Transcript week-3
Lecture Three Digital & Analogue Sound Overview • By the end of this lecture you will be familiar with: – – – – – Theory behind sound Analogue Vs Digital sound Implications for multimedia Sampling audio Anatomy of sound cards 2 Sound theory What is Sound? • Frequency, e.g. Middle C: 256 Hz - High C: 512 Hz 4 What is Sound cont.? • Amplitude - loudness (usually measured in dB) • Pure sound -v- musical instruments • Harmonics 5 Analogue Sound • Continuously varying waveform • ‘Records’ & tape cassettes are analogue media • Voltages recorded and regenerated • Still the preference of purists 6 Digital Sound • Sound recorded as 1’s and 0’s (as usual) • Compact discs and mini discs are digital media • Recording: analogue to digital converter (ADC) – changes voltages to digital stream of 1’s and 0’s • Playback: digital to analogue converter (DAC) – changes digital stream back to voltages • Limited in quality by sampling settings 7 Implications • Copying of analogue media - loss of quality • Copying of digital media - perfect replication • Concerns over copyright theft • Digital is Robust • Digital is Compact • No ‘graceful degradation’ with digital 8 Quality of Digital Sound Features of Sound Cards 9 Quality & Sampling Frequencies • Nyquist Sampling Theorem – faithful sampling requires a sampling frequency twice that of the highest frequency in the signal • Human ear: 20Hz to 20kHz (approximately) • Audio CD sampled at 44.1kHz • Two issues: sampling frequency number of bits per sample 10 Number of bits per sample • Affects the quality of the digital sound Dynamic Range • 8 bits => range split into 256 steps • Issue also of mono and stereo (2 tracks) 11 Example 1 • CD-quality sound recording – 44.1kHz sampling frequency, 16-bits per sample, stereo sound 1 second has 44,100 samples of 2 bytes each = 88,200 bytes x 2 channels = 176,400 bytes / second (or 172.3kB/s) 12 Example 2 • Telephone speech has a narrow bandwidth of around 3.6kHz 1 second has 7200 samples at 8 bits mono = 7200 bytes / second = 7kB/s (and could be much less if compression is used) 13 Example 3 • A CD-ROM can store 650MB of data or 74 minutes of stereo music at 44.1kHz. What is the sample size? 650 / 74 = 8.78 MB / minute = 150kB/s = 150*1024*8 bits/s 88,200 samples per second So , 150 * 1024 * 8 / 88200 = 14 bits per sample 14 Final Example • One minute of sound must be recorded using a maximum of 2.5MB of memory. What are the sampling options? Available for 1 second is 2.5*1024/60 = 42.67kB OPTION 1: 8-bit & 42.67kHz rate, mono OPTION 2: 16-bit & 21.33kHz rate, mono OPTION 3: 8-bit & 21.33kHz rate, stereo OPTION 4: 16-bit & 10.67kHz rate, stereo 15 Features of Sound Cards Anatomy of a Sound Card 17 Digital Audio Replay & Recording • Matched pair of D-to-A and A-to-D converters • Programmable sample rate generator – rates usually fractions of 44.1kHz – latest cards are using fractions of 48kHz (DAT) – best cards are using 96kHz (professional systems) • Full-duplex operation only on better cards – needs 2 DMA channels => complex installation 18 MIDI • Musical Instrument Data Interface • Records: – which instrument, e.g. flute – which note, e.g. middle C – duration, tempo, attack/decay, volume • Allows for 16 simultaneous ‘voices’ • Recorded from MIDI-compatible instruments 19 MIDI Synthesiser • FM synthesis – mathematical descriptions of the characteristics of musical instruments – get a facsimile of the instrument playing the note • Wave-table synthesis – much more realistic (c.f. a piano sound on each) – uses samples of actual sounds made by real musical instruments 20 Wave-tables • One or more samples for each instrument. • Sound cards vary in their polyphony, i.e. the maximum number of notes which can be played at once, e.g. 8 or 16. • Wave-table can be held in ROM or RAM (the latter allowing download from the host). • DSP chip may offer other effects, e.g. reverb. 21 MOD files • A de-facto standard in computer games industry • File contains the sample data (c.f. MIDI) + song • Song is specified as series of steps (16/bar) – pitch of the note – which sample is to be used – modifications to the sample, e.g. vibrato • Ability to loop sections, e.g. for rhythm 22 Questions? 23