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Technical Reference Guide
Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition–- September1998
6-7
6.2.2 FM SYNTHESIS AUDIO PROCESSING
The audio subsystem supports playback of MIDI (.MID) files. A .MID file does not contain
audio information in the same way that .WAV files do. In .MID files, audio data consists of note
on/off, tone type, and amplification information. Audio stored in the .MID file format has the
benefit of taking up far less space than audio stored as .WAV files.
The ES1869 controller includes a 20-voice, four-operator frequency modulated (FM) synthesizer.
In FM synthesis, one signal (the carrier) is forced to vary from it’s center frequency by another
signal (the modulator) resulting in a sideband or “harmonic” frequency. The frequency of the
harmonic is determined by the original carrier frequency and the modulating frequency. The
number of harmonics generated is determined by the strength (amplitude) of the modulating
signal. The microsystem that produces the FM signal is called a patch (Figure 6-5).
Figure 6–5.
FM Synthesis Patch
Note that while an analog representation is shown in Figure 6-6. Synthesis occurs as a digital
operation with the results being sent to the DAC.
The FM synthesis process is a playback-only operation involving the writing of .MID data to the
audio subsystem over the ISA bus. The only reads involve checking the controller for status.
Figure 6-6 shows the ISA bus transaction for FM synthesis. Note that if a succeeding data byte is
meant for the same location as the previous byte, the address does not need to be re-written.
Figure 6–6.
Audio Car-to-ISA Bus FM Audio Data Format
Modulator
Oscillator
Carrier
Oscillator
Frequency Cntrl
Amplitude Cntrl
Frequency Cntrl
Modulating Signal
FM Signal
Address Data
Address
or
Data
A0
IOR-
or
IOC-
Data 7..0