Rainbow Electronics APR6016 - Manual (Page 14)

Preliminary
APR6016 Data Sheet
Page 14 Voice Recording & Playback Device
Revision 1.0
ment needs to be selected, and do so before the SAC signal
returns high. Failing to specify the next command before the
current segment is exhausted (either during recording or
playback) will result in a noticeable gap during playback or
recording.
The /BUSY pin indicates when the device is performing either
a play, record, DIG_WRITE, DIG_READ or fast forward func-
tion. The host microprocessor can monitor the busy pin to
confirm the status of these commands. The /BUSY pin is nor-
mally high and goes low while the device is busy. The low
time is governed by the length of recording or playback spec-
ified 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 aliasing errors is equal to half the
sampling frequency. The APR6016 automatically filters its
input, based on the selected sampling frequency, to meet this
requirement.
Higher sampling rates increase recording bandwidth, and
hence voice quality, but also use more memory cells for the
same amount of recording time. The APR6016 ccommo-
dates sampling rates as high as 8kHz.
Lower sampling rates use less memory cells and effectively
increase the duration capabilities of the device, but also
reduce recording bandwidth. The
APR6016 allows sampling
rates as low as 4 kHz.
Designers can thus control the quality/duration trade-off by
controlling the sampling frequency. Sampling frequency can
be controlled by using the PWRUP command. This command
can change sampling frequency regardless of whether the
internal oscillator is used or an external clock is used.
The APR6016 derives its sampling clock from one of two
sources: internal or external. If a clocking signal is present on
the EXTCLK input the device will automatically use this signal
as the sampling clock source. If no input is present on the
EXTCLK input the device automatically 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 sig-
nals can be divided down to a desired sampling rate. This
allows high frequency signals of up to 10 MHz to be fed into
the EXTCLK pin. Using this feature simplifies designs by
allowing use of a clock already present in the system, as
opposed to having to generate or externally divide down a
custom clock. Details for programing the clock divider are
described in the SPI interface section under the PWRUP
paragraph.
The default power up condition for the APR6016 is to use the
internal oscillator at a sampling frequency of 6.4 kHz.
Storage Technology
The APR6016 stores voice signals by sampling incoming
voice data and storing the sampled signals directly into
FLASH memory cells. Each FLASH cell can support voltage
ranges from 1 to 256 levels. These 256 discrete voltage lev-
els are the equivalent of eight (2
8
=256) bit binary encoded
values. During playback the stored signals are retrieved from
memory, smoothed to form a continuous signal and finally
amplified before being fed to an external speaker amplifier.
Squelch
The APR6016 is equipped with an internal squelch feature.
The Squelch circuit automatically attenuates the output signal
by 6 dB during quiet passages in the playback material. Mut-
ing the output signal during quiet passages helps eliminate
background noise. Background noise may enter the system
in a number of ways including: present in the original signal,
natural noise present in some power amplifier designs, or
induced through a poorly filtered power supply.
The response time of the squelch 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
squelch feature can be disabled by connecting the SQLCAP
pin to VCC.
The active low output /SQLOUT goes low whenever the inter-
nal squelch activates. This signal can be used to squelch the
output power amplifier. Squelching the output amplifier
results in further reduction of noise; especially when the
power amplifier is running at high gain & loud volumes.