Converting voltages to PCM audio (programming related)

Hello everyone,

I'm recording audio with a data acquisition card from the speaker
cables. When recording, I get an array of unsigned short integer
values. These I need to convert to PCM audio. Everything works quite
nicely, except the fact that a lot of noise occurs in the PCM file. I'm
using Measurement Computings PCI-DAS6036 -card, and I have tried this
also with a simulated National Instruments PCI-4462 -card (the
simulated card only gives a sine wave).

What I have done is that first I reduce the received unsigned short
values (1-65535) to scale them on the signed short value range (these
are from MCC card, NI card gives voltages which I multiply to match the
signed short value range). After that I write these values to a file as
16bit signed integers in little-endian format.

And when this so called audio is played, it has a _lot_ of noise in it
that can not be heard from the speakers or when recording from sound
card. Waveform and amplitude are correct and you can hear the actual
audio playing. So, is there something I'm missing? Can I convert the
received values directly to PCM format? I would not like to filter the
signal (nor could I since my knowledge of actual DSP is almost
non-existent).

And last, but not least, please forgive me if this was an inappropriate
choice for a newsgroup to ask about this matter. If any of you should
know a more appropriate newsgroup, I really would like to know.

Best regards,
vesse

.

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