From Synthesis to Sampling

Transcription

1 #1 Lec # 66 Fall From Synthesis to Sampling Evolution of PC Audio Proskauer Yakovlev

2 #2 Lec # 66 Fall Overview Role of an audio interface History of audio interfaces PC Speaker (1981) to Intel HD Audio (2004) Shift from synthesized audio to sampled audio Modern audio devices Entirely sample-based Move toward integrated onboard audio External DACs and amps

3 #3 Lec # 66 Fall Role of an Audio Interface Perform audio recording and playback processing Move the load of audio processing away from the CPU Signal conversion DAC for playback ADC for recording

4 #4 Lec # 66 Fall Synthesis Synthesis is the process of electrically creating sound There are many modes of synthesis Frequency Modulation (FM) synthesis The frequency of a carrier wave is modulated by another oscillator Complex and unique waveforms can be produced Particularly good at metallic and percussive sounds Sample-based synthesis Similar to synthesis using the standard waveforms but the seed waves are sampled sounds or instruments

5 #5 Lec # 66 Fall Sampling and Pulse Code Modulation Analog signals are continuous and have a specific value at every given moment in time This type of data is not conducive to the way computers operate Sampling is the process of approximating an analog signal by reading the value at a given interval The quality of the sample is determined by the sample rate and the bit depth (resolution) Pulse Code Modulation (PCM) is a method to digitally represent sampled audio

6 #6 Lec # 66 Fall PC Speaker (1981) First appeared in the IBM 5051 as a means of indicating motherboard error codes Produced only a single square wave at a given frequency Entirely CPU controlled, required lots of CPU performance to operate Rapidly switching between frequencies could give the illusion of multiple voices

7 #7 Lec # 66 Fall PC Speaker PWM Pulse-width Modulation could be used to reproduce rudimentary sampled audio The Programmable Interval Timer (PIT) would be configured to produce a PWM signal at a frequency higher than the speaker could reproduce Adjusting the pulse width created a DC offset, extruding the speaker a specific amount 6 bits of freedom Very CPU intensive, required nearly all the CPU time Graphics would lock up while audio was playing

8 #8 Lec # 66 Fall PCjr/Tandy 1000 (1983) PCjr was IBM s first step into the home micro market Included a Texas Instruments SN76496 programmable sound generator (PSG) Capable of producing 3 square wave channels and one noise channel 16 volume levels for each channel Similar to the MOS Technologies SID chips on the Commodore 64

9 #9 Lec # 66 Fall AdLib (1987) First major improvement in PC audio Featured the Yamaha YM3812 (OPL2) FM chip 6 melodic voices with 5 percussion voices, or 9 melodic voices 2 FM operators Full ADSR envelope for each voice Very high support in games from the time

10 #10 Lec # 66 Fall Creative Music System / Game Blaster (1988) First attempt from Creative Labs to produce a card Featured two Philips SAA1099 PSGs 12 channels of square waves Marketed through RadioShack as the Game Blaster Did not sell well since the AdLib card offered far better music quality

11 #11 Lec # 66 Fall Sound Blaster (1989) Designed to directly compete with the AdLib card First card to feature sampling 22.05kHz 8-bit mono playback 13kHz 8-bit mono recording Included the same Yamaha music chip as the AdLib Commercial success Integrated game port Massive library of supported games Pushed AdLib out of the market within 2 years

12 #12 Lec # 66 Fall Roland MT-32 (1987) External MIDI synthesizer module 32 voices 128 instruments 30 percussion 10 digital effects Not originally intended for use in PC gaming Very cost prohibitive, required MIDI interface card (typically the MPU-401) Was created by Roland for amateur musicians as a budget external synthesizer Best sounding audio solution at the time, supported by games of the time

13 #13 Lec # 66 Fall Gravis Ultrasound (1991) First PC card to support sample-based synthesis Gravis GF1 256kB RAM 32 voices 16-bit playback 8-bit record 44.1kHz Stereo Supported hardware mixing, which was usually performed by the CPU Did not contain FM synthesis chip, used the GF1 to emulate the OPL2

14 #14 Lec # 66 Fall AC 97 (1997) Audio codec developed by Intel in response to ubiquity of sound cards Digital controller (DC97) Built into southbridge of motherboard Interfaced with CPU and system memory AC-Link Digital interface between DC97 and the codec Codec Third party chip that interfaced with the AC-Link and contained the DAC components

15 #15 Lec # 66 Fall Intel High Definition Audio (2004) Successor to AC 97 Same controller/link/codec architecture as AC 97 Specs 15 input, 15 output streams 16 PCM channels/stream 8-32 bit sample resolution 6-192kHz sample rate Audio jack detection and retasking

16 #16 Lec # 66 Fall Sound Cards Today Onboard solutions have sufficient isolation to prevent EMI. Placed far from other components Discrete sound cards are less common Discrete cards typically use PCIe ports Offer more channels Support for higher impedance headphones Less EMI than onboard audio

17 #17 Lec # 66 Fall External DACs and Amplifiers Used typically in music production and professional audio work Situated outside of a computer Offer lowest possible EMI for clean sound Amplifiers may be necessary for high impedance speakers Minimizes THD

18 #18 Lec # 66 Fall Conclusion Early computers were not powerful enough to reproduce sampled audio Synthesis was used to offload the audio generation workload off of the CPU Improvements in memory, processing power, and architecture bits allowed for dedicated sound cards to reproduce sampled audio Continued CPU performance improvements allowed audio generation to return to the CPU and motherboardbased chips

19 #19 Lec # 66 Fall References Audio Codec 97, Intel Corporation, Revision 2.3, April, Ahlzen, Lars, and Clarence Song. The Sound Blaster Live! Book: A Complete Guide to the World's Most Sound Card. No Starch Press, Books24x7. Web. Dec. 5, Popular Göhler, Stefan. Phenomenal. Crossfire Designs, Mar. 12, 2003, Grosse, Robert. Specifications of the IBM 5150 PC. University of Saskatchewan Computer Museum, University of Saskatchewan, 22 Feb. 2012, computermuseum.usask.ca/articles-and-press/ibm Specifications.pdf. High Definition Audio Specification, Intel Corporation, Revision 1.0a, June 17, Latimer, Joey. Hit It, Maestro! Compute, Apr. 1990, pp Leonard, Jim. IBM PC Ramblings. That Oldskool Beat, Oldskool.org, 17 June 2016, Murray, David. How Oldschool Sound/Music worked. YouTube, YouTube, 5 Oct. 2015, Electromagnetic Interference. Wikipedia, Wikimedia Foundation, 6 Dec. 2017, en.wikipedia.org/wiki/electromagnetic_interference.