1Phys 1240: Sound and MusicLAST: wind instrumentsTODAY: Jeff Merkel on “sound in thestudio”NEXT: Wrapping up - wind instruments,voices, sonograms, …No new reading! No more assignmentsafter CAPA #9 (tonight)Final Exam : Sat 7:30-10. See web!Chris and David’s office hoursaccordingly rescheduled:Chris Tuesday (usual time, 2-4)David Tuesday (usual time, 4-6)Digital AudioRecording, Mixing, andDelivery“Capturing” audiowaves withmicrophones• Microphone Transducessound, turning acousticpressure waves into anelectrical signal in a verysimilar way as our ears• We now can record voltage asa function of time.• We’ve seen this voltage vs.time many times now, which isa waveform.• But how do we actuallyrecord?RecordingLots of different ways to recorddigital data, but the most commonfull-fidelity is PCM, or Pulse CodeModulationAudio CDs, .wav, .aiffA/D (Analog to Digital) conversionBasically “connect the dots” on agrid systemTimeAmplitude2Digital SamplingWe now need to decide how often to“sample” the voltage signal.timeConcept Question 1I have a 220 Hz wave and a1000 Hz wave. Which one needsa higher (more samples persecond) sample rate to beresolved?A)220 HzB)1000 HzC)They use the sameNyquist TheoremNyquist’s theorem states that ananalog signal wave form can bereconstructed without error fromsamples taken at equal timeintervals. The sampling ratemust be greater than or equalto twice the highest frequencycomponent of the analogsignal.Concept Question 2If humans can generally hear upto 20,000 Hz, what rate must thesound be sampled at to achievethe full frequency range?A)10,000 Hz (10 kHz)B)20,000 Hz (20 kHz)C)40,000 Hz (40 kHz)D)No freakin’ idea3125-2,000chicken200-12,000owl250-8,000cockatiel200-8,500parakeet250-8,000canary50-4,000tree frog100-3,000bullfrog50-1,100tuna50-4,000catfish20-3,000goldfish75-150,000porpoise16-12,000elephant1,000-123,000beluga whale2,000-110,000bat90-22,800chinchilla500-64,000opossum16-44,000ferret100-40,000raccoon250-45,000hedgehog54-50,000guinea pig100-60,000gerbil1,000-91,000mouse200-76,000rat360-42,000rabbit100-30,000sheep55-33,500horse23-35,000cow45-64,000cat67-45,000dog64-23,000humanApproximate Range (Hz)SpeciesConcept Question 3For a dog that can hear up to45,000 Hz, how high must thesample rate be for it to hearfull fidelity?A)22,500 HzB)45,000 HzC)90,000 HzD)No ideaConcept Question 4Common CD’s have a samplerate of 44,100 Hz. What is thehighest frequency that a CD canaccurately play back?A)22,050 HzB)44,100 HzC)88,200 HzD)Not a clueBit DepthWe now need to work on the y-axis ofour grid4The Binary SystemIn the digital world there’s only 1’s and 0’sThe Binary series:0, 1, 2, 4, 6, 16, 32, 64, 128, 256,…1-bit can be 0 or 1 so there are 21 combinations.2-bit 00 001 1 4 combos, 2210 211 33-bit 000 0001 1 8 combos, 23010 2011 3100 4101 5110 6111 716-bit = 216 = 65,536 combinations24-bit = 224 = 16,777,216 combinationsThis now gives us a numberassociation with the y-axis02nn = number of bitsBecause waveform amplitudes are analogous todisplacement from zero, let’s adjust the scale so that thecenter is 0, and the outer bounds are + and – of half thenumber of combinations.-32,76832,7680So for a 16-bit system, there are 65,536 combinations. Theupper bound corresponds to 32,768 and lower to -32,768Concept Question 5A 24-bit digital audio system has16,777,216 combinations. Abouthow many steps are therebetween the zero point and oneof the outer bounds?A)70,000B)8,400,000C)17,000,000D)34,000,000E)Way too many to countSo a 24-bit system represents a 256ximprovement over a 16-bit system!5What happens if you gobeyond the maximums?There simply isn’t data outsidethe outer bounds. Clipping ofthe wave occurs.Results in nasty sounding clipping in the form of severeharmonic distortion. Also, any high frequency data on the crestsof the waves is lost.What if the wave doesn’texactly correspond to anumber on the y-axis?This is called Quantization Error.Because the wave is slightlydistorted from the original, fidelityis lost.Dithering introduces randomnesswhen lowering bit depth whichactually increases perceivedsmoothness.We now have a “plot” ofour x and y axis. Howmuch data are we usingup?For CD quality audio, which is 16-bit, 44.1kHz:16 bits x 2 (stereo) x 44,100 Hz x1byte/8bits =176,400 bytes/sec176,400 bytes/sec x 60 sec/min =10,584,000 bytes/min, or 10.6 Mb/minMixingEach instrument/mic is recordedto it’s own track.The streams of audio are thenprocessed and combined.Mixing multiple tracks down toa stereo two track compilationis called “mixing down”6Time to make a CD!CD’s areMasteringMastering is the art of compiling allthe songs into a single project andcreating the actual master that willbe mass replicated.Make the music sound good on manydifferent playback systems.Listening to music in terms offrequency.Quality control issues.Let’s make CD’s!Compact Discs work on thesame binary principals asregularly stored data, but donein an optical fashion.Pits diffuse the laser while non-pits reflect.Compressed DataAudioMP3, Ogg Vorbis, WMA, etc.Work on the psychoacousticphenomenon that there’s a lotof stuff you can’t hear that canbe removed.Masking is the main principalbehind it. Masking in time andfrequency.7Surround FormatsDolby Digital 5.1DTS 5.1SDDS 7.1THX – a quality standarddeveloped by TomlinsonHollman/George
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