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Chapter 6 Audio Subsystem
Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition - September 1998
6-6
6.2.1.3 PCM Configuration Modes
PCM operation can be configured for compatible ( common sound board functionality) mode or
set up for extended mode, which has some performance advantages. Table 6-1 lists the
differences between the modes of operation.
Table 6–1.
Audio Mode Differences
Table 6-1.
Audio Mode Differences
Function
Compatibility
Mode
Extended
Mode
FIFO Size Available 64 bytes (SW Control) 256 bytes (HW Control)
Mono 8-bit ADC, DAC 44 KHz Max Sampling 44 KHz Max Sampling
Mono 16-bit ADC, DAC 22 KHz Max Sampling 44 KHz Max Sampling
Stereo 8-bit ADC, DAC 22 KHz Max Sampling 44 KHz Max Sampling
Stereo 16-bit ADC n/a 44 KHz Max Sampling
Stereo 16-bit DAC 11 KHz Max Sampling 44 KHz Max Sampling
Signed/Unsigned Control No Yes
AGC During Capture Mono Only (22 KHz) No
Programmed I/O Block Transfer No Yes
FIFO Status Flags No Yes
Auto Reload DMA Yes Yes
Time Base for Programmable Time 1 MHz or 1.5 MHz 800 KHz or 400 KHz
ADC/DAC Jitter +/- 2 usec None
Sound Blaster Pro Compatible Yes No
6.2.1.4 PCM Bus Cycles
The I/O and DMA cycles used by PCM operations to process .WAV data follow standard ISA bus
conventions. All bus transfers occur at the bytes level. Programmed I/O cycles are always used
for programming the control registers and may also be used for transferring audio data to and
from the audio subsystem as well. Quantized audio data is built using the “little endian” format
(LSB occupies the lowest memory address). Data transfers over the ISA occur as shown below.
Figure 6–4.
Audio Subsystem-to-ISA Bus PCM Audio Data Formats / Byte Ordering
8-bit Mono
8-bit Stereo
16-bit Mono
16-bit Stereo
Sample 6 Sample 5 Sample 4 Sample 2Sample 3 Sample 1
Sample 3 Sample 3 Sample 2 Sample 1Sample 2 Sample 1
Sample 3 Sample 2 Sample 1
Right Left Right RightLeft Left
Mono Hi
Left Hi
Sample 3 Sample 2
Sample 1
Mono LoMono HiMono LoMono HiMono Lo
Left LoLeft LoLeft Hi Right Hi Right Lo
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