iSight Programming Guide: Audio Unit

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Audio Unit

The audio unit is identified in the Configuration ROM as a Unit Directory with Unit Specification ID 0x000A27 and Unit Software Version 0x000010. Like the video unit, the audio unit is controlled by a register file. The base address of the register file is specified in the Unit Directory of the audio unit. The register file is organized as shown in “Table 1-6.”

**Table 1-6** The audio unit register file
Offset	Name	Description
0x000	AudioEnable	Write 0x80000000 to enable audio transmission (or 0 to disable)
0x204	Default Audio Gain	Read-only
0x210	Gain: Raw Start	Read-only Scale factor for Gain (at 0x500)
0x214	Gain: Raw End	Read-only Scale factor for Gain (at 0x500)
0x218	Gain: Decibels Start	Read-only Signed 32-bit value
0x21C	Gain: Decibels End	Read-only Signed 32-bit value
0x280	Sample Rate Inquiry	Read-only (see 0x400)
0x300	Isochronous TX Config	Isoch Channel + (Speed << 16)
0x400	Sample Rate	Write 0x80000000 to select 48K samples/sec
0x500	Gain	Read/Write values within range given by Raw Start/Raw End registers
0x504	Mute	Write any non-zero value to mute audio without stopping isochronous stream

Audio is sent in large chunks during gaps in the video, according to the format shown in “Table 1-7.” As explained in the “Introduction,” an audio packet will never be sent in the same isochronous cycle as a video packet. When 640 x 480 YUV 4:1:1 30 fps video is selected, the audio packets will be about twice as large (and half as numerous) as in any other video mode.

**Table 1-7** Format of audio chunks
Byte Offset	Name	Description of Quadlet value
0x000	1394 Packet Header	See 1394 Chapter 6; Length and Channel #
0x004	1394 Header CRC*	CRC for header quadlet only
0x008	Audio Sample Count	Count of audio samples in this packet
0x00C	Audio Signature	ASCII “sght” (0x73676874)
0x010	Audio Sample Total	Total count of all previous samples
0x014	Reserved	Reserved
0x018-0xNNN	Audio Data	Two-channel audio data (see “Table 1-8”). Up to 1900 bytes (depending on video format)
0xNNN+4	1394 Data CRC*	CRC for payload quadlets
* Most Link hardware (such as OHCI) removes these CRCs as the packet is received

Each audio sample is a four-byte value containing two signed 16-bit samples. Viewed on 1394, the sample is structured as shown in “Table 1-8.”

**Table 1-8** Audio sample structures
Byte 0 (bits 0-7)	Byte 1 (bits 8-15)	Byte 2 (bits 16-23)	Byte 3 (bits 24-31)
16-bit audio sample (Front/”Left”)		16-bit audio sample (Rear/”Right”)

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Last updated: 2004-10-05

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