MIDI Makeouts!
The architecture of the 
can be divided into three elements – the MIDI interpreter, the pitch generator, and the effects processor. The MIDI interpreter, the real brains of the device, receives the MIDI serial data stream and parses it into real MIDI commands and data. Once received, it interprets them and supplies control data to the pitch generator and effects processor engines. Such information includes sample numbers, pitch tuning, and vibrato depth.
For the effects processor, the generated parameters include channel volume, pan left and right, reverb, and chorus depth. By coordinating the initialization of all these parameters, the MIDI interpreter engine supplies an overall control mechanism for the sound synthesis.
The pitch-generator engine reads the raw data samples from the sample ROM at a rate determined by the requested MIDI note number, taking into account the tuning, vibrato, and pitch-bend effects. After the samples are read, they are fed to a sample-rate converter that interpolates the original sample rates to a constant 44.1 kHz. This synchronizes all the samples for the effects-processing engine. The interpolated samples are stored in a buffer that provides communications between the pitch generator and the effects processor.
The effects processor takes the raw interpolated samples from the pitch generator and adds effects such as volume, pan, chorus, and reverb. At the same time, the processor enhances the overall quality of the reproduction using envelope generation and filtering.
Designed completely with digital logic, the 9236 is a full General MIDI wavetable music synthesizer. The MIDI interpreter, synthesis engine, effects processing, and RAM and ROM are all integrated into the same package. The part receives a standard serial MIDI data stream at 31.25 kbits/s and outputs a stereo 16-bit digital-audio stream at a 44.1-ksample/s sampling rate. The digital-audio output is directly compatible with Crystal’s CS4237B/38B multimedia codecs, as well as the company’s CS4333 digital-to-analog converter (DAC). The 9236 operates from a single 3.3-V supply, although the inputs are 5-V tolerant. As a result, the IC can be connected directly to parts that are powered by 3.3- or 5-V supplies.
When the 9236 is deployed with the 4237/38, it supplies a complete ISA plug-and-play-compatible sound system. In this scenario, the 9236′s digital output goes directly to the codec in a digital format, thereby eliminating the need for a separate DAC. Another benefit of this configuration is that a separate quartz crystal circuit is not required because the codec’s 16.9344-MHz clock output drives the 9236′s clock input.
MIDI messages are sent from the host PC to the 9236 over the ISA bus. The 4237/38 offers an MPU-401 UART-mode-compatible ISA MIDI interface. As a result, the MIDI data is sent over the ISA bus to the 9236 in a serial format. The synthesizer interprets that data and generates the associated musical sounds, which are outputed serially.
Applications that require an analog output could make use of the 4333 DAC to convert the signals. In this configuration, the 9236′s digital-audio outputs are sent to the 4333, which converts the data streams into left and right analog audio inputs.
In addition to receiving the clock from an external master clock source, the timing signals can be generated using the on-chip oscillator circuit with an external crystal. Using the master clock, the incoming signal is connected to the 9236′s 5-V master-clock input pin, and the 3-V crystal input is grounded. When the on-chip oscillator helps generate the timing, the quartz crystal is connected between the 9236′s 3-V crystal input and the crystal output pin, and the MCLK5I pin is grounded.
The 9236 supplies a fully static power-down mode, which is initiated either by asserting the Power Down signal, or by gating off the 5-V master-clock input. In the power-down mode, clock signals to nearly all internal circuits are gated off. Because of its static nature, all internal states and register values are retained as long as power is applied to the device and the Reset signal remains inactive. The power-down mode is terminated by de-asserting the Power Down signal, or by gating back on the 5-V master-clock input.
The 9236 can operate in a Multimode format. In other words, the MIDI controller contains 16 receiver channels, one for each of the MIDI channels. Each channel receiver gets MIDI messages on its dedicated MIDI channel.