Lpc2000

Hi Tom,

I made a similar application a long time ago (using a 8051 with an external 
audio DAC and unpacking 16-bit ADPCM in real time) and found out that these 
things are very, very sensitve to jitter. So a variation of about 1 uP 
cycle (about 1us) on playing a sound sampled at 11kHz definitely could be 
heard (!) so I could not rely on the timer interrupt but hat to poll the 
timer instead and insert a nop depending on the even / uneven state of the 
timer when I begun start polling. All this will not be as bad with the 
LPX2xxx, as this device is about 50 times faster, but you have to take care 
on jitter anyway.

The second very important thing is to have a good low-pass filter between 
the DAC output and the audio input. You may use a circuit with one or two 
op-amps (I don't remember, but I think it was a 4th or 6th order filter I 
used) otherwise you will hear the quantisation noice - a simple R/C 
combination will not be enough definitely.

I hope that helps.

Regards
Herbert


At 18:53 19.01.2006, you wrote:
>I am trying to use the LPC2138 part as a simple voice device (play 16
>khz voice).  Currently I am using a Keil MCB2130 eval board and
>running a simple example program that plays 16 bit 8khz wav files via
>the D/A port through a LM386 to the on board speaker (the wav file has
>no hiss on my PC).
>
>The process works but there is a low level hiss or static in the
>audio.  I have take the Aout and filtered it with a RLC filter to get
>rid of the tinness from the audio but still has the hiss.  I also
>tried using an off board amp and speaker but still have the
>hiss/static in the audio.
>
>If the D/A is not changing then the hiss/static goes away so it does
>not seem to be noise coupling over.
>
>I tried looking at the signal on the scope but can not see anything
>above the scope noise.
>
>I am weak on audio stuff and am hoping to get some pointers.
>
>1. Is this hiss just from the D/A S/N ratio and I am stuck with it??
>
>2. Is caused by the Vref and V3A being tied with the Digital Vcc?  (If
>this I would assume it would still get the noise when the D/A is not
>changing)
>
>3. Do I need to filter the Aout better?
>
>4. Any other test I should try??
>
>Thanks
>Tom




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