Physics 1240 1/24/06Today: Waveforms and digital samplingNext time: Sources of musical soundLast timeOnly a vibration that repeats itself at regular intervals produces the sensation of a distinct pitchPeriodic motion -- repeats itself at regular intervalsfrequency = 1 / periodFrequency (physical) determines pitch (sensation)Amplitude - how large the oscillation isOutlineAudible rangeBasics of digital sound editingSampling rateBit depth or dynamic rangeEstimates of size of digital sound filesCT3.1Should we count the clicker scores today?A) YesB) NoC) Abstain (Abstain: to refrain deliberately and often with an effort of self-denial from an action or practice)CT3.2If we double the period of a musical vibration what would happento the pitch?A) It would go up by a factor of 4B) It would go up by a factor of 2C) the pitch would not changeD) the pitch would go downCT3.3What happens to the musical pitch in the above question?(Octave - a change of a factor of two in frequency)A) It goes up one octaveB) It goes down one octaveC) It does not changeCT3.4What object is a good example of wave motion?A) pendulum at large amplitudesB) mass on a spring at small amplitudesC) a tuning forkD) none of the aboveCT3.5Suppose a steady tone is played through a speaker. Is the location of the speaker cone plotted as a function of time a wave?A) yesB) noC) it’s time to drop the courseAudible Range: 20 Hz – 20,000 Hz(Frequencies the human ear can hear)CT3.6A mechanical engineer designs and builds a “nano-guitar” with silicon strings that are 1 micron in length and produce a steadytone at 30 MHz (1 MHz=1,000,000 Hz). Can the human ear hear this tone?A) NoB) Yes, with appropriate amplification.Digital RecordingDigital SamplingA “sample” of the sound is taken at very small time intervalsSampling rate is how many samples per secondCD Quality – this is a standardStereo – 2 channelsSample rate is 44,100 Hz or one sample every 0.0000227CT3.7 What type of waveform is this?A) semi-periodic waveB) sine waveC) sound sphereCT3.8 What is the period? A) 2.2 secB) 10 HzC) 0.1 sec2.1 sec2.2 secCT3.9 What is the frequency? A) 10 HzB) 1 HzC) 0.1 Hz1.002 sec 1.003 secCT3.10 What is the frequency of this waveform?A) 100 HzB) 500 HzC) 1,000 HzCT3.11How did Scott know that a 44,100 Hz sampling rate corresponds to a time interval between samples of 0.0000227 sec?A) he knows frequency = 1/periodB) he knows the sound of speed AND frequency = speed/ wavelengthC) he is aware of the ANSI MPEG3 standardBit Depth (or Dynamic Range) is the range of possible valuesCD Quality0 to 216or from 0 to 65,536 or from -32,768 to +32,768CD quality is called “16-bit sound” or a “bit depth of 16”Other common bit depths8 bit 0 to 28(low quality digital sound) 24 bit 0 to 224(high quality digital recording) 32 bit 0 to 232CT3.12Your Aunt is Chief Financial Officer at steel company in Pittsburg. She has hired you part-time to post the audio of their quarterly announcement of earnings on the company web site. This is a recording of a teleconference. What might be an appropriate bitdepth? A) 8-bitB) 16-bitC) 24-bitD) 32-bitCD Quality Sample Size Estimates44,100 samples x 2 channels x 16 bits = 1,411,200 bits/secThere are 8 bits per "byte", so1,411,200 bits / 8 = 176,400 bytes/secCDs hold about 74 minutes or 176,400 bytes/sec x 74 minutes x 60 sec/min = 783,000,000 bytes = 783 MbytesCD Quality Sample Size – 74 min 44,100 samples x 2 channels x 16 bits 1 byte / (8 bits) x74 min x 60 sec/min = 783 million bytesCT3.13How big would a 74 minute mono 16-bit 44,100 Hz sample be?A) 783 million bytesB) 392 million bytesC) 705,600 bytes(How about a stereo 8-bit 8,000 Hz sample? Invent your own problems
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