I get the feeling this thread has gone
off in totally the wrong direction. You are talking about high quality
sound as you have mentioned MD and recording music so I would forget about
8 or 10 bit conversion. With the 10bit convertors in the AVRs I would guess
you will get one or two bits of noise so you are reduced to around 50dB
signal to noise or dynamic range.
Using the internal convertors also ignores
the plusses that you have lost throwing away an external ADC or Codec and
that is that you need a good quality microphone amplifier with a decent
amount of gain. Another problem you create by using the internal ADC is
the enormous amount of filtering you need to prevent aliasing problems
particularly when using sampling rates in the audio band. If you are using
the internal ADC running at 8kHz with just 8 bits of real resolution you
are going to need at least 4 poles of filtering if not more at 16kHz to
prevent high freqencies sounding like a robot or a greetings card!
I haven't got a feel for budget from
your earlier emails but some of the sound card codecs should be available
with mic amp and will give you sample sizes which can be easily reduced
to 16 bit at sample rates that can be set to suit your memory size. They
use very high frequency convertors and digital filters intenrally which
mean you have to do little or no external audio filtering to prevent aliasing
problems. Your choice of the Analogue Devices part will give you far better
quality than internal ADC and PWM output and for $3 is a bargain but still
borderline quality for music if you are thinking MD or CD quality. If you
want to put music through it you might need to look at the Analog Devices
Soundmax Codecs but you would then need an external mic amp.
For example have a look at the WM8974
from Wolfson (I dont work for them!) which for under $4 gives a mic amp,
high quality CODEC and a speaker driver (might be useful). Will do all
that down to 8kHz if stuck for data throughput although I still wouldnt
recommend it with music. It also gives a bare minimum dynamic range for
"tape recorder/MD"
quality.
http://www.wolfson.co.uk/products/digital_audio/codecs/WM8974/
Dave Pallant
AVR-Chat@yahoogroups.com wrote on 22/08/2005 14:06:07:
> Would I be able to get better sound using a 10bit R2R network than
using a
> 16bit PWM timer?
>
> Regards
> Lasse Madsen
>
>
>
> -----Original Message-----
> From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On
Behalf
> Of Thomas Keller
> Sent: 22. august 2005 15:02
> To: AVR-Chat@yahoogroups.com
> Subject: RE: [AVR-Chat] OT: Sound record/playback
>
> On Mon, 2005-08-22 at 12:45 +0200, Lasse Madsen wrote:
> > Raw sampling is a possibility I'm considering but then it's not
> > possible to load the file into a PC for example without storing
it as
> > WAV or something similar.
>
> � �AIUI. "WAV" *IS* raw sampled audio.
>
> > I think 8 KHz sampling rate is not enough to create a sound like
one
> > would expect from a tape recorder/MD or similar device.
> > What if I "only" needed 16 KHz sampling from a 16bit
DAC connected to
> > the SPI or XMEM interface and stored that onto the card on the
fly and
> > then loaded one of the 16bit timers with the recorded result
and
> > forwarded through the "record buffer" with 16KHz wouldn't
that work ?
>
> � �Using a 16 or 20 Mhz AVR, this shouldn't be a problem.
�The actual
> speed isn;'t going to be as big a problem for you in terms of fidelity
> as the size of your samples. �8 bit samples are going to resultin
a
> pretty "grainy" sound. �10 bit samples will be marginally
better.
>
> > I have never tried to use the onboard 16bit timer for audio generation
> > Is it possible to get a decent "sound" out of an RC
filter being
> > pulsed with PWM?
>
> � �PWM actual.y offers a better solution than plain analog,
in many
> audio applications. �The higher speed AVRs are plenty fast enough
to do
> audio PWM (aka Class D) without difficulty. �You may want to
consider
> driving your output through a half-bridge FET circuit, and using
> polarity switching, to get a true AC audio signal out. �The other
> possibility, if you aren;'t restriced to single chip, is to use an
> external DAC, but that is expensive, and given the 10 bit nature of
your
> sound (assuming you use 10 bit as opposed to 8) youwon't gain much.
>
> tom
>
>
>
>
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