User guide

Page 14 Voice Recording & Playback Device
Revision 2.1
low edge of the SAC signal as an indicator that the next seg-
ment needs to be selected, and do so before the SAC si
gnal
returns hi
gh. Failing to specify the next command before the
current se
gment is exhausted (either during recording or
pla
yback) will result in a noticeable gap during playback.
The /BUSY pin indicates when the device is performin
g either
a pla
y, record or fast forward function. The host microproces-
sor can monitor the bus
y pin to confirm the status of these
commands. The Bus
y pin is normally high and goes low while
the device is bus
y. The low time is governed by the length of
recordin
g or playback specified by the user.
Sampling Rate and Voice Quality
The Nyquist Sampling Theorem requires that the highest fre-
quency component a sampling system can accommodate
without the introduction of aliasin
g errors is equal to half the
samplin
g frequency. The APR6008 automatically filters its
input, based on the selected samplin
g frequency, to meet this
re
quirement.
Hi
gher sampling rates increase recording bandwidth, and
hence voice
quality, but also use more memory cells for the
same amount of recordin
g time. The APR6008 accommo-
dates samplin
g rates as high as 8kHz.
Lower samplin
g rates use less memory cells and effectively
increase the duration capabilities of the device, but also
reduce recordin
g bandwidth. The APR6008 allows sampling
rates as low as 4 kHz.
Desi
gners can thus control the quality/duration trade-off by
controlling the sampling frequency. Sampling frequency can
be controlled b
y using the PWRUP command. This command
can chan
ge sampling frequency regardless of whether the
internal oscillator is used or an external clock is used.
The APR6008 derives its samplin
g clock from one of two
sources; internal or external. If a clockin
g signal is present on
the EXTCLK input the device will automaticall
y use this signal
as the samplin
g clock source. If no input is present on the
EXTCLK input the device automaticall
y defaults to the inter-
nal clock source. When the EXTCLK pin is not used it should
be tied to GND.
An internal clock divider is provided so that external clock si
g-
nals can be divided down to a desired samplin
g rate. This
allows hi
gh frequency signals of up to 10 MHz to be fed into
the EXTCLK pin. Usin
g this feature simplifies designs by
allowing use of a clock already present in the system, as
opposed to havin
g to generate or externally divide down a
custom clock. Details for pro
graming the clock divider are
described in the SPI interface section under the PWRUP
para
graph.
The default power up condition for the APR6008 is to use the
internal oscillator at a samplin
g frequency of 6.4 kHz.
Storage Technology
The APR6008 stores voice signals by sampling incoming
voice data and storing the sampled signals directly into
FLASH memor
y cells. Each FLASH cell can support voltage
ran
ges from 1 to 256 levels. These 256 discrete voltage lev-
els are the e
quivalent of eight (2
8
=256) bit binary encoded
values. Durin
g playback the stored signals are retrieved from
memor
y, smoothed to form a continuous signal and finally
amplified before being fed to an external speaker amplifier.
Squelch
The APR6008 is equipped with an internal squelch feature.
The S
quelch circuit automatically attenuates the output signal
b
y 6 db during quiet passages in the playback material. Mut-
in
g the output signal during quiet passages helps eliminate
back
ground noise. Background noise may enter the system
in a number of wa
ys including: present in the original signal,
natural noise present in some power amplifier desi
gns, or
induced throu
gh a poorly filtered power supply.
The response time of the s
quelch circuit is controlled by the
time constant of the capacitor connected to the SQLCAP pin.
The recommended value of this capacitor is 1.0 uF. The
s
quelch feature can be disabled by connecting the SQLCAP
pin to VCC.
The active low output /SQL
goes low whenever the internal
s
quelch activates. This signal can be used to squelch the out-
put power amplifier. S
quelching the output amplifier results in
further reduction of noise; especiall
y when the power ampli-
fier is run at hi
gh gain & loud volumes.
APR6008