AHI Device

About AHI

AHI is a retargetable audio subsystem which provides standardized operating system support for wide range of audio hardware. AHI offers improved functionality not available through the AmigaOS audio device driver, such as seamless audio playback from a user selected audio device, standardized functionality for audio recording and efficient software mixing routines for combining multiple sound channels.

Definitions

Terms used in the following discussions may be unfamiliar. Some of the more important ones are defined below.

Sample

A sample is one binary number, representing the amplitude at a fixed point in time. A sample is often stored as an 8 bit signed integer, a 16 bit signed integer, a 32 bit floating point number etc. AHI supports only integers.

Sample frame

In mono environment, a sample frame is the same as a sample. In stereo environment, a sample frame is a tuple of two samples. The first member is for the left channel and the second member is for the right channel.

Sound

Many sample frames stored in sequence as an array can be called a sound. A sound is, however, not limited to being formed by samples. It can also be parameters to an analog synth or a MIDI instrument, or be white noise. AHI supports only sounds formed by samples.

AHI Device

AHI device has two APIs: a library-like function interface for low-level access and device interface for high-level access.

AHI Device Commands and Functions

Command	Command Operation
CMD_FLUSH	Abort all current requests, both active and waiting, even other programs requests!
CMD_READ	Read raw samples from audio input.
CMD_RESET	Restore device to a known state.
CMD_START	Start device processing (like ^Q).
CMD_STOP	Stop device processing (like ^S).
CMD_WRITE	Write raw samples to audio output.
NSCMD_DEVICEQUERY	Query the device for its capabilities.

Device Interface

struct AHIRequest
{
    struct IOStdReq ahir_Std;       /* Standard IO request     */
    UWORD  ahir_Version;            /* Needed version          */
    UWORD  ahir_Pad1;
    ULONG  ahir_Private[2];         /* Hands off!              */
    ULONG  ahir_Type;               /* Sample format           */
    ULONG  ahir_Frequency;          /* Sample/Record frequency */
    Fixed  ahir_Volume;             /* Sample volume           */
    Fixed  ahir_Position;           /* Stereo position         */
    struct AHIRequest *ahir_Link;   /* For double buffering    */
};