Analog Devices ee-54 Application Notes

Engineer To Engineer Note EE-54

Notes on using Analog Devices’ DSP, audio, & video components from the Computer Products Division

Phone: (800) ANALOG-D or [phone], FAX: [phone], EMAIL: [e-mail]

HOW TO USE AD1819A
VARIABLE SAMPLE RATE
SUPPORT

Last Modified: 5/12/98

Overview

This Engineer’s Note will cover the AD1819A's variable
sample rate support. One advantage the AD1819A has
over other AC `97 codecs is that it can support any
sample rate between 7 kHz and 48 kHz with one Hz
precision. Two different sample rates may be used, and
either may be assigned to any of the four converter
channels in the AD1819A. In addition, one of the sample
rates may be multiplied by a constant factor of 10/7 or 8/7
for generation of irrational sample rates used in V.34
modems. The variable sample rate support in the
AD1819A eliminates the burden of digital re-sampling
imposed on DC `97 controllers by the AC `97
specification, without any loss in audio performance.

Sample rate changes take effect immediately after either
register is written.

Miscellaneous Control Bits Register

Register 76h contains four bits which assign one of the
two SRGs (Sample Rate Generators) to each conversion
channel, and two bits to enable the 10/7 or 8/7 multipliers
for irrational modem rates.

Bit Name Description

------------------------------------------------------------------------------------------­0 ARSR ADC right sample rate generator select
2 DRSR DAC right sample rate generator select
5 SRX8D7 multiply SR1 rate by 8/7
6 SRX10D7 multiply SR1 rate by 10/7
8 ALSR ADC left sample rate generator select
10 DLSR DAC left sample rate generator select

Other bits are reserved or used for other purposes, so this
register should be read, the result modified as necessary,
and then written back.

If the ALSR, ARSR, DLSR, or DRSR bits are zero then
the corresponding converter channels are assigned to
SRG0, and use the sample rate in
register 78h. If any of those bits are ones then
corresponding channels are assigned to SRG1, and use the
sample rate in register 7Ah. The default state of all of
these bits is zero in the AD1819A,
but that may change in later parts. Hence, software
should always set these bits instead of assuming they are
reset to a fixed value.

REGISTERS

Four vendor-defined registers in the AD1819A control its
pair of sample rate generators (SRGs). Two (78h and
7Ah) contain the sample rates to be generated. Another
(76h) assigns one of the two sample rates to each
conversion channel, and enables the 10/7 or 8/7
multipliers for modem support. The last (74h) enables
DAC request
bits, which are required for proper transfer of DAC data at
sample rates less than the 48 kHz serial frame rate.

Sample Rate registers

Registers 78h and 7Ah contain the sample rates currently
assigned to each SRG in Hz. Behavior of the SRGs is
undefined if sample rates less than 7 kHz (1B58h) or
greater than the default rate of 48 kHz (BB80h) are used.

If the SRX8D7 or SRX10D7 bits are set then the sample
rate generated by SRG1 is multiplied by a constant factor
of 8/7 or 10/7, respectively. The rate generated by SRG0
is unaffected by these bits. The result is undefined if both
bits are set. Care must be taken to ensure that neither the
base sample rate (if both SRX bits are clear) nor the
multiplied sample rate (if one of the SRX bits is set) are
ever outside of the allowed 7 kHz to 48 kHz range; this
should not be a problem for any of the V.34 irrational
rates.

Serial Configuration register

Register 74h contains one bit to enable DAC requests, and
six read-only bits for the DAC request bits themselves.

Bit Name Description

-------------------------------------------------------------------------------------------­0 DRRQ0 Master codec DAC right request
1 DRRQ1 Slave 1 codec DAC right request
2 DRRQ2 Slave 2 codec DAC right request
8 DLRQ0 Master codec DAC left request
9 DLRQ1 Slave 1 codec DAC left request

a

10 DLRQ2 Slave 2 codec DAC left request
11 DRQEN Enable DAC requests in serial frame

Other bits are reserved or used for other purposes, so this
register should be read, the result modified as necessary,
and then written back. If there is only one codec in the
system then it is the master
codec; multi-codec systems are covered elsewhere.

DAC request bits are asserted whenever the associated
DAC channel is able to accept data in the next serial
frame. This information is required because it is possible
to provide more samples to a DAC channel than it can
accept when the DAC channel is running at a rate
less than the serial frame rate of 48 kHz. If a DAC
request bit is deasserted then the controller should not send
a DAC sample for that channel in the next frame.

If the DRQEN bit is set then the AD1819A produces
DAC request bits at two different points in the input serial
frame. Those two points are
in the bottom half of the status address slot, and in the
status data slot.

The DAC request bits in the status address slot are
compatible with the recently released AC `97 2.0
extensions. They are active-low (send sample if zero), and
are output in the same order as the output slots for the
corresponding DAC channels.

Slot Bit SDATA_IN Controls slot

----------------------------------------------------------

---­1 19 0
1 18:12 status address
1 11 DLRQ0# 3
1 10 DRRQ0# 4
1 9 DLRQ1# 5
1 8 DRRQ1# 6
1 7 DLRQ2# 7
1 6 DRRQ2# 8
1 5:0 0

DAC requests are also produced in the status address and
data slots by emulating register reads of register 74h.
Once DRQEN is set, every
input frame (except those following an output frame
containing a read command) will contain 74h in the status
address slot and the contents
of register 74h in the status data slot. This mode is
provided for use with controllers that cannot propagate
data from the low half of the status address slot. It is
preferable to use the DAC request bits from the status
address slot instead of those from the status data slot,
because the latter are obscured whenever a real register read

command is issued.

DIFFERENCES BETWEEN THE AD1819A

AND AC `97 2.0

Many features of the AC `97 2.0 variable sample rate
extensions are based on features introduced by Analog
Devices in the AD1819A, but there are some differences
with which systems using the AD1819A must cope.

First is the sample rate registers themselves. The
AD1819A's sample rate registers are at indices 78h and
7Ah instead of at 2Ch through 34h, and there is no fixed
mapping of converter channels to SRGs. Also, the
AD1819A relies on the user not to write sample rates
outside of the acceptable range to the sample rate registers;
AC `97 2.0 codecs handle invalid settings by forcing the
register to the nearest acceptable value.

AC `97 2.0 codecs require that a bit (VRA, register 2Ah)
be set before sample rates may be changed, while the
AD1819A's variable sample rate support is always
enabled. VRA also enables DAC request bits in the status
address slot on AC `97 2.0 codecs, while the AD1819A
uses the DRQEN bit in register 74h to enable DAC
requests in both the status address and status data slots.

Future AC `97 codecs from Analog Devices will comply
with the AC `97 2.0 variable sample rate extensions,
while retaining as much compatibility with the AD1819A
as possible.

SYNCHRONIZATION OF AUDIO DATA

Some controllers or applications require that all stereo
ADC or DAC samples be delivered in (left, right) pairs.
The AD1819A will deliver ADC samples and DAC
requests in pairs as long as some simple rules are
followed.

The ADCs will deliver samples in pairs as long as both
ADC channels are assigned to the same SRG (ALSR =
ARSR in register 76h) and loopback mode is disabled
(LPBK = 0 in register 20h). The ADCs must be powered
down and back up again whenever ALSR and ARSR are
changed or LPBK is cleared in order to guarantee that
samples will be delivered in pairs.

DAC requests will be issued in pairs as long as both DAC
channels are assigned to the same SRG (DLSR = DRSR
in register 76h), loopback mode

EE-54 Page 2

Notes on using Analog Devices’ DSP, audio, & video components from the Computer Products Division

Phone: (800) ANALOG-D or [phone], FAX: [phone], EMAIL: [e-mail]