It is a result of decades of experience by the Dolby laboratories in developing a signal processing system that will exploit the characteristics of the human hearing. A Dolby digital audio is audible from a laser disc, DVD video disc, DVD ROM discs which are used for the computers, digital cable TV, broadcast satellites and a digital broadcast TVs. The generally the discs are marked with the logo which is shown into the figure 5. Now what is a 5.1 channel Dolby digital? A Dolby digital programs can deliver a surround sound with the 5 discrete full range channels left channel, center channel, right channel, left surround and right surround channel and plus it has a 6th channel for the low frequency effect that are more heard into the movie theater as it covers only about the one-tenth of the audible bandwidth of other channel the LFE is referred as a 0.1 channel. As you can see that so this device may be the DVD player or the set-top box which will provide the sound to the Dolby decoder and the Dolby decoder is connected to the speakers. The decoder will act as a 5.1 channel driver which will drive the left speaker so this is right speaker a center speaker which are generally used for the dialogues come these are the surround speakers which are used for the background noise and LFE speaker is used for the low frequency effect. Dolby digital bit stream deliver a full frequency bandwidth main channel from 3 hertz to the 20 kilo hertz and it also deliver a limited frequency bandwidth that is LFE channel from 3 to 120 hertz. Current a Dolby digital encoder accept the word length of 16 18 or 20 beats at the sampling rate of 30 to 44.1 or 48 kilo hertz. The Dolby digital algorithm provides 6 24-bit resolution and a future version may extend up to the sampling rate to 96 kilo hertz. The logo Dolby digital on a DVD video disc or the other program source indicate that it has one or more Dolby digital encoded soundtrack but it does not indicate the number of channels. In order to identify how many channels are present in that we can use a symbol such as this is used for the mono this is used for the stereo this will identify the 2.1 surround system and this will identify 5.1 channel Dolby digital. Basically the surround channels in Dolby digital ranges from 20 hertz to 20 kilo hertz which is stereo in case of the Dolby surround it is limited to mono from the range of 100 hertz to 7 kilo hertz. Low frequency effect channel for the Dolby digital is available in the range from 20 hertz to 120 hertz whereas in case of the Dolby surround it is not available. The panning options in case of the Dolby digital the sound flows in a multiple way whereas in case of the Dolby surround it is it flows from left to right right to left right to back and back to front. In order to see the effect of 5.1 channel please use the headphone to see the video. This is the left channel we have the center channel over here is the right channel this is the left surround channel the next sound you hear is the LFE channel in this way in this way the 5.1 channel is heard by the listener. So, these are the references for the session. Thank you.