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Sound with Etrax



Hello!

To be able to control devices connected to the devboard_82 by a telephone, I
will attach a GSM-modem and a DTMF-decoder so that the user may call the
Axis-Board and give commands via pressing the phone-buttons.
For this purpose, I want the Axis-board to provide a spoken "menu".

My idea for this:
- upload of sound-samples in PCM-Format to Devboard (8kHz sampling-rate,
8bit/sample, in total not more than 60sec)
- connecting a DAC to a sync serial port
- generating the appropriate data-stream output on the sync serial port
- feeding the mic-port of the modem with the analogue signal from the DAC

Now there are several questions:
1) Is there a better way? I think it should be possible to generate directly
PWM-signals on the serial port and low-pass filter them to achieve an
analogue audio signal. The sound should not be high-quality, just
understandable.
Another idea would be to use a text-to-speech module. Any experiences?

2) The DAC needs at least 18bit/sample, and works in stereo mode (two
channels). As the sync serial port only supports 16 or 24 bit/frame, it
seems that I have to use 24 bit-mode.
In this case, the clock had to be 8kHz*24*2=384kHz, which is not derivable
from the 4.096 MHz clock nor from the 3.125 MHz clock. I think I do have to
change the sample-rate or use an external clock with 3.072 MHz?

3) For the I2S-bus Audio-Data-Stream the Frame-signal is half the time high
and half the time low, for selecting right or left channel. The data-stream
should be valid in each cycle. Is this possible? It seems that in normal
operation mode the data-stream is only valid in one of the Frame-signal
states (either high OR low). In this case, I would never be able to use
stereo-sound.

4) Is there an audio-decompression-tool available, running on the Etrax, to
use less memory for audio-samples? 60s PCM-data use about 500kB.

5) I would like to have also 2 additional RS232-Ports. Is this possible, or
do I have to forget about one RS232 if I connect the DAC?

Well, many questions, but I would appreciate any hint!

Best regards,
Martin