APR6016 datasheet(14/24 Pages) APLUS

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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 (28=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.

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